Prepare documentation for Promptkit v0.4.0

README.md

@@ -1,18 +1,50 @@
 # Promptkit
 
-Promptkit is the reusable Go prompt-execution framework being separated from
+Promptkit is a reusable Go library for preparing and executing prompt-defined
-Scriptorium. Its module path is:
+LLM workflows. Its module path is:
 
 ```text
 gitea.maximumdirect.net/eric/promptkit
 ```
 
-The repository currently provides the independent Go module and its root public
+The root `promptkit` package provides the supported public engine. Consumers
-package boundary. Framework behavior and consumer APIs have not yet been
+can configure filesystem or in-memory prompt, profile, and schema sources,
-extracted, so there is no installation or usage example at this time.
+prepare requests without generation, run requests with the built-in
+OpenAI-compatible client, or inject their own model client and artifact reader.
+
+## Quickstart
+
+Run the maintained offline preparation example from the repository root:
+
+```sh
+go run ./examples/go-library/prepare
+```
+
+It loads a repository-local prompt, supplies an in-memory profile and inline
+input, and prints deterministic preparation metadata without contacting a
+provider or requiring credentials. Read the
+[example source](examples/go-library/prepare/main.go), the
+[Go package consumer guide](docs/consumers/pkg-promptkit.md), and the
+[framework format reference](docs/formats.md) to build a consumer workflow.
 
 Contributors should start with the [development guide](docs/development.md).
 The [architecture policy](docs/policy/architecture.md) defines the library
-boundary and constraints that future framework work must preserve.
+boundary and constraints that framework work must preserve.
+
+## Release Guidance
+
+Consumers upgrading from `v0.3.0` to `v0.4.0` should read the
+[v0.4.0 changelog and adoption guide](docs/releases/v0.4.0.md).
+
+Consumers upgrading from `v0.2.0` to `v0.3.0` should read the
+[v0.3.0 changelog](docs/releases/v0.3.0.md).
+
+Consumers moving from `v0.1.0` to `v0.2.0` should read the
+[v0.2.0 changelog and migration guide](docs/releases/v0.2.0.md).
+
+## Related Project
+
+[Scriptorium](https://gitea.maximumdirect.net/eric/scriptorium) is the CLI and
+HTTP application built on Promptkit.
 
 Promptkit is licensed under the [GNU General Public License version 3](LICENSE).

architecture_test.go

@@ -0,0 +1,89 @@
+package promptkit_test
+
+import (
+	"go/ast"
+	"go/parser"
+	"go/token"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"strconv"
+	"strings"
+	"testing"
+)
+
+const formerModulePath = "gitea.maximumdirect.net/eric/" + "scrip" + "torium"
+
+func TestRepositoryDoesNotImportFormerModule(t *testing.T) {
+	violations, err := findFormerModuleImports(".")
+	if err != nil {
+		t.Fatalf("inspect repository imports: %v", err)
+	}
+	if len(violations) > 0 {
+		t.Fatalf("repository imports the former module:\n%s", strings.Join(violations, "\n"))
+	}
+}
+
+func TestFormerModuleGuardFindsNestedImport(t *testing.T) {
+	root := t.TempDir()
+	nested := filepath.Join(root, "nested", "package")
+	if err := os.MkdirAll(nested, 0o755); err != nil {
+		t.Fatalf("create nested package: %v", err)
+	}
+
+	sourcePath := filepath.Join(nested, "violation.go")
+	source := "package nested\n\nimport _ " + strconv.Quote(formerModulePath+"/internal/domain") + "\n"
+	if err := os.WriteFile(sourcePath, []byte(source), 0o600); err != nil {
+		t.Fatalf("write nested source: %v", err)
+	}
+
+	violations, err := findFormerModuleImports(root)
+	if err != nil {
+		t.Fatalf("inspect nested imports: %v", err)
+	}
+	if len(violations) != 1 {
+		t.Fatalf("violations = %v, want one nested import", violations)
+	}
+	if !strings.Contains(violations[0], "violation.go") ||
+		!strings.Contains(violations[0], formerModulePath+"/internal/domain") {
+		t.Fatalf("violation = %q, want file and import path", violations[0])
+	}
+}
+
+func findFormerModuleImports(root string) ([]string, error) {
+	var violations []string
+	err := filepath.WalkDir(root, func(path string, entry fs.DirEntry, walkErr error) error {
+		if walkErr != nil {
+			return walkErr
+		}
+		if entry.IsDir() {
+			switch entry.Name() {
+			case ".git", "generated", "vendor":
+				return filepath.SkipDir
+			}
+			return nil
+		}
+		if filepath.Ext(path) != ".go" {
+			return nil
+		}
+
+		file, err := parser.ParseFile(token.NewFileSet(), path, nil, parser.ImportsOnly|parser.ParseComments)
+		if err != nil {
+			return err
+		}
+		if ast.IsGenerated(file) {
+			return nil
+		}
+		for _, spec := range file.Imports {
+			importPath, err := strconv.Unquote(spec.Path.Value)
+			if err != nil {
+				return err
+			}
+			if importPath == formerModulePath || strings.HasPrefix(importPath, formerModulePath+"/") {
+				violations = append(violations, path+": "+importPath)
+			}
+		}
+		return nil
+	})
+	return violations, err
+}

artifact_reader.go

@@ -0,0 +1,39 @@
+package promptkit
+
+import (
+	"context"
+	"errors"
+
+	artifactadapter "gitea.maximumdirect.net/eric/promptkit/internal/artifact"
+	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
+)
+
+var errNilArtifactReaderResponse = errors.New("artifact reader returned nil artifact without error")
+
+type publicArtifactReaderAdapter struct {
+	reader ArtifactReader
+}
+
+var _ artifactadapter.Reader = publicArtifactReaderAdapter{}
+
+func (a publicArtifactReaderAdapter) Read(ctx context.Context, ref domain.ArtifactRef) (*domain.Artifact, error) {
+	artifact, err := a.reader.Read(ctx, ArtifactRef{
+		Type: ArtifactRefType(ref.Type),
+		URI:  ref.URI,
+		Body: ref.Body,
+	})
+	if err != nil {
+		return nil, err
+	}
+	if artifact == nil {
+		return nil, errNilArtifactReaderResponse
+	}
+	return &domain.Artifact{
+		Name:        artifact.Name,
+		ContentType: artifact.ContentType,
+		Body:        copyBytes(artifact.Body),
+		URI:         artifact.URI,
+		Size:        artifact.Size,
+		Hash:        artifact.Hash,
+	}, nil
+}

artifact_reader_internal_test.go

@@ -0,0 +1,36 @@
+package promptkit
+
+import (
+	"context"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
+)
+
+func TestPublicArtifactReaderAdapterCopiesBody(t *testing.T) {
+	reader := internalArtifactReaderFake{
+		artifact: &Artifact{Body: []byte("original")},
+	}
+	adapter := publicArtifactReaderAdapter{reader: &reader}
+
+	artifact, err := adapter.Read(context.Background(), domain.ArtifactRef{
+		Type: domain.ArtifactRefInline,
+		URI:  "memory://input",
+		Body: "input",
+	})
+	if err != nil {
+		t.Fatalf("read artifact: %v", err)
+	}
+	artifact.Body[0] = 'X'
+	if got := string(reader.artifact.Body); got != "original" {
+		t.Fatalf("reader artifact body was mutated: %q", got)
+	}
+}
+
+type internalArtifactReaderFake struct {
+	artifact *Artifact
+}
+
+func (r *internalArtifactReaderFake) Read(context.Context, ArtifactRef) (*Artifact, error) {
+	return r.artifact, nil
+}

backends.go

@@ -0,0 +1,99 @@
+package promptkit
+
+import (
+	"gitea.maximumdirect.net/eric/promptkit/internal/backend"
+	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
+)
+
+// BackendOpenRouter is the reserved ID of Promptkit's built-in OpenRouter
+// backend.
+const BackendOpenRouter = backend.OpenRouterID
+
+// BackendLocal is the case-sensitive conventional ID used by [LocalBackend].
+// It is not a built-in or reserved backend and must be registered with
+// [WithBackend].
+const BackendLocal = "local"
+
+// Backend configures one engine-scoped OpenAI-compatible backend.
+//
+// Backend has no stable JSON representation. Use keyed literals so additions
+// to this configuration value do not break source compatibility.
+type Backend struct {
+	// ID is the stable, case-sensitive registry key. NewEngine trims it and
+	// requires a non-blank value. BackendOpenRouter is reserved.
+	ID string
+	// Endpoint is the OpenAI-compatible base endpoint. NewEngine trims it and
+	// requires an absolute HTTP or HTTPS URL with a host and without user
+	// information, a query string, or a fragment. Paths are allowed.
+	Endpoint string
+	// APIKeyEnv optionally names the environment variable containing the API
+	// key. NewEngine trims it and requires the portable form
+	// [A-Za-z_][A-Za-z0-9_]*. Store only the name, never a credential value.
+	APIKeyEnv string
+	// ExtraParams contains backend-wide request defaults. Values must be
+	// JSON-compatible, finite, acyclic, and keyed by non-empty strings. Keys
+	// must not be model, session_id, messages, temperature, max_tokens, top_p,
+	// service_tier, reasoning_effort, or response_format. An empty map supplies
+	// no defaults. NewEngine deeply copies the map.
+	ExtraParams map[string]any
+	// ConcurrencyLimit is the maximum number of simultaneous model-generation
+	// calls allowed for this backend within one Engine. Zero leaves the backend
+	// unlimited. A negative value makes NewEngine fail with ErrInvalidConfig.
+	ConcurrencyLimit int
+	// QueueCapacity controls how many additional Run or RunPrepared calls may
+	// be admitted beyond ConcurrencyLimit. Nil uses 1024 when ConcurrencyLimit
+	// is positive; a pointer uses its exact value, including zero. The pointed-to
+	// value must be non-negative, and QueueCapacity must be nil when
+	// ConcurrencyLimit is zero. Their sum must fit in an int. WithBackend copies
+	// the value and does not retain the pointer.
+	QueueCapacity *int
+}
+
+// LocalBackend returns a caller-owned Backend for a conventional local
+// OpenAI-compatible endpoint. It sets ID to BackendLocal and copies endpoint
+// and concurrencyLimit into Endpoint and ConcurrencyLimit without
+// normalization or validation. APIKeyEnv, ExtraParams, and QueueCapacity keep
+// their zero values.
+//
+// LocalBackend does not read environment variables, register the value, or
+// mutate engine or package state. Supply the returned value through
+// [WithBackend]; [NewEngine] then applies the ordinary backend validation and
+// concurrency semantics, including default queue capacity for a positive
+// limit, unlimited behavior for zero, and ErrInvalidConfig for a negative
+// limit.
+func LocalBackend(endpoint string, concurrencyLimit int) Backend {
+	return Backend{
+		ID:               BackendLocal,
+		Endpoint:         endpoint,
+		ConcurrencyLimit: concurrencyLimit,
+	}
+}
+
+// WithBackend adds one Backend registration to the constructed Engine.
+//
+// Registrations accumulate in option order. Every normalized ID must be unique
+// across consumer registrations and built-ins; a duplicate or invalid
+// definition makes NewEngine fail with ErrInvalidConfig. In particular,
+// BackendOpenRouter cannot be replaced. The immutable registration is scoped
+// to the resulting Engine and cannot be enumerated, replaced, removed, or
+// mutated after construction. WithBackend does not install package-global
+// state.
+func WithBackend(backend Backend) Option {
+	queueCapacity := 0
+	queueCapacitySet := backend.QueueCapacity != nil
+	if queueCapacitySet {
+		queueCapacity = *backend.QueueCapacity
+	}
+	return optionFunc(func(options *engineOptions) error {
+		options.backends = append(options.backends, domain.Backend{
+			ID:               backend.ID,
+			Endpoint:         backend.Endpoint,
+			APIKeyEnv:        backend.APIKeyEnv,
+			ExtraParams:      backend.ExtraParams,
+			ConcurrencyLimit: backend.ConcurrencyLimit,
+			QueueCapacity:    queueCapacity,
+			QueueCapacitySet: queueCapacitySet,
+		})
+		return nil
+	})
+}

capacity_contract_test.go

@@ -0,0 +1,409 @@
+package promptkit_test
+
+import (
+	"context"
+	"errors"
+	"sync"
+	"testing"
+	"time"
+
+	"gitea.maximumdirect.net/eric/promptkit"
+)
+
+func TestEngineLimitsInjectedClientConcurrency(t *testing.T) {
+	release := make(chan struct{})
+	client := newCapacityGateClient(release, 8)
+	engine := newBackendCapacityEngine(t, client, 2, capacityInt(4), nil)
+
+	results := make(chan capacityRunResult, 6)
+	for i := 0; i < 6; i++ {
+		go runCapacityRequest(engine, context.Background(), promptkit.RunRequest{
+			PromptID: "prompt",
+			Execution: &promptkit.ExecutionTargetOverride{
+				Endpoint: "http://request.example/v1",
+			},
+		}, results)
+	}
+
+	first := awaitCapacityRequest(t, client.started)
+	second := awaitCapacityRequest(t, client.started)
+	if first.Target.BackendID != "limited" || second.Target.BackendID != "limited" {
+		t.Fatalf("endpoint override changed backend pool: first=%q second=%q",
+			first.Target.BackendID, second.Target.BackendID)
+	}
+	if active, peak, _ := client.snapshot(); active != 2 || peak != 2 {
+		t.Fatalf("client concurrency before release=(active=%d peak=%d), want 2", active, peak)
+	}
+
+	close(release)
+	for i := 0; i < 6; i++ {
+		outcome := awaitCapacityRun(t, results)
+		if outcome.err != nil || outcome.result == nil {
+			t.Fatalf("run outcome=(%+v, %v), want success", outcome.result, outcome.err)
+		}
+	}
+	if _, peak, calls := client.snapshot(); peak > 2 || calls != 6 {
+		t.Fatalf("client observations=(peak=%d calls=%d), want peak <= 2 and 6 calls", peak, calls)
+	}
+}
+
+func TestEngineRejectsRunBeforeCompletionWhenAdmissionIsFull(t *testing.T) {
+	artifactRelease := make(chan struct{})
+	reader := &capacityArtifactReader{
+		entered: make(chan struct{}, 2),
+		release: artifactRelease,
+	}
+	client := newCapacityGateClient(closedCapacityChannel(), 2)
+	engine := newBackendCapacityEngine(t, client, 1, capacityInt(0), reader)
+	firstResult := make(chan capacityRunResult, 1)
+	go runCapacityRequest(engine, context.Background(), capacityInputRequest("http://first.example/v1"), firstResult)
+
+	awaitCapacitySignal(t, reader.entered, "first artifact read")
+
+	canceledContext, cancel := context.WithCancel(context.Background())
+	cancel()
+	result, err := engine.Run(canceledContext, capacityInputRequest("http://canceled.example/v1"))
+	if result != nil || !errors.Is(err, context.Canceled) {
+		t.Fatalf("canceled capacity admission=(%+v, %v), want context cancellation", result, err)
+	}
+	var canceledCapacityErr *promptkit.CapacityError
+	if errors.Is(err, promptkit.ErrCapacityExceeded) || errors.As(err, &canceledCapacityErr) {
+		t.Fatalf("canceled admission exposed capacity rejection: %v", err)
+	}
+
+	result, err = engine.Run(context.Background(), capacityInputRequest("http://second.example/v1"))
+	if result != nil {
+		t.Fatalf("capacity rejection returned partial result: %+v", result)
+	}
+	if !errors.Is(err, promptkit.ErrCapacityExceeded) {
+		t.Fatalf("capacity rejection=%v, want ErrCapacityExceeded", err)
+	}
+	if errors.Is(err, promptkit.ErrInvalidRequest) || errors.Is(err, promptkit.ErrLLMGenerate) {
+		t.Fatalf("capacity rejection had an unrelated category: %v", err)
+	}
+	var capacityErr *promptkit.CapacityError
+	if !errors.As(err, &capacityErr) || capacityErr == nil {
+		t.Fatalf("capacity rejection=%v, want CapacityError", err)
+	}
+	if capacityErr.BackendID != "limited" {
+		t.Fatalf("capacity backend ID=%q, want limited", capacityErr.BackendID)
+	}
+	capacityErr.BackendID = "changed"
+
+	result, err = engine.Run(context.Background(), capacityInputRequest("http://third.example/v1"))
+	var subsequentCapacityErr *promptkit.CapacityError
+	if result != nil || !errors.As(err, &subsequentCapacityErr) ||
+		subsequentCapacityErr == nil || subsequentCapacityErr.BackendID != "limited" {
+		t.Fatalf("subsequent capacity rejection=(%+v, %v), want independent limited CapacityError", result, err)
+	}
+	if calls := reader.callCount(); calls != 1 {
+		t.Fatalf("artifact calls=%d, want only the admitted run", calls)
+	}
+	if _, _, calls := client.snapshot(); calls != 0 {
+		t.Fatalf("client calls=%d before admitted run was released, want 0", calls)
+	}
+
+	close(artifactRelease)
+	outcome := awaitCapacityRun(t, firstResult)
+	if outcome.err != nil || outcome.result == nil {
+		t.Fatalf("first run outcome=(%+v, %v), want success", outcome.result, outcome.err)
+	}
+}
+
+func TestBackendCapacityIsIndependentBetweenEngines(t *testing.T) {
+	firstRelease := make(chan struct{})
+	firstClient := newCapacityGateClient(firstRelease, 1)
+	firstEngine := newBackendCapacityEngine(t, firstClient, 1, capacityInt(0), nil)
+	secondClient := newCapacityGateClient(closedCapacityChannel(), 1)
+	secondEngine := newBackendCapacityEngine(t, secondClient, 1, capacityInt(0), nil)
+
+	firstResult := make(chan capacityRunResult, 1)
+	go runCapacityRequest(firstEngine, context.Background(), promptkit.RunRequest{PromptID: "prompt"}, firstResult)
+	awaitCapacityRequest(t, firstClient.started)
+
+	result, err := secondEngine.Run(context.Background(), promptkit.RunRequest{PromptID: "prompt"})
+	if err != nil || result == nil {
+		t.Fatalf("second engine run=(%+v, %v), want independent success", result, err)
+	}
+	if _, _, calls := secondClient.snapshot(); calls != 1 {
+		t.Fatalf("second engine client calls=%d, want 1", calls)
+	}
+
+	close(firstRelease)
+	outcome := awaitCapacityRun(t, firstResult)
+	if outcome.err != nil || outcome.result == nil {
+		t.Fatalf("first engine run=(%+v, %v), want success", outcome.result, outcome.err)
+	}
+}
+
+func TestUnlimitedBackendsRetainInjectedClientConcurrency(t *testing.T) {
+	tests := []struct {
+		name      string
+		configure func(*testing.T, promptkit.LLMClient) *promptkit.Engine
+	}{
+		{
+			name: "custom backend",
+			configure: func(t *testing.T, client promptkit.LLMClient) *promptkit.Engine {
+				return newBackendCapacityEngine(t, client, 0, nil, nil)
+			},
+		},
+		{
+			name: "endpoint-only profile",
+			configure: func(t *testing.T, client promptkit.LLMClient) *promptkit.Engine {
+				engine, err := promptkit.NewEngine(promptkit.Config{},
+					promptkit.WithPromptFS(contractPromptFS("prompt", "profile", "message"), "."),
+					promptkit.WithProfiles(promptkit.Profile{
+						ID: "profile", Endpoint: "http://endpoint.example/v1", Model: "model",
+					}),
+					promptkit.WithLLMClient(client),
+				)
+				if err != nil {
+					t.Fatalf("construct endpoint-only engine: %v", err)
+				}
+				return engine
+			},
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			release := make(chan struct{})
+			client := newCapacityGateClient(release, 2)
+			engine := tc.configure(t, client)
+			results := make(chan capacityRunResult, 2)
+			for i := 0; i < 2; i++ {
+				go runCapacityRequest(
+					engine,
+					context.Background(),
+					promptkit.RunRequest{PromptID: "prompt"},
+					results,
+				)
+			}
+
+			awaitCapacityRequest(t, client.started)
+			awaitCapacityRequest(t, client.started)
+			if active, peak, _ := client.snapshot(); active != 2 || peak != 2 {
+				t.Fatalf("unlimited concurrency=(active=%d peak=%d), want 2", active, peak)
+			}
+			close(release)
+			for i := 0; i < 2; i++ {
+				outcome := awaitCapacityRun(t, results)
+				if outcome.err != nil || outcome.result == nil {
+					t.Fatalf("run outcome=(%+v, %v), want success", outcome.result, outcome.err)
+				}
+			}
+		})
+	}
+}
+
+func TestCapacityExceededSentinelContract(t *testing.T) {
+	if promptkit.ErrCapacityExceeded == nil {
+		t.Fatal("ErrCapacityExceeded is nil")
+	}
+	var nilCapacityErr *promptkit.CapacityError
+	zeroCapacityErr := &promptkit.CapacityError{}
+	populatedCapacityErr := &promptkit.CapacityError{BackendID: "limited"}
+	for _, capacityErr := range []error{nilCapacityErr, zeroCapacityErr} {
+		if !errors.Is(capacityErr, promptkit.ErrCapacityExceeded) {
+			t.Fatalf("capacity error=%v, want ErrCapacityExceeded", capacityErr)
+		}
+	}
+	var discoveredCapacityErr *promptkit.CapacityError
+	if !errors.As(populatedCapacityErr, &discoveredCapacityErr) || discoveredCapacityErr != populatedCapacityErr {
+		t.Fatalf("populated capacity error is not discoverable: %v", populatedCapacityErr)
+	}
+
+	for _, unrelated := range []error{
+		promptkit.ErrInvalidConfig,
+		promptkit.ErrInvalidRequest,
+		promptkit.ErrLLMGenerate,
+		promptkit.ErrValidation,
+	} {
+		if errors.Is(promptkit.ErrCapacityExceeded, unrelated) ||
+			errors.Is(unrelated, promptkit.ErrCapacityExceeded) ||
+			errors.Is(populatedCapacityErr, unrelated) {
+			t.Fatalf("ErrCapacityExceeded aliases unrelated sentinel %v", unrelated)
+		}
+	}
+}
+
+type capacityRunResult struct {
+	result *promptkit.RunResult
+	err    error
+}
+
+func runCapacityRequest(
+	engine *promptkit.Engine,
+	ctx context.Context,
+	request promptkit.RunRequest,
+	results chan<- capacityRunResult,
+) {
+	result, err := engine.Run(ctx, request)
+	results <- capacityRunResult{result: result, err: err}
+}
+
+func newBackendCapacityEngine(
+	t *testing.T,
+	client promptkit.LLMClient,
+	limit int,
+	queueCapacity *int,
+	reader promptkit.ArtifactReader,
+) *promptkit.Engine {
+	t.Helper()
+	promptFS := contractPromptFS("prompt", "profile", "message")
+	if reader != nil {
+		promptFS = contractInputPromptFS()
+	}
+	options := []promptkit.Option{
+		promptkit.WithPromptFS(promptFS, "."),
+		promptkit.WithBackend(promptkit.Backend{
+			ID:               "limited",
+			Endpoint:         "http://backend.example/v1",
+			ConcurrencyLimit: limit,
+			QueueCapacity:    queueCapacity,
+		}),
+		promptkit.WithProfiles(promptkit.Profile{
+			ID: "profile", BackendID: "limited", Model: "model",
+		}),
+		promptkit.WithLLMClient(client),
+	}
+	if reader != nil {
+		options = append(options, promptkit.WithArtifactReader(reader))
+	}
+	engine, err := promptkit.NewEngine(promptkit.Config{}, options...)
+	if err != nil {
+		t.Fatalf("construct capacity engine: %v", err)
+	}
+	return engine
+}
+
+func capacityInputRequest(endpoint string) promptkit.RunRequest {
+	return promptkit.RunRequest{
+		PromptID: "input-prompt",
+		Inputs: map[string]promptkit.ArtifactRef{
+			"input": promptkit.Inline("input"),
+		},
+		Execution: &promptkit.ExecutionTargetOverride{Endpoint: endpoint},
+	}
+}
+
+type capacityGateClient struct {
+	mu      sync.Mutex
+	active  int
+	peak    int
+	calls   int
+	started chan promptkit.GenerateRequest
+	release <-chan struct{}
+}
+
+func newCapacityGateClient(release <-chan struct{}, buffer int) *capacityGateClient {
+	return &capacityGateClient{
+		started: make(chan promptkit.GenerateRequest, buffer),
+		release: release,
+	}
+}
+
+func (c *capacityGateClient) Generate(
+	ctx context.Context,
+	request promptkit.GenerateRequest,
+) (*promptkit.GenerateResponse, error) {
+	c.mu.Lock()
+	c.calls++
+	c.active++
+	if c.active > c.peak {
+		c.peak = c.active
+	}
+	c.mu.Unlock()
+	defer func() {
+		c.mu.Lock()
+		c.active--
+		c.mu.Unlock()
+	}()
+
+	c.started <- request
+	select {
+	case <-c.release:
+		return &promptkit.GenerateResponse{Content: "ok"}, nil
+	case <-ctx.Done():
+		return nil, ctx.Err()
+	}
+}
+
+func (c *capacityGateClient) snapshot() (active, peak, calls int) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	return c.active, c.peak, c.calls
+}
+
+type capacityArtifactReader struct {
+	mu      sync.Mutex
+	calls   int
+	entered chan struct{}
+	release <-chan struct{}
+}
+
+func (r *capacityArtifactReader) Read(
+	ctx context.Context,
+	_ promptkit.ArtifactRef,
+) (*promptkit.Artifact, error) {
+	r.mu.Lock()
+	r.calls++
+	r.mu.Unlock()
+	r.entered <- struct{}{}
+	select {
+	case <-r.release:
+		return &promptkit.Artifact{Body: []byte("input")}, nil
+	case <-ctx.Done():
+		return nil, ctx.Err()
+	}
+}
+
+func (r *capacityArtifactReader) callCount() int {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	return r.calls
+}
+
+func awaitCapacityRequest(
+	t *testing.T,
+	requests <-chan promptkit.GenerateRequest,
+) promptkit.GenerateRequest {
+	t.Helper()
+	select {
+	case request := <-requests:
+		return request
+	case <-time.After(5 * time.Second):
+		t.Fatal("timed out waiting for client invocation")
+		return promptkit.GenerateRequest{}
+	}
+}
+
+func awaitCapacityRun(t *testing.T, results <-chan capacityRunResult) capacityRunResult {
+	t.Helper()
+	select {
+	case result := <-results:
+		return result
+	case <-time.After(5 * time.Second):
+		t.Fatal("timed out waiting for Run")
+		return capacityRunResult{}
+	}
+}
+
+func awaitCapacitySignal(t *testing.T, signal <-chan struct{}, name string) {
+	t.Helper()
+	select {
+	case <-signal:
+	case <-time.After(5 * time.Second):
+		t.Fatalf("timed out waiting for %s", name)
+	}
+}
+
+func capacityInt(value int) *int {
+	return &value
+}
+
+func closedCapacityChannel() <-chan struct{} {
+	channel := make(chan struct{})
+	close(channel)
+	return channel
+}

capacity_error.go

@@ -0,0 +1,40 @@
+package promptkit
+
+import (
+	"fmt"
+	"strings"
+)
+
+// CapacityError reports bounded admission rejected for a selected backend.
+//
+// Engine-produced values identify only rejection at Promptkit's bounded
+// [Engine.Run] or [Engine.RunPrepared] admission boundary. BackendID is the
+// normalized registered backend ID used for routing and capacity; endpoint
+// overrides do not change it. Every engine-produced value is nonnil and has a
+// nonblank BackendID. Provider errors, active-generation waiting, and caller
+// cancellation are not represented by this type.
+//
+// Callers own returned values and may mutate BackendID without affecting engine
+// state or another error. CapacityError and its default Go encoding have no
+// stable JSON contract. Consumer-constructed values do not establish that an
+// engine rejected work.
+type CapacityError struct {
+	// BackendID is the normalized registered backend ID whose admission was
+	// rejected.
+	BackendID string
+}
+
+// Error returns diagnostic wording that is not a parsing contract. It is safe
+// to call on a nil receiver or a value with a blank BackendID.
+func (e *CapacityError) Error() string {
+	if e == nil || strings.TrimSpace(e.BackendID) == "" {
+		return ErrCapacityExceeded.Error()
+	}
+	return fmt.Sprintf("backend %q admission: %v", e.BackendID, ErrCapacityExceeded)
+}
+
+// Unwrap returns ErrCapacityExceeded so errors.Is and errors.As can be used
+// together. It is safe to call on a nil receiver or a zero value.
+func (e *CapacityError) Unwrap() error {
+	return ErrCapacityExceeded
+}

convert.go

@@ -0,0 +1,453 @@
+package promptkit
+
+import (
+	"reflect"
+
+	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
+	"gitea.maximumdirect.net/eric/promptkit/internal/jsonvalue"
+)
+
+func toDomainRunRequest(req RunRequest) (domain.RunRequest, error) {
+	execution, err := toDomainExecutionTargetOverride(req.Execution)
+	if err != nil {
+		return domain.RunRequest{}, err
+	}
+	return domain.RunRequest{
+		PromptID:      req.PromptID,
+		PromptVersion: req.PromptVersion,
+		ProfileID:     req.ProfileID,
+		SessionID:     req.SessionID,
+		APIKey:        req.APIKey,
+		Inputs:        toDomainArtifactRefMap(req.Inputs),
+		Vars:          copyStringMap(req.Vars),
+		Execution:     execution,
+		Validation:    toDomainOutputContractPtr(req.Validation),
+	}, nil
+}
+
+func fromDomainPreparedRun(prepared *domain.PreparedRun) *PreparedRun {
+	if prepared == nil {
+		return nil
+	}
+	return &PreparedRun{
+		PromptID:             prepared.PromptID,
+		PromptVersion:        prepared.PromptVersion,
+		PromptHash:           prepared.PromptHash,
+		SelectedProfileID:    prepared.SelectedProfileID,
+		SelectedBackendID:    prepared.SelectedBackendID,
+		EffectiveModelParams: fromDomainExecutionTarget(prepared.EffectiveModelParams),
+		OutputContract:       fromDomainOutputContract(prepared.OutputContract),
+		StructuredOutput:     fromDomainStructuredOutputSpec(prepared.StructuredOutput),
+		InputHashes:          copyStringMap(prepared.InputHashes),
+		SessionID:            prepared.SessionID,
+		RenderedPromptHash:   prepared.RenderedPromptHash,
+		Messages:             fromDomainRenderedMessages(prepared.Messages),
+		StartTime:            prepared.StartTime,
+		EndTime:              prepared.EndTime,
+		DurationMS:           prepared.DurationMS,
+	}
+}
+
+func fromDomainRunResult(result *domain.RunResult) *RunResult {
+	if result == nil {
+		return nil
+	}
+	return &RunResult{
+		RunID:                result.RunID,
+		Artifact:             fromDomainArtifact(result.Artifact),
+		RawOutput:            result.RawOutput,
+		Validation:           fromDomainValidationResult(result.Validation),
+		PromptID:             result.PromptID,
+		PromptVersion:        result.PromptVersion,
+		PromptHash:           result.PromptHash,
+		SessionID:            result.SessionID,
+		RenderedPromptHash:   result.RenderedPromptHash,
+		SelectedProfileID:    result.SelectedProfileID,
+		SelectedBackendID:    result.SelectedBackendID,
+		ModelName:            result.ModelName,
+		Endpoint:             result.Endpoint,
+		EffectiveModelParams: fromDomainExecutionTarget(result.EffectiveModelParams),
+		InputHashes:          copyStringMap(result.InputHashes),
+		Usage:                fromDomainTokenUsage(result.Usage),
+		StartTime:            result.StartTime,
+		EndTime:              result.EndTime,
+		Duration:             result.Duration,
+	}
+}
+
+func fromDomainGenerateRequest(req domain.GenerateRequest) GenerateRequest {
+	return GenerateRequest{
+		Prompt:           fromDomainRenderedPrompt(req.Prompt),
+		Target:           fromDomainExecutionTarget(req.Target),
+		TargetPresence:   fromDomainExecutionTargetPresence(req.TargetPresence),
+		StructuredOutput: fromDomainStructuredOutputSpec(req.StructuredOutput),
+		APIKey:           req.Target.APIKey,
+	}
+}
+
+func toDomainGenerateResponse(resp *GenerateResponse) *domain.GenerateResponse {
+	if resp == nil {
+		return nil
+	}
+	return &domain.GenerateResponse{
+		Content: resp.Content,
+		Usage:   toDomainTokenUsage(resp.Usage),
+	}
+}
+
+func fromDomainRenderedPrompt(prompt domain.RenderedPrompt) RenderedPrompt {
+	return RenderedPrompt{
+		SessionID: prompt.SessionID,
+		Messages:  fromDomainRenderedMessages(prompt.Messages),
+	}
+}
+
+func toDomainArtifactRefMap(src map[string]ArtifactRef) map[string]domain.ArtifactRef {
+	if src == nil {
+		return nil
+	}
+	out := make(map[string]domain.ArtifactRef, len(src))
+	for k, v := range src {
+		out[k] = toDomainArtifactRef(v)
+	}
+	return out
+}
+
+func toDomainArtifactRef(ref ArtifactRef) domain.ArtifactRef {
+	return domain.ArtifactRef{
+		Type: domain.ArtifactRefType(ref.Type),
+		URI:  ref.URI,
+		Body: ref.Body,
+	}
+}
+
+func fromDomainArtifact(artifact domain.Artifact) Artifact {
+	return Artifact{
+		Name:        artifact.Name,
+		ContentType: artifact.ContentType,
+		Body:        copyBytes(artifact.Body),
+		URI:         artifact.URI,
+		Size:        artifact.Size,
+		Hash:        artifact.Hash,
+	}
+}
+
+func toDomainExecutionTargetOverride(override *ExecutionTargetOverride) (*domain.ExecutionTargetOverride, error) {
+	if override == nil {
+		return nil, nil
+	}
+	extraParams, err := jsonvalue.CopyMap(override.ExtraParams)
+	if err != nil {
+		return nil, err
+	}
+	return &domain.ExecutionTargetOverride{
+		Endpoint:        override.Endpoint,
+		Model:           override.Model,
+		Temperature:     copyFloat64Ptr(override.Temperature),
+		MaxTokens:       copyIntPtr(override.MaxTokens),
+		TopP:            copyFloat64Ptr(override.TopP),
+		TimeoutSeconds:  copyIntPtr(override.TimeoutSeconds),
+		ServiceTier:     override.ServiceTier,
+		ReasoningEffort: copyStringPtr(override.ReasoningEffort),
+		APIKeyEnv:       override.APIKeyEnv,
+		ExtraParams:     extraParams,
+	}, nil
+}
+
+func fromDomainExecutionTarget(target domain.ExecutionTarget) ExecutionTarget {
+	return ExecutionTarget{
+		BackendID:       target.BackendID,
+		Endpoint:        target.Endpoint,
+		Model:           target.Model,
+		Temperature:     target.Temperature,
+		MaxTokens:       target.MaxTokens,
+		TopP:            target.TopP,
+		TimeoutSeconds:  target.TimeoutSeconds,
+		ServiceTier:     target.ServiceTier,
+		ReasoningEffort: target.ReasoningEffort,
+		APIKeyEnv:       target.APIKeyEnv,
+		ExtraParams:     copyAnyMap(target.ExtraParams),
+	}
+}
+
+func fromDomainProfileInspection(inspection *domain.ProfileInspection) *ProfileInspection {
+	if inspection == nil {
+		return nil
+	}
+	return &ProfileInspection{
+		ProfileID:            inspection.ProfileID,
+		EffectiveModelParams: fromDomainExecutionTarget(inspection.EffectiveModelParams),
+		APIKeyRequired:       inspection.APIKeyRequired,
+	}
+}
+
+func fromDomainPromptInspection(inspection *domain.PromptInspection) *PromptInspection {
+	if inspection == nil {
+		return nil
+	}
+	inputs := make([]PromptInputDefinition, len(inspection.Inputs))
+	for i, input := range inspection.Inputs {
+		inputs[i] = PromptInputDefinition{
+			Name:        input.Name,
+			Required:    input.Required,
+			ContentType: input.ContentType,
+			Description: input.Description,
+		}
+	}
+	return &PromptInspection{
+		PromptID:         inspection.PromptID,
+		PromptVersion:    inspection.PromptVersion,
+		PromptHash:       inspection.PromptHash,
+		DefaultProfileID: inspection.DefaultProfileID,
+		Inputs:           inputs,
+		OutputContract:   fromDomainOutputContract(inspection.OutputContract),
+	}
+}
+
+func fromDomainExecutionTargetPresence(presence domain.ExecutionTargetPresence) ExecutionTargetPresence {
+	return ExecutionTargetPresence{
+		Temperature:    presence.Temperature,
+		MaxTokens:      presence.MaxTokens,
+		TopP:           presence.TopP,
+		TimeoutSeconds: presence.TimeoutSeconds,
+	}
+}
+
+func toDomainOutputContractPtr(contract *OutputContract) *domain.OutputContract {
+	if contract == nil {
+		return nil
+	}
+	out := toDomainOutputContract(*contract)
+	return &out
+}
+
+func toDomainOutputContract(contract OutputContract) domain.OutputContract {
+	return domain.OutputContract{
+		Format:         domain.OutputFormat(contract.Format),
+		ValidationMode: domain.ValidationMode(contract.ValidationMode),
+		SchemaPath:     contract.SchemaPath,
+		RepairAttempts: contract.RepairAttempts,
+	}
+}
+
+func fromDomainOutputContract(contract domain.OutputContract) OutputContract {
+	return OutputContract{
+		Format:         OutputFormat(contract.Format),
+		ValidationMode: ValidationMode(contract.ValidationMode),
+		SchemaPath:     contract.SchemaPath,
+		RepairAttempts: contract.RepairAttempts,
+	}
+}
+
+func fromDomainValidationResult(result domain.ValidationResult) ValidationResult {
+	return ValidationResult{
+		Status:         ValidationStatus(result.Status),
+		Mode:           ValidationMode(result.Mode),
+		Errors:         copyStringSlice(result.Errors),
+		SchemaPath:     result.SchemaPath,
+		RepairAttempts: result.RepairAttempts,
+		IsValid:        result.IsValid,
+	}
+}
+
+func fromDomainTokenUsage(usage domain.TokenUsage) TokenUsage {
+	return TokenUsage{
+		PromptTokens:     usage.PromptTokens,
+		CompletionTokens: usage.CompletionTokens,
+		TotalTokens:      usage.TotalTokens,
+		CachedTokens:     usage.CachedTokens,
+		CacheWriteTokens: usage.CacheWriteTokens,
+	}
+}
+
+func toDomainTokenUsage(usage TokenUsage) domain.TokenUsage {
+	return domain.TokenUsage{
+		PromptTokens:     usage.PromptTokens,
+		CompletionTokens: usage.CompletionTokens,
+		TotalTokens:      usage.TotalTokens,
+		CachedTokens:     usage.CachedTokens,
+		CacheWriteTokens: usage.CacheWriteTokens,
+	}
+}
+
+func fromDomainRenderedMessages(messages []domain.RenderedMessage) []RenderedMessage {
+	if messages == nil {
+		return nil
+	}
+	out := make([]RenderedMessage, len(messages))
+	for i, msg := range messages {
+		out[i] = RenderedMessage{
+			Role:         msg.Role,
+			Content:      msg.Content,
+			CacheControl: fromDomainCacheControl(msg.CacheControl),
+		}
+	}
+	return out
+}
+
+func fromDomainCacheControl(cacheControl *domain.CacheControl) *CacheControl {
+	if cacheControl == nil {
+		return nil
+	}
+	return &CacheControl{
+		Type: CacheControlType(cacheControl.Type),
+		TTL:  cacheControl.TTL,
+	}
+}
+
+func fromDomainStructuredOutputSpec(spec *domain.StructuredOutputSpec) *StructuredOutputSpec {
+	if spec == nil {
+		return nil
+	}
+	out := &StructuredOutputSpec{
+		Type: StructuredOutputType(spec.Type),
+	}
+	if spec.JSONSchema != nil {
+		out.JSONSchema = &StructuredOutputJSONSpec{
+			Name:   spec.JSONSchema.Name,
+			Strict: spec.JSONSchema.Strict,
+			Schema: copyAny(spec.JSONSchema.Schema),
+		}
+	}
+	return out
+}
+
+func copyStringMap(src map[string]string) map[string]string {
+	if src == nil {
+		return nil
+	}
+	out := make(map[string]string, len(src))
+	for k, v := range src {
+		out[k] = v
+	}
+	return out
+}
+
+func copyAnyMap(src map[string]any) map[string]any {
+	if src == nil {
+		return nil
+	}
+	out := make(map[string]any, len(src))
+	for k, v := range src {
+		out[k] = copyAny(v)
+	}
+	return out
+}
+
+func copyAny(value any) any {
+	if value == nil {
+		return nil
+	}
+	switch v := value.(type) {
+	case map[string]any:
+		return copyAnyMap(v)
+	case []any:
+		out := make([]any, len(v))
+		for i, item := range v {
+			out[i] = copyAny(item)
+		}
+		return out
+	case []string:
+		return copyStringSlice(v)
+	case []byte:
+		return copyBytes(v)
+	default:
+		return copyReflectValue(reflect.ValueOf(value)).Interface()
+	}
+}
+
+func copyReflectValue(value reflect.Value) reflect.Value {
+	if !value.IsValid() {
+		return value
+	}
+
+	switch value.Kind() {
+	case reflect.Interface:
+		if value.IsNil() {
+			return reflect.Zero(value.Type())
+		}
+		copied := copyReflectValue(value.Elem())
+		if copied.IsValid() && copied.Type().AssignableTo(value.Type()) {
+			return copied
+		}
+		out := reflect.New(value.Type()).Elem()
+		out.Set(copied)
+		return out
+	case reflect.Pointer:
+		if value.IsNil() {
+			return reflect.Zero(value.Type())
+		}
+		out := reflect.New(value.Type().Elem())
+		out.Elem().Set(copyReflectValue(value.Elem()))
+		return out
+	case reflect.Map:
+		if value.IsNil() {
+			return reflect.Zero(value.Type())
+		}
+		out := reflect.MakeMapWithSize(value.Type(), value.Len())
+		iter := value.MapRange()
+		for iter.Next() {
+			out.SetMapIndex(copyReflectValue(iter.Key()), copyReflectValue(iter.Value()))
+		}
+		return out
+	case reflect.Slice:
+		if value.IsNil() {
+			return reflect.Zero(value.Type())
+		}
+		out := reflect.MakeSlice(value.Type(), value.Len(), value.Cap())
+		for i := 0; i < value.Len(); i++ {
+			out.Index(i).Set(copyReflectValue(value.Index(i)))
+		}
+		return out
+	case reflect.Array:
+		out := reflect.New(value.Type()).Elem()
+		for i := 0; i < value.Len(); i++ {
+			out.Index(i).Set(copyReflectValue(value.Index(i)))
+		}
+		return out
+	default:
+		return value
+	}
+}
+
+func copyStringSlice(src []string) []string {
+	if src == nil {
+		return nil
+	}
+	out := make([]string, len(src))
+	copy(out, src)
+	return out
+}
+
+func copyBytes(src []byte) []byte {
+	if src == nil {
+		return nil
+	}
+	out := make([]byte, len(src))
+	copy(out, src)
+	return out
+}
+
+func copyFloat64Ptr(src *float64) *float64 {
+	if src == nil {
+		return nil
+	}
+	v := *src
+	return &v
+}
+
+func copyStringPtr(src *string) *string {
+	if src == nil {
+		return nil
+	}
+	v := *src
+	return &v
+}
+
+func copyIntPtr(src *int) *int {
+	if src == nil {
+		return nil
+	}
+	v := *src
+	return &v
+}

doc.go

@@ -1,2 +1,64 @@
-// Package promptkit defines the public package boundary for the Promptkit Go module.
+// Package promptkit provides an embeddable engine for preparing and executing
+// prompt-defined LLM workflows.
+//
+// Applications construct an [Engine] with [NewEngine], select filesystem or
+// in-memory sources and optional engine-scoped [Backend] registrations, and
+// call [Engine.InspectPrompt], [Engine.InspectProfile], [Engine.Prepare],
+// [Engine.PrepareExecution], [Engine.Run], or [Engine.RunPrepared]. Concrete
+// registries, repositories, validators, and the built-in OpenAI-compatible
+// client remain internal implementation details.
+//
+// # Concurrency and ownership
+//
+// An Engine supports concurrent InspectPrompt, InspectProfile, Prepare,
+// PrepareExecution, Run, and RunPrepared calls. Engine-local backend policies
+// bound admitted Run and RunPrepared calls and model generations where
+// configured, while different backend pools and unlimited backends continue
+// independently. An injected [LLMClient] or [ArtifactReader] can therefore
+// still receive concurrent calls and must be safe for that use.
+//
+// NewEngine copies in-memory profiles and backend definitions. Prepare,
+// PrepareExecution, and Run copy request maps, slices, pointer values, and
+// JSON-compatible extra parameters before using them. InspectPrompt and
+// InspectProfile return copied inspection values. Returned values and values
+// passed to extension interfaces are likewise isolated from engine state.
+// Callers own those copies and may mutate them after the call that supplied or
+// returned them. Returned structured errors are likewise caller-owned and may
+// be mutated without affecting engine state or another error.
+//
+// # Security and sensitive data
+//
+// The default artifact reader treats [File] paths as caller-selected operating
+// system paths. It does not restrict them to an application root or impose an
+// inbound request-size policy. Promptkit is not an inbound request or
+// untrusted-input security boundary. Applications must validate and restrict
+// untrusted input before constructing a request, or install an [ArtifactReader]
+// that enforces their filesystem, authorization, and size policies.
+//
+// Rendered messages, input and output [Artifact] bodies, [RunResult.RawOutput],
+// and [ValidationResult.Errors] may contain sensitive data. Credential
+// exclusion and redaction do not sanitize those values. Applications and
+// injected collaborators are responsible for access control, retention,
+// logging, and secret handling appropriate to their data.
+//
+// # JSON
+//
+// Stable JSON representations are provided for [PreparedRun], [RunResult],
+// [Artifact], [ExecutionTarget], [OutputContract], [ValidationResult],
+// [TokenUsage], [RenderedPrompt], [RenderedMessage], [CacheControl],
+// [StructuredOutputSpec], [StructuredOutputJSONSpec], [GenerateRequest],
+// [GenerateResponse], [ExecutionTargetPresence], and the string value types
+// used by those values.
+//
+// Construction, inspection, handle, and error values, including [Config],
+// [Backend], [RunRequest], [ArtifactRef], [ExecutionTargetOverride], [Profile],
+// [OpenAICompatibleProfileConfig], [ProfileInspection],
+// [PromptInputDefinition], [PromptInspection], [PreparedExecution], and
+// [CapacityError], do not have stable JSON representations. Direct API keys
+// are nevertheless excluded from JSON for every public value.
+//
+// JSON timestamps use time.Time's RFC 3339 encoding and are omitted when zero.
+// PreparedRun and RunResult durations are encoded as integer milliseconds in
+// duration_ms and omitted when zero. Run IDs and all exposed hashes are opaque:
+// their spelling, length, character set, and algorithm are not API contracts.
 package promptkit

docs/consumers/pkg-promptkit.md

@@ -0,0 +1,401 @@
+# Package `promptkit`
+
+## Purpose
+
+This guide helps Go consumers assemble Promptkit and choose the main
+preparation or execution workflow. The declarations and GoDoc in the
+[root package](../../doc.go) own exact field, option, serialization,
+concurrency, ownership, failure, and cancellation semantics. The
+[framework format reference](../formats.md) owns prompt, profile, and schema
+file contracts.
+
+Import the package as:
+
+```go
+import "gitea.maximumdirect.net/eric/promptkit"
+```
+
+The following Go fragments are illustrative and omit surrounding package,
+import, and error-handling code. Use the maintained examples for complete
+programs.
+
+## Construct An Engine
+
+Create an engine with
+[`NewEngine`](../../engine.go). A directory-backed setup supplies a prompt
+directory and may supply profile and schema directories:
+
+```go
+engine, err := promptkit.NewEngine(promptkit.Config{
+	PromptDir:  "prompts",
+	ProfileDir: "profiles",
+	SchemaDir:  "schemas",
+})
+```
+
+Options support single-file or `fs.FS` sources, in-memory profiles,
+engine-scoped backends, and injected artifact or model clients. Consult the
+[constructor and option GoDoc](../../engine.go) for composition, precedence,
+validation, and default transport behavior. Source discovery, format
+validation, and profile precedence are defined by the
+[framework format reference](../formats.md).
+
+## Inspect A Prompt Before Preparation
+
+Use [`Engine.InspectPrompt`](../../engine.go) to check one configured prompt's
+declared inputs and output workflow without creating placeholder inputs or
+resolving a profile:
+
+```go
+inspection, err := engine.InspectPrompt(ctx, "meeting.summary", "")
+if err != nil {
+	return err
+}
+
+for _, input := range inspection.Inputs {
+	// Compare the declared input with application configuration.
+}
+```
+
+Use this configuration-time boundary when the application needs only the
+declared prompt interface. Use `InspectProfile` separately when it must also
+check a configured profile. Use `Prepare` when it needs inputs, schemas, or
+rendered messages, and use prepared execution when that work must remain tied
+to later execution. The method's [GoDoc](../../engine.go) owns exact fields,
+hash, ownership, and error semantics.
+
+## Prepare Without Model Execution
+
+[`Engine.Prepare`](../../engine.go) resolves the selected prompt and profile,
+loads inputs and any structured-output schema, and renders messages without
+calling a model client. Choose it when the prepared value is the final
+inspection or persistence result and no later execution must be tied to that
+exact snapshot:
+
+```go
+prepared, err := engine.Prepare(ctx, promptkit.RunRequest{
+	PromptID: "meeting.summary",
+	Inputs: map[string]promptkit.ArtifactRef{
+		"note": promptkit.Inline("Synthetic meeting notes"),
+	},
+})
+```
+
+The maintained
+[offline preparation example](../../examples/go-library/prepare/main.go)
+shows a complete runnable setup with a prompt file, in-memory profile, and
+inline input. Exact request requirements and prepared-result fields belong to
+the [`RunRequest` and `PreparedRun` GoDoc](../../types.go).
+
+## Prepare Now And Execute The Same Snapshot Later
+
+Use [`Engine.PrepareExecution`](../../engine.go) when an application must
+inspect or persist preflight details before deciding whether to start model
+work, while ensuring that later execution uses those exact rendered messages,
+inputs, target settings, and validation resources:
+
+```go
+preparedExecution, err := engine.PrepareExecution(ctx, promptkit.RunRequest{
+	PromptID: "meeting.summary",
+	Inputs: map[string]promptkit.ArtifactRef{
+		"note": promptkit.Inline("Synthetic meeting notes"),
+	},
+})
+if err != nil {
+	return err
+}
+defer preparedExecution.Discard()
+
+details := preparedExecution.Details()
+// Inspect or persist an application-selected safe subset of details.
+
+result, err := engine.RunPrepared(ctx, preparedExecution)
+```
+
+Preparation does not call the model or reserve backend capacity.
+`RunPrepared` executes from the retained snapshot rather than reloading
+consumer sources. The handle is opaque in-process state, while `Details`
+contains rendered content and remains subject to the application's data
+handling policy. The
+[`PreparedExecution` and method GoDoc](../../prepared_execution.go) and
+[engine operation GoDoc](../../engine.go) own exact lifecycle, engine-binding,
+credential, cancellation, timing, and error semantics.
+
+## Execute And Validate
+
+[`Engine.Run`](../../engine.go) performs the same preparation, invokes the
+configured model client, classifies the generated artifact, and validates the
+content in one call. Choose it when the application does not need a preflight
+boundary tied to the eventual execution. A completed content check may return
+`ValidationFailed` in the result; an operational inability to validate returns
+an error.
+
+The maintained
+[offline execution example](../../examples/go-library/run/main.go) injects a
+deterministic model client and exercises `Run` without credentials, network
+access, or paid calls. It is intentionally separate from the preparation
+example so each workflow and its small prompt fixture can be copied and run on
+its own.
+
+Use the [`RunResult` and `ValidationResult` GoDoc](../../types.go) for the
+returned data and the `Engine.Run` GoDoc for failure and cancellation
+semantics. The
+[OpenAI-compatible integration contract](../integrations/openai-compatible-chat.md)
+owns the built-in client's outbound HTTP behavior.
+
+## Inputs, Profiles, And Overrides
+
+Use `File`, `Inline`, or `InlineWithURI` to construct request inputs. A request
+can select a profile explicitly or use the prompt's default profile, and can
+replace execution settings or the complete output contract.
+
+The [public value GoDoc](../../types.go) defines nil, empty, zero, replacement,
+copy, and credential behavior. The
+[framework format reference](../formats.md) defines how those request values
+interact with prompt definitions, file-backed profiles, built-ins, schemas,
+and framework defaults.
+
+For programmatic profiles,
+[`OpenAICompatibleProfile`](../../profiles.go) converts ordinary
+OpenAI-compatible settings into a value accepted by `WithProfiles`.
+
+### Inspect A Profile Before Prompt Work
+
+Use [`Engine.InspectProfile`](../../engine.go) to validate one configured
+profile without constructing a synthetic prompt or placeholder inputs. It
+resolves the profile's effective target but does not prepare or execute a
+prompt:
+
+```go
+inspection, err := engine.InspectProfile(ctx, profileID)
+if err != nil {
+	return err
+}
+
+target := inspection.EffectiveModelParams
+if target.APIKeyEnv != "" {
+	// Apply application policy for the named environment variable.
+} else if inspection.APIKeyRequired {
+	// Arrange a direct credential before later execution.
+}
+```
+
+Use this configuration-time boundary when only the profile and its target need
+checking. Use `Prepare` when the application also needs prompt, input, schema,
+or rendering work; use prepared execution when that work must remain tied to a
+later execution. Inspection reports credential requirements but leaves the
+timing of credential enforcement to the application. The method's
+[GoDoc](../../engine.go) owns its exact result and error contract.
+
+### Set A Per-Run Session And Reasoning
+
+Supply a direct session ID when one prompt should be correlated with a
+consumer-managed conversation or workflow without changing prompt variables:
+
+```go
+reasoning := "high"
+result, err := engine.Run(ctx, promptkit.RunRequest{
+	PromptID: "meeting.summary",
+	SessionID: "conversation-42",
+	Inputs: map[string]promptkit.ArtifactRef{
+		"note": promptkit.Inline("Synthetic meeting notes"),
+	},
+	Execution: &promptkit.ExecutionTargetOverride{
+		ReasoningEffort: &reasoning,
+	},
+})
+```
+
+A nil reasoning pointer inherits the selected profile, a pointer to a
+nonblank string replaces it, and a pointer to a blank string disables
+reasoning for that run. Session IDs are correlation metadata, not credentials;
+use stable, non-secret values that are safe to expose to collaborators and
+providers. The
+[`RunRequest` and `ExecutionTargetOverride` GoDoc](../../types.go) owns the
+exact normalization, precedence, error, copying, and exposure contract.
+
+### Configure A Local OpenAI-Compatible Endpoint
+
+Choose the smallest configuration that fits how the endpoint will be reused.
+
+#### Use An Endpoint-Only Profile
+
+Put the endpoint directly on an in-memory profile when only that profile needs
+it and shared backend identity or capacity policy is unnecessary:
+
+```go
+engine, err := promptkit.NewEngine(promptkit.Config{
+	PromptDir: "prompts",
+},
+	promptkit.WithProfiles(promptkit.Profile{
+		ID:       "local-summary",
+		Endpoint: "http://localhost:8000/v1",
+		Model:    "example-model",
+	}),
+)
+```
+
+Endpoint-only profiles have an empty backend ID and remain unrestricted by
+backend capacity policy.
+
+#### Use The Conventional Local Backend
+
+Use `LocalBackend` when profiles should share the conventional `local`
+identity, endpoint, and concurrency limit:
+
+```go
+engine, err := promptkit.NewEngine(promptkit.Config{
+	PromptDir: "prompts",
+},
+	promptkit.WithBackend(
+		promptkit.LocalBackend("http://localhost:8000/v1", 2),
+	),
+	promptkit.WithProfiles(promptkit.Profile{
+		ID:        "local-summary",
+		BackendID: promptkit.BackendLocal,
+		Model:     "example-model",
+	}),
+)
+```
+
+The helper is explicit: it does not pre-register a backend or read environment
+variables. Supplying a positive limit leaves queue capacity omitted, so normal
+backend registration selects the existing default waiting capacity of 1024.
+The returned value still enters the engine through `WithBackend`.
+
+#### Configure A Complete Backend
+
+Use a keyed `Backend` value for authentication, extra request parameters, an
+explicit queue capacity, a custom ID, or multiple local endpoints:
+
+```go
+noWaiting := 0
+engine, err := promptkit.NewEngine(promptkit.Config{
+	PromptDir: "prompts",
+},
+	promptkit.WithBackend(promptkit.Backend{
+		ID:               "local-gpu",
+		Endpoint:         "http://gpu-host:8000/v1",
+		APIKeyEnv:        "LOCAL_GPU_API_KEY",
+		ExtraParams:      map[string]any{"provider_option": "enabled"},
+		ConcurrencyLimit: 2,
+		QueueCapacity:    &noWaiting,
+	}),
+	promptkit.WithProfiles(promptkit.Profile{
+		ID:        "gpu-summary",
+		BackendID: "local-gpu",
+		Model:     "example-model",
+	}),
+)
+```
+
+Use distinct custom IDs when registering multiple local endpoints.
+Registrations belong to one engine and custom IDs cannot replace built-ins.
+The [`Backend`, `LocalBackend`, and `WithBackend` GoDoc](../../backends.go)
+defines exact construction, validation, copying, uniqueness, concurrency, and
+request-default behavior.
+
+Both file-backed and in-memory profiles select a registration through
+`backend` or `Profile.BackendID`. Profile and request endpoint overrides retain
+that routing and capacity identity. `PreparedRun.SelectedBackendID`,
+`RunResult.SelectedBackendID`, and the effective `ExecutionTarget.BackendID`
+expose it to consumers and injected model clients. Endpoint-only profiles
+remain supported and expose an empty backend ID.
+
+### Limit Backend Concurrency
+
+Set `Backend.ConcurrencyLimit` when a backend needs protection from too many
+simultaneous model calls. Leaving `QueueCapacity` nil, as in the local-backend
+example above, selects the default waiting capacity of 1024.
+
+To accept no waiting backlog beyond the active calls, provide an explicit
+zero:
+
+```go
+noWaiting := 0
+backend := promptkit.Backend{
+	ID:               "local-gpu",
+	Endpoint:         "http://gpu-host:8000/v1",
+	ConcurrencyLimit: 2,
+	QueueCapacity:    &noWaiting,
+}
+```
+
+The pointer distinguishes an explicit zero from omission. Keep using keyed
+`Backend` literals so additive configuration fields remain source-compatible.
+Capacity belongs to one engine and the selected backend ID; endpoint-only
+profiles and custom backends without a configured limit remain unrestricted.
+Exact validation, defaulting, ownership, and concurrency semantics belong to
+the [`Backend` GoDoc](../../backends.go).
+
+## Credentials
+
+File-backed profiles name an environment variable; in-memory profiles can
+require a direct request key. Direct keys are request-scoped and are excluded
+from supported JSON values and the package's `String` and `GoString`
+summaries. The exact precedence and redaction guarantees belong to
+[`RunRequest`, `GenerateRequest`, and the profile GoDoc](../../types.go).
+
+## Protect Files And Generated Data
+
+The default artifact reader opens a `File` reference as a caller-selected
+operating-system path. It does not constrain paths to an application root,
+impose an inbound request-size policy, or establish an untrusted-input security
+boundary. Applications must validate and restrict untrusted paths and payloads
+before constructing a request, or inject an artifact reader that enforces
+their filesystem, authorization, and size policies.
+
+Rendered messages, input and output artifact bodies, raw model output, and
+validation diagnostics can contain sensitive data. API-key redaction does not
+sanitize those values. Treat prepared values, results, collaborator requests,
+errors, and logs according to the application's data-access, retention, and
+secret-handling policies.
+
+## Extension Interfaces
+
+Inject an [`LLMClient` or `ArtifactReader`](../../types.go) when the built-in
+behavior does not fit the application. Their GoDoc defines concurrent use,
+context handling, ownership of copied values, nil responses, and preservation
+of collaborator errors. Implementations must honor cancellation, safely manage
+copies they retain, avoid unsafe logging of content or credentials, and enforce
+the application policy that motivated the injection.
+
+## Handle Errors
+
+Use `errors.Is` with the
+[public error sentinels and operation GoDoc](../../engine.go). The declarations
+distinguish invalid construction, invalid requests, absent sources,
+source-loading failures, collaborator failures, and operational validation
+failures. Specific request conditions may also match the broader
+`ErrInvalidRequest`, and injected collaborator identities are preserved where
+documented. Invalid or duplicate backend registrations match
+`ErrInvalidConfig`; selecting an unknown backend matches `ErrProfileLoad`.
+
+When a limited backend has admitted all active and waiting calls, handle
+`ErrCapacityExceeded` separately from request errors and provider failures:
+
+```go
+result, err := engine.Run(ctx, request)
+if errors.Is(err, promptkit.ErrCapacityExceeded) {
+	var capacityErr *promptkit.CapacityError
+	if errors.As(err, &capacityErr) {
+		// Record capacityErr.BackendID using application-owned diagnostics.
+	}
+
+	// Apply application policy: shed work, report overload, or retry later.
+}
+```
+
+A rejected call returns no partial result and does not invoke the model
+client. Promptkit does not prescribe retries or map this error to an HTTP
+status; those choices remain with the consuming application. The
+[`CapacityError` GoDoc](../../capacity_error.go) owns the exact typed-error
+contract, while the [`Engine.Run` and error GoDoc](../../engine.go) owns broad
+error and cancellation identities.
+
+## Application Boundary
+
+Promptkit is an importable library. It does not own a command, inbound HTTP
+API, process configuration, or deployment policy. Applications map the root
+package's results and errors into those concerns, including inbound size and
+trust policy.

docs/development.md

@@ -31,11 +31,14 @@ Start with:
 
 | Task | Read before changing |
 | --- | --- |
-| Documentation or examples | The [documentation policy](policy/documentation.md) and the canonical owner of the affected contract. |
+| Root public API | The [architecture policy](policy/architecture.md), [consumer guide](consumers/pkg-promptkit.md), [testing policy](policy/testing.md), and existing GoDoc. |
-| Tests or test fixtures | The [testing policy](policy/testing.md), the owning package, and any focused internal document listed by the component overview. |
+| Prompt, profile, or schema formats | The [framework format reference](formats.md), owning parser or validator package, and [documentation policy](policy/documentation.md). |
-| Root public API, once implemented | The [architecture policy](policy/architecture.md), [root package declaration](../doc.go), [testing policy](policy/testing.md), and existing GoDoc. |
+| Source loading or validation | The [framework format reference](formats.md), [internal source document](internal/sources.md), and owning package tests. |
-| Internal package implementation, once introduced | The [architecture policy](policy/architecture.md), [internal component overview](internal/overview.md), and any focused internal document that the overview lists for that package. |
+| Model-client behavior | The [OpenAI-compatible integration contract](integrations/openai-compatible-chat.md), [internal model-client document](internal/llm.md), and owning package tests. |
-| Integration behavior, once introduced | The [architecture policy](policy/architecture.md), [documentation policy](policy/documentation.md), and the integration's owning contract under `docs/integrations/`. |
+| Internal package implementation | The [architecture policy](policy/architecture.md), [internal component overview](internal/overview.md), and focused internal document listed for that package. |
+| Tests or test fixtures | The [testing policy](policy/testing.md), owning package, and focused internal document listed by the component overview. |
+| Maintained example | The [example](../examples/go-library/prepare/main.go), [consumer guide](consumers/pkg-promptkit.md), [framework format reference](formats.md), and [documentation policy](policy/documentation.md). |
+| Documentation | The [documentation policy](policy/documentation.md) and canonical owner of every affected contract. |
 | Release preparation or publication | The [release procedure](release.md). |
 
 For cross-cutting changes, follow every applicable row. Do not create
@@ -50,8 +53,10 @@ validation from the Promptkit repository root:
 
 ```sh
 go test ./...
+go test -race ./...
 go vet ./...
 go build ./...
+go run ./examples/go-library/prepare
 ```
 
 Check formatting across every tracked Go file:
@@ -75,7 +80,7 @@ behavior checked by the module.
 ## Focused Validation
 
 Use focused checks while iterating, then run the complete validation sequence
-before accepting the change. The root package currently supports:
+before accepting the change. The root package supports:
 
 ```sh
 go test .
@@ -83,17 +88,16 @@ go vet .
 go build .
 ```
 
-Filter tests by name without assuming a future package layout:
+Filter tests by name without assuming a fixed internal package layout:
 
 ```sh
 go test ./... -run 'TestName'
 ```
 
-Replace `TestName` with a useful regular expression. When internal packages are
+Replace `TestName` with a useful regular expression. Target only paths that
-introduced, target only paths that actually exist, such as
+exist, and consult the internal component overview for their owning
-`go test ./path/to/package`, and consult the internal component overview for
+documentation. A filtered or package-specific run does not replace the
-their owning documentation. A filtered or package-specific run does not replace
+complete repository validation.
-the complete repository validation.
 
 ## Coordinated Work With Scriptorium
 

docs/formats.md

@@ -0,0 +1,308 @@
+# Framework Format Reference
+
+## Purpose
+
+This document is the canonical contract for Promptkit prompt-definition,
+profile, and schema files. The [Go package consumer guide](consumers/pkg-promptkit.md)
+explains how to select these sources and invoke the engine. The
+[OpenAI-compatible integration contract](integrations/openai-compatible-chat.md)
+owns the resulting outbound wire behavior.
+
+Prompt and profile sources recursively discover files ending in `.yaml` or
+`.yml`. YAML decoding is strict: unknown fields are errors for the selected
+definition. Definitions are selected by their YAML `id`, not their file name
+or directory.
+
+## Prompt Definitions
+
+A prompt definition describes inputs, Go-template messages, an optional
+default profile, and an output contract.
+
+```yaml
+id: meeting.summary
+version: "1.0.0"
+default_profile: local-summary
+description: Summarize a synthetic meeting note.
+session_id: '{{.session}}'
+inputs:
+  - name: note
+    required: true
+    content_type: text/plain
+    description: Meeting note to summarize.
+messages:
+  - role: system
+    content: Return a concise summary.
+    cache_control:
+      type: ephemeral
+      ttl: 1h
+  - role: user
+    content_file: ./summary.user.md
+output:
+  format: markdown
+  validation_mode: basic
+  repair_attempts: 0
+```
+
+| Field | Required | Meaning |
+| --- | --- | --- |
+| `id` | yes | Non-empty prompt identifier used by `RunRequest.PromptID`. |
+| `version` | yes | Non-empty version selected by an optional `RunRequest.PromptVersion`. |
+| `default_profile` | no | Non-empty profile ID used when the request omits `ProfileID`. |
+| `description` | no | Human-readable description. |
+| `session_id` | no | Go template rendered from request variables and input helpers. |
+| `inputs` | no | Declared input metadata. |
+| `messages` | yes | One or more chat-message templates. |
+| `output` | yes | Output format and validation settings. |
+
+When a request omits a version, the selected prompt ID must identify exactly
+one definition. When it supplies a version, the ID and version pair must be
+unique.
+
+Exact prompt inspection uses this same configured source, strict decoding,
+referenced content-file resolution, and ID/version selection. It reports the
+selected definition's declared metadata without changing the prompt format or
+executing the definition.
+
+### Inputs
+
+Each `inputs` item has these fields:
+
+| Field | Required | Meaning |
+| --- | --- | --- |
+| `name` | yes | Non-empty name used by the request input map and `input` template helper. Names must be unique. |
+| `required` | no | When true, preparation fails if the request omits the input. The default is false. |
+| `content_type` | no | Expected media-type metadata. |
+| `description` | no | Human-readable input description. |
+
+Requests supply inputs as inline or file-backed `ArtifactRef` values. Declared
+required inputs must be present. A template reference also requires the named
+input to exist, whether or not it was declared. Extra request inputs are
+allowed.
+
+### Messages And Templates
+
+Each message has a non-empty `role` and exactly one of:
+
+- `content`, containing an inline Go template; or
+- `content_file`, naming a file whose contents are the Go template.
+
+For directory and `fs.FS` prompt sources, `content_file` resolves relative to
+the prompt file and remains within the source root. `WithPromptFile` also
+resolves it relative to that file.
+
+Request variables are the template data, so a variable named `audience` is
+referenced as `{{.audience}}`. The `{{input "note"}}` helper renders the body
+of a named input. Missing variables and input references are errors.
+
+The optional `session_id` uses the same template data and input helper. Its
+rendered value is trimmed, omitted when empty, and limited to 256 Unicode code
+points. A nonblank direct request session ID bypasses this template completely;
+a blank direct value leaves the template behavior unchanged.
+
+### Cache Control
+
+`cache_control` is optional and has these fields:
+
+| Field | Required | Values |
+| --- | --- | --- |
+| `type` | yes | `ephemeral` |
+| `ttl` | no | Empty or `1h` |
+
+Promptkit preserves cache-control metadata on the rendered message. The
+outbound integration determines its wire representation.
+
+### Output Contract
+
+| Field | Required | Values or behavior |
+| --- | --- | --- |
+| `format` | yes | `text`, `markdown`, or `json`. |
+| `validation_mode` | yes | `none`, `basic`, `json`, or `json_schema`. |
+| `schema_path` | for `json_schema` | Path to a schema in the configured schema source. |
+| `repair_attempts` | no | Integer zero or greater; omitted means zero. |
+
+The validation modes behave as follows:
+
+- `none` skips content validation;
+- `basic` requires non-empty generated content;
+- `json` requires valid JSON; and
+- `json_schema` requires valid JSON that satisfies the selected schema.
+
+`format` controls output artifact metadata. JSON Schema mode also supplies the
+schema to compatible model clients as structured-output metadata. The public
+engine does not install an output repairer, so its validation is single-pass
+even when a positive `repair_attempts` value is present.
+
+A request-level `OutputContract` replaces the complete prompt output contract.
+It does not merge individual fields. If its format is empty, Promptkit uses
+`text`.
+
+## Profile Definitions
+
+A profile supplies model execution settings:
+
+```yaml
+id: local-summary
+backend: openrouter
+model: example-model
+temperature: 0.2
+max_tokens: 500
+top_p: 0.95
+timeout_seconds: 90
+service_tier: flex
+reasoning_effort: medium
+extra_params:
+  provider_option: enabled
+```
+
+| Field | Required | Meaning |
+| --- | --- | --- |
+| `id` | yes | Non-empty profile identifier. IDs must be unique within one source. |
+| `backend` | unless `endpoint` is present | Backend registry ID. It is trimmed and registry membership is checked when the profile is prepared or inspected. |
+| `endpoint` | unless `backend` is present | Non-empty OpenAI-compatible base URL, including an API version path when required. When both connection fields are present, this overrides the backend endpoint without changing backend identity. |
+| `model` | yes | Non-empty provider model name. |
+| `temperature` | no | Number from 0 through 2. |
+| `max_tokens` | no | Integer zero or greater. |
+| `top_p` | no | Number from 0 through 1. |
+| `timeout_seconds` | no | Per-generation deadline in whole seconds; integer zero or greater. |
+| `service_tier` | no | Provider-specific request tier. |
+| `reasoning_effort` | no | Provider-specific reasoning setting. |
+| `api_key_env` | no | Name of an environment variable containing the API key. |
+| `extra_params` | no | JSON-compatible provider-specific outbound fields. |
+
+Raw `api_key` is prohibited in profile YAML. Store only an environment
+variable name in `api_key_env`.
+
+Promptkit does not infer a backend from a model or endpoint. Endpoint-only
+profiles remain supported and have no effective backend ID.
+The engine always provides the built-in `openrouter` ID. Consumers can add
+engine-scoped IDs with
+[`WithBackend`](../backends.go); exact registration validation belongs to its
+GoDoc.
+
+`extra_params` accepts null, booleans, finite numbers, strings, arrays, and
+objects with string keys. Keys must be non-empty. With the built-in client,
+they also cannot collide with the standard fields listed in the
+[outbound request contract](integrations/openai-compatible-chat.md#request-body).
+
+### Defaults And Overrides
+
+Execution settings resolve in this order:
+
+1. framework defaults;
+2. the selected backend, when the profile names one;
+3. the selected profile; and
+4. request `ExecutionTargetOverride` values.
+
+The framework defaults are:
+
+| Setting | Default |
+| --- | --- |
+| `temperature` | `0` |
+| `max_tokens` | `0` |
+| `top_p` | `1` |
+| `timeout_seconds` | `600` |
+
+Numeric zero in a file or in-memory profile means that the profile does not
+replace the framework default. Numeric request overrides use pointers, so an
+explicit zero is preserved. In particular, an explicit request
+`timeout_seconds` of zero disables the per-generation deadline while leaving
+the caller context and transport timeout intact.
+
+Non-empty profile strings replace backend defaults, and non-empty request
+strings replace both. Request reasoning is the exception: a nil
+`ReasoningEffort` pointer inherits the profile, a pointer to a nonblank string
+trims and replaces it, and a pointer to a blank string clears it. Backend
+identity is retained when either layer overrides the endpoint, so the override
+also retains any engine-local capacity policy configured for that backend.
+Capacity configuration belongs to the Go
+[`Backend` API](../backends.go), not prompt or profile YAML. A non-empty
+`extra_params` map at each layer replaces the entire lower-precedence map
+rather than merging keys.
+The [outbound integration contract](integrations/openai-compatible-chat.md)
+defines how the effective settings are serialized.
+
+### Source And Profile Precedence
+
+An explicit request profile ID takes precedence over the prompt's
+`default_profile`. If neither is present, preparation fails. Exact profile
+inspection instead takes one explicit profile ID and does not use a prompt
+default.
+
+Profile sources resolve matching IDs in this order:
+
+1. in-memory profiles supplied with `WithProfiles`;
+2. a profile file, `fs.FS`, or configured profile directory; and
+3. embedded built-in profiles.
+
+A higher-precedence source falls back only when the profile is absent. An
+invalid matching profile is an error and does not fall back. In-memory
+`Profile` values follow the same ranges as YAML profiles. They use
+`APIKeyRequired` for request-scoped credentials instead of `api_key_env`.
+Preparation and exact profile inspection use this same source precedence.
+
+## Built-In Profile Catalog
+
+Every built-in selects the `openrouter` backend. The engine's built-in backend
+registry supplies `https://openrouter.ai/api/v1` and the environment-variable
+name `OPENROUTER_API_KEY`, so individual profiles contain only model and
+generation settings. Built-in profile files do not repeat those connection
+values. A custom or in-memory profile with the same profile ID takes
+precedence.
+
+| Provider | ID | Model |
+| --- | --- | --- |
+| aion-labs | `aion-2` | `aion-labs/aion-2.0` |
+| anthropic | `claude-fable-latest` | `~anthropic/claude-fable-latest` |
+| anthropic | `claude-haiku-latest` | `~anthropic/claude-haiku-latest` |
+| anthropic | `claude-opus-latest` | `~anthropic/claude-opus-latest` |
+| anthropic | `claude-sonnet-latest` | `~anthropic/claude-sonnet-latest` |
+| deepseek | `deepseek-3-2` | `deepseek/deepseek-v3.2` |
+| deepseek | `deepseek-4-flash` | `deepseek/deepseek-v4-flash` |
+| deepseek | `deepseek-4-pro` | `deepseek/deepseek-v4-pro` |
+| google | `gemini-2-flash` | `google/gemini-2.5-flash` |
+| google | `gemini-2-flash-lite` | `google/gemini-2.5-flash-lite` |
+| google | `gemini-2-pro` | `google/gemini-2.5-pro` |
+| google | `gemini-3-flash-lite` | `google/gemini-3.1-flash-lite` |
+| google | `gemini-flash-latest` | `~google/gemini-flash-latest` |
+| google | `gemini-pro-latest` | `~google/gemini-pro-latest` |
+| google | `gemma-4-31b` | `google/gemma-4-31b-it:exacto` |
+| minimax | `minimax-m2` | `minimax/minimax-m2.5` |
+| minimax | `minimax-m3` | `minimax/minimax-m3` |
+| mistral | `mistral-large-2512` | `mistralai/mistral-large-2512` |
+| mistral | `mistral-medium-3-5` | `mistralai/mistral-medium-3-5` |
+| mistral | `mistral-small-3` | `mistralai/mistral-small-3.2-24b-instruct` |
+| mistral | `mistral-small-4` | `mistralai/mistral-small-2603` |
+| nvidia | `nemotron-3-ultra` | `nvidia/nemotron-3-ultra-550b-a55b` |
+| openai | `gpt-5-mini` | `openai/gpt-5.4-mini` |
+| openai | `gpt-5-nano` | `openai/gpt-5.4-nano` |
+
+## Schemas
+
+Schemas are JSON documents selected by a prompt or request
+`schema_path`. For a directory or `fs.FS` source, paths resolve within the
+configured source root. Referenced nested schemas resolve relative to the
+owning schema document. `WithSchemaFile` exposes one schema, addressed by its
+base name.
+
+An unreadable, invalid, or unresolvable schema produces an operational
+validation error. Generated content that is valid JSON but does not satisfy the
+schema produces a failed validation result.
+
+## Credentials
+
+Credential values belong at the request or environment boundary, never in
+prompt, profile, schema, or example files:
+
+- a file profile names an environment variable with `api_key_env`;
+- an in-memory profile may set `APIKeyRequired`;
+- a request can provide a direct `APIKey` or override `APIKeyEnv`; and
+- a direct request key takes precedence over environment lookup.
+
+After a direct request key, the credential-source precedence is request
+`APIKeyEnv`, profile `api_key_env`, then the backend default. An in-memory
+profile with `APIKeyRequired` clears an inherited backend environment name and
+requires a direct key unless the request explicitly supplies `APIKeyEnv`.
+Promptkit validates required credential availability during preparation.
+Direct keys are excluded from JSON results and redacted by public string
+formatters. Environment-variable names may appear in prepared metadata, but
+their values do not.

docs/integrations/openai-compatible-chat.md

@@ -0,0 +1,106 @@
+# OpenAI-Compatible Chat Integration
+
+## Purpose
+
+This document defines the outbound HTTP behavior implemented by Promptkit's
+internal OpenAI-compatible model client. The
+[internal model-client document](../internal/llm.md) owns implementation flow,
+errors, and test ownership. The root Promptkit engine uses this client by
+default unless a consumer injects another implementation. The
+[framework format reference](../formats.md) owns the profile and prompt values
+that produce these outbound settings.
+
+## Endpoint And Method
+
+Generation sends an HTTP `POST` with `Content-Type: application/json`.
+Before the client is called, the engine resolves framework, backend, profile,
+and request values into one execution target. A non-empty endpoint from that
+target overrides the client's configured base URL. After trailing slashes are
+removed, `/chat/completions` is appended. Generation fails before sending when
+neither source supplies an endpoint.
+
+The target's backend ID is routing metadata for prepared values, results, and
+injected clients. The built-in client does not derive the URL from that ID and
+does not serialize it in the provider request.
+
+## Authentication
+
+A non-empty API key supplied directly on the execution target takes
+precedence. Otherwise, when an API-key environment-variable name is supplied,
+the client reads that variable and requires a non-empty value. The selected
+key is sent as `Authorization: Bearer <key>`. No authorization header is sent
+when neither mechanism is configured.
+
+The target contains the already resolved environment-variable name: an
+explicit request override takes precedence over profile metadata, which takes
+precedence over the backend default. Only the name reaches prepared metadata;
+the environment value is read just before the provider call and is never added
+to the JSON body.
+
+## Request Body
+
+The request body always contains `model` and `messages`. The execution
+target's model takes precedence over the client's configured model, and one
+must be available.
+
+Each ordinary message contains its `role` and string `content`. A
+cache-controlled message instead uses a text content block containing `type`,
+`text`, and `cache_control`; an empty cache-control TTL is omitted.
+
+The effective direct or prompt-rendered session ID is trimmed, limited to 256
+Unicode code points, and sent when nonempty as top-level `session_id`. It is
+never also sent as a session header.
+
+The client conditionally includes:
+
+- `temperature`, `max_tokens`, and `top_p` when non-zero or explicitly
+  present;
+- non-empty `service_tier` and effective `reasoning_effort`; an explicitly
+  disabled reasoning setting is empty and therefore omitted; and
+- `response_format` for JSON Schema structured output, including its name,
+  strict flag, and schema document.
+
+The engine resolves backend, profile, and request extra-parameter maps by
+whole-map replacement rather than key merging. The resulting effective map is
+then merged directly into the top-level body after JSON serialization is
+verified. Empty keys and collisions with these reserved fields are rejected
+before any provider call:
+
+- `model`
+- `session_id`
+- `messages`
+- `temperature`
+- `max_tokens`
+- `top_p`
+- `service_tier`
+- `reasoning_effort`
+- `response_format`
+
+`backend_id`, `api_key_env`, and resolved credential values are not provider
+request fields.
+
+## Response Handling
+
+Any 2xx response is decoded as an OpenAI-compatible chat response. The client
+returns the first choice's non-empty message content and maps prompt,
+completion, total, cached, and cache-write token counts.
+
+Invalid JSON, absent choices, and empty first-choice content are malformed
+responses. For a non-2xx status, the error includes the status code but never
+the provider response body.
+
+## Timeout And Cancellation
+
+Timeouts are layered:
+
+- the caller context remains the outer cancellation boundary;
+- a positive generation timeout adds a request context deadline;
+- zero adds no generation-specific deadline;
+- a negative generation timeout is invalid; and
+- the cloned `http.Client` supplies the whole-request transport cap, retaining
+  a positive supplied-client timeout or applying the configured/default
+  timeout when the supplied value is not positive.
+
+The earliest applicable caller, generation, or transport deadline controls the
+request. Constructing the internal client does not mutate a supplied
+`http.Client`.

docs/internal/capacity.md

@@ -0,0 +1,113 @@
+# Internal Capacity Management
+
+## Purpose
+
+This document describes the implemented engine-local capacity coordination in
+`internal/capacity`. The [architecture policy](../policy/architecture.md) owns
+component boundaries, the [backend GoDoc](../../backends.go) owns exact public
+configuration semantics, and the
+[internal runner document](runner.md) owns orchestration around admission.
+
+Capacity scheduling is outside the provider wire contract. It does not add
+fields to execution targets, generated requests, prompt or profile YAML, or
+stable JSON values.
+
+## Construction And Pool Lifecycle
+
+Each root `NewEngine` call obtains a normalized capacity-policy snapshot from
+its immutable backend registry and constructs a new `Manager`. The manager
+creates one pool for each limited backend ID. It has no package-global mutable
+state, background workers, shutdown protocol, or persistence, so engines with
+the same registrations still have independent capacity.
+
+Unlimited registered backends and endpoint-only profiles have no pool. Their
+admission and generation calls take the unrestricted fast path. An endpoint
+override does not change the selected backend ID and therefore does not change
+the pool.
+
+One pool owns immutable active and total limits plus mutex-protected admission
+count, active count, and ordered waiter list. Pool state exists only for the
+lifetime of its engine.
+
+## Bounded Execution Admission
+
+For ordinary `Run`, the runner asks the manager to admit after resolving the
+prompt, profile, selected backend, effective execution target, credentials, and
+output contract, but before schema loading, artifact loading, or rendering.
+`PrepareExecution` performs no admission. `RunPrepared` claims its handle,
+rechecks credential availability, and then asks the manager to admit the
+frozen backend before generation.
+
+Admission is immediate: a limited pool either reserves a slot or returns only
+the internal `ErrCapacityExceeded` identity. The runner attaches the selected
+backend identity at its use-case boundary, and the root facade translates that
+typed value without treating it as an invalid request or generation failure.
+
+The total admitted bound is the active-generation limit plus its configured
+waiting capacity. The returned release function is idempotent. The runner
+defers it as soon as admission succeeds. An ordinary run holds the lease across
+remaining preparation, initial generation, validation, every repair attempt,
+and all failure or cancellation exits. Prepared execution holds the normal
+lease across generation, validation, every internal repair attempt, and all
+execution exits. A repair is part of its original admission and does not
+reserve another bounded slot.
+
+## FIFO Generation Permits
+
+`NewClient` wraps the engine's selected internal model client after public
+client adaptation or built-in client construction. Initial generation and the
+default repairer receive the same wrapper.
+
+For each `Generate` call, the wrapper selects a pool from the request's
+effective backend ID. An unlimited call passes directly to the next client. A
+limited call acquires an active permit, invokes the next client, and defers
+permit release so ordinary returns and panic unwinding both restore capacity.
+Preparation and validation never hold an active permit.
+
+When all active permits are occupied, calls join a mutex-protected FIFO waiter
+list. Releasing a permit transfers it directly to the oldest remaining waiter
+before making it generally available. Pools do not order work relative to
+other backend IDs.
+
+The wrapper passes generation requests, responses, and collaborator errors
+through unchanged. It owns scheduling only; the concrete model client remains
+responsible for provider transport behavior.
+
+## Cancellation And Release
+
+Admission checks the caller context before reserving a slot. A call canceled
+while waiting for an active permit removes its waiter under the same pool lock
+used to grant permits. If cancellation removes the waiter first, the wrapped
+client is not invoked. If a concurrent grant wins first, the call owns the
+permit and invokes the client with the original context, allowing the client
+to observe cancellation normally.
+
+This grant-or-cancel decision prevents lost and double-released permits.
+Admission leases and active permits are released after success, collaborator
+errors, validation failures, cancellation, and panic unwinding. Canceled
+waiters are unlinked so their contexts and requests are not retained by the
+pool.
+
+## Test Ownership
+
+The [manager tests](../../internal/capacity/manager_test.go) own policy
+validation, bounded admission, idempotent release, context handling, and
+unlimited admission. The
+[client tests](../../internal/capacity/client_test.go) own peak enforcement,
+FIFO transfer, canceled-waiter removal, grant/cancel races, independent pools,
+unlimited calls, passthrough behavior, and panic release.
+
+The [runner tests](../../internal/usecase/runner_test.go) own ordinary early
+admission, lease lifetime, failure release, and shared initial/repair
+scheduling. The
+[prepared-execution use-case tests](../../internal/usecase/prepared_execution_test.go)
+own deferred admission, credential ordering, and prepared-execution lease
+release. The
+[external package capacity tests](../../capacity_contract_test.go) own the
+assembled public-engine behavior for configured limits, capacity errors,
+endpoint identity, engine independence, and injected clients. The
+[prepared-execution contract tests](../../prepared_execution_contract_test.go)
+own the public prepared-capacity boundary. The
+[root error-boundary tests](../../errors_internal_test.go) own preservation of
+the public generation category and context identity when generation is
+canceled.

docs/internal/llm.md

@@ -0,0 +1,81 @@
+# Internal Model Client
+
+## Purpose
+
+This document describes Promptkit's internal model-client implementation. The
+[architecture policy](../policy/architecture.md) owns the library boundary,
+and the
+[OpenAI-compatible chat integration](../integrations/openai-compatible-chat.md)
+owns the observable outbound HTTP contract. The
+[framework format reference](../formats.md) owns the profile and prompt
+settings consumed by the client.
+
+The concrete client remains under `internal/llm`. The root engine assembles it
+as the default implementation behind Promptkit's public client boundary.
+
+## Components And Flow
+
+`Client` is the provider-neutral generation boundary consumed by later
+orchestration. `OpenAICompatibleClient` is the built-in implementation. It
+uses internal domain values for rendered prompts, execution targets,
+structured output, responses, and token usage.
+
+The runner supplies a fully resolved target after applying backend, profile,
+and request precedence. The client uses its endpoint, credential metadata,
+generation fields, and extra parameters. `BackendID` remains routing metadata
+for the generation boundary and is not mapped into the provider payload.
+
+Construction validates the configured base URL and clones any supplied
+`http.Client` so Promptkit can apply its timeout default without mutating the
+caller's client. Generation then:
+
+1. validates request-level timeout and endpoint requirements;
+2. maps the internal request into the OpenAI-compatible chat payload;
+3. validates and merges extra parameters;
+4. resolves authentication;
+5. performs the outbound request under the applicable deadlines; and
+6. decodes the first response choice and token usage.
+
+`internal/llm` owns the set of reserved OpenAI-compatible request fields used
+when validating extra parameters. Backend registration consumes the same rule
+without making the model client depend on registry configuration.
+
+The implementation has no retry loop, tool-call support, provider catalog,
+inbound HTTP behavior, or durable session store.
+
+## Prepared Generation
+
+For [`RunPrepared`](../../engine.go), the runner supplies the model client with
+the target, rendered messages, and structured-output constraint retained by
+executable preparation. Execution does not reopen or rerender consumer
+sources.
+
+Before backend admission, the runner rechecks that the frozen credential
+environment-variable name is available. The handle does not retain the
+environment value; the model client resolves the value visible when generation
+begins. A direct request key remains in private execution state only until the
+claimed execution finishes or an unclaimed handle is discarded. Exact public
+ownership and redaction semantics belong to the
+[`PreparedExecution` GoDoc](../../prepared_execution.go).
+
+## Failure Categories
+
+The package preserves distinct error identities for invalid client
+configuration, invalid generation requests, request execution failures,
+non-success provider statuses, and malformed successful responses. Provider
+response bodies are not included in non-success errors.
+
+Caller cancellation and deadline failures during the outbound request are
+reported as request execution failures. The runner classifies these identities
+without depending on HTTP status mapping.
+
+## Test Ownership
+
+The
+[OpenAI-compatible client tests](../../internal/llm/openai_compatible_client_test.go)
+own configuration, client cloning, deterministic deadline precedence,
+authentication, request and response mapping, malformed data, error identity,
+cancellation, and response-body suppression. The root transport contract test
+also verifies that resolved backend settings reach this client without
+serializing backend identity. All use local test servers or test transports;
+the default suite makes no live or paid provider requests.

docs/internal/overview.md

@@ -11,14 +11,30 @@ contributor workflow and validation.
 
 | Component | Implemented responsibility | References |
 | --- | --- | --- |
-| Root `promptkit` package | Establishes the public package boundary for the Go module. It does not yet provide migrated framework behavior or exported APIs. | [Package declaration](../../doc.go) |
+| Root `promptkit` package | Provides the supported engine facade, source, backend-registration, and injection options, public request, result, prompt-inspection, and profile-inspection values, opaque prepared-execution handles, profile construction, extension interfaces, value conversion, redacted formatting, typed capacity errors, public error mapping, and engine-local assembly. | [Package GoDoc](../../doc.go), [prepared execution](../../prepared_execution.go), [backend API](../../backends.go), [engine assembly](../../engine.go) |
+| `examples/go-library/prepare` | Demonstrates an offline downstream consumer using a prompt file, in-memory profile, inline input, and `Prepare`. It is not a public library package. | [Example program](../../examples/go-library/prepare/main.go) |
+| `examples/go-library/run` | Demonstrates an offline downstream consumer using a prompt file, in-memory profile, inline input, an injected deterministic model client, and `Run`. It is not a public library package. | [Example program](../../examples/go-library/run/main.go) |
+| `internal/backend` | Constructs each engine's immutable registry from the built-in OpenRouter definition and consumer additions, validates and defensively copies definitions through the shared JSON-value package, and consumes the LLM-owned OpenAI-compatible reserved request-field rule. | [Backend registry](../../internal/backend/registry.go) |
+| `internal/capacity` | Owns engine-local bounded execution admission and FIFO model-generation permits for limited backend IDs, including cancellation-safe waiter removal and client wrapping. | [Internal capacity management](capacity.md) |
+| `internal/domain` | Defines internal framework values for requests, artifacts, prompt definitions, profiles, execution targets, rendering, generation, and validation. | [Domain declarations](../../internal/domain/domain.go) |
+| `internal/defaults` | Defines application-neutral framework constants and constructs the default execution target. It contains no CLI, server, or inbound HTTP limits. | [Framework defaults](../../internal/defaults/defaults.go) |
+| `internal/filecatalog` | Provides deterministic YAML discovery and path helpers for operating-system filesystems and `fs.FS` sources. | [File catalog](../../internal/filecatalog/catalog.go) |
+| `internal/jsonvalue` | Validates and deeply copies JSON-compatible extra-parameter and prepared-schema trees while preserving supported concrete value types. | [JSON values](../../internal/jsonvalue/jsonvalue.go) |
+| `internal/promptdef` | Loads strictly decoded, validated prompt definitions from filesystem and `fs.FS` sources, including version selection and contained file-backed message content. | [Framework formats](../formats.md), [prompt-definition repository](../../internal/promptdef/filesystem_repository.go) |
+| `internal/profile` | Loads strictly decoded, validated execution profiles, including backend selection, from filesystem and `fs.FS` sources and composes repositories with error-preserving fallback. | [Framework formats](../formats.md), [profile repositories](../../internal/profile/filesystem_repository.go) |
+| `internal/profile/builtin` | Embeds the built-in profile catalog, whose entries select OpenRouter, and combines it with an optional primary repository. | [Built-in catalog](../formats.md#built-in-profile-catalog), [repository](../../internal/profile/builtin/repository.go) |
+| `internal/prompt` | Renders prompt messages from Go templates with artifact, variable, session, and cache-control data. | [Go-template renderer](../../internal/prompt/go_renderer.go) |
+| `internal/artifact` | Resolves ordinary inline and unrestricted caller-selected file references into copied artifacts with metadata and hashes. | [Internal sources and validation](sources.md) |
+| `internal/validate` | Validates basic, JSON, and JSON Schema output using operating-system filesystem or `fs.FS` schema sources and creates frozen validation plans for prepared execution. | [Framework formats](../formats.md#schemas), [internal sources and validation](sources.md) |
+| `internal/llm` | Defines the internal generation boundary and implements outbound OpenAI-compatible chat requests from resolved execution targets, including response decoding, authentication, deadline handling, and ownership of the OpenAI-compatible reserved request-field policy. | [Internal model client](llm.md) |
+| `internal/usecase` | Resolves prompt definitions and hashes, profiles, backends, and targets for exact inspection and request settings for preparation, and coordinates ordinary execution and one-attempt prepared execution across internal sources, rendering, artifact loading, generation, validation, capacity, and optional repair. | [Internal runner](runner.md), [prepared-execution implementation](../../internal/usecase/prepared_execution.go) |
 
-The root `promptkit` package is the sole implemented Go package. No internal
+The root package assembles these internal components without exposing their
-framework packages exist yet.
+representations. Consumers depend only on the root facade.
 
 ## Maintenance
 
-Update this inventory as framework extraction introduces packages or changes
+Update this inventory as implementation adds packages or changes component
-component responsibilities. List only implemented components; proposed package
+responsibilities. List only implemented components; proposed package
 boundaries belong in temporary planning documents until their implementation
 lands.

docs/internal/runner.md

@@ -0,0 +1,189 @@
+# Internal Runner
+
+## Purpose
+
+This document describes Promptkit's implemented internal orchestration. The
+[architecture policy](../policy/architecture.md) owns dependency and consumer
+boundaries. The [source and validation document](sources.md) owns repository,
+artifact, rendering, and validation behavior, while the
+[model-client document](llm.md) owns generation behavior and failure
+categories.
+
+The runner remains under `internal/usecase` and is assembled by the root
+Promptkit engine. Its concrete type is not part of the public API.
+The [framework format reference](../formats.md) owns prompt, profile, schema,
+and override semantics consumed by the runner.
+
+## Collaborators
+
+`Runner` coordinates narrow internal interfaces for prompt definitions,
+profiles, backend resolution, artifacts, rendering, model generation, and
+validation. The root engine supplies one immutable registry containing the
+built-in backend and validated consumer additions, one engine-local run
+admitter, and a model client wrapped by the same capacity manager. Schema
+documents are loaded through the validator's optional schema-loader interface.
+An output repairer can be injected internally, but the ordinary runner
+constructor does not enable one.
+
+Each invocation carries its state in request, prepared-run, and result values.
+The runner has no durable run or session store.
+
+## Shared Prompt Selection
+
+The runner uses one prompt-selection and hashing boundary for ordinary
+preparation and exact prompt inspection. Preparation retains its early
+request-ID check before direct-session normalization; both operations then use
+the configured prompt repository to select one definition, load referenced
+message content, and calculate the same prompt hash.
+
+Inspection stops after that structural lookup. It does not parse templates or
+touch profile, artifact, schema, renderer, validator, admission, or model
+collaborators. The root [`Engine.InspectPrompt`](../../engine.go) GoDoc owns
+the public operation's exact contract.
+
+## Shared Profile Selection
+
+The runner uses one profile-selection and target-resolution boundary for
+ordinary preparation and exact profile inspection. Preparation first selects a
+request profile or a prompt default; inspection begins with its required
+explicit profile ID. Both then apply the ordinary source precedence, resolve a
+named backend, and construct the effective target from framework, backend, and
+profile values.
+
+Inspection stops after the resulting endpoint and model are structurally
+validated. It does not check credential availability or perform prompt,
+artifact, schema, rendering, admission, or model-client work. The root
+[`Engine.InspectProfile`](../../engine.go) GoDoc owns the public operation's
+exact contract.
+
+## Shared Preparation Pipeline
+
+`Prepare` and `Run` share one private preparation pipeline split at the point
+where a run can be assigned to its selected backend pool. The resolution phase
+performs only the work needed to validate routing and admission:
+
+1. validate the required prompt selection and normalize any direct session ID;
+2. load the prompt definition and hash the original definition;
+3. select the request profile or the prompt's default profile;
+4. resolve the profile's backend ID, when present;
+5. resolve application-neutral defaults, backend defaults, profile values,
+   and explicit request overrides in that order;
+6. validate endpoint, model, numeric overrides, and credential requirements;
+7. resolve the effective output contract without loading its schema; and
+8. retain the definition, source identities, effective settings, output
+   contract, and preparation start time in invocation-local state.
+
+The completion phase consumes that state without reloading the prompt,
+profile, or backend:
+
+1. load structured-output schema metadata when required;
+2. load and hash input artifacts;
+3. render messages and the prompt-defined session;
+4. apply any direct session ID;
+5. hash the effective rendered prompt; and
+6. construct the prepared value and preparation timing.
+
+`Prepare` runs both phases consecutively and never performs capacity admission.
+`Run` performs backend admission between the phases. This structure preserves
+one execution-precedence and error-ordering implementation while allowing a
+full backend pool to reject work before expensive schema, artifact, and
+rendering operations.
+
+Pointer-based numeric overrides preserve an explicit zero. Invalid negative or
+out-of-range values fail as invalid requests. Endpoint overrides do not change
+the selected backend identity. Non-empty extra-parameter maps replace whole
+lower-precedence maps. A direct API key takes precedence over environment
+lookup; otherwise request, profile, and backend environment-variable names
+apply in that order. A profile requiring a direct key clears an inherited
+backend environment name unless the request supplies its own name. Secret
+values remain excluded from serialized metadata.
+
+Reasoning overrides are tri-state: nil inherits the profile, a pointer to a
+nonblank string trims and replaces it, and a pointer to a blank string clears
+it. A nonblank direct session is normalized before source loading, bypasses
+the prompt session template, and is applied after ordinary message rendering.
+A blank direct value retains prompt-template behavior. The runner clears the
+template only on a value copy of the definition, so the definition hash always
+describes the original source while the rendered-prompt hash includes the
+effective direct or rendered session.
+
+The registry is read-only after engine construction. Concurrent `Prepare` and
+`Run` calls resolve independent defensive backend values and keep all
+invocation state local.
+
+## Run Flow
+
+`Run` records its start time, performs the shared resolution phase, and asks
+its `RunAdmitter` to reserve capacity for the effective backend ID. A nil
+admitter is an internal unlimited fallback. After successful admission, `Run`
+immediately defers the returned release function, performs the completion
+phase, makes one initial generation call, builds the named output artifact,
+and validates that artifact. Invalid generated content remains a validation
+result; an inability to perform validation is an operational error.
+
+The admission lease covers completion-phase preparation, initial generation,
+validation, every repair, and every exit. It bounds accepted work without
+serializing preparation or validation behind the active-generation limit.
+The wrapped model client separately acquires a FIFO active permit only around
+each actual generation call.
+
+When an internal repairer is present, a JSON or JSON Schema content failure can
+trigger bounded repair attempts. Repair receives the effective execution
+target and session ID, validation errors, prior output, and structured-output
+specification. The default repairer uses the same wrapped client as initial
+generation, so each repair reacquires the selected backend's active permit
+while remaining inside its original admission lease. Repair never performs a
+second bounded admission. This capability remains internal and is not a public
+option.
+
+A successful result includes the output artifact and raw output, validation
+state, effective session ID, prompt and rendered-prompt hashes, selected
+profile and backend, effective settings, input hashes, token usage, a generated
+run identifier, and UTC timing. The same effective session reaches initial
+generation and any repair attempt through the rendered prompt. The same
+effective target, including backend identity, reaches generation and any
+repair attempt.
+
+## Failure Categories
+
+Package errors distinguish invalid requests, required profile selection,
+credential failures, and prompt, profile, artifact, rendering, generation, and
+validation failures. Wrapping preserves the package identities mapped by the
+public facade and retains collaborator identities where they are part of the
+internal contract.
+
+Admission capacity exhaustion retains the internal capacity identity. At the
+use-case boundary, the runner attaches the selected backend ID in an internal
+typed error, and the root facade copies that value into the public
+[`CapacityError`](../../capacity_error.go) without parsing diagnostic text. It
+is not recategorized as an invalid request or generation failure, and no
+partial result is returned. A context already done at admission retains its
+context identity directly. Cancellation while waiting for an active generation
+permit prevents client invocation when it wins the grant race; the model-client
+boundary then preserves the context error through the generation-failure
+category. Deferred release restores the admission lease on preparation,
+generation, validation, repair, and cancellation failures.
+
+Other context cancellation propagates through the invoked collaborator and is
+classified by the owning operation.
+An overlong direct session is an invalid request before source loading, while
+an invalid or overlong prompt session template remains a prompt-render failure.
+An unknown selected backend, or a selected backend with no configured resolver,
+is classified as a profile-load failure.
+
+## Test Ownership And Changes
+
+The [runner tests](../../internal/usecase/runner_test.go) own preparation order,
+selection and override precedence, the two-phase boundary, early admission,
+lease lifetime and release, direct-session resolution, schema-before-generation
+behavior, hashing, generation and validation outcomes, backend propagation,
+bounded repair, shared initial/repair capacity, credentials and redaction,
+error categories, artifact metadata, usage, and timing. The
+[capacity subsystem document](capacity.md) identifies the focused pool,
+waiter, and wrapped-client tests.
+
+Changes to orchestration should continue to use the existing package
+interfaces, keep request state local to an invocation, and preserve the shared
+resolution and completion pipeline. Source, renderer, validator, or
+model-client contract changes belong first in their owning package and
+document.

docs/internal/sources.md

@@ -0,0 +1,99 @@
+# Internal Sources And Validation
+
+## Purpose
+
+This document describes Promptkit's implemented internal source, artifact,
+rendering, and output-validation behavior. The
+[architecture policy](../policy/architecture.md) owns the library boundary and
+dependency rules. None of these internal packages is a supported consumer API,
+and the root engine assembles them behind its public source options and values.
+The [framework format reference](../formats.md) owns the exact file fields,
+validation modes, built-in catalog, and source precedence.
+
+## Prompt Definitions
+
+`internal/promptdef` discovers YAML deterministically, decodes and validates
+definitions, selects an ID and optional version, and resolves file-backed
+message content within the selected operating-system or `fs.FS` source.
+
+Exact prompt inspection performs one point-in-time lookup through that same
+repository and validates referenced message content before returning declared
+metadata. It does not parse templates or read profile, input, or schema
+sources, and it does not retain the definition for a later execution.
+
+Its package tests own prompt selection, strict decoding, definition validation,
+duplicate detection, and source containment:
+[prompt-definition repository tests](../../internal/promptdef/repository_test.go).
+
+## Profiles And Built-Ins
+
+`internal/profile` loads and validates execution profiles from an
+operating-system filesystem or an `fs.FS`. It supports a primary repository
+with fallback only when the primary reports that a profile is absent. Strict
+YAML decoding recognizes the optional `backend` field, trims its value, and
+requires a model plus at least one non-blank backend or endpoint. Loading does
+not check registry membership because the available registry belongs to the
+assembled engine; the runner checks membership during preparation and exact
+profile inspection.
+
+Exact profile inspection performs one point-in-time lookup through those
+profile sources and checks the resolved target without reading prompt, input,
+or schema sources. It does not retain that lookup for a later execution.
+
+`internal/profile/builtin` embeds the maintained built-in profile catalog and
+can place a caller-selected repository ahead of that catalog. Every embedded
+profile selects `openrouter` and inherits its endpoint and credential
+environment-variable name from the built-in backend registry rather than
+repeating those values. Profile behavior is owned by the
+[profile repository tests](../../internal/profile/repository_test.go), while
+catalog completeness, the backend-selection invariant, duplicate IDs, and
+overlay behavior are owned by the
+[built-in repository tests](../../internal/profile/builtin/repository_test.go).
+
+## Ordinary Artifacts
+
+`internal/artifact` resolves inline references and unrestricted,
+caller-selected file paths. It copies content into an artifact, records
+metadata and a content hash, applies a content-type fallback, and honors
+context cancellation.
+
+This ordinary reader does not implement an inbound HTTP security boundary. In
+particular, it does not constrain files to an application root or impose an
+HTTP request-size policy. Scriptorium's restricted HTTP reader remains an
+application concern outside Promptkit. The
+[artifact reader tests](../../internal/artifact/reader_test.go) own the
+implemented reader behavior and failures.
+
+## Rendering
+
+`internal/prompt` renders definition messages as Go templates using named
+artifacts and variables. It carries message roles, session IDs, and cache
+control into the rendered prompt. The
+[renderer tests](../../internal/prompt/renderer_test.go) own rendering behavior.
+
+## Schemas And Output Validation
+
+`internal/validate` provides validators backed by an operating-system
+filesystem or an `fs.FS`. Invalid generated content is returned as a validation
+result; inability to load, register, or compile a schema is an operational
+error.
+
+For executable preparation, the built-in validators create a frozen validation
+plan. None, basic, and JSON modes retain the effective output contract without
+source access. JSON Schema mode loads the root document, resolves and compiles
+every transitive reference during preparation, and retains the compiled
+validator. The provider-facing structured-output metadata uses that same
+captured root document.
+
+`PrepareExecution` also completes prompt and profile selection, artifact
+loading and hashing, session and message rendering, and target resolution.
+`RunPrepared` uses the retained source-derived state and validation plan; it
+does not reopen prompt, profile, input, or schema sources and does not rerender
+the request. By contrast, ordinary `Prepare` produces a preparation value only:
+a later `Run` performs its own source resolution and preparation.
+
+The [validator tests](../../internal/validate/standard_validator_test.go) own
+basic, JSON, JSON Schema, source resolution, schema loading, compilation,
+frozen-reference behavior, and content-failure behavior. Prepared execution
+orchestration is owned by the
+[use-case tests](../../internal/usecase/prepared_execution_test.go).

docs/policy/architecture.md

@@ -9,15 +9,65 @@ implemented packages without redefining these rules.
 
 ## System Shape
 
-Promptkit is an importable Go library. It does not provide a runnable command,
+Promptkit is an importable Go library. It does not ship a command, an HTTP
-an HTTP service, or another application process.
+service, or another application process. Repository examples demonstrate
+library use but are not Promptkit applications or release artifacts.
 
-The module root contains package `promptkit`, which is the public facade and the
+The module root contains package `promptkit`, which is the public facade. It
-only implemented Go package in the current repository foundation. It declares
+provides the supported engine, configuration and source options, requests,
-the module's public package boundary but does not yet provide migrated framework
+results, public values, extension interfaces, profiles, and error sentinels.
-behavior or exported APIs. No internal framework packages currently exist.
 
-Future framework extraction must follow this dependency direction:
+The implemented internal components consist of:
+
+- `internal/domain`, which owns framework data values shared by later internal
+  components;
+- `internal/backend`, which owns validated immutable OpenAI-compatible backend
+  definitions and the built-in OpenRouter definition;
+- `internal/capacity`, which owns engine-local bounded run admission and
+  model-generation scheduling for limited backends;
+- `internal/defaults`, which owns application-neutral framework defaults and
+  constructs the default execution target;
+- `internal/filecatalog`, which discovers YAML files and provides source-path
+  helpers for filesystem and `fs.FS` consumers;
+- `internal/jsonvalue`, which validates and defensively copies JSON-compatible
+  extra-parameter trees;
+- `internal/promptdef`, which loads and validates prompt definitions from
+  filesystem and `fs.FS` sources;
+- `internal/profile`, which loads, validates, and overlays execution profiles
+  from filesystem and `fs.FS` sources;
+- `internal/profile/builtin`, which embeds the built-in execution profile
+  catalog;
+- `internal/prompt`, which renders prompt messages from Go templates;
+- `internal/artifact`, which resolves ordinary inline and unrestricted
+  caller-selected file references;
+- `internal/validate`, which validates basic, JSON, and JSON Schema output
+  using filesystem and `fs.FS` schema sources;
+- `internal/llm`, which defines the provider-neutral generation boundary and
+  implements outbound OpenAI-compatible chat requests; and
+- `internal/usecase`, which coordinates preparation and execution across the
+  internal framework components.
+
+The `examples/go-library/prepare` and `examples/go-library/run` packages are
+maintained downstream consumers of the root facade. They do not expose library
+packages or participate in internal assembly.
+
+The root facade assembles one immutable backend registry, one capacity manager,
+the internal repositories, renderer, validator, outbound client, and use-case
+runner while translating public values and errors at the library boundary. The
+registry contains built-ins plus validated engine-scoped consumer additions.
+The facade constructs the capacity manager from the registry's immutable
+policy snapshot, wraps the selected built-in or injected model client, and
+supplies bounded admission to the runner. The defaults and renderer depend on
+the domain model. Prompt-definition and profile repositories use the domain
+model, file catalog, and YAML decoder. The built-in profile repository supplies
+an embedded `fs.FS` to the profile package. Artifact reading uses the domain
+model and application-neutral defaults. Validation uses the domain model, file
+catalog, and JSON Schema implementation. The model client uses the domain
+model, application-neutral defaults, and an injected or standard-library HTTP
+client. The use-case runner depends on the narrow interfaces owned by each
+internal component, including backend lookup and run admission.
+
+The current implementation follows this dependency direction:
 
 ```text
 downstream consumers, including Scriptorium
@@ -32,14 +82,19 @@ downstream consumers, including Scriptorium
         narrow injected abstractions
 ```
 
-The facade may coordinate internal components. Internal components must depend
+The backend registry depends on the domain model and shared JSON-value
-on narrow abstractions for behavior supplied from outside the library; they
+validation, has no mutation API after construction, and consumes the
-must not depend on consumers or on Scriptorium. This diagram constrains future
+OpenAI-compatible reserved request-field rule owned by the model client. The
-work and does not assert that the internal components already exist.
+capacity component depends on the domain model and the narrow internal
+model-client boundary, not on provider transport implementation. The model
+client does not depend on registry or capacity configuration. The facade
+coordinates internal components and adapts the supported public extension
+interfaces to narrow internal abstractions. Internal components must not depend
+on consumers or on Scriptorium.
 
 ## Repository And Consumer Boundary
 
-Scriptorium is a downstream application that will consume Promptkit through
+Scriptorium is a downstream application that consumes Promptkit through
 the supported public facade. It is not a Promptkit package and must not become
 an internal dependency.
 
@@ -112,7 +167,6 @@ state.
 
 ## Current-State Maintenance
 
-This policy distinguishes present implementation from constraints on future
+Do not list planned packages as implemented components. When implementation
-framework extraction. Do not list planned packages as implemented components.
+introduces a package, update the internal inventory and the owning contract or
-When extraction introduces a package, update the internal inventory and the
+subsystem document in the same change.
-owning contract or subsystem document in the same change.

docs/policy/documentation.md

@@ -75,18 +75,20 @@ mechanisms, not secret values.
 | Testing policy | `docs/policy/testing.md` | Test philosophy, risk-based sufficiency, test boundaries, doubles, coverage guidance, regression policy, and test maintenance. | Subsystem behavior, exact public contracts, subsystem-specific test inventories, and implementation plans. |
 | Release procedure | `docs/release.md`, when present | Required release validation, version and tag procedure, release ordering, and post-publication checks. | General contributor workflow, public API semantics, and decision history. |
 | Exact exported Go API | Go declarations and GoDoc, as APIs are implemented | Exported names, signatures, types, values, errors, and exact behavioral contracts. | Task-oriented consumer walkthroughs, implementation details, and future API proposals. |
+| Framework file formats | `docs/formats.md` | Prompt-definition and profile YAML fields, schema references, defaults, validation modes, built-in profiles, credentials, and file-to-request precedence. | Exported Go declarations, outbound wire behavior, internal parsing mechanics, and application configuration. |
 | Consumer guidance | `docs/consumers/`, when consumer workflows require dedicated guidance | Task-oriented use of implemented public APIs, minimal examples, and consumer responsibilities. | Exact exported declarations and internal mechanics. |
 | Durable integration contracts | `docs/integrations/`, when integrations exist | External formats and protocols, compatibility behavior, and upstream or downstream responsibilities. | Internal transformations and public Go declarations. |
+| Supplemental release guidance | None. `docs/releases/` may be used when a release benefits from a changelog or migration guide. | No canonical content. These files may briefly summarize release-specific changes, compatibility, and consumer migration paths, and may be corrected, consolidated, archived, or removed when no longer useful. | Public API and behavior contracts, formats, integrations, architecture, release procedure, and the authoritative annotated-tag release record. |
 | Implemented component inventory | `docs/internal/overview.md` | Current packages and components, their implemented responsibilities, and links to focused internal documents. | Normative architecture, contributor workflow, external contracts, and proposed components. |
 | Internal subsystem behavior | Other files under `docs/internal/`, when a subsystem needs durable detail | Implementation flow, internal collaborators and state transitions, package-local guarantees and failures, and relevant tests. | Global architecture invariants, public API definitions, and future package plans. |
 | Architectural decision history | `docs/adr/`, when repository-local decisions require records | Significant decisions, context, alternatives, rationale, consequences, and supersession history. | Current behavior reference, implementation status, and task sequencing. |
 | Temporary feature roadmaps | `docs/roadmap/`, while planned work needs coordination | Proposed or accepted scope, sequencing, gates, and implementation status. | Implemented behavior reference and durable decision rationale. |
-| Complete copyable artifacts | `examples/`, when maintained examples exist | Valid inputs, Go programs, and other files intended to be copied or run. | Field-by-field reference, exact API declarations, and prose explanation. |
+| Complete copyable artifacts | `examples/` | Valid inputs, Go programs, and other files intended to be copied or run. | Field-by-field reference, exact API declarations, and prose explanation. |
 
 Conditional owners do not require placeholder files or directories. Create a
-consumer, integration, subsystem, ADR, roadmap, or example document only when
+consumer, integration, release, subsystem, ADR, roadmap, or example document
-the corresponding implemented interface, decision, planned effort, or
+only when the corresponding implemented interface, release, decision, planned
-maintained artifact exists.
+effort, or maintained artifact exists.
 
 ## Boundary Rules
 
@@ -105,6 +107,26 @@ for a task. Internal documents explain how Promptkit implements them. Internal
 documentation may identify a public type or external format as a dependency,
 but must link to its canonical definition rather than restate it.
 
+The [framework format reference](../formats.md) owns exact prompt, profile, and
+schema-file contracts. Integration documents own external wire formats.
+
+### Supplemental Release Guidance
+
+Files under `docs/releases/` may provide changelog-style summaries and
+migration guidance for a particular release. They are navigation and
+orientation aids, not canonical owners of public APIs, behavior, formats,
+integrations, architecture, release procedure, or other durable facts. When a
+reader needs detail beyond a short release-specific note, the release document
+must link to the applicable canonical documentation rather than reproduce its
+contract.
+
+The annotated tag message required by the
+[release procedure](../release.md#write-the-release-note) remains the
+authoritative release record. Supplemental release documents may be corrected,
+consolidated, archived, or removed at any time when they are no longer useful,
+provided maintained documentation does not depend on them and the annotated
+tag record remains intact.
+
 ### Security Topics
 
 This policy owns what documentation and examples may contain. Architecture owns
@@ -156,6 +178,10 @@ durable owners, update incoming links, and archive or remove the roadmap
 according to repository practice. Do not preserve completed roadmaps as a
 second current-state reference.
 
+Supplemental release documents may likewise be removed without preserving a
+replacement. Before removal, update maintained incoming links so current
+documentation does not depend on an optional historical guide.
+
 Before completing documentation work:
 
 - verify affected behavior and examples;

docs/policy/testing.md

@@ -55,6 +55,15 @@ documentation-link, and repository-hygiene checks before accepting changes.
 Introducing hosted CI later would supplement, not silently redefine, this
 documented validation model.
 
+The complete test sequence includes ordinary and race-enabled package tests.
+The maintained offline consumer workflow is also run from the repository root:
+
+```sh
+go test ./...
+go test -race ./...
+go run ./examples/go-library/prepare
+```
+
 Tests in the default suite must be deterministic, offline, and independent of
 real credentials. They must not invoke paid APIs, use live network
 dependencies, or depend on mutable external services. Tests that require live
@@ -77,6 +86,10 @@ Use each test type where it protects a distinct risk:
   relied upon by consumers.
 - Integration tests use real collaborators when correctness depends on their
   interaction, while replacing live or nondeterministic external boundaries.
+- External-package root tests exercise the public facade as a Go consumer,
+  while internal package tests own focused implementation behavior.
+- The maintained offline preparation example protects one representative
+  assembled consumer workflow without contacting a model provider.
 - Fixtures should be minimal, synthetic, versioned with the behavior they
   exercise, and free of credentials or private data.
 - Golden files are appropriate only when the complete output is intentionally
@@ -154,7 +167,8 @@ A test failing is not the same as a test needing to be edited. Many tests may co
 
 Configurable thresholds and defaults must not be duplicated throughout the test suite.
 
-For example, do not encode an internal concurrency limit indirectly:
+The following fragments are illustrative rather than standalone Go programs.
+Do not encode an internal concurrency limit indirectly:
 
 ```go
 // Production policy:

docs/release.md

@@ -7,26 +7,91 @@ tags. It does not publish runnable binaries or binary packages and does not
 currently use hosted CI. The release maintainer performs and records the
 required validation.
 
-The first planned release is `v0.1.0`. Do not create that tag until the
+`v0.1.0` is the initial published release. Later releases use semantic
-framework has been extracted and the resulting public library has passed this
+`vMAJOR.MINOR.PATCH` tags. Before `v1`, minor releases may change the public
-procedure. Later tags use the `vMAJOR.MINOR.PATCH` form. While Promptkit remains
+API and patch releases preserve compatibility within their minor line. Every
-pre-`v1`, release notes must identify intentional public API changes and any
+pre-`v1` release note must summarize compatibility, identify public API
-consumer migration required by them.
+changes, and state any action required of consumers.
 
-## Prepare The Release
+Promptkit releases are source-only. The annotated tag message is the release
+note; there is no separate hosted release or binary packaging step.
 
-Work from a clean checkout of the intended release commit, outside any Go
+## Establish The Candidate
-workspace and without a local module replacement. Confirm the source commit is
-already published through the normal branch workflow.
 
-From the Promptkit repository root, verify the checkout:
+Choose a version that has not been published and export it as
+`RELEASE_VERSION`. Run every command in this procedure from the Promptkit
+repository root in the same POSIX shell. Do not reuse `v0.1.0` or another
+existing version.
+
+The following guard derives the release commit from `HEAD` and stops on a
+missing or malformed version, a checkout other than synchronized `main`,
+uncommitted changes, an active Go workspace, a module replacement, a vendor
+tree, or an existing local or remote tag:
 
 ```sh
-test -z "$(go env GOWORK)"
+set -eu
-test -z "$(git status --short)"
+
-git fetch --tags origin
+: "${RELEASE_VERSION:?export an unpublished vMAJOR.MINOR.PATCH version}"
+if ! printf '%s\n' "$RELEASE_VERSION" |
+    grep -Eq '^v(0|[1-9][0-9]*)\.(0|[1-9][0-9]*)\.(0|[1-9][0-9]*)$'
+then
+    printf '%s\n' "invalid release version: $RELEASE_VERSION" >&2
+    exit 1
+fi
+
+RELEASE_COMMIT=$(git rev-parse --verify 'HEAD^{commit}')
+export RELEASE_COMMIT
+
+check_release_candidate() {
+    test "$(git branch --show-current)" = main
+    test -z "$(git status --porcelain)"
+
+    gowork_value=$(go env GOWORK)
+    case "$gowork_value" in
+        ''|off) ;;
+        *)
+            printf '%s\n' "active Go workspace: $gowork_value" >&2
+            return 1
+            ;;
+    esac
+
+    test -z "$(git ls-files go.work go.work.sum)"
+    test ! -e vendor
+    if grep -Eq '^[[:space:]]*replace([[:space:]]|\()' go.mod
+    then
+        printf '%s\n' 'go.mod contains a replacement' >&2
+        return 1
+    fi
+
+    git fetch origin main --tags
+    test "$RELEASE_COMMIT" = \
+        "$(git rev-parse --verify 'refs/remotes/origin/main^{commit}')"
+
+    if git show-ref --verify --quiet "refs/tags/$RELEASE_VERSION"
+    then
+        printf '%s\n' "local tag already exists: $RELEASE_VERSION" >&2
+        return 1
+    fi
+    if test -n "$(
+        git ls-remote --tags origin \
+            "refs/tags/$RELEASE_VERSION" \
+            "refs/tags/$RELEASE_VERSION^{}"
+    )"
+    then
+        printf '%s\n' "remote tag already exists: $RELEASE_VERSION" >&2
+        return 1
+    fi
+}
+
+check_release_candidate
 ```
 
+Do not continue unless the guard completes successfully. In particular, push
+the intended commit through the normal `main` branch workflow before release;
+the tag procedure is not a substitute for publishing the source commit.
+
+## Validate The Candidate
+
 Confirm the module and root package metadata:
 
 ```sh
@@ -41,93 +106,169 @@ gitea.maximumdirect.net/eric/promptkit 1.25.5
 promptkit gitea.maximumdirect.net/eric/promptkit
 ```
 
-Run the same default Go validation required by the
+Run the complete maintainer validation required by the
 [development guide](development.md):
 
 ```sh
 go test ./...
+go test -race ./...
 go vet ./...
 go build ./...
+go run ./examples/go-library/prepare
 ```
 
-Check every tracked Go file and repository whitespace:
+Check every tracked Go file. This command must produce no output:
 
 ```sh
-gofmt -l $(git ls-files '*.go')
+unformatted=$(
+    git ls-files '*.go' |
+        while IFS= read -r go_file
+        do
+            gofmt -l "$go_file"
+        done
+)
+test -z "$unformatted"
+```
+
+Follow every maintained Markdown link and confirm that its local or published
+target exists. Review the repository for generated binaries, test or coverage
+output, credentials, template residue, downloaded assets, and other files that
+do not belong in source control.
+
+Recheck module and repository hygiene, whitespace, and the clean checkout:
+
+```sh
+test -z "$(git ls-files go.work go.work.sum)"
+test ! -e vendor
+if grep -Eq '^[[:space:]]*replace([[:space:]]|\()' go.mod
+then
+    printf '%s\n' 'go.mod contains a replacement' >&2
+    exit 1
+fi
 git diff --check
+test -z "$(git status --porcelain)"
 ```
 
-The formatting command must produce no paths. Follow every maintained Markdown
+## Write The Release Note
-link and confirm its target exists. Review the repository for generated
-binaries, test or coverage output, credentials, template residue, and other
-files that do not belong in source control.
 
-Confirm that no workspace override is tracked and that `go.mod` contains no
+Prepare a plain-text annotated-tag message outside the repository and export
-`replace` directive:
+its path as `RELEASE_NOTES_FILE`. Use this form, replacing each summary with
+release-specific text; write `None.` when there are no public API changes or
+consumer actions:
 
-```sh
+```text
-git ls-files go.work go.work.sum
+Promptkit vMAJOR.MINOR.PATCH
-rg -n '^replace\b' go.mod
+
+Validated commit: full commit ID
+Compatibility: compatibility summary
+Public API changes: changes or None.
+Consumer action: required action or None.
 ```
 
-Both commands must produce no output. Re-run `git status --short` and require a
+After writing it, require all release-note fields, the selected version, and
-clean result after every validation and review check.
+the validated commit to be present:
-
-## Create And Publish The Tag
-
-Choose the semantic version from the intended compatibility change. Record the
-release commit before tagging:
 
 ```sh
-release_version=v0.1.0
+: "${RELEASE_NOTES_FILE:?export the path to the release-note file}"
-release_commit=$(git rev-parse HEAD)
+test -f "$RELEASE_NOTES_FILE"
+test -s "$RELEASE_NOTES_FILE"
+grep -F "Promptkit $RELEASE_VERSION" "$RELEASE_NOTES_FILE"
+grep -F "Validated commit: $RELEASE_COMMIT" "$RELEASE_NOTES_FILE"
+grep -F 'Compatibility:' "$RELEASE_NOTES_FILE"
+grep -F 'Public API changes:' "$RELEASE_NOTES_FILE"
+grep -F 'Consumer action:' "$RELEASE_NOTES_FILE"
 ```
 
-Replace the example version for later releases and keep both values in the same
+Inspect the complete message and confirm that it accurately records the
-shell for the remaining commands. Confirm the tag does not already exist
+compatibility impact, public API changes, and required consumer action.
-locally or remotely:
+
+## Create And Inspect The Tag
+
+Run the candidate guard again immediately before tag creation. This ensures
+that validation or release-note preparation did not change the checkout and
+that the commit is still published and untagged:
 
 ```sh
-test -z "$(git tag --list "$release_version")"
+check_release_candidate
-test -z "$(git ls-remote --tags origin "refs/tags/$release_version")"
 ```
 
-Create an annotated tag whose message identifies the release and records that
+Create the annotated tag from the prepared release note and bind it explicitly
-the documented validation passed for the tagged commit:
+to the validated commit:
 
 ```sh
-git tag --annotate "$release_version" \
+git tag --annotate "$RELEASE_VERSION" \
-  --message "Promptkit $release_version; documented validation passed for $release_commit"
+    --file "$RELEASE_NOTES_FILE" \
+    "$RELEASE_COMMIT"
 ```
 
-Inspect the tag before publication:
+Inspect both the tag message and its source commit before publication:
 
 ```sh
-git show --no-patch --decorate "$release_version"
+test "$(git cat-file -t "refs/tags/$RELEASE_VERSION")" = tag
-test "$(git rev-list -n 1 "$release_version")" = "$release_commit"
+git show --no-patch --decorate "refs/tags/$RELEASE_VERSION"
+test "$(
+    git rev-parse --verify "refs/tags/$RELEASE_VERSION^{commit}"
+)" = "$RELEASE_COMMIT"
 ```
 
-Publish the tag without relying on a hosting-provider-specific release
+If inspection finds an error, delete the unpublished local tag, correct the
-interface:
+release note or candidate, and repeat the guards. Never move or recreate a tag
+that has been published.
+
+## Publish The Selected Tag
+
+Push only the selected tag ref. Do not use `git push --tags`:
 
 ```sh
-git push origin "refs/tags/$release_version"
+git push origin \
+    "refs/tags/$RELEASE_VERSION:refs/tags/$RELEASE_VERSION"
 ```
 
 ## Verify Publication
 
-Confirm that the remote tag object matches the local annotated tag and still
+Compare the remote annotated-tag object with the local object, then compare the
-resolves to the intended source commit:
+remote peeled source commit with the validated commit:
 
 ```sh
-remote_tag=$(git ls-remote --tags origin "refs/tags/$release_version" | awk '{print $1}')
+remote_tag=$(
-test "$remote_tag" = "$(git rev-parse "refs/tags/$release_version")"
+    git ls-remote --tags origin "refs/tags/$RELEASE_VERSION" |
-test "$(git rev-list -n 1 "refs/tags/$release_version")" = "$release_commit"
+        awk 'NR == 1 { print $1 }'
+)
+remote_commit=$(
+    git ls-remote --tags origin "refs/tags/$RELEASE_VERSION^{}" |
+        awk 'NR == 1 { print $1 }'
+)
+test -n "$remote_tag"
+test "$remote_tag" = \
+    "$(git rev-parse --verify "refs/tags/$RELEASE_VERSION")"
+test "$remote_commit" = "$RELEASE_COMMIT"
 ```
 
-Promptkit must publish the required tag before Scriptorium or another consumer
+Finally, resolve the version as an ordinary Go module in a temporary module
-publishes a release that depends on that version. Released consumer modules
+outside this repository and without a workspace or replacement:
-must not use a local replacement or unpublished Promptkit revision.
+
+```sh
+resolution_dir=$(mktemp -d)
+(
+    trap 'rm -rf "$resolution_dir"' 0 1 2 15
+    cd "$resolution_dir"
+    GOWORK=off go mod init example.com/promptkit-release-check
+    GOWORK=off go mod download \
+        "gitea.maximumdirect.net/eric/promptkit@$RELEASE_VERSION"
+    resolved_version=$(
+        GOWORK=off go list -m -f '{{.Version}}' \
+            "gitea.maximumdirect.net/eric/promptkit@$RELEASE_VERSION"
+    )
+    test "$resolved_version" = "$RELEASE_VERSION"
+)
+```
+
+Promptkit must publish and verify the required version before Scriptorium or
+another consumer publishes a release that depends on it. This ordering does
+not replace the consumer project's own release procedure. Released consumers
+must select the published Promptkit tag through ordinary module resolution,
+without a workspace, replacement, vendored Promptkit source, or unpublished
+revision.
 
 ## Policy Changes
 

docs/releases/v0.2.0.md

@@ -0,0 +1,243 @@
+# Promptkit v0.2.0
+
+This supplemental changelog and migration guide summarizes the consumer-facing
+changes from `v0.1.0` to `v0.2.0`. The annotated `v0.2.0` tag is the
+authoritative release record. Exact current contracts belong to the linked
+GoDoc and durable documentation.
+
+## Summary
+
+`v0.2.0` adds three major capabilities:
+
+- an engine-scoped registry for reusable OpenAI-compatible backend
+  definitions;
+- bounded, backend-specific run admission and model-generation concurrency;
+  and
+- direct per-run session IDs and tri-state reasoning-effort overrides.
+
+Existing endpoint-only profiles remain supported. Consumers can adopt backend
+registration and runtime overrides incrementally rather than rewriting all
+profiles during the upgrade.
+
+## Compatibility At A Glance
+
+Promptkit remains pre-`v1`, and this minor release includes source-level and
+behavioral changes that deserve review.
+
+| Area | `v0.1.0` consumer impact |
+| --- | --- |
+| Endpoint-only profiles | Continue to work without migration. |
+| Built-in profiles | Continue to use OpenRouter and `OPENROUTER_API_KEY`; they now select the built-in `openrouter` backend. |
+| Custom backends | Registration is optional. Existing profiles may keep their endpoint and credential configuration. |
+| Reasoning overrides | String assignments must migrate to the new pointer field. |
+| `RunRequest.Metadata` | Removed; delete assignments to this field. |
+| OpenRouter concurrency | Now limited to 16 active generations with waiting capacity of 1024 per engine. |
+| Public JSON | `v0.2.0` formalizes supported JSON representations; consumers relying on `v0.1.0` encodings should review the notes below. |
+| Unkeyed public struct literals | May require updates because fields were added. Keyed literals are recommended. |
+
+## Upgrade
+
+After the `v0.2.0` tag is published, update the module dependency with:
+
+```sh
+go get gitea.maximumdirect.net/eric/promptkit@v0.2.0
+go mod tidy
+```
+
+Run the consuming project's ordinary tests and race-enabled tests after the
+upgrade, especially if it calls one engine concurrently or persists Promptkit
+JSON values.
+
+## Backend Registry
+
+Consumers may now register reusable OpenAI-compatible backend definitions with
+`WithBackend`, then select them by ID from file-backed or in-memory profiles.
+A backend can supply its endpoint, API-key environment-variable name,
+request-wide extra parameters, and optional capacity policy.
+
+Registrations are immutable and belong to one engine. Consumer registrations
+can add new IDs but cannot replace Promptkit's reserved `openrouter` backend.
+Profiles that select a backend may still override its endpoint without losing
+the backend's routing or capacity identity.
+
+An existing endpoint-only in-memory profile remains valid:
+
+```go
+promptkit.Profile{
+	ID:       "local",
+	Endpoint: "http://localhost:8000/v1",
+	Model:    "example-model",
+}
+```
+
+Adopting the registry is optional and can be done when several profiles should
+share connection or capacity settings:
+
+```go
+engine, err := promptkit.NewEngine(
+	promptkit.Config{PromptDir: "prompts"},
+	promptkit.WithBackend(promptkit.Backend{
+		ID:        "local",
+		Endpoint:  "http://localhost:8000/v1",
+		APIKeyEnv: "LOCAL_LLM_API_KEY",
+	}),
+	promptkit.WithProfiles(promptkit.Profile{
+		ID:        "local-summary",
+		BackendID: "local",
+		Model:     "example-model",
+	}),
+)
+```
+
+See the
+[local-endpoint consumer guide](../consumers/pkg-promptkit.md#configure-a-local-openai-compatible-endpoint)
+for task-oriented usage. The
+[`Backend` and `WithBackend` GoDoc](../../backends.go) owns exact registration,
+validation, copying, defaulting, and uniqueness semantics. The
+[framework format reference](../formats.md) owns the profile `backend` field
+and execution precedence.
+
+## Backend-Specific Concurrency
+
+Each registered backend may now define:
+
+- an active model-generation limit; and
+- a bounded number of additional admitted `Run` calls.
+
+Promptkit owns scheduling for both its built-in model client and an injected
+`LLMClient`. `Run` remains synchronous: an admitted caller waits for its
+ordinary result, while a call beyond the bounded admission capacity returns
+`ErrCapacityExceeded`. Capacity is engine-local and keyed by backend ID.
+Endpoint-only profiles and custom backends without a configured limit remain
+unlimited.
+
+The built-in OpenRouter backend now permits 16 active generations and 1024
+additional admitted calls per engine. Applications that can exceed this bound
+should handle capacity exhaustion separately from provider and request
+failures:
+
+```go
+result, err := engine.Run(ctx, request)
+if errors.Is(err, promptkit.ErrCapacityExceeded) {
+	// Apply application-specific overload or retry policy.
+}
+```
+
+Promptkit does not prescribe retries or map this error to an HTTP status. See
+the
+[concurrency consumer guidance](../consumers/pkg-promptkit.md#limit-backend-concurrency)
+and the [`Backend` GoDoc](../../backends.go) for the canonical configuration
+contract. Runtime behavior and public error identities belong to the
+[`Engine.Run` GoDoc](../../engine.go).
+
+## Per-Run Session IDs
+
+`RunRequest.SessionID` can now supply a consumer-managed correlation ID for one
+`Prepare` or `Run` invocation. A nonblank direct value overrides the prompt's
+session template and is exposed in prepared values, results, injected-client
+requests, and provider observability. Session IDs should therefore be stable,
+non-secret values.
+
+```go
+result, err := engine.Run(ctx, promptkit.RunRequest{
+	PromptID: "meeting.summary",
+	SessionID: "conversation-42",
+})
+```
+
+The built-in OpenAI-compatible client sends a nonempty effective session as the
+top-level `session_id` request-body field, not as an `x-session-id` header. See
+the
+[session and reasoning consumer guide](../consumers/pkg-promptkit.md#set-a-per-run-session-and-reasoning),
+the [`RunRequest` GoDoc](../../types.go), and the
+[OpenAI-compatible request contract](../integrations/openai-compatible-chat.md#request-body)
+for exact normalization, length, exposure, and wire behavior.
+
+## Per-Run Reasoning Effort
+
+`ExecutionTargetOverride.ReasoningEffort` changed from `string` to `*string` so
+one request can distinguish inheritance, replacement, and explicit clearing.
+
+Update a `v0.1.0` override like this:
+
+```go
+// v0.1.0
+Execution: &promptkit.ExecutionTargetOverride{
+	ReasoningEffort: "high",
+}
+```
+
+to:
+
+```go
+// v0.2.0
+reasoning := "high"
+Execution: &promptkit.ExecutionTargetOverride{
+	ReasoningEffort: &reasoning,
+}
+```
+
+The three states are:
+
+- `nil` inherits the selected profile's value;
+- a pointer to a nonblank string replaces it for that invocation; and
+- a pointer to an empty or whitespace-only string clears it for that
+  invocation.
+
+This allows consumers to consolidate profiles that differed only by reasoning
+effort. The [`ExecutionTargetOverride` GoDoc](../../types.go) owns the exact
+override contract.
+
+## Other Migration Notes
+
+### Remove `RunRequest.Metadata`
+
+`RunRequest.Metadata` is no longer part of the public request. Remove any
+assignment to that field. Use application-owned state keyed by `RunResult.RunID`
+or a direct `SessionID` when correlation is needed; these identifiers have
+different purposes, so choose according to the application's lifecycle.
+
+### Review Persisted JSON
+
+`v0.2.0` defines stable JSON representations for the public result, artifact,
+execution, validation, and model-client values listed in the
+[package documentation](../../doc.go). Consumers that treated `v0.1.0`
+reflection-derived encodings as stable should update fixtures and stored-data
+adapters.
+
+In particular:
+
+- `RunResult` encodes elapsed time as integer milliseconds in `duration_ms`
+  instead of encoding `time.Duration` under `duration`;
+- result JSON can include the new `session_id` and `selected_backend_id`
+  fields;
+- execution-target JSON can include `backend_id`; and
+- artifact and target-presence fields now use their documented lower-case
+  names.
+
+The `v0.2.0` `RunResult` decoder reads `duration_ms`; it does not translate a
+persisted `v0.1.0` `duration` field. Transform old payloads before decoding
+when preserving their elapsed duration matters.
+
+### Prefer Keyed Struct Literals
+
+New fields were added to several public structs. Replace positional composite
+literals with keyed literals so future additive fields do not cause another
+source migration.
+
+## Migration Checklist
+
+- Update the module dependency and run the consumer's tests.
+- Change reasoning overrides from strings to pointers.
+- Remove uses of `RunRequest.Metadata`.
+- Review unkeyed Promptkit struct literals.
+- Decide whether shared endpoints should move into registered backends.
+- If using built-in OpenRouter profiles at high concurrency, handle
+  `ErrCapacityExceeded` and review the new engine-local bound.
+- Review stored JSON, fixtures, and downstream decoders.
+- Optionally replace profile-specific session or reasoning variants with
+  per-run overrides.
+
+For complete consumer workflows, use the
+[package consumer guide](../consumers/pkg-promptkit.md) and maintained
+[offline execution example](../../examples/go-library/run/main.go).

docs/releases/v0.3.0.md

@@ -0,0 +1,74 @@
+# Promptkit v0.3.0
+
+This supplemental changelog summarizes the consumer-facing changes from
+`v0.2.0` to `v0.3.0`. The annotated `v0.3.0` tag is the authoritative release
+record. Exact current contracts belong to the linked GoDoc and durable
+documentation.
+
+## Summary
+
+`v0.3.0` adds a concise way to register the common local OpenAI-compatible
+backend configuration:
+
+- `BackendLocal` provides the conventional, non-reserved backend ID `"local"`;
+  and
+- `LocalBackend` constructs an ordinary `Backend` from an endpoint and
+  concurrency limit.
+
+The helper is explicit and additive. It does not pre-register a backend, read
+environment variables, select a model, or replace the complete `Backend`
+configuration interface.
+
+## Compatibility
+
+Existing `v0.2.0` consumers require no migration. Endpoint-only profiles,
+complete custom `Backend` values, the built-in OpenRouter backend, and existing
+registrations using the literal ID `"local"` continue to work unchanged.
+
+## Upgrade
+
+Update the module dependency with:
+
+```sh
+go get gitea.maximumdirect.net/eric/promptkit@v0.3.0
+go mod tidy
+```
+
+Run the consuming project's ordinary tests and race-enabled tests after the
+upgrade.
+
+## Configure A Local Backend
+
+Register the convenience value through the existing `WithBackend` option and
+select it from one or more profiles:
+
+```go
+engine, err := promptkit.NewEngine(
+	promptkit.Config{PromptDir: "prompts"},
+	promptkit.WithBackend(
+		promptkit.LocalBackend("http://localhost:8000/v1", 2),
+	),
+	promptkit.WithProfiles(promptkit.Profile{
+		ID:        "local-summary",
+		BackendID: promptkit.BackendLocal,
+		Model:     "example-model",
+	}),
+)
+```
+
+Use an endpoint-only profile when shared backend identity and capacity policy
+are unnecessary. Continue to use a complete keyed `Backend` value for custom
+IDs, authentication, extra request parameters, explicit queue capacity, or
+multiple local endpoints.
+
+See the
+[local-endpoint consumer guide](../consumers/pkg-promptkit.md#configure-a-local-openai-compatible-endpoint)
+for task-oriented configuration choices. The
+[`BackendLocal`, `LocalBackend`, and `WithBackend` GoDoc](../../backends.go)
+owns their exact construction, registration, validation, and concurrency
+semantics.
+
+## Consumer Action
+
+None. Adopt the convenience constructor when it simplifies local endpoint
+configuration.

docs/releases/v0.4.0.md

@@ -0,0 +1,189 @@
+# Promptkit v0.4.0
+
+This supplemental changelog and adoption guide summarizes the consumer-facing
+changes from `v0.3.0` to `v0.4.0`. The annotated `v0.4.0` tag is the
+authoritative release record. Exact current contracts belong to the linked
+GoDoc and durable documentation.
+
+## Summary
+
+`v0.4.0` adds four complementary capabilities:
+
+- opaque prepared-execution handles for preparing once, inspecting safe
+  details, and executing the same frozen snapshot;
+- exact prompt-definition inspection without profile resolution or execution;
+- exact profile inspection without selecting a prompt or checking credential
+  availability; and
+- structured backend identity on engine admission-capacity rejection.
+
+These APIs let consumers perform more precise preflight work and retain useful
+operational context without reproducing Promptkit's internal resolution logic.
+
+## Compatibility
+
+The release is additive for `v0.3.0` consumers. Existing uses of `Prepare`,
+`Run`, backend registration, endpoint-only profiles, local-backend helpers,
+runtime overrides, public JSON values, and error sentinels continue to work
+without migration.
+
+Capacity rejection now returns a structured error while continuing to match
+`ErrCapacityExceeded` through `errors.Is`. Error-string wording and direct
+sentinel equality were not public contracts.
+
+The new inspection values, capacity error, and prepared-execution handle do not
+have stable JSON representations. `PreparedExecution.Details` returns the
+existing stable `PreparedRun` value.
+
+## Upgrade
+
+Update the module dependency with:
+
+```sh
+go get gitea.maximumdirect.net/eric/promptkit@v0.4.0
+go mod tidy
+```
+
+Run the consuming project's ordinary and race-enabled tests after upgrading.
+No source migration is required.
+
+## Prepare Once And Execute The Same Snapshot
+
+Consumers that need to persist preparation details before generation can now
+prepare an opaque, engine-bound execution:
+
+```go
+prepared, err := engine.PrepareExecution(ctx, request)
+if err != nil {
+	// Handle preparation failure.
+}
+defer prepared.Discard()
+
+details := prepared.Details()
+// Persist a consumer-selected, appropriately protected preparation record.
+
+result, err := engine.RunPrepared(ctx, prepared)
+```
+
+Preparation freezes the selected sources, rendered messages, effective
+settings, input content, structured-output metadata, and validation resources
+needed by execution. `Details` returns a fresh, caller-owned,
+credential-redacted `PreparedRun`.
+
+A handle belongs to its creating engine and permits one execution attempt.
+`RunPrepared` consumes that attempt on success and on operational failure.
+`Discard` is idempotent and releases an unclaimed handle's execution-only
+state. Consumers should discard handles they will not execute, particularly
+when a direct request API key may be retained privately until claim or
+discard.
+
+Prepared execution does not reserve backend admission during preparation.
+Credential availability and backend admission are checked when execution
+begins. The execution context is independent of the preparation context.
+
+See the
+[prepared-execution consumer guide](../consumers/pkg-promptkit.md#prepare-now-and-execute-the-same-snapshot-later),
+the [`PreparedExecution` GoDoc](../../prepared_execution.go), and the
+[`Engine.PrepareExecution` and `Engine.RunPrepared` GoDoc](../../engine.go)
+for the exact lifecycle, ownership, cancellation, capacity, timing, and
+failure contracts.
+
+## Inspect A Prompt
+
+`Engine.InspectPrompt` resolves one prompt ID and optional version through the
+engine's configured prompt source:
+
+```go
+inspection, err := engine.InspectPrompt(ctx, "report.summary", "")
+```
+
+The result includes prompt identity, the opaque prompt hash, declared default
+profile ID, declared input metadata, and normalized output contract. It
+structurally loads the selected definition and referenced message content but
+does not resolve a profile, load schemas or artifacts, render templates,
+reserve capacity, or contact a model.
+
+Use inspection for exact configuration checks and metadata discovery. Use
+`PrepareExecution` rather than relying on a prior inspection when later
+execution must freeze one exact source state, because filesystem-backed
+inspection is only a point-in-time lookup.
+
+See the
+[prompt-inspection consumer guide](../consumers/pkg-promptkit.md#inspect-a-prompt-before-preparation)
+and [`Engine.InspectPrompt` GoDoc](../../engine.go) for exact selection,
+ownership, and error behavior.
+
+## Inspect A Profile
+
+`Engine.InspectProfile` resolves one explicit profile independently of a
+prompt:
+
+```go
+inspection, err := engine.InspectProfile(ctx, "report-production")
+```
+
+The result includes the resolved effective execution target and whether a
+later request must provide a direct credential. Environment-variable names may
+be reported, but inspection does not read credential values or require the
+named variable to be populated.
+
+Inspection applies the ordinary configured and built-in profile precedence and
+resolves any selected backend. It does not load a prompt, render content,
+reserve capacity, or contact a model.
+
+See the
+[profile-inspection consumer guide](../consumers/pkg-promptkit.md#inspect-a-profile-before-prompt-work)
+and [`Engine.InspectProfile` GoDoc](../../engine.go) for the exact resolution,
+credential, ownership, and error contracts.
+
+## Identify Capacity-Rejected Backends
+
+Calls rejected at Promptkit's bounded engine admission boundary continue to
+match `ErrCapacityExceeded`. Consumers can additionally obtain the selected
+registered backend ID without parsing diagnostic text:
+
+```go
+result, err := engine.Run(ctx, request)
+if errors.Is(err, promptkit.ErrCapacityExceeded) {
+	var capacityErr *promptkit.CapacityError
+	if errors.As(err, &capacityErr) {
+		// Record capacityErr.BackendID using application-owned diagnostics.
+	}
+
+	// Apply application-owned overload or retry policy.
+}
+```
+
+The structured error applies to `Run` and `RunPrepared` admission rejection.
+It does not represent provider throttling, quota exhaustion, cancellation
+while waiting for generation capacity, or another model-client failure.
+Promptkit does not prescribe retry timing or transport status mapping.
+
+See the
+[error-handling consumer guide](../consumers/pkg-promptkit.md#handle-errors),
+the [`CapacityError` GoDoc](../../capacity_error.go), and the
+[`ErrCapacityExceeded` GoDoc](../../engine.go) for the canonical contracts.
+
+## Public API Additions
+
+The release adds:
+
+- `Engine.PrepareExecution`;
+- `Engine.RunPrepared`;
+- `PreparedExecution`, including `Details`, `Discard`, `String`, and
+  `GoString`;
+- `Engine.InspectPrompt`;
+- `PromptInspection`;
+- `PromptInputDefinition`;
+- `Engine.InspectProfile`;
+- `ProfileInspection`; and
+- `CapacityError`.
+
+No public API was removed.
+
+## Consumer Action
+
+None. Existing `v0.3.0` workflows may upgrade without adopting the new APIs.
+
+Consumers that adopt prepared execution should discard unused handles.
+Consumers that need backend-specific capacity diagnostics may add an
+`errors.As` check while retaining their existing `errors.Is` classification.

docs/roadmap/future.md

@@ -0,0 +1,51 @@
+# Future Feature Ideas
+
+## Purpose
+
+This document catalogs reasonably specific ideas that may be useful in future
+Promptkit development. It is an idea pool, not a commitment, schedule, or
+description of current behavior.
+
+Ideas belong here while they are worth retaining but have not been selected
+for active development. Keep each entry at the level of intended capability,
+consumer value, and important scope boundaries. Defer API design,
+implementation details, sequencing, and acceptance criteria until an idea is
+selected.
+
+## Using This Catalog
+
+- Add an idea when its purpose and likely value can be stated clearly.
+- Keep entries independent enough that maintainers can evaluate and select
+  them individually.
+- Note significant dependencies or boundary concerns, but do not turn entries
+  into implementation plans.
+- Treat inclusion as an invitation to evaluate, not as approval or priority.
+- When an idea is selected, move its active planning to a focused roadmap or,
+  when it requires a durable architectural decision, an ADR. Update
+  current-state documentation only when implementation lands.
+- Remove ideas that are no longer relevant. Retain a rejected idea only when
+  its rationale is likely to prevent repeated reconsideration.
+
+Future capabilities must continue to respect the
+[architecture policy](../policy/architecture.md), particularly Promptkit's
+role as an application-neutral library and its boundary with downstream
+consumers.
+
+## Ideas
+
+No ideas currently await selection.
+
+## Entry Format
+
+Use a short heading followed by a concise summary. Add focused bullets when
+they help preserve important scope boundaries without becoming an
+implementation plan:
+
+```markdown
+### Idea name
+
+Describe the intended capability, who benefits, and the most important scope
+boundary or dependency.
+
+- Optionally record an important behavior or boundary.
+```

docs/roadmap/notarius-promptkit-wishlist.md

@@ -0,0 +1,245 @@
+# Notarius PromptKit Wishlist
+
+## Purpose
+
+This document records features and interface changes that would be useful
+additions to PromptKit from the perspective of the maintainers of Notarius, a
+downstream application that consumes PromptKit.
+
+PromptKit now provides the capabilities Notarius currently needs. The
+remaining deferred ideas are optional opportunities to improve checkpointing
+and operational observability.
+
+The examples are API sketches intended to communicate the desired capability,
+not prescriptive names or finalized Go contracts.
+
+## Priority 1: Atomic Execution With Prepared Details
+
+**Disposition:** Implemented through [`Engine.PrepareExecution` and
+`Engine.RunPrepared`](../../engine.go). See the
+[consumer guidance](../consumers/pkg-promptkit.md#prepare-now-and-execute-the-same-snapshot-later).
+A separate `RunDetailed` method is not cataloged.
+
+### Downstream need
+
+Notarius needs both:
+
+- the completed `RunResult`; and
+- the rendered messages, effective output contract, hashes, and other
+  preparation details exposed by `PreparedRun`.
+
+Notarius uses the prepared details to construct redaction-aware debug bundles
+and retain enough information to diagnose model behavior.
+
+### Implemented behavior
+
+Notarius can prepare one frozen execution snapshot, retain a caller-owned and
+credential-redacted `Details` value for its debug bundle, and execute the same
+snapshot through `RunPrepared`. The opaque handle is engine-bound and
+single-use; an unused handle can be released with `Discard`. The consumer
+guide and exported GoDoc own the exact lifecycle and failure contracts.
+
+### Value to Notarius
+
+This removes duplicate work from PromptKit-backed calls and ensures that
+retained debug material corresponds atomically to the actual execution.
+
+## Priority 2: Prompt-Independent Profile Inspection
+
+**Disposition:** Implemented as
+[`Engine.InspectProfile`](../../engine.go). See the
+[consumer guidance](../consumers/pkg-promptkit.md#inspect-a-profile-before-prompt-work).
+
+### Downstream need
+
+Notarius validates configured pipeline profile IDs before beginning a run. It
+needs to determine whether:
+
+- a profile exists;
+- its referenced backend is registered;
+- its execution target can be resolved; and
+- it declares a credential requirement that the application may need to
+  enforce.
+
+This validation should not require model generation.
+
+### Previous integration
+
+Before profile inspection was available, Notarius constructed a synthetic
+prompt using `testing/fstest.MapFS`, supplied a dummy transcript, and called
+`Engine.Prepare` solely to exercise profile and backend resolution.
+
+### Value to Notarius
+
+The implemented interface eliminates a synthetic production-only prompt
+fixture and establishes a direct, supported contract for configuration-time
+profile and backend validation.
+
+## Priority 3: Semantic Execution-Target Fingerprints
+
+**Disposition:** Deferred pending a separate semantic-equality design for
+resolved execution targets.
+
+### Downstream need
+
+Notarius checkpoints model-backed pipeline stages. A checkpoint must not be
+reused when generation-affecting PromptKit configuration changes.
+
+Notarius therefore needs a stable equality signal for the effective profile
+and backend target used by a pipeline.
+
+### Current integration
+
+Notarius currently constructs this identity itself from:
+
+- a manually maintained marker for the PromptKit release and built-in profile
+  catalog;
+- raw hashes of configured profile files; and
+- a separate hash of the configured conventional local-backend endpoint.
+
+This is safe but conservative and coupled to PromptKit details. Raw file
+hashing also invalidates checkpoints for semantically irrelevant YAML changes,
+such as comments or formatting.
+
+### Requested capability
+
+Expose an opaque semantic digest for a resolved profile and its effective
+generation target. It could be returned by the proposed profile-resolution
+API:
+
+```go
+type ResolvedProfile struct {
+	ProfileID       string
+	BackendID       string
+	EffectiveTarget ExecutionTarget
+	ExecutionDigest string
+}
+```
+
+Alternatively, PromptKit could expose a dedicated method such as
+`ProfileExecutionDigest(profileID)`.
+
+### Desired equality semantics
+
+The digest should change when generation-affecting state changes, including:
+
+- resolved model and endpoint;
+- backend routing identity;
+- backend request defaults and extra parameters;
+- profile generation parameters; and
+- the semantic identity of any selected built-in profile.
+
+The digest should not incorporate:
+
+- credential values;
+- concurrency or queue capacity;
+- filesystem source paths;
+- YAML comments or formatting; or
+- other settings that affect scheduling or source representation without
+  changing the generation target.
+
+The credential environment-variable name may need to participate if changing
+it can select a materially different provider account or target. PromptKit
+should define this deliberately while continuing to exclude the resolved
+secret value.
+
+### Design considerations
+
+- Treat the digest as an opaque equality value rather than a public encoding
+  of internal structures.
+- Document which categories of change affect equality.
+- Include a versioned semantic marker internally so PromptKit can deliberately
+  invalidate old digests when its resolution semantics change.
+- Prefer a per-profile digest over a digest of every profile known to an
+  engine. Notarius generally knows which profiles a resolved pipeline uses.
+- Do not require consumers to know PromptKit's built-in catalog version.
+
+### Value to Notarius
+
+This would let Notarius remove its PromptKit release marker and raw
+profile-source fingerprinting, reduce unnecessary checkpoint invalidation, and
+delegate execution-target equality to the component that owns target
+resolution.
+
+## Priority 4: Structured Capacity Errors
+
+**Disposition:** Implemented behavior. See the consumer guide's
+[Handle Errors](../consumers/pkg-promptkit.md#handle-errors) section.
+
+### Downstream need
+
+Notarius translates PromptKit backend-capacity rejection into a
+provider-neutral application error. When multiple backends are active,
+operators would benefit from knowing which backend rejected admission without
+parsing an error string or exposing endpoint details.
+
+### Implemented behavior
+
+PromptKit now retains the broad capacity classification while allowing
+Notarius to obtain the selected backend ID without parsing diagnostic text.
+The consumer guide owns the application workflow, including retry and backoff
+policy.
+
+### Value to Notarius
+
+This improves operational diagnostics and future metrics while preserving
+the provider-neutral error boundary used by Notarius.
+
+## Capabilities PromptKit Already Provides Well
+
+The current PromptKit boundary is sufficient for Notarius's implemented
+behavior. In particular, PromptKit already provides:
+
+- filesystem, `fs.FS`, and programmatic prompt, profile, and schema sources;
+- offline preparation without model execution;
+- structured output and content validation;
+- direct session propagation;
+- tri-state per-run reasoning overrides;
+- selected profile, backend, model, endpoint, effective parameters, hashes, and
+  token-usage provenance;
+- endpoint-only profiles;
+- the conventional `local` backend helper;
+- arbitrary engine-scoped `Backend` registrations;
+- backend authentication environment names, extra parameters, concurrency
+  limits, and queue-capacity policies;
+- provider-client and artifact-reader extension interfaces;
+- context cancellation; and
+- useful public error sentinels, including profile absence and capacity
+  exhaustion.
+
+The wishlist does not imply that Notarius needs PromptKit to broaden its core
+responsibilities. It primarily asks for more direct access to information and
+operations that PromptKit already computes internally.
+
+## Responsibilities That Should Remain In Notarius
+
+The following concerns belong to the downstream application and should not
+move into PromptKit for the sake of Notarius:
+
+- pipeline staging, dependencies, and generated references;
+- application-wide scheduling across providers and backends;
+- module and validation retry policy;
+- checkpoints, resume, and recomputation;
+- durable run artifacts and manifests;
+- D&D prompts, schemas, extractors, validators, and normalizers;
+- Notarius configuration-file parsing and precedence;
+- domain-specific prompt-cache prefix policy; and
+- application-specific redaction, retention, and debug-bundle policy.
+
+PromptKit's complete `Backend` API already supports custom IDs, multiple local
+endpoints, authentication, extra parameters, and explicit queue policies.
+Whether Notarius exposes those capabilities in its own configuration is an
+application-policy decision, not an upstream PromptKit gap.
+
+## Suggested Upstream Sequence
+
+For downstream adoption and any remaining upstream work, the useful order is:
+
+1. Adopt prepared execution for atomic details and results.
+2. Add a semantic execution-target digest, preferably alongside profile
+   inspection.
+3. Use the implemented typed capacity error where backend admission diagnostics
+   are needed.
+
+The first removes the concrete execution workaround. The second would improve
+checkpoint correctness and reduce coupling. The third is operational polish.

docs/roadmap/weatherreporter-promptkit-wishlist.md

@@ -0,0 +1,331 @@
+# Weatherreporter PromptKit Wishlist
+
+## Purpose
+
+This document records features and interface changes that would be useful
+additions to PromptKit from the perspective of the maintainers of
+Weatherreporter, a downstream application planning to replace its Scriptorium
+CLI integration with PromptKit.
+
+PromptKit now provides the capabilities Weatherreporter needs for the
+migration. The remaining deferred ideas are optional opportunities to validate
+configuration earlier and improve durable failure diagnostics.
+
+The examples are API sketches intended to communicate the desired capability,
+not prescriptive names or finalized Go contracts. The related
+[Notarius PromptKit wishlist](notarius-promptkit-wishlist.md) proposes several
+overlapping features from another downstream consumer's perspective.
+
+## Priority 1: Executable Preparation Handles
+
+**Disposition:** Implemented as [`Engine.PrepareExecution` and
+`Engine.RunPrepared`](../../engine.go). See the
+[consumer guidance](../consumers/pkg-promptkit.md#prepare-now-and-execute-the-same-snapshot-later).
+
+### Downstream need
+
+Weatherreporter treats prompt preparation as a durable preflight boundary. It
+needs to:
+
+1. prepare the exact request that will be executed;
+2. persist a safe preparation record before starting the provider call; and
+3. execute without reloading or rerendering prompt, profile, schema, or input
+   sources.
+
+Persisting preflight before generation leaves useful evidence when a provider
+call fails or the process is interrupted during generation.
+
+### Implemented behavior
+
+Weatherreporter can prepare one frozen execution snapshot, persist a
+caller-owned and credential-redacted `Details` value, and execute that same
+snapshot through `RunPrepared`. The opaque handle is engine-bound and
+single-use; an unused handle can be released with `Discard`. The consumer
+guide and exported GoDoc own the exact lifecycle, credential, cancellation,
+and capacity contracts.
+
+### Value to Weatherreporter
+
+This preserves Weatherreporter's durable preflight behavior, removes duplicate
+work, eliminates the source-consistency window, and ensures that persisted
+provenance describes the actual execution.
+
+## Priority 2: Prompt-Definition Inspection
+
+**Disposition:** Implemented as
+[`Engine.InspectPrompt`](../../engine.go). See the
+[consumer guidance](../consumers/pkg-promptkit.md#inspect-a-prompt-before-preparation).
+
+### Downstream need
+
+Weatherreporter has a fixed registry of seven report definitions. Each report
+selects a prompt ID and one of two output workflows:
+
+- direct Markdown; or
+- structured generated text followed by application-owned domain validation
+  and Markdown template rendering.
+
+Weatherreporter will embed the PromptKit prompt definitions and private
+response schemas that implement those reports. It needs to validate that the
+report registry and embedded prompt corpus agree before weather collection or
+provider execution.
+
+### Previous integration option
+
+Before prompt inspection was available, Weatherreporter could maintain
+synthetic data-package fixtures and call `Engine.Prepare` for every report
+prompt during tests. Runtime validation could also occur through the ordinary
+per-report preparation stage.
+
+This required complete placeholder inputs and profile resolution when the
+application primarily wanted to inspect prompt identity and declared contracts.
+
+### Value to Weatherreporter
+
+The implemented interface lets Weatherreporter directly verify that every
+report prompt exists, requires the curated `data_package` input, and declares
+the expected Markdown or JSON Schema output contract. It reduces synthetic
+test setup and moves failures ahead of weather collection.
+
+## Priority 3: Prompt-Independent Profile Inspection
+
+**Disposition:** Implemented as
+[`Engine.InspectProfile`](../../engine.go). See the
+[consumer guidance](../consumers/pkg-promptkit.md#inspect-a-profile-before-prompt-work).
+
+### Downstream need
+
+Weatherreporter will allow operators to select an external PromptKit profile
+source and may allow an explicit profile override. It needs to reject a missing
+profile, unknown backend, or malformed execution target before collecting
+weather data or writing report artifacts, and to apply its own policy to a
+reported credential requirement.
+
+### Value to Weatherreporter
+
+The implemented interface improves fail-fast configuration validation and gives
+operator-facing errors direct profile and backend context. It remains optional
+for the initial migration.
+
+## Priority 4: Eager Source Validation
+
+**Disposition:** Deferred until prompt and profile inspection have been used
+to determine whether a broader engine-wide validation operation is still
+needed.
+
+### Downstream need
+
+PromptKit deliberately defers reading and validating filesystem and `fs.FS`
+prompt, profile, and schema content until a request needs it. Weatherreporter
+has a small fixed embedded prompt corpus and one optional external profile
+source. It would benefit from an explicit offline validation operation for
+tests, startup diagnostics, and configuration checks.
+
+### Current integration option
+
+Weatherreporter can prepare every report prompt with fixture inputs and inspect
+any explicit profiles individually. That provides strong coverage but requires
+consumer-maintained traversal and synthetic material.
+
+### Requested capability
+
+Consider an opt-in source-validation operation:
+
+```go
+type SourceValidationOptions struct {
+	RequireCredentials bool
+}
+
+func (e *Engine) ValidateSources(
+	ctx context.Context,
+	opts SourceValidationOptions,
+) error
+```
+
+The operation should eagerly discover and structurally validate the configured
+prompt, profile, and schema sources without model generation.
+
+### Design considerations
+
+- Keep deferred validation as the normal `NewEngine` behavior.
+- Make eager validation an explicit consumer choice.
+- Validate duplicate IDs and versions, strict YAML decoding, referenced content
+  files, profile/backend membership, schema syntax, and schema references.
+- Distinguish structural credential declarations from current environment
+  availability.
+- Do not read or expose credential values when credential availability is not
+  requested.
+- Preserve source-specific public error identities and useful path context.
+- Respect context cancellation during filesystem discovery and schema work.
+- Consider whether exact prompt and profile inspection APIs already provide a
+  smaller sufficient surface before adding an engine-wide operation.
+
+### Value to Weatherreporter
+
+This would simplify offline corpus checks and catch malformed operator profile
+sources before report work begins. It is helpful but lower priority than exact
+prompt and profile inspection.
+
+## Priority 5: Structured Generation Errors
+
+**Disposition:** Deferred pending stronger downstream demand and a narrower
+design that does not duplicate prepared provenance or impose HTTP-specific
+fields on injected model clients.
+
+### Downstream need
+
+Weatherreporter preserves redacted, inspectable failure receipts for report
+runs. When model generation fails operationally, it needs to classify the
+failure and retain safe execution context without parsing error prose.
+
+Prompt preparation already supplies selected profile, backend, and model
+identity. Provider status classification would add useful operator context,
+especially when the built-in OpenAI-compatible client receives a non-success
+HTTP status.
+
+### Current integration option
+
+PromptKit exposes `ErrLLMGenerate` and preserves injected client errors through
+`errors.Is`. Weatherreporter can reliably classify generation failure and use
+its preparation record for profile, backend, and model provenance. Any further
+diagnostic detail remains a redacted error string.
+
+### Requested capability
+
+Consider a typed generation error that continues to match `ErrLLMGenerate`:
+
+```go
+type GenerationError struct {
+	BackendID string
+	Model     string
+	StatusCode int
+}
+```
+
+The exact fields may differ. The useful contract is safe structured context
+available through `errors.As`, while `errors.Is(err, ErrLLMGenerate)` remains
+compatible.
+
+### Design considerations
+
+- Include only fields that PromptKit knows reliably and can expose safely.
+- Treat an HTTP status as optional because injected model clients may not use
+  HTTP.
+- Do not expose provider response bodies, endpoints, credential environment
+  names, credential values, request content, or generated content.
+- Do not make a structured error a second source of prompt/profile provenance
+  already present in a prepared execution.
+- Preserve injected client error identity.
+- Keep retry and backoff policy with the consuming application.
+
+### Value to Weatherreporter
+
+This would improve durable failure receipts and troubleshooting, particularly
+for built-in transport failures. It is not required if preparation details and
+the existing sentinel remain available.
+
+## Lower-Priority Shared Wishlist Items
+
+### Structured Capacity Errors
+
+**Disposition:** Implemented behavior. See the consumer guide's
+[Handle Errors](../consumers/pkg-promptkit.md#handle-errors) section.
+
+PromptKit now exposes the stable backend ID for capacity rejection without
+requiring Weatherreporter to parse error text.
+
+Weatherreporter currently generates batch reports sequentially and constructs
+one engine per invocation, so engine-local capacity exhaustion is unlikely in
+the initial design. The typed error becomes more valuable if report generation
+later becomes concurrent or PromptKit engines become longer-lived.
+It should not block adoption.
+
+### Semantic Execution-Target Fingerprints
+
+**Disposition:** Deferred pending a separate semantic-equality design for
+resolved execution targets.
+
+The semantic target digest proposed by the
+[Notarius wishlist](notarius-promptkit-wishlist.md#priority-3-semantic-execution-target-fingerprints)
+would provide a compact equality signal for audit metadata.
+
+Weatherreporter does not currently reuse LLM-dependent checkpoints. Its Recent
+Changes behavior compares deterministic module snapshots rather than generated
+reports, so the digest has no immediate cache-correctness role. Existing
+PromptKit result metadata is sufficient for the initial integration. A digest
+would still be useful provenance and future-proofing, but it is not a
+migration priority.
+
+## Capabilities PromptKit Already Provides Well
+
+PromptKit already provides the essential Weatherreporter integration surface:
+
+- importable in-process engine construction;
+- filesystem, `fs.FS`, and programmatic prompt, profile, and schema sources;
+- offline preparation without model execution;
+- prepared execution handles for a durable preflight boundary;
+- exact prompt and profile inspection;
+- versioned prompt selection;
+- text, Markdown, JSON, and JSON Schema output contracts;
+- single-pass output validation with raw output retained after completed
+  validation failure;
+- inline input artifacts with provenance URIs and input hashes;
+- selected profile, backend, model, effective target, prompt hashes, timing,
+  and token-usage provenance;
+- endpoint-only profiles and engine-scoped backend registration;
+- injected model-client and artifact-reader interfaces;
+- caller cancellation, generation timeout, and transport timeout behavior; and
+- public error sentinels for configuration, prompt, profile, artifact,
+  validation, capacity, and generation failures; and
+- structured backend identity for capacity rejection.
+
+These capabilities are sufficient for Weatherreporter to adopt PromptKit
+without waiting for new upstream work.
+
+## Responsibilities That Should Remain In Weatherreporter
+
+The following concerns belong to Weatherreporter and should not move into
+PromptKit:
+
+- report definitions, valid periods, batches, and output naming;
+- application prompt content and private report response schemas;
+- deterministic weather facts, modules, and Recent Changes;
+- curated `data_package` construction and persistence;
+- generated-text domain validation and Markdown template rendering;
+- managed artifact paths, atomic writes, metadata, and inspection commands;
+- preparation, execution, raw-output, and failure-receipt schemas;
+- CLI configuration loading and precedence;
+- debug enablement, redaction, placement, sensitivity, and retention;
+- distributor notification;
+- batch continuation and any future retry policy; and
+- application-level compatibility and migration policy.
+
+## Suggested Upstream Sequence
+
+For downstream adoption and any remaining upstream work, the useful order is:
+
+1. Adopt the implemented executable preparation handles.
+2. Consider eager source validation after evaluating whether the two exact
+   inspection APIs are sufficient.
+3. Add structured generation errors.
+4. Use structured capacity errors and consider semantic execution-target
+   fingerprints as lower-priority operational improvements.
+
+The first item removes the material integration workaround. Prompt and profile
+inspection improve fail-fast validation. The remaining items are optional
+ergonomic and diagnostic improvements.
+
+## Adoption Sequencing
+
+Weatherreporter should not wait for the deferred wishlist items. The current
+PromptKit interface is sufficient when Weatherreporter:
+
+- embeds immutable prompt and schema assets;
+- supplies immutable inline data-package bytes;
+- constructs one engine per CLI invocation;
+- prepares an execution, persists selected `Details`, and calls
+  `RunPrepared`; and
+- keeps PromptKit behind a weatherreporter-owned adapter contract.
+
+Prompt inspection, profile inspection, source validation, structured errors,
+capacity details, and semantic fingerprints should not gate adoption.

engine.go

@@ -0,0 +1,658 @@
+package promptkit
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"io/fs"
+	"net/http"
+	"os"
+	"path/filepath"
+	"strings"
+	"time"
+
+	artifactadapter "gitea.maximumdirect.net/eric/promptkit/internal/artifact"
+	"gitea.maximumdirect.net/eric/promptkit/internal/backend"
+	"gitea.maximumdirect.net/eric/promptkit/internal/capacity"
+	"gitea.maximumdirect.net/eric/promptkit/internal/defaults"
+	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
+	"gitea.maximumdirect.net/eric/promptkit/internal/llm"
+	"gitea.maximumdirect.net/eric/promptkit/internal/profile"
+	"gitea.maximumdirect.net/eric/promptkit/internal/profile/builtin"
+	"gitea.maximumdirect.net/eric/promptkit/internal/prompt"
+	"gitea.maximumdirect.net/eric/promptkit/internal/promptdef"
+	"gitea.maximumdirect.net/eric/promptkit/internal/usecase"
+	"gitea.maximumdirect.net/eric/promptkit/internal/validate"
+)
+
+// ErrInvalidConfig identifies invalid engine construction, including missing
+// required configuration, invalid options or backend registrations, and a nil
+// Engine receiver.
+var ErrInvalidConfig = errors.New("invalid engine configuration")
+
+var (
+	// ErrInvalidRequest identifies a request whose required values, overrides,
+	// credentials, or effective settings are invalid.
+	ErrInvalidRequest = errors.New("invalid run request")
+	// ErrPromptNotFound identifies a requested prompt ID or version that is not
+	// present in the selected prompt source. It does not also match
+	// ErrPromptLoad.
+	ErrPromptNotFound = errors.New("prompt not found")
+	// ErrProfileNotFound identifies a selected profile ID that is absent from
+	// every configured profile source. It does not also match ErrProfileLoad.
+	ErrProfileNotFound = errors.New("profile not found")
+	// ErrProfileRequired identifies a request for which neither RunRequest.ProfileID
+	// nor the selected prompt's default profile is present. Such an error also
+	// matches ErrInvalidRequest.
+	ErrProfileRequired = errors.New("profile selection is required")
+	// ErrPromptLoad identifies a failure to read, decode, validate, select, or
+	// hash a prompt definition, except for the not-found case represented by
+	// ErrPromptNotFound.
+	ErrPromptLoad = errors.New("failed to load prompt definition")
+	// ErrProfileLoad identifies a failure to read, decode, validate, or select
+	// an execution profile or resolve its backend, except for the profile
+	// not-found case represented by ErrProfileNotFound.
+	ErrProfileLoad = errors.New("failed to load execution profile")
+	// ErrAPIKeyEnvMissing identifies an APIKeyEnv whose environment variable is
+	// unset or empty when no direct RunRequest.APIKey takes precedence. Such an
+	// error also matches ErrInvalidRequest.
+	ErrAPIKeyEnvMissing = errors.New("api_key_env points to an unset environment variable")
+	// ErrArtifactLoad identifies a failure to resolve an input artifact. Errors
+	// returned by an injected ArtifactReader remain available through errors.Is.
+	ErrArtifactLoad = errors.New("failed to load artifact")
+	// ErrPromptRender identifies a failure to render prompt messages or the
+	// session ID from the resolved inputs and variables.
+	ErrPromptRender = errors.New("failed to render prompt")
+	// ErrCapacityExceeded identifies a Run or RunPrepared rejected because the
+	// selected backend already admitted ConcurrencyLimit + QueueCapacity calls.
+	// A [CapacityError] reports the selected backend ID. It is not an invalid
+	// request, an LLM or provider rate-limit response, or ErrLLMGenerate.
+	ErrCapacityExceeded = errors.New("backend capacity exceeded")
+	// ErrLLMGenerate identifies a model-client failure or a nil successful
+	// response. Errors returned by an injected LLMClient remain available
+	// through errors.Is.
+	ErrLLMGenerate = errors.New("failed to generate output")
+	// ErrValidation identifies an operational failure to load or compile a
+	// schema or validate output. A completed validation whose Status is
+	// ValidationFailed is returned in RunResult without this error.
+	ErrValidation = errors.New("failed to validate output")
+)
+
+// Engine inspects prompts and profiles and prepares and runs Promptkit prompt
+// requests.
+//
+// An Engine is safe for concurrent calls to [Engine.InspectPrompt],
+// [Engine.InspectProfile], [Engine.Prepare], [Engine.PrepareExecution],
+// [Engine.Run], and [Engine.RunPrepared]. Each Engine owns independent
+// backend-capacity pools that coordinate Run and RunPrepared admission and
+// model generation. Injected collaborators may still be invoked concurrently
+// across different backend pools or for unlimited backends.
+type Engine struct {
+	runner *usecase.Runner
+}
+
+// Config selects the directory-backed sources and built-in model-client
+// transport used by [NewEngine]. Config has no stable JSON representation.
+type Config struct {
+	// PromptDir is the directory searched recursively for prompt definitions.
+	// It is required unless a WithPromptFS or WithPromptFile option supplies the
+	// prompt source.
+	PromptDir string
+	// ProfileDir is an optional directory whose profiles take precedence over
+	// embedded built-in profiles. An empty value selects only built-ins unless
+	// profile options are also supplied.
+	ProfileDir string
+	// SchemaDir is the root for JSON Schema files. An empty value uses the
+	// current directory. WithSchemaFS or WithSchemaFile replaces this source.
+	SchemaDir string
+	// Timeout is the transport-wide safety cap for the built-in LLM client
+	// when HTTPClient is absent or has a non-positive timeout. A zero or negative
+	// value selects the 10-minute default.
+	Timeout time.Duration
+	// HTTPClient is cloned for the built-in LLM client. Its positive Timeout
+	// takes precedence over Timeout. A zero or negative client Timeout inherits
+	// Timeout or the 10-minute default. The supplied client is not mutated. This
+	// field is ignored when WithLLMClient is used.
+	HTTPClient *http.Client
+}
+
+// Option customizes engine construction.
+//
+// NewEngine applies options in argument order and ignores nil options. Within
+// each prompt-source, profile-source, in-memory-profile, schema-source,
+// model-client, and artifact-reader category, the last non-nil valid option
+// replaces earlier options in that category. WithBackend is the additive
+// exception: unique registrations accumulate, and a repeated backend ID is an
+// error rather than a replacement. An invalid option fails construction even
+// if a later option would replace it.
+type Option interface {
+	apply(*engineOptions) error
+}
+
+type optionFunc func(*engineOptions) error
+
+func (f optionFunc) apply(options *engineOptions) error {
+	return f(options)
+}
+
+type engineOptions struct {
+	llmClient       llm.Client
+	artifactReader  artifactadapter.Reader
+	promptDefs      promptdef.Repository
+	profiles        profile.Repository
+	memoryProfiles  profile.Repository
+	backends        []domain.Backend
+	validator       validate.Validator
+	promptSource    bool
+	profileSource   bool
+	memorySource    bool
+	validatorSource bool
+	artifactSource  bool
+}
+
+// WithLLMClient replaces the built-in model client used by [Engine.Run] and
+// [Engine.RunPrepared].
+//
+// A nil client makes NewEngine fail with ErrInvalidConfig. The Engine schedules
+// Generate calls according to the selected backend's capacity policy, but the
+// client may still be called concurrently across different backend pools or for
+// unlimited backends. The client is not used by [Engine.Prepare] or
+// [Engine.PrepareExecution].
+func WithLLMClient(client LLMClient) Option {
+	return optionFunc(func(options *engineOptions) error {
+		if client == nil {
+			return ErrInvalidConfig
+		}
+		options.llmClient = publicLLMClientAdapter{client: client}
+		return nil
+	})
+}
+
+// WithArtifactReader replaces the default reader for every input artifact
+// reference, regardless of its ArtifactRef.Type.
+//
+// A nil reader makes NewEngine fail with ErrInvalidConfig. The reader may be
+// called concurrently.
+func WithArtifactReader(reader ArtifactReader) Option {
+	return optionFunc(func(options *engineOptions) error {
+		if reader == nil {
+			return ErrInvalidConfig
+		}
+		options.artifactReader = publicArtifactReaderAdapter{reader: reader}
+		options.artifactSource = true
+		return nil
+	})
+}
+
+// WithPromptFS loads prompt definitions from fsys under root.
+//
+// The source uses the same strict prompt YAML rules as configured prompt
+// directories, and prompt content_file paths resolve within this source.
+// fsys must be non-nil and root must be non-empty; otherwise NewEngine fails
+// with ErrInvalidConfig. This option replaces Config.PromptDir and earlier
+// prompt-source options.
+func WithPromptFS(fsys fs.FS, root string) Option {
+	return optionFunc(func(options *engineOptions) error {
+		if fsys == nil {
+			return ErrInvalidConfig
+		}
+		if strings.TrimSpace(root) == "" {
+			return ErrInvalidConfig
+		}
+		options.promptDefs = promptdef.NewFSRepository(fsys, root)
+		options.promptSource = true
+		return nil
+	})
+}
+
+// WithPromptFile loads prompt definitions from the single prompt file at path.
+//
+// Relative prompt content_file paths resolve from the file's directory. path
+// must name an existing non-directory file when NewEngine applies the option.
+// This option replaces Config.PromptDir and earlier prompt-source options.
+func WithPromptFile(path string) Option {
+	return optionFunc(func(options *engineOptions) error {
+		fsys, root, err := fileSource(path)
+		if err != nil {
+			return err
+		}
+		options.promptDefs = promptdef.NewFSRepository(fsys, root)
+		options.promptSource = true
+		return nil
+	})
+}
+
+// WithProfileFS loads execution profiles from fsys under root.
+//
+// Profiles from this source overlay built-in profiles. Profile YAML must use
+// api_key_env for environment-based credentials; raw API keys are rejected.
+// fsys must be non-nil and root must be non-empty; otherwise NewEngine fails
+// with ErrInvalidConfig. This option replaces Config.ProfileDir and earlier
+// file or FS profile-source options, but remains below WithProfiles in
+// precedence.
+func WithProfileFS(fsys fs.FS, root string) Option {
+	return optionFunc(func(options *engineOptions) error {
+		if fsys == nil {
+			return ErrInvalidConfig
+		}
+		if strings.TrimSpace(root) == "" {
+			return ErrInvalidConfig
+		}
+		options.profiles = profile.NewFSRepository(fsys, root)
+		options.profileSource = true
+		return nil
+	})
+}
+
+// WithProfileFile loads execution profiles from the single profile file at path.
+//
+// The profile overlays built-in profiles. Profile YAML must use api_key_env for
+// environment-based credentials; raw API keys are rejected. path must name an
+// existing non-directory file when NewEngine applies the option. This option
+// replaces Config.ProfileDir and earlier file or FS profile-source options,
+// but remains below WithProfiles in precedence.
+func WithProfileFile(path string) Option {
+	return optionFunc(func(options *engineOptions) error {
+		fsys, root, err := fileSource(path)
+		if err != nil {
+			return err
+		}
+		options.profiles = profile.NewFSRepository(fsys, root)
+		options.profileSource = true
+		return nil
+	})
+}
+
+// WithProfiles configures in-memory profiles that take precedence over
+// configured profile files and built-in profiles.
+//
+// NewEngine validates and copies every profile. IDs must be unique within one
+// call. An invalid profile, duplicate ID, or unsupported ExtraParams value
+// makes construction fail with ErrInvalidConfig. Repeating WithProfiles
+// replaces the complete earlier in-memory set rather than merging it.
+func WithProfiles(profiles ...Profile) Option {
+	return optionFunc(func(options *engineOptions) error {
+		repo, err := newMemoryProfileRepository(profiles)
+		if err != nil {
+			return err
+		}
+		options.memoryProfiles = repo
+		options.memorySource = true
+		return nil
+	})
+}
+
+// WithSchemaFS loads JSON Schema documents from fsys under root.
+//
+// Prompt schema_path values resolve within this source when schema validation
+// or structured output is requested. fsys must be non-nil and root must be
+// non-empty; otherwise NewEngine fails with ErrInvalidConfig. This option
+// replaces Config.SchemaDir and earlier schema-source options.
+func WithSchemaFS(fsys fs.FS, root string) Option {
+	return optionFunc(func(options *engineOptions) error {
+		if fsys == nil {
+			return ErrInvalidConfig
+		}
+		if strings.TrimSpace(root) == "" {
+			return ErrInvalidConfig
+		}
+		options.validator = validate.NewFSValidator(fsys, root)
+		options.validatorSource = true
+		return nil
+	})
+}
+
+// WithSchemaFile loads JSON Schema documents from the single schema file at path.
+//
+// Prompt schema_path values refer to the file's base name. path must name an
+// existing non-directory file when NewEngine applies the option. This option
+// replaces Config.SchemaDir and earlier schema-source options.
+func WithSchemaFile(path string) Option {
+	return optionFunc(func(options *engineOptions) error {
+		fsys, root, err := fileSource(path)
+		if err != nil {
+			return err
+		}
+		options.validator = validate.NewFSValidator(fsys, root)
+		options.validatorSource = true
+		return nil
+	})
+}
+
+// NewEngine constructs an Engine from configuration and options.
+//
+// Options are applied in order according to [Option]. PromptDir is required
+// unless a prompt-source option is present. Construction validates option
+// arguments, in-memory profiles, and backend registrations but defers reading
+// and validating prompt, file-backed profile, and schema contents until Prepare
+// or Run needs them.
+//
+// NewEngine returns an error matching ErrInvalidConfig for invalid
+// configuration, options, or backend-capacity policies. Each constructed
+// Engine has independent backend-capacity pools. Construction does not perform
+// model requests or require credentials.
+func NewEngine(cfg Config, opts ...Option) (*Engine, error) {
+	var options engineOptions
+	for _, opt := range opts {
+		if opt == nil {
+			continue
+		}
+		if err := opt.apply(&options); err != nil {
+			return nil, fmt.Errorf("%w: %v", ErrInvalidConfig, err)
+		}
+	}
+
+	promptDefs := options.promptDefs
+	if !options.promptSource {
+		if strings.TrimSpace(cfg.PromptDir) == "" {
+			return nil, fmt.Errorf("%w: prompt directory is required", ErrInvalidConfig)
+		}
+		promptDefs = promptdef.NewFilesystemRepository(cfg.PromptDir)
+	}
+
+	profiles := builtin.NewRepositoryWithDirectory(cfg.ProfileDir)
+	if options.profileSource {
+		profiles = builtin.NewRepositoryWithPrimary(options.profiles)
+	}
+	if options.memorySource {
+		profiles = profile.NewOverlayRepository(options.memoryProfiles, profiles)
+	}
+
+	backendRegistry, err := backend.NewRegistry(options.backends)
+	if err != nil {
+		return nil, fmt.Errorf("%w: failed to construct backend registry: %v", ErrInvalidConfig, err)
+	}
+
+	capacityManager, err := capacity.NewManager(backendRegistry.CapacityPolicies())
+	if err != nil {
+		return nil, fmt.Errorf("%w: failed to construct backend capacity manager: %v", ErrInvalidConfig, err)
+	}
+
+	validator := options.validator
+	if !options.validatorSource {
+		schemaDir := cfg.SchemaDir
+		if strings.TrimSpace(schemaDir) == "" {
+			schemaDir = defaults.SchemaDirDefault
+		}
+		validator = validate.NewStandardValidator(schemaDir)
+	}
+
+	llmClient := options.llmClient
+	if llmClient == nil {
+		var err error
+		llmClient, err = llm.NewOpenAICompatibleClient(llm.OpenAICompatibleConfig{
+			Timeout:    cfg.Timeout,
+			HTTPClient: cfg.HTTPClient,
+		})
+		if err != nil {
+			return nil, fmt.Errorf("%w: %v", ErrInvalidConfig, err)
+		}
+	}
+	llmClient = capacity.NewClient(capacityManager, llmClient)
+
+	artifacts := options.artifactReader
+	if !options.artifactSource {
+		artifacts = artifactadapter.NewCompositeReader()
+	}
+
+	return &Engine{
+		runner: usecase.NewRunner(
+			promptDefs,
+			profiles,
+			backendRegistry,
+			artifacts,
+			prompt.NewGoRenderer(),
+			llmClient,
+			validator,
+			capacityManager,
+		),
+	}, nil
+}
+
+func fileSource(name string) (fs.FS, string, error) {
+	cleanName := strings.TrimSpace(name)
+	if cleanName == "" {
+		return nil, "", ErrInvalidConfig
+	}
+	dir := filepath.Dir(cleanName)
+	base := filepath.Base(cleanName)
+	if base == "." || base == string(filepath.Separator) || strings.TrimSpace(base) == "" {
+		return nil, "", ErrInvalidConfig
+	}
+	info, err := os.Stat(cleanName)
+	if err != nil {
+		return nil, "", fmt.Errorf("%w: failed to access source file %q: %v", ErrInvalidConfig, cleanName, err)
+	}
+	if info.IsDir() {
+		return nil, "", fmt.Errorf("%w: source path %q must be a file", ErrInvalidConfig, cleanName)
+	}
+	return os.DirFS(dir), filepath.ToSlash(base), nil
+}
+
+// InspectPrompt resolves one explicit prompt definition without selecting a
+// profile or starting execution work.
+//
+// InspectPrompt requires a nonblank promptID. It passes nonblank promptID and
+// promptVersion values unchanged to the engine's ordinary, case-sensitive
+// prompt selection. An empty version succeeds only when that source has one
+// selected ID; a nonempty version selects one exact ID/version pair. The
+// configured prompt source is used without merging, fallback, or enumeration.
+//
+// A successful result proves that the selected definition and any referenced
+// message content files were structurally loaded. Inputs are returned in
+// definition order. DefaultProfileID is declared metadata only and is not
+// resolved. OutputContract is the normalized declared contract, with a JSON
+// Schema path when declared but without loading or compiling that schema.
+// PromptHash is the same opaque equality value as PreparedRun.PromptHash for
+// the selected definition and observed source state; its spelling, length,
+// encoding, algorithm, and security properties are not contracts.
+//
+// This method does not return prompt bodies, templates, source paths, schemas,
+// rendered messages, or execution settings. It does not resolve a profile or
+// credential, read artifacts or schemas, render, validate, admit capacity,
+// contact a provider, or generate model output. The returned PromptInspection
+// and its input slice are caller-owned. Filesystem-backed inspection is a
+// point-in-time lookup and does not freeze a definition for later execution.
+//
+// A nil Engine returns an error matching ErrInvalidConfig. A blank prompt ID
+// matches ErrInvalidRequest. An absent exact ID or version matches
+// ErrPromptNotFound and not ErrPromptLoad. Malformed, unreadable, duplicate,
+// ambiguous, referenced-content, or hashing failures match ErrPromptLoad.
+// Cancellation during lookup matches ErrPromptLoad while preserving the
+// context error. InspectPrompt returns no partial result on error.
+func (e *Engine) InspectPrompt(
+	ctx context.Context,
+	promptID string,
+	promptVersion string,
+) (*PromptInspection, error) {
+	if e == nil || e.runner == nil {
+		return nil, fmt.Errorf("%w: engine is nil", ErrInvalidConfig)
+	}
+
+	inspection, err := e.runner.InspectPrompt(ctx, promptID, promptVersion)
+	if err != nil {
+		return nil, mapPublicError(err)
+	}
+	return fromDomainPromptInspection(inspection), nil
+}
+
+// InspectProfile resolves one explicit profile without selecting a prompt or
+// starting execution work.
+//
+// InspectProfile trims surrounding whitespace from profileID and looks up the
+// resulting nonblank ID exactly and case-sensitively through the engine's
+// ordinary in-memory, configured-source, and built-in profile precedence. It
+// applies framework defaults, the selected backend, and then the selected
+// profile to EffectiveModelParams without a request override. BackendID is
+// empty for an endpoint-only profile.
+//
+// APIKeyEnv in the returned target is an environment-variable name, never its
+// value. APIKeyRequired instead reports a direct credential requirement and is
+// mutually exclusive with a nonblank APIKeyEnv. InspectProfile neither derives
+// an ID from a prompt default_profile nor checks credential availability, so an
+// absent or blank named environment variable is not an error.
+//
+// The returned ProfileInspection and all nested mutable values are
+// caller-owned. Filesystem-backed inspection is a point-in-time lookup and
+// does not freeze the profile for a later execution. This method does not load
+// a prompt, render, read artifacts or schemas, admit backend capacity, contact
+// a provider, or generate model output.
+//
+// A nil Engine returns an error matching ErrInvalidConfig. A blank profile ID
+// matches ErrInvalidRequest. An absent exact ID matches ErrProfileNotFound and
+// not ErrProfileLoad. Malformed or unreadable profile data, an unknown backend,
+// or an invalid resolved target matches ErrProfileLoad. Cancellation during
+// profile loading matches ErrProfileLoad while preserving the context error.
+// InspectProfile returns no partial result on error.
+func (e *Engine) InspectProfile(ctx context.Context, profileID string) (*ProfileInspection, error) {
+	if e == nil || e.runner == nil {
+		return nil, fmt.Errorf("%w: engine is nil", ErrInvalidConfig)
+	}
+
+	inspection, err := e.runner.InspectProfile(ctx, profileID)
+	if err != nil {
+		return nil, mapPublicError(err)
+	}
+	return fromDomainProfileInspection(inspection), nil
+}
+
+// Prepare resolves and renders a prompt request without calling an LLM.
+//
+// Prepare selects the prompt and profile, resolves any selected backend and
+// effective execution settings, resolves the output contract, loads and hashes
+// inputs, loads structured-output schema metadata when required, and renders
+// the session ID and messages. The returned PreparedRun is owned by the caller
+// and never contains a resolved API-key value, model output, or validation
+// result.
+//
+// A nil Engine returns an error matching ErrInvalidConfig. Request and
+// preparation failures may match ErrInvalidRequest, ErrPromptNotFound,
+// ErrPromptLoad, ErrProfileNotFound, ErrProfileLoad, ErrProfileRequired,
+// ErrAPIKeyEnvMissing, ErrArtifactLoad, ErrPromptRender, or ErrValidation as
+// applicable. Cancellation is passed to the active collaborator and is
+// reported in the applicable operation category; no general errors.Is
+// relationship to ctx.Err is promised. Prepare returns no partial result on
+// error.
+func (e *Engine) Prepare(ctx context.Context, req RunRequest) (*PreparedRun, error) {
+	if e == nil || e.runner == nil {
+		return nil, fmt.Errorf("%w: engine is nil", ErrInvalidConfig)
+	}
+
+	domainReq, err := toDomainRunRequest(req)
+	if err != nil {
+		return nil, fmt.Errorf("%w: %v", ErrInvalidRequest, err)
+	}
+
+	prepared, err := e.runner.Prepare(ctx, domainReq)
+	if err != nil {
+		return nil, mapPublicError(err)
+	}
+	return fromDomainPreparedRun(prepared), nil
+}
+
+// PrepareExecution completely prepares a prompt request without calling the
+// configured LLMClient or reserving backend admission capacity.
+//
+// The returned opaque handle is bound to this Engine and permits one
+// [Engine.RunPrepared] invocation. Preparation freezes the selected sources,
+// rendered messages, effective settings, inputs, provider structured-output
+// metadata, and validation resources needed by that invocation. The handle
+// retains a direct RunRequest.APIKey only in private execution state;
+// [PreparedExecution.Details] is credential-redacted.
+//
+// The context governs preparation only. Cancellation after this method
+// returns does not invalidate the handle or propagate to RunPrepared.
+// PrepareExecution returns the same error categories as [Engine.Prepare] and
+// returns no handle on error. A nil Engine returns an error matching
+// ErrInvalidConfig.
+func (e *Engine) PrepareExecution(ctx context.Context, req RunRequest) (*PreparedExecution, error) {
+	if e == nil || e.runner == nil {
+		return nil, fmt.Errorf("%w: engine is nil", ErrInvalidConfig)
+	}
+
+	domainReq, err := toDomainRunRequest(req)
+	if err != nil {
+		return nil, fmt.Errorf("%w: %v", ErrInvalidRequest, err)
+	}
+
+	prepared, err := e.runner.PrepareExecution(ctx, domainReq)
+	if err != nil {
+		return nil, mapPublicError(err)
+	}
+	return &PreparedExecution{internal: prepared}, nil
+}
+
+// Run prepares a request, invokes the configured LLMClient, and validates the
+// generated output.
+//
+// A content-validation failure is a successful run whose
+// RunResult.Validation has Status ValidationFailed. An inability to perform
+// validation returns an error matching ErrValidation and no partial result.
+// The public Engine does not perform output repair, so validation is
+// single-pass even when OutputContract.RepairAttempts is positive.
+//
+// Run can return every error category documented by [Engine.Prepare], plus
+// ErrCapacityExceeded and ErrLLMGenerate. An engine admission rejection is
+// discoverable as [CapacityError] and still matches ErrCapacityExceeded. It
+// occurs before artifacts, schemas, rendering, or model generation because the
+// selected backend's admission capacity is full; it does not match
+// ErrInvalidRequest or ErrLLMGenerate. Errors from injected clients remain
+// available through errors.Is. Cancellation while waiting for model-generation
+// capacity matches both ErrLLMGenerate and the context error. Cancellation
+// otherwise follows the active collaborator's documented behavior. A nil
+// Engine returns ErrInvalidConfig. Run returns no partial result on error.
+func (e *Engine) Run(ctx context.Context, req RunRequest) (*RunResult, error) {
+	if e == nil || e.runner == nil {
+		return nil, fmt.Errorf("%w: engine is nil", ErrInvalidConfig)
+	}
+
+	domainReq, err := toDomainRunRequest(req)
+	if err != nil {
+		return nil, fmt.Errorf("%w: %v", ErrInvalidRequest, err)
+	}
+
+	result, err := e.runner.Run(ctx, domainReq)
+	if err != nil {
+		return nil, mapPublicError(err)
+	}
+	return fromDomainRunResult(result), nil
+}
+
+// RunPrepared atomically claims and executes a handle created by
+// [Engine.PrepareExecution].
+//
+// A valid owning-Engine invocation consumes the handle's one attempt before
+// credential revalidation, backend admission, generation, or validation.
+// Cancellation, capacity rejection, generation failure, operational
+// validation failure, and success all leave the handle unusable. A nil,
+// zero-value, foreign-Engine, discarded, claimed, or used handle returns an
+// error matching ErrInvalidRequest; a nil Engine returns ErrInvalidConfig and
+// does not claim the handle.
+//
+// The supplied context governs this execution attempt independently of the
+// preparation context. It covers credential revalidation, admission,
+// generation, validation, and any internal repair. Result timing begins after
+// the claim and excludes preparation and consumer-held delay.
+//
+// RunPrepared can return ErrInvalidRequest, ErrAPIKeyEnvMissing,
+// ErrCapacityExceeded, ErrLLMGenerate, or ErrValidation as applicable while
+// preserving documented collaborator and context identities. An engine
+// admission rejection is discoverable as [CapacityError] and still matches
+// ErrCapacityExceeded. A completed content-validation rejection is returned
+// in RunResult, not as an operational error. An operational error returns no
+// partial RunResult.
+func (e *Engine) RunPrepared(ctx context.Context, prepared *PreparedExecution) (*RunResult, error) {
+	if e == nil || e.runner == nil {
+		return nil, fmt.Errorf("%w: engine is nil", ErrInvalidConfig)
+	}
+
+	var internal *usecase.PreparedExecution
+	if prepared != nil {
+		internal = prepared.internal
+	}
+	result, err := e.runner.RunPrepared(ctx, internal)
+	if err != nil {
+		return nil, mapPublicError(err)
+	}
+	return fromDomainRunResult(result), nil
+}

errors.go

@@ -0,0 +1,69 @@
+package promptkit
+
+import (
+	"errors"
+	"fmt"
+	"strings"
+
+	"gitea.maximumdirect.net/eric/promptkit/internal/capacity"
+	"gitea.maximumdirect.net/eric/promptkit/internal/profile"
+	"gitea.maximumdirect.net/eric/promptkit/internal/promptdef"
+	"gitea.maximumdirect.net/eric/promptkit/internal/usecase"
+)
+
+func mapPublicError(err error) error {
+	if err == nil {
+		return nil
+	}
+	var internalCapacityError *usecase.CapacityError
+	if errors.As(err, &internalCapacityError) && internalCapacityError != nil &&
+		strings.TrimSpace(internalCapacityError.BackendID) != "" {
+		return &CapacityError{BackendID: internalCapacityError.BackendID}
+	}
+	publicErr := publicErrorFor(err)
+	if publicErr == nil {
+		return err
+	}
+	return fmt.Errorf("%w: %w", publicErr, err)
+}
+
+func publicErrorFor(err error) error {
+	switch {
+	case errors.Is(err, promptdef.ErrPromptDefinitionNotFound):
+		return ErrPromptNotFound
+	case errors.Is(err, profile.ErrProfileNotFound):
+		return ErrProfileNotFound
+	case errors.Is(err, usecase.ErrProfileRequired):
+		return errors.Join(ErrInvalidRequest, ErrProfileRequired)
+	case errors.Is(err, usecase.ErrPromptLoad):
+		return ErrPromptLoad
+	case errors.Is(err, usecase.ErrProfileLoad):
+		return ErrProfileLoad
+	case errors.Is(err, promptdef.ErrInvalidYAML), errors.Is(err, promptdef.ErrInvalidPromptDefinition):
+		return ErrPromptLoad
+	case isProfileLoadCause(err):
+		return ErrProfileLoad
+	case errors.Is(err, usecase.ErrAPIKeyEnvMissing):
+		return errors.Join(ErrInvalidRequest, ErrAPIKeyEnvMissing)
+	case errors.Is(err, capacity.ErrCapacityExceeded):
+		return ErrCapacityExceeded
+	case errors.Is(err, usecase.ErrArtifactLoad):
+		return ErrArtifactLoad
+	case errors.Is(err, usecase.ErrPromptRender):
+		return ErrPromptRender
+	case errors.Is(err, usecase.ErrLLMGenerate):
+		return ErrLLMGenerate
+	case errors.Is(err, usecase.ErrValidation):
+		return ErrValidation
+	case errors.Is(err, usecase.ErrInvalidRequest):
+		return ErrInvalidRequest
+	default:
+		return nil
+	}
+}
+
+func isProfileLoadCause(err error) bool {
+	return errors.Is(err, profile.ErrInvalidYAML) ||
+		errors.Is(err, profile.ErrInvalidProfile) ||
+		errors.Is(err, profile.ErrRawAPIKeyNotAllowed)
+}

errors_internal_test.go

@@ -0,0 +1,50 @@
+package promptkit
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/promptkit/internal/usecase"
+)
+
+func TestMapPublicErrorPreservesGenerationCancellation(t *testing.T) {
+	internalErr := fmt.Errorf("%w: %w", usecase.ErrLLMGenerate, context.Canceled)
+
+	err := mapPublicError(internalErr)
+	if !errors.Is(err, ErrLLMGenerate) {
+		t.Fatalf("mapped error=%v, want ErrLLMGenerate", err)
+	}
+	if !errors.Is(err, context.Canceled) {
+		t.Fatalf("mapped error=%v, want context.Canceled", err)
+	}
+}
+
+func TestMapPublicErrorTranslatesCapacityError(t *testing.T) {
+	internalErr := &usecase.CapacityError{BackendID: "limited"}
+
+	err := mapPublicError(internalErr)
+	var publicErr *CapacityError
+	if !errors.As(err, &publicErr) || publicErr == nil {
+		t.Fatalf("mapped error=%v, want public CapacityError", err)
+	}
+	if publicErr.BackendID != "limited" {
+		t.Fatalf("mapped backend ID=%q, want limited", publicErr.BackendID)
+	}
+	if !errors.Is(err, ErrCapacityExceeded) {
+		t.Fatalf("mapped error=%v, want ErrCapacityExceeded", err)
+	}
+	if errors.Is(err, ErrInvalidRequest) || errors.Is(err, ErrLLMGenerate) {
+		t.Fatalf("mapped capacity error has an unrelated category: %v", err)
+	}
+	var leakedInternalErr *usecase.CapacityError
+	if errors.As(err, &leakedInternalErr) {
+		t.Fatalf("mapped error exposes internal CapacityError: %v", err)
+	}
+
+	internalErr.BackendID = "changed"
+	if publicErr.BackendID != "limited" {
+		t.Fatalf("mapped backend ID changed with source error: %q", publicErr.BackendID)
+	}
+}

examples/go-library/prepare/main.go

@@ -0,0 +1,60 @@
+package main
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"os"
+
+	"gitea.maximumdirect.net/eric/promptkit"
+)
+
+type summary struct {
+	PromptID        string `json:"prompt_id"`
+	PromptVersion   string `json:"prompt_version"`
+	SelectedProfile string `json:"selected_profile"`
+	Model           string `json:"model"`
+	MessageCount    int    `json:"message_count"`
+}
+
+func main() {
+	engine, err := promptkit.NewEngine(
+		promptkit.Config{},
+		promptkit.WithPromptFile("examples/go-library/prepare/prompt.yaml"),
+		promptkit.WithProfiles(promptkit.Profile{
+			ID:       "offline-example",
+			Endpoint: "https://example.invalid/v1",
+			Model:    "offline-model",
+		}),
+	)
+	if err != nil {
+		exit(err)
+	}
+
+	prepared, err := engine.Prepare(context.Background(), promptkit.RunRequest{
+		PromptID: "example.prepare",
+		Inputs: map[string]promptkit.ArtifactRef{
+			"note": promptkit.Inline("Ada finished the migration review."),
+		},
+	})
+	if err != nil {
+		exit(err)
+	}
+
+	encoder := json.NewEncoder(os.Stdout)
+	encoder.SetIndent("", "  ")
+	if err := encoder.Encode(summary{
+		PromptID:        prepared.PromptID,
+		PromptVersion:   prepared.PromptVersion,
+		SelectedProfile: prepared.SelectedProfileID,
+		Model:           prepared.EffectiveModelParams.Model,
+		MessageCount:    len(prepared.Messages),
+	}); err != nil {
+		exit(err)
+	}
+}
+
+func exit(err error) {
+	fmt.Fprintln(os.Stderr, err)
+	os.Exit(1)
+}

examples/go-library/prepare/prompt.yaml

@@ -0,0 +1,16 @@
+id: example.prepare
+version: "1.0.0"
+default_profile: offline-example
+description: Prepare a prompt without contacting a model provider.
+inputs:
+  - name: note
+    required: true
+    content_type: text/plain
+messages:
+  - role: system
+    content: Summarize the note in one sentence.
+  - role: user
+    content: '{{input "note"}}'
+output:
+  format: text
+  validation_mode: basic

examples/go-library/run/main.go

@@ -0,0 +1,81 @@
+package main
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"os"
+
+	"gitea.maximumdirect.net/eric/promptkit"
+)
+
+type deterministicClient struct{}
+
+func (deterministicClient) Generate(
+	ctx context.Context,
+	_ promptkit.GenerateRequest,
+) (*promptkit.GenerateResponse, error) {
+	if err := ctx.Err(); err != nil {
+		return nil, err
+	}
+
+	return &promptkit.GenerateResponse{
+		Content: "Ada finished the migration review.",
+		Usage: promptkit.TokenUsage{
+			PromptTokens:     12,
+			CompletionTokens: 6,
+			TotalTokens:      18,
+		},
+	}, nil
+}
+
+type summary struct {
+	Output           string                     `json:"output"`
+	ValidationStatus promptkit.ValidationStatus `json:"validation_status"`
+	IsValid          bool                       `json:"is_valid"`
+	Model            string                     `json:"model"`
+	TotalTokens      int                        `json:"total_tokens"`
+}
+
+func main() {
+	engine, err := promptkit.NewEngine(
+		promptkit.Config{},
+		promptkit.WithPromptFile("examples/go-library/run/prompt.yaml"),
+		promptkit.WithProfiles(promptkit.Profile{
+			ID:       "offline-example",
+			Endpoint: "https://example.invalid/v1",
+			Model:    "offline-model",
+		}),
+		promptkit.WithLLMClient(deterministicClient{}),
+	)
+	if err != nil {
+		exit(err)
+	}
+
+	result, err := engine.Run(context.Background(), promptkit.RunRequest{
+		PromptID: "example.run",
+		Inputs: map[string]promptkit.ArtifactRef{
+			"note": promptkit.Inline("Ada finished the migration review."),
+		},
+	})
+	if err != nil {
+		exit(err)
+	}
+
+	encoder := json.NewEncoder(os.Stdout)
+	encoder.SetIndent("", "  ")
+	if err := encoder.Encode(summary{
+		Output:           result.RawOutput,
+		ValidationStatus: result.Validation.Status,
+		IsValid:          result.Validation.IsValid,
+		Model:            result.ModelName,
+		TotalTokens:      result.Usage.TotalTokens,
+	}); err != nil {
+		exit(err)
+	}
+}
+
+func exit(err error) {
+	fmt.Fprintln(os.Stderr, err)
+	os.Exit(1)
+}

examples/go-library/run/prompt.yaml

@@ -0,0 +1,16 @@
+id: example.run
+version: "1.0.0"
+default_profile: offline-example
+description: Run a prompt with a deterministic injected model client.
+inputs:
+  - name: note
+    required: true
+    content_type: text/plain
+messages:
+  - role: system
+    content: Summarize the note in one sentence.
+  - role: user
+    content: '{{input "note"}}'
+output:
+  format: text
+  validation_mode: basic

formatting.go

@@ -0,0 +1,56 @@
+package promptkit
+
+import "fmt"
+
+// String returns a concise request summary without exposing the direct API key
+// or input and variable contents. Reflection-based formatting does not carry
+// this guarantee.
+func (r RunRequest) String() string {
+	return r.redactedString()
+}
+
+// GoString returns a concise request summary without exposing the direct API
+// key or input and variable contents. Reflection-based formatting does not
+// carry this guarantee.
+func (r RunRequest) GoString() string {
+	return r.redactedString()
+}
+
+func (r RunRequest) redactedString() string {
+	return fmt.Sprintf(
+		"promptkit.RunRequest{PromptID:%q PromptVersion:%q ProfileID:%q APIKeySet:%t Inputs:%d Vars:%d ExecutionSet:%t ValidationSet:%t}",
+		r.PromptID,
+		r.PromptVersion,
+		r.ProfileID,
+		r.APIKey != "",
+		len(r.Inputs),
+		len(r.Vars),
+		r.Execution != nil,
+		r.Validation != nil,
+	)
+}
+
+// String returns a concise request summary without exposing direct API keys or
+// rendered prompt content. Reflection-based formatting does not carry this
+// guarantee.
+func (r GenerateRequest) String() string {
+	return r.redactedString()
+}
+
+// GoString returns a concise request summary without exposing direct API keys or
+// rendered prompt content. Reflection-based formatting does not carry this
+// guarantee.
+func (r GenerateRequest) GoString() string {
+	return r.redactedString()
+}
+
+func (r GenerateRequest) redactedString() string {
+	return fmt.Sprintf(
+		"promptkit.GenerateRequest{Messages:%d Model:%q APIKeySet:%t StructuredOutputSet:%t ExtraParams:%d}",
+		len(r.Prompt.Messages),
+		r.Target.Model,
+		r.APIKey != "",
+		r.StructuredOutput != nil,
+		len(r.Target.ExtraParams),
+	)
+}

go.mod

@@ -1,3 +1,10 @@
 module gitea.maximumdirect.net/eric/promptkit
 
 go 1.25.5
+
+require (
+	github.com/santhosh-tekuri/jsonschema/v6 v6.0.2
+	gopkg.in/yaml.v3 v3.0.1
+)
+
+require golang.org/x/text v0.14.0 // indirect

go.sum

@@ -0,0 +1,10 @@
+github.com/dlclark/regexp2 v1.11.0 h1:G/nrcoOa7ZXlpoa/91N3X7mM3r8eIlMBBJZvsz/mxKI=
+github.com/dlclark/regexp2 v1.11.0/go.mod h1:DHkYz0B9wPfa6wondMfaivmHpzrQ3v9q8cnmRbL6yW8=
+github.com/santhosh-tekuri/jsonschema/v6 v6.0.2 h1:KRzFb2m7YtdldCEkzs6KqmJw4nqEVZGK7IN2kJkjTuQ=
+github.com/santhosh-tekuri/jsonschema/v6 v6.0.2/go.mod h1:JXeL+ps8p7/KNMjDQk3TCwPpBy0wYklyWTfbkIzdIFU=
+golang.org/x/text v0.14.0 h1:ScX5w1eTa3QqT8oi6+ziP7dTV1S2+ALU0bI+0zXKWiQ=
+golang.org/x/text v0.14.0/go.mod h1:18ZOQIKpY8NJVqYksKHtTdi31H5itFRjB5/qKTNYzSU=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
+gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=

internal/artifact/reader.go

@@ -0,0 +1,122 @@
+package artifact
+
+import (
+	"context"
+	"crypto/sha256"
+	"errors"
+	"fmt"
+	"io"
+	"mime"
+	"os"
+	"path/filepath"
+
+	"gitea.maximumdirect.net/eric/promptkit/internal/defaults"
+	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
+)
+
+var (
+	ErrUnsupportedRefType = errors.New("unsupported artifact reference type")
+	ErrMissingInlineBody  = errors.New("missing body for inline artifact")
+	ErrMissingFilePath    = errors.New("missing file path for file artifact")
+)
+
+// Reader resolves artifact references into actual artifacts.
+type Reader interface {
+	Read(ctx context.Context, ref domain.ArtifactRef) (*domain.Artifact, error)
+}
+
+// CompositeReader routes artifact resolution based on the reference type.
+type CompositeReader struct {
+	inlineReader *inlineReader
+	fileReader   Reader
+}
+
+func NewCompositeReader() Reader {
+	return &CompositeReader{
+		inlineReader: &inlineReader{},
+		fileReader:   &fileReader{},
+	}
+}
+
+func (c *CompositeReader) Read(ctx context.Context, ref domain.ArtifactRef) (*domain.Artifact, error) {
+	select {
+	case <-ctx.Done():
+		return nil, ctx.Err()
+	default:
+	}
+
+	switch ref.Type {
+	case domain.ArtifactRefInline:
+		return c.inlineReader.Read(ctx, ref)
+	case domain.ArtifactRefFile:
+		return c.fileReader.Read(ctx, ref)
+	default:
+		return nil, fmt.Errorf("%w: %s", ErrUnsupportedRefType, ref.Type)
+	}
+}
+
+type inlineReader struct{}
+
+func (r *inlineReader) Read(ctx context.Context, ref domain.ArtifactRef) (*domain.Artifact, error) {
+	select {
+	case <-ctx.Done():
+		return nil, ctx.Err()
+	default:
+	}
+
+	if ref.Body == "" {
+		return nil, ErrMissingInlineBody
+	}
+
+	body := []byte(ref.Body)
+	return &domain.Artifact{
+		ContentType: defaults.ContentTypeTextPlain,
+		Body:        body,
+		Size:        int64(len(body)),
+		Hash:        fmt.Sprintf("%x", sha256.Sum256(body)),
+		URI:         ref.URI,
+	}, nil
+}
+
+type fileReader struct{}
+
+func (r *fileReader) Read(ctx context.Context, ref domain.ArtifactRef) (*domain.Artifact, error) {
+	select {
+	case <-ctx.Done():
+		return nil, ctx.Err()
+	default:
+	}
+
+	if ref.URI == "" {
+		return nil, ErrMissingFilePath
+	}
+
+	return readFileArtifact(ref.URI)
+}
+
+func readFileArtifact(path string) (*domain.Artifact, error) {
+	file, err := os.Open(path)
+	if err != nil {
+		return nil, fmt.Errorf("failed to read file %s: %w", path, err)
+	}
+	defer file.Close()
+
+	data, err := io.ReadAll(file)
+	if err != nil {
+		return nil, fmt.Errorf("failed to read file %s: %w", path, err)
+	}
+
+	contentType := mime.TypeByExtension(filepath.Ext(path))
+	if contentType == "" {
+		contentType = defaults.ContentTypeTextPlain
+	}
+
+	return &domain.Artifact{
+		Name:        filepath.Base(path),
+		ContentType: contentType,
+		Body:        data,
+		URI:         path,
+		Size:        int64(len(data)),
+		Hash:        fmt.Sprintf("%x", sha256.Sum256(data)),
+	}, nil
+}

internal/artifact/reader_test.go

@@ -0,0 +1,180 @@
+package artifact
+
+import (
+	"context"
+	"errors"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
+)
+
+func TestCompositeReader_Read(t *testing.T) {
+	reader := NewCompositeReader()
+	ctx := context.Background()
+
+	t.Run("inline artifact", func(t *testing.T) {
+		ref := domain.ArtifactRef{
+			Type: domain.ArtifactRefInline,
+			Body: "hello world",
+		}
+		art, err := reader.Read(ctx, ref)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if string(art.Body) != "hello world" {
+			t.Errorf("expected 'hello world', got %s", string(art.Body))
+		}
+		if art.ContentType != "text/plain" {
+			t.Errorf("expected text/plain content type, got %q", art.ContentType)
+		}
+		if art.Hash != "b94d27b9934d3e08a52e52d7da7dabfac484efe37a5380ee9088f7ace2efcde9" {
+			t.Errorf("unexpected hash: %s", art.Hash)
+		}
+		if art.Size != int64(len(ref.Body)) {
+			t.Errorf("expected size %d, got %d", len(ref.Body), art.Size)
+		}
+	})
+
+	t.Run("inline artifact missing body", func(t *testing.T) {
+		ref := domain.ArtifactRef{
+			Type: domain.ArtifactRefInline,
+			Body: "",
+		}
+		_, err := reader.Read(ctx, ref)
+		if !errors.Is(err, ErrMissingInlineBody) {
+			t.Errorf("expected ErrMissingInlineBody, got %v", err)
+		}
+	})
+
+	t.Run("unsupported ref type", func(t *testing.T) {
+		ref := domain.ArtifactRef{
+			Type: domain.ArtifactRefType("unsupported"),
+			URI:  "unsupported://bucket/key",
+		}
+		_, err := reader.Read(ctx, ref)
+		if !errors.Is(err, ErrUnsupportedRefType) {
+			t.Error("expected error for unsupported type")
+		}
+	})
+}
+
+func TestCompositeReaderCopiesInlineData(t *testing.T) {
+	reader := NewCompositeReader()
+	ref := domain.ArtifactRef{
+		Type: domain.ArtifactRefInline,
+		Body: "hello",
+		URI:  "inline:greeting",
+	}
+
+	first, err := reader.Read(context.Background(), ref)
+	if err != nil {
+		t.Fatalf("read first artifact: %v", err)
+	}
+	first.Body[0] = 'j'
+
+	second, err := reader.Read(context.Background(), ref)
+	if err != nil {
+		t.Fatalf("read second artifact: %v", err)
+	}
+	if got := string(second.Body); got != ref.Body {
+		t.Fatalf("expected an independent body %q, got %q", ref.Body, got)
+	}
+	if second.URI != ref.URI {
+		t.Fatalf("expected URI %q, got %q", ref.URI, second.URI)
+	}
+}
+
+func TestCompositeReaderHonorsCancellation(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+
+	_, err := NewCompositeReader().Read(ctx, domain.ArtifactRef{
+		Type: domain.ArtifactRefInline,
+		Body: "ignored",
+	})
+	if !errors.Is(err, context.Canceled) {
+		t.Fatalf("expected context cancellation, got %v", err)
+	}
+}
+
+func TestFileReader_Read(t *testing.T) {
+	content := []byte("test file content")
+	filePath := filepath.Join(t.TempDir(), "artifact.txt")
+	if err := os.WriteFile(filePath, content, 0o600); err != nil {
+		t.Fatal(err)
+	}
+
+	reader := NewCompositeReader()
+	ctx := context.Background()
+
+	t.Run("file artifact loading", func(t *testing.T) {
+		ref := domain.ArtifactRef{
+			Type: domain.ArtifactRefFile,
+			URI:  filePath,
+		}
+		art, err := reader.Read(ctx, ref)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if string(art.Body) != string(content) {
+			t.Errorf("expected %s, got %s", string(content), string(art.Body))
+		}
+		if art.Name != filepath.Base(filePath) {
+			t.Errorf("expected name %q, got %q", filepath.Base(filePath), art.Name)
+		}
+		if !strings.HasPrefix(art.ContentType, "text/plain") {
+			t.Errorf("expected text content type, got %q", art.ContentType)
+		}
+		if art.URI != filePath {
+			t.Errorf("expected URI %q, got %q", filePath, art.URI)
+		}
+		if art.Size != int64(len(content)) {
+			t.Errorf("expected size %d, got %d", len(content), art.Size)
+		}
+		if art.Hash != "60f5237ed4049f0382661ef009d2bc42e48c3ceb3edb6600f7024e7ab3b838f3" {
+			t.Errorf("unexpected hash: %s", art.Hash)
+		}
+	})
+
+	t.Run("missing file path", func(t *testing.T) {
+		ref := domain.ArtifactRef{
+			Type: domain.ArtifactRefFile,
+			URI:  "",
+		}
+		_, err := reader.Read(ctx, ref)
+		if !errors.Is(err, ErrMissingFilePath) {
+			t.Errorf("expected ErrMissingFilePath, got %v", err)
+		}
+	})
+
+	t.Run("missing file", func(t *testing.T) {
+		ref := domain.ArtifactRef{
+			Type: domain.ArtifactRefFile,
+			URI:  filepath.Join(t.TempDir(), "missing.txt"),
+		}
+		if _, err := reader.Read(ctx, ref); err == nil {
+			t.Fatal("expected missing file error")
+		}
+	})
+
+	t.Run("unknown extension uses text fallback", func(t *testing.T) {
+		path := filepath.Join(t.TempDir(), "artifact.unknownextension")
+		if err := os.WriteFile(path, content, 0o600); err != nil {
+			t.Fatal(err)
+		}
+
+		art, err := reader.Read(ctx, domain.ArtifactRef{
+			Type: domain.ArtifactRefFile,
+			URI:  path,
+		})
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if art.ContentType != "text/plain" {
+			t.Errorf("expected text/plain fallback, got %q", art.ContentType)
+		}
+	})
+}

internal/backend/registry.go

@@ -0,0 +1,212 @@
+// Package backend owns validated, immutable OpenAI-compatible backend
+// definitions.
+package backend
+
+import (
+	"errors"
+	"fmt"
+	"net/url"
+	"regexp"
+	"sort"
+	"strings"
+
+	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
+	"gitea.maximumdirect.net/eric/promptkit/internal/jsonvalue"
+	"gitea.maximumdirect.net/eric/promptkit/internal/llm"
+)
+
+const (
+	// OpenRouterID is the reserved ID of Promptkit's built-in OpenRouter
+	// backend.
+	OpenRouterID = "openrouter"
+
+	openRouterEndpoint  = "https://openrouter.ai/api/v1"
+	openRouterAPIKeyEnv = "OPENROUTER_API_KEY"
+
+	openRouterConcurrencyLimit = 16
+	defaultQueueCapacity       = 1024
+)
+
+// ErrBackendNotFound identifies a registry lookup for an unknown backend ID.
+var ErrBackendNotFound = errors.New("backend not found")
+
+var environmentVariableName = regexp.MustCompile(`^[A-Za-z_][A-Za-z0-9_]*$`)
+
+// Registry is an immutable collection of validated backend definitions.
+type Registry struct {
+	backends map[string]domain.Backend
+}
+
+// NewRegistry constructs a registry containing the built-in OpenRouter
+// definition followed by the supplied additions. Every ID must be unique.
+func NewRegistry(additions []domain.Backend) (*Registry, error) {
+	registry := &Registry{
+		backends: make(map[string]domain.Backend, len(additions)+1),
+	}
+
+	definitions := make([]domain.Backend, 0, len(additions)+1)
+	definitions = append(definitions, domain.Backend{
+		ID:               OpenRouterID,
+		Endpoint:         openRouterEndpoint,
+		APIKeyEnv:        openRouterAPIKeyEnv,
+		ConcurrencyLimit: openRouterConcurrencyLimit,
+	})
+	definitions = append(definitions, additions...)
+
+	for _, definition := range definitions {
+		definition.ID = strings.TrimSpace(definition.ID)
+		if definition.ID == "" {
+			return nil, errors.New("backend ID must not be blank")
+		}
+		if _, exists := registry.backends[definition.ID]; exists {
+			return nil, fmt.Errorf("backend ID %q is already registered", definition.ID)
+		}
+
+		normalized, err := normalizeBackend(definition)
+		if err != nil {
+			return nil, err
+		}
+		registry.backends[normalized.ID] = normalized
+	}
+
+	return registry, nil
+}
+
+// GetBackend returns a defensive copy of the backend registered with id.
+func (r *Registry) GetBackend(id string) (domain.Backend, error) {
+	if r == nil {
+		return domain.Backend{}, fmt.Errorf("%w: %q", ErrBackendNotFound, id)
+	}
+	definition, ok := r.backends[id]
+	if !ok {
+		return domain.Backend{}, fmt.Errorf("%w: %q", ErrBackendNotFound, id)
+	}
+	extraParams, err := jsonvalue.CopyMap(definition.ExtraParams)
+	if err != nil {
+		return domain.Backend{}, fmt.Errorf("copy backend %q: %w", id, err)
+	}
+	definition.ExtraParams = extraParams
+	return definition, nil
+}
+
+// CapacityPolicies returns a copy of the normalized policies for limited
+// backends.
+func (r *Registry) CapacityPolicies() map[string]domain.BackendCapacityPolicy {
+	policies := make(map[string]domain.BackendCapacityPolicy)
+	if r == nil {
+		return policies
+	}
+	for id, definition := range r.backends {
+		if definition.ConcurrencyLimit == 0 {
+			continue
+		}
+		policies[id] = domain.BackendCapacityPolicy{
+			ConcurrencyLimit: definition.ConcurrencyLimit,
+			QueueCapacity:    definition.QueueCapacity,
+		}
+	}
+	return policies
+}
+
+func normalizeBackend(definition domain.Backend) (domain.Backend, error) {
+	definition.Endpoint = strings.TrimSpace(definition.Endpoint)
+	if err := validateEndpoint(definition.Endpoint); err != nil {
+		return domain.Backend{}, fmt.Errorf("backend %q endpoint: %w", definition.ID, err)
+	}
+
+	definition.APIKeyEnv = strings.TrimSpace(definition.APIKeyEnv)
+	if definition.APIKeyEnv != "" && !environmentVariableName.MatchString(definition.APIKeyEnv) {
+		return domain.Backend{}, fmt.Errorf(
+			"backend %q api key environment variable %q is invalid",
+			definition.ID,
+			definition.APIKeyEnv,
+		)
+	}
+
+	if definition.ConcurrencyLimit < 0 {
+		return domain.Backend{}, fmt.Errorf(
+			"backend %q concurrency limit must not be negative",
+			definition.ID,
+		)
+	}
+	if definition.QueueCapacity < 0 {
+		return domain.Backend{}, fmt.Errorf(
+			"backend %q queue capacity must not be negative",
+			definition.ID,
+		)
+	}
+	if definition.ConcurrencyLimit == 0 {
+		if definition.QueueCapacitySet {
+			return domain.Backend{}, fmt.Errorf(
+				"backend %q queue capacity requires a positive concurrency limit",
+				definition.ID,
+			)
+		}
+		definition.QueueCapacity = 0
+	} else {
+		if !definition.QueueCapacitySet {
+			definition.QueueCapacity = defaultQueueCapacity
+			definition.QueueCapacitySet = true
+		}
+		maxInt := int(^uint(0) >> 1)
+		if definition.QueueCapacity > maxInt-definition.ConcurrencyLimit {
+			return domain.Backend{}, fmt.Errorf(
+				"backend %q total capacity overflows int",
+				definition.ID,
+			)
+		}
+	}
+
+	keys := make([]string, 0, len(definition.ExtraParams))
+	for key := range definition.ExtraParams {
+		keys = append(keys, key)
+	}
+	sort.Strings(keys)
+	for _, key := range keys {
+		if key == "" {
+			return domain.Backend{}, fmt.Errorf("backend %q extra parameter key must not be empty", definition.ID)
+		}
+		if llm.IsReservedOpenAIChatRequestField(key) {
+			return domain.Backend{}, fmt.Errorf(
+				"backend %q extra parameter %q collides with a reserved request field",
+				definition.ID,
+				key,
+			)
+		}
+	}
+
+	extraParams, err := jsonvalue.CopyMap(definition.ExtraParams)
+	if err != nil {
+		return domain.Backend{}, fmt.Errorf("backend %q extra parameters: %w", definition.ID, err)
+	}
+	definition.ExtraParams = extraParams
+	return definition, nil
+}
+
+func validateEndpoint(endpoint string) error {
+	if endpoint == "" {
+		return errors.New("must not be blank")
+	}
+	if strings.Contains(endpoint, "#") {
+		return errors.New("must not contain a fragment")
+	}
+
+	parsed, err := url.Parse(endpoint)
+	if err != nil {
+		return fmt.Errorf("must be a valid URL: %w", err)
+	}
+	scheme := strings.ToLower(parsed.Scheme)
+	if scheme != "http" && scheme != "https" {
+		return errors.New("must use http or https")
+	}
+	if !parsed.IsAbs() || parsed.Hostname() == "" {
+		return errors.New("must be absolute and include a host")
+	}
+	if parsed.User != nil {
+		return errors.New("must not contain user information")
+	}
+	if parsed.RawQuery != "" || parsed.ForceQuery {
+		return errors.New("must not contain a query string")
+	}
+	return nil
+}

internal/backend/registry_test.go

@@ -0,0 +1,368 @@
+package backend_test
+
+import (
+	"errors"
+	"strings"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/promptkit/internal/backend"
+	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
+)
+
+const validEndpoint = "https://backend.example/v1"
+
+func TestRegistryIncludesExactOpenRouterDefinition(t *testing.T) {
+	registry, err := backend.NewRegistry(nil)
+	if err != nil {
+		t.Fatalf("construct registry: %v", err)
+	}
+
+	definition, err := registry.GetBackend(backend.OpenRouterID)
+	if err != nil {
+		t.Fatalf("look up OpenRouter: %v", err)
+	}
+	if definition.ID != "openrouter" ||
+		definition.Endpoint != "https://openrouter.ai/api/v1" ||
+		definition.APIKeyEnv != "OPENROUTER_API_KEY" ||
+		definition.ConcurrencyLimit != 16 ||
+		definition.QueueCapacity != 1024 ||
+		!definition.QueueCapacitySet ||
+		definition.ExtraParams != nil {
+		t.Fatalf("unexpected OpenRouter definition: %#v", definition)
+	}
+	policies := registry.CapacityPolicies()
+	if len(policies) != 1 ||
+		policies["openrouter"] != (domain.BackendCapacityPolicy{
+			ConcurrencyLimit: 16,
+			QueueCapacity:    1024,
+		}) {
+		t.Fatalf("unexpected OpenRouter capacity policies: %#v", policies)
+	}
+}
+
+func TestRegistryNormalizesUniqueAdditionsAndIsolatesMutations(t *testing.T) {
+	nested := map[string]int{"limit": 2}
+	extraParams := map[string]any{
+		"count":  int64(7),
+		"nested": nested,
+	}
+	registry, err := backend.NewRegistry([]domain.Backend{
+		{
+			ID:               " custom ",
+			Endpoint:         " https://custom.example/openai/v1 ",
+			APIKeyEnv:        " CUSTOM_API_KEY ",
+			ExtraParams:      extraParams,
+			ConcurrencyLimit: 3,
+			QueueCapacity:    2,
+			QueueCapacitySet: true,
+		},
+		{
+			ID:       "Custom",
+			Endpoint: validEndpoint,
+		},
+	})
+	if err != nil {
+		t.Fatalf("construct registry: %v", err)
+	}
+
+	nested["limit"] = 99
+	extraParams["added"] = true
+
+	got, err := registry.GetBackend("custom")
+	if err != nil {
+		t.Fatalf("look up custom backend: %v", err)
+	}
+	if got.ID != "custom" ||
+		got.Endpoint != "https://custom.example/openai/v1" ||
+		got.APIKeyEnv != "CUSTOM_API_KEY" ||
+		got.ConcurrencyLimit != 3 ||
+		got.QueueCapacity != 2 ||
+		!got.QueueCapacitySet {
+		t.Fatalf("unexpected normalized definition: %#v", got)
+	}
+	if count, ok := got.ExtraParams["count"].(int64); !ok || count != 7 {
+		t.Fatalf("integer type or value changed: %#v", got.ExtraParams["count"])
+	}
+	gotNested, ok := got.ExtraParams["nested"].(map[string]int)
+	if !ok || gotNested["limit"] != 2 {
+		t.Fatalf("container type or value changed: %#v", got.ExtraParams["nested"])
+	}
+	if _, exists := got.ExtraParams["added"]; exists {
+		t.Fatalf("registry retained caller map: %#v", got.ExtraParams)
+	}
+
+	gotNested["limit"] = 100
+	got.ExtraParams["added"] = true
+	again, err := registry.GetBackend("custom")
+	if err != nil {
+		t.Fatalf("look up custom backend again: %v", err)
+	}
+	if again.ExtraParams["nested"].(map[string]int)["limit"] != 2 {
+		t.Fatalf("lookup exposed registry nested map: %#v", again.ExtraParams)
+	}
+	if _, exists := again.ExtraParams["added"]; exists {
+		t.Fatalf("lookup exposed registry map: %#v", again.ExtraParams)
+	}
+
+	if _, err := registry.GetBackend("Custom"); err != nil {
+		t.Fatalf("backend IDs should be case-sensitive: %v", err)
+	}
+
+	policies := registry.CapacityPolicies()
+	if len(policies) != 2 {
+		t.Fatalf("unexpected capacity policy count: %#v", policies)
+	}
+	policies["custom"] = domain.BackendCapacityPolicy{}
+	delete(policies, backend.OpenRouterID)
+	againPolicies := registry.CapacityPolicies()
+	if againPolicies["custom"] != (domain.BackendCapacityPolicy{
+		ConcurrencyLimit: 3,
+		QueueCapacity:    2,
+	}) {
+		t.Fatalf("capacity policy map mutated registry state: %#v", againPolicies)
+	}
+	if _, ok := againPolicies[backend.OpenRouterID]; !ok {
+		t.Fatalf("capacity policy deletion mutated registry state: %#v", againPolicies)
+	}
+}
+
+func TestNewRegistryNormalizesCapacityPolicy(t *testing.T) {
+	maxInt := int(^uint(0) >> 1)
+	tests := []struct {
+		name       string
+		definition domain.Backend
+		want       domain.BackendCapacityPolicy
+		wantSet    bool
+		wantError  bool
+	}{
+		{
+			name:       "unlimited when omitted",
+			definition: domain.Backend{},
+		},
+		{
+			name: "default queue",
+			definition: domain.Backend{
+				ConcurrencyLimit: 2,
+			},
+			want: domain.BackendCapacityPolicy{
+				ConcurrencyLimit: 2,
+				QueueCapacity:    1024,
+			},
+			wantSet: true,
+		},
+		{
+			name: "explicit zero queue",
+			definition: domain.Backend{
+				ConcurrencyLimit: 2,
+				QueueCapacitySet: true,
+			},
+			want: domain.BackendCapacityPolicy{
+				ConcurrencyLimit: 2,
+			},
+			wantSet: true,
+		},
+		{
+			name: "negative concurrency limit",
+			definition: domain.Backend{
+				ConcurrencyLimit: -1,
+			},
+			wantError: true,
+		},
+		{
+			name: "negative queue capacity",
+			definition: domain.Backend{
+				ConcurrencyLimit: 1,
+				QueueCapacity:    -1,
+				QueueCapacitySet: true,
+			},
+			wantError: true,
+		},
+		{
+			name: "queue without limit",
+			definition: domain.Backend{
+				QueueCapacitySet: true,
+			},
+			wantError: true,
+		},
+		{
+			name: "total overflow",
+			definition: domain.Backend{
+				ConcurrencyLimit: maxInt,
+				QueueCapacity:    1,
+				QueueCapacitySet: true,
+			},
+			wantError: true,
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			tc.definition.ID = "custom"
+			tc.definition.Endpoint = validEndpoint
+			registry, err := backend.NewRegistry([]domain.Backend{tc.definition})
+			if tc.wantError {
+				if err == nil {
+					t.Fatal("expected invalid capacity policy error")
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("construct registry: %v", err)
+			}
+
+			definition, err := registry.GetBackend("custom")
+			if err != nil {
+				t.Fatalf("look up custom backend: %v", err)
+			}
+			if definition.ConcurrencyLimit != tc.want.ConcurrencyLimit ||
+				definition.QueueCapacity != tc.want.QueueCapacity ||
+				definition.QueueCapacitySet != tc.wantSet {
+				t.Fatalf("normalized capacity=(%d, %d, %t), want (%d, %d, %t)",
+					definition.ConcurrencyLimit,
+					definition.QueueCapacity,
+					definition.QueueCapacitySet,
+					tc.want.ConcurrencyLimit,
+					tc.want.QueueCapacity,
+					tc.wantSet,
+				)
+			}
+			policies := registry.CapacityPolicies()
+			got, ok := policies["custom"]
+			if ok != tc.wantSet || got != tc.want {
+				t.Fatalf("capacity policy=(%#v, %t), want (%#v, %t)", got, ok, tc.want, tc.wantSet)
+			}
+		})
+	}
+}
+
+func TestNewRegistryRejectsDuplicateIDs(t *testing.T) {
+	tests := []struct {
+		name      string
+		additions []domain.Backend
+		wantID    string
+	}{
+		{
+			name: "built-in collision after normalization",
+			additions: []domain.Backend{{
+				ID: " openrouter ",
+			}},
+			wantID: "openrouter",
+		},
+		{
+			name: "consumer collision after normalization",
+			additions: []domain.Backend{
+				{ID: "custom", Endpoint: validEndpoint},
+				{ID: " custom ", Endpoint: "https://other.example/v1"},
+			},
+			wantID: "custom",
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			_, err := backend.NewRegistry(tc.additions)
+			if err == nil {
+				t.Fatal("expected duplicate ID error")
+			}
+			if !strings.Contains(err.Error(), tc.wantID) {
+				t.Fatalf("expected error to identify %q, got %v", tc.wantID, err)
+			}
+		})
+	}
+}
+
+func TestNewRegistryValidatesIDs(t *testing.T) {
+	for _, id := range []string{"", " \t\n "} {
+		t.Run(id, func(t *testing.T) {
+			_, err := backend.NewRegistry([]domain.Backend{{
+				ID:       id,
+				Endpoint: validEndpoint,
+			}})
+			if err == nil {
+				t.Fatal("expected blank ID error")
+			}
+		})
+	}
+}
+
+func TestNewRegistryValidatesEndpoints(t *testing.T) {
+	tests := []struct {
+		name     string
+		endpoint string
+	}{
+		{name: "blank", endpoint: ""},
+		{name: "relative", endpoint: "/v1"},
+		{name: "missing host", endpoint: "https:///v1"},
+		{name: "unsupported scheme", endpoint: "ftp://backend.example/v1"},
+		{name: "user information", endpoint: "https://user@backend.example/v1"},
+		{name: "query", endpoint: "https://backend.example/v1?mode=chat"},
+		{name: "empty query", endpoint: "https://backend.example/v1?"},
+		{name: "fragment", endpoint: "https://backend.example/v1#chat"},
+		{name: "empty fragment", endpoint: "https://backend.example/v1#"},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			_, err := backend.NewRegistry([]domain.Backend{{
+				ID:       "custom",
+				Endpoint: tc.endpoint,
+			}})
+			if err == nil {
+				t.Fatal("expected invalid endpoint error")
+			}
+		})
+	}
+}
+
+func TestNewRegistryValidatesEnvironmentVariableNames(t *testing.T) {
+	for _, name := range []string{"1API_KEY", "API-KEY", "API KEY", "ÅPI_KEY"} {
+		t.Run(name, func(t *testing.T) {
+			_, err := backend.NewRegistry([]domain.Backend{{
+				ID:        "custom",
+				Endpoint:  validEndpoint,
+				APIKeyEnv: name,
+			}})
+			if err == nil {
+				t.Fatal("expected invalid environment-variable name error")
+			}
+		})
+	}
+}
+
+func TestNewRegistryRejectsInvalidAndReservedExtraParameters(t *testing.T) {
+	tests := []struct {
+		name        string
+		extraParams map[string]any
+	}{
+		{name: "unsupported value", extraParams: map[string]any{"value": make(chan int)}},
+		{name: "reserved key", extraParams: map[string]any{"model": "override"}},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			_, err := backend.NewRegistry([]domain.Backend{{
+				ID:          "custom",
+				Endpoint:    validEndpoint,
+				ExtraParams: tc.extraParams,
+			}})
+			if err == nil {
+				t.Fatal("expected invalid extra parameters error")
+			}
+		})
+	}
+}
+
+func TestRegistryLookupReportsNotFound(t *testing.T) {
+	registry, err := backend.NewRegistry(nil)
+	if err != nil {
+		t.Fatalf("construct registry: %v", err)
+	}
+
+	_, err = registry.GetBackend("missing")
+	if !errors.Is(err, backend.ErrBackendNotFound) {
+		t.Fatalf("expected ErrBackendNotFound, got %v", err)
+	}
+	if !strings.Contains(err.Error(), "missing") {
+		t.Fatalf("expected error to identify backend, got %v", err)
+	}
+}

internal/capacity/client.go

@@ -0,0 +1,40 @@
+package capacity
+
+import (
+	"context"
+
+	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
+	"gitea.maximumdirect.net/eric/promptkit/internal/llm"
+)
+
+type client struct {
+	manager *Manager
+	next    llm.Client
+}
+
+// NewClient wraps next with configured active-generation limits. A nil manager
+// leaves next unchanged.
+func NewClient(manager *Manager, next llm.Client) llm.Client {
+	if manager == nil {
+		return next
+	}
+	return &client{
+		manager: manager,
+		next:    next,
+	}
+}
+
+func (c *client) Generate(
+	ctx context.Context,
+	req domain.GenerateRequest,
+) (*domain.GenerateResponse, error) {
+	pool := c.manager.getPool(req.Target.BackendID)
+	if pool == nil {
+		return c.next.Generate(ctx, req)
+	}
+	if err := pool.acquire(ctx); err != nil {
+		return nil, err
+	}
+	defer pool.releaseActive()
+	return c.next.Generate(ctx, req)
+}

internal/capacity/client_test.go

@@ -0,0 +1,517 @@
+package capacity
+
+import (
+	"context"
+	"errors"
+	"reflect"
+	"runtime"
+	"sync"
+	"sync/atomic"
+	"testing"
+	"time"
+
+	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
+	"gitea.maximumdirect.net/eric/promptkit/internal/llm"
+)
+
+type generateResult struct {
+	response *domain.GenerateResponse
+	err      error
+}
+
+type clientFunc func(
+	context.Context,
+	domain.GenerateRequest,
+) (*domain.GenerateResponse, error)
+
+func (f clientFunc) Generate(
+	ctx context.Context,
+	req domain.GenerateRequest,
+) (*domain.GenerateResponse, error) {
+	return f(ctx, req)
+}
+
+type blockingClient struct {
+	mu       sync.Mutex
+	active   int
+	peak     int
+	calls    map[string]int
+	started  chan string
+	releases map[string]chan struct{}
+}
+
+func newBlockingClient(releases map[string]chan struct{}) *blockingClient {
+	return &blockingClient{
+		calls:    make(map[string]int),
+		started:  make(chan string, 64),
+		releases: releases,
+	}
+}
+
+func (c *blockingClient) Generate(
+	ctx context.Context,
+	req domain.GenerateRequest,
+) (*domain.GenerateResponse, error) {
+	id := req.Prompt.SessionID
+	c.mu.Lock()
+	c.active++
+	if c.active > c.peak {
+		c.peak = c.active
+	}
+	c.calls[id]++
+	c.mu.Unlock()
+	defer func() {
+		c.mu.Lock()
+		c.active--
+		c.mu.Unlock()
+	}()
+
+	c.started <- id
+	if release := c.releases[id]; release != nil {
+		select {
+		case <-release:
+		case <-ctx.Done():
+			return nil, ctx.Err()
+		}
+	}
+	return &domain.GenerateResponse{Content: id}, nil
+}
+
+func (c *blockingClient) callCount(id string) int {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	return c.calls[id]
+}
+
+func (c *blockingClient) peakConcurrency() int {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	return c.peak
+}
+
+func generateAsync(
+	client llm.Client,
+	ctx context.Context,
+	backendID string,
+	id string,
+) <-chan generateResult {
+	result := make(chan generateResult, 1)
+	go func() {
+		response, err := client.Generate(ctx, domain.GenerateRequest{
+			Prompt: domain.RenderedPrompt{SessionID: id},
+			Target: domain.ExecutionTarget{BackendID: backendID},
+		})
+		result <- generateResult{response: response, err: err}
+	}()
+	return result
+}
+
+func waitForWaiterCount(t *testing.T, manager *Manager, backendID string, want int) {
+	t.Helper()
+	pool := manager.pools[backendID]
+	deadline := time.Now().Add(2 * time.Second)
+	for {
+		pool.mu.Lock()
+		got := pool.waiters.Len()
+		pool.mu.Unlock()
+		if got == want {
+			return
+		}
+		if time.Now().After(deadline) {
+			t.Fatalf("waiter count=%d, want %d", got, want)
+		}
+		runtime.Gosched()
+	}
+}
+
+func receiveStarted(t *testing.T, started <-chan string) string {
+	t.Helper()
+	select {
+	case id := <-started:
+		return id
+	case <-time.After(2 * time.Second):
+		t.Fatal("timed out waiting for wrapped client invocation")
+		return ""
+	}
+}
+
+func receiveResult(t *testing.T, result <-chan generateResult) generateResult {
+	t.Helper()
+	select {
+	case got := <-result:
+		return got
+	case <-time.After(2 * time.Second):
+		t.Fatal("timed out waiting for generation result")
+		return generateResult{}
+	}
+}
+
+func newTestManager(t *testing.T, policies map[string]domain.BackendCapacityPolicy) *Manager {
+	t.Helper()
+	manager, err := NewManager(policies)
+	if err != nil {
+		t.Fatalf("construct manager: %v", err)
+	}
+	return manager
+}
+
+func TestClientLimitsPeakConcurrencyAndServesWaitersFIFO(t *testing.T) {
+	manager := newTestManager(t, map[string]domain.BackendCapacityPolicy{
+		"limited": {ConcurrencyLimit: 1},
+	})
+	firstRelease := make(chan struct{})
+	secondRelease := make(chan struct{})
+	thirdRelease := make(chan struct{})
+	next := newBlockingClient(map[string]chan struct{}{
+		"first":  firstRelease,
+		"second": secondRelease,
+		"third":  thirdRelease,
+	})
+	client := NewClient(manager, next)
+
+	first := generateAsync(client, context.Background(), "limited", "first")
+	if got := receiveStarted(t, next.started); got != "first" {
+		t.Fatalf("first invocation=%q, want first", got)
+	}
+	second := generateAsync(client, context.Background(), "limited", "second")
+	waitForWaiterCount(t, manager, "limited", 1)
+	third := generateAsync(client, context.Background(), "limited", "third")
+	waitForWaiterCount(t, manager, "limited", 2)
+
+	close(firstRelease)
+	if got := receiveResult(t, first); got.err != nil {
+		t.Fatalf("first generation: %v", got.err)
+	}
+	if got := receiveStarted(t, next.started); got != "second" {
+		t.Fatalf("second invocation=%q, want second", got)
+	}
+	close(secondRelease)
+	if got := receiveResult(t, second); got.err != nil {
+		t.Fatalf("second generation: %v", got.err)
+	}
+	if got := receiveStarted(t, next.started); got != "third" {
+		t.Fatalf("third invocation=%q, want third", got)
+	}
+	close(thirdRelease)
+	if got := receiveResult(t, third); got.err != nil {
+		t.Fatalf("third generation: %v", got.err)
+	}
+	if peak := next.peakConcurrency(); peak != 1 {
+		t.Fatalf("peak concurrency=%d, want 1", peak)
+	}
+}
+
+func TestClientPeakConcurrencyDoesNotExceedConfiguredLimit(t *testing.T) {
+	const limit = 2
+	manager := newTestManager(t, map[string]domain.BackendCapacityPolicy{
+		"limited": {ConcurrencyLimit: limit},
+	})
+	gate := make(chan struct{})
+	releases := make(map[string]chan struct{})
+	for i := range 5 {
+		releases[string(rune('a'+i))] = gate
+	}
+	next := newBlockingClient(releases)
+	client := NewClient(manager, next)
+
+	results := make([]<-chan generateResult, 0, len(releases))
+	for id := range releases {
+		results = append(results, generateAsync(client, context.Background(), "limited", id))
+	}
+	for range limit {
+		receiveStarted(t, next.started)
+	}
+	waitForWaiterCount(t, manager, "limited", len(releases)-limit)
+
+	close(gate)
+	for _, result := range results {
+		if got := receiveResult(t, result); got.err != nil {
+			t.Fatalf("generation: %v", got.err)
+		}
+	}
+	if peak := next.peakConcurrency(); peak != limit {
+		t.Fatalf("peak concurrency=%d, want %d", peak, limit)
+	}
+}
+
+func TestClientRemovesCanceledWaiters(t *testing.T) {
+	tests := []struct {
+		name      string
+		cancelID  string
+		wantOrder []string
+	}{
+		{name: "first waiter", cancelID: "one", wantOrder: []string{"two", "three"}},
+		{name: "middle waiter", cancelID: "two", wantOrder: []string{"one", "three"}},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			manager := newTestManager(t, map[string]domain.BackendCapacityPolicy{
+				"limited": {ConcurrencyLimit: 1},
+			})
+			holderRelease := make(chan struct{})
+			releases := map[string]chan struct{}{
+				"holder": holderRelease,
+				"one":    make(chan struct{}),
+				"two":    make(chan struct{}),
+				"three":  make(chan struct{}),
+			}
+			next := newBlockingClient(releases)
+			client := NewClient(manager, next)
+
+			holder := generateAsync(client, context.Background(), "limited", "holder")
+			if got := receiveStarted(t, next.started); got != "holder" {
+				t.Fatalf("initial invocation=%q, want holder", got)
+			}
+
+			contexts := make(map[string]context.Context)
+			cancels := make(map[string]context.CancelFunc)
+			results := make(map[string]<-chan generateResult)
+			for _, id := range []string{"one", "two", "three"} {
+				contexts[id], cancels[id] = context.WithCancel(context.Background())
+				results[id] = generateAsync(client, contexts[id], "limited", id)
+				waitForWaiterCount(t, manager, "limited", len(results))
+			}
+
+			cancels[tc.cancelID]()
+			if got := receiveResult(t, results[tc.cancelID]); !errors.Is(got.err, context.Canceled) {
+				t.Fatalf("canceled waiter error=%v, want context.Canceled", got.err)
+			}
+			waitForWaiterCount(t, manager, "limited", 2)
+
+			close(holderRelease)
+			if got := receiveResult(t, holder); got.err != nil {
+				t.Fatalf("holder generation: %v", got.err)
+			}
+			for _, id := range tc.wantOrder {
+				if got := receiveStarted(t, next.started); got != id {
+					t.Fatalf("next invocation=%q, want %q", got, id)
+				}
+				close(releases[id])
+				if got := receiveResult(t, results[id]); got.err != nil {
+					t.Fatalf("%s generation: %v", id, got.err)
+				}
+			}
+			if calls := next.callCount(tc.cancelID); calls != 0 {
+				t.Fatalf("canceled waiter invoked wrapped client %d times", calls)
+			}
+			for _, cancel := range cancels {
+				cancel()
+			}
+		})
+	}
+}
+
+func TestClientGrantCancellationRaceDoesNotLeakPermit(t *testing.T) {
+	const iterations = 200
+	for i := range iterations {
+		manager := newTestManager(t, map[string]domain.BackendCapacityPolicy{
+			"limited": {ConcurrencyLimit: 1},
+		})
+		holderRelease := make(chan struct{})
+		var waiterCalls atomic.Int64
+		next := clientFunc(func(
+			_ context.Context,
+			req domain.GenerateRequest,
+		) (*domain.GenerateResponse, error) {
+			if req.Prompt.SessionID == "holder" {
+				<-holderRelease
+			} else if req.Prompt.SessionID == "waiter" {
+				waiterCalls.Add(1)
+			}
+			return &domain.GenerateResponse{Content: req.Prompt.SessionID}, nil
+		})
+		client := NewClient(manager, next)
+
+		holder := generateAsync(client, context.Background(), "limited", "holder")
+		waitForActiveCount(t, manager, "limited", 1)
+		ctx, cancel := context.WithCancel(context.Background())
+		waiterResult := generateAsync(client, ctx, "limited", "waiter")
+		waitForWaiterCount(t, manager, "limited", 1)
+
+		start := make(chan struct{})
+		var race sync.WaitGroup
+		race.Add(2)
+		go func() {
+			defer race.Done()
+			<-start
+			cancel()
+		}()
+		go func() {
+			defer race.Done()
+			<-start
+			close(holderRelease)
+		}()
+		close(start)
+		race.Wait()
+
+		if got := receiveResult(t, holder); got.err != nil {
+			t.Fatalf("iteration %d holder generation: %v", i, got.err)
+		}
+		got := receiveResult(t, waiterResult)
+		switch calls := waiterCalls.Load(); {
+		case calls == 0 && errors.Is(got.err, context.Canceled):
+		case calls == 1 && got.err == nil:
+		default:
+			t.Fatalf("iteration %d waiter calls=%d error=%v", i, calls, got.err)
+		}
+
+		probe := generateAsync(client, context.Background(), "limited", "probe")
+		if got := receiveResult(t, probe); got.err != nil {
+			t.Fatalf("iteration %d probe generation: %v", i, got.err)
+		}
+		waitForActiveCount(t, manager, "limited", 0)
+		waitForWaiterCount(t, manager, "limited", 0)
+	}
+}
+
+func waitForActiveCount(t *testing.T, manager *Manager, backendID string, want int) {
+	t.Helper()
+	pool := manager.pools[backendID]
+	deadline := time.Now().Add(2 * time.Second)
+	for {
+		pool.mu.Lock()
+		got := pool.active
+		pool.mu.Unlock()
+		if got == want {
+			return
+		}
+		if time.Now().After(deadline) {
+			t.Fatalf("active count=%d, want %d", got, want)
+		}
+		runtime.Gosched()
+	}
+}
+
+func TestClientUsesIndependentPoolsAndUnlimitedFastPaths(t *testing.T) {
+	manager := newTestManager(t, map[string]domain.BackendCapacityPolicy{
+		"alpha": {ConcurrencyLimit: 1},
+		"beta":  {ConcurrencyLimit: 1},
+	})
+	alphaRelease := make(chan struct{})
+	betaRelease := make(chan struct{})
+	next := newBlockingClient(map[string]chan struct{}{
+		"alpha": alphaRelease,
+		"beta":  betaRelease,
+	})
+	client := NewClient(manager, next)
+
+	alpha := generateAsync(client, context.Background(), "alpha", "alpha")
+	beta := generateAsync(client, context.Background(), "beta", "beta")
+	started := map[string]bool{
+		receiveStarted(t, next.started): true,
+		receiveStarted(t, next.started): true,
+	}
+	if !started["alpha"] || !started["beta"] {
+		t.Fatalf("independent pools did not both start: %#v", started)
+	}
+	close(alphaRelease)
+	close(betaRelease)
+	if got := receiveResult(t, alpha); got.err != nil {
+		t.Fatalf("alpha generation: %v", got.err)
+	}
+	if got := receiveResult(t, beta); got.err != nil {
+		t.Fatalf("beta generation: %v", got.err)
+	}
+
+	for _, backendID := range []string{"", "unknown"} {
+		response, err := client.Generate(context.Background(), domain.GenerateRequest{
+			Prompt: domain.RenderedPrompt{SessionID: backendID},
+			Target: domain.ExecutionTarget{BackendID: backendID},
+		})
+		if err != nil || response == nil {
+			t.Fatalf("unlimited backend %q response=(%#v, %v)", backendID, response, err)
+		}
+	}
+	if got := NewClient(nil, next); got != next {
+		t.Fatal("nil manager did not return the wrapped client unchanged")
+	}
+}
+
+func TestClientPreservesRequestsResponsesAndErrors(t *testing.T) {
+	manager := newTestManager(t, map[string]domain.BackendCapacityPolicy{
+		"limited": {ConcurrencyLimit: 1},
+	})
+	request := domain.GenerateRequest{
+		Prompt: domain.RenderedPrompt{
+			SessionID: "session",
+			Messages: []domain.RenderedMessage{
+				{Role: "user", Content: "content"},
+			},
+		},
+		Target: domain.ExecutionTarget{
+			BackendID:   "limited",
+			Model:       "model",
+			ExtraParams: map[string]any{"key": "value"},
+		},
+	}
+	response := &domain.GenerateResponse{
+		Content: "output",
+		Usage:   domain.TokenUsage{TotalTokens: 7},
+	}
+	collaboratorErr := errors.New("collaborator failure")
+	tests := []struct {
+		name     string
+		response *domain.GenerateResponse
+		err      error
+	}{
+		{name: "successful response", response: response},
+		{name: "nil response"},
+		{name: "collaborator error", response: response, err: collaboratorErr},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			var captured domain.GenerateRequest
+			next := clientFunc(func(
+				_ context.Context,
+				req domain.GenerateRequest,
+			) (*domain.GenerateResponse, error) {
+				captured = req
+				return tc.response, tc.err
+			})
+			gotResponse, gotErr := NewClient(manager, next).Generate(context.Background(), request)
+			if !reflect.DeepEqual(captured, request) {
+				t.Fatalf("request changed: %#v", captured)
+			}
+			if gotResponse != tc.response || gotErr != tc.err {
+				t.Fatalf("response=(%p, %v), want (%p, %v)",
+					gotResponse, gotErr, tc.response, tc.err)
+			}
+		})
+	}
+}
+
+func TestClientReleasesPermitDuringPanicUnwinding(t *testing.T) {
+	manager := newTestManager(t, map[string]domain.BackendCapacityPolicy{
+		"limited": {ConcurrencyLimit: 1},
+	})
+	var calls atomic.Int64
+	next := clientFunc(func(
+		_ context.Context,
+		_ domain.GenerateRequest,
+	) (*domain.GenerateResponse, error) {
+		if calls.Add(1) == 1 {
+			panic("test panic")
+		}
+		return &domain.GenerateResponse{Content: "recovered"}, nil
+	})
+	client := NewClient(manager, next)
+	request := domain.GenerateRequest{
+		Target: domain.ExecutionTarget{BackendID: "limited"},
+	}
+
+	func() {
+		defer func() {
+			if recover() == nil {
+				t.Fatal("expected wrapped client panic")
+			}
+		}()
+		_, _ = client.Generate(context.Background(), request)
+	}()
+
+	response, err := client.Generate(context.Background(), request)
+	if err != nil || response == nil || response.Content != "recovered" {
+		t.Fatalf("generation after panic=(%#v, %v)", response, err)
+	}
+}

internal/capacity/manager.go

@@ -0,0 +1,160 @@
+// Package capacity coordinates engine-local run admission and model-generation
+// concurrency for configured backends.
+package capacity
+
+import (
+	"container/list"
+	"context"
+	"errors"
+	"fmt"
+	"strings"
+	"sync"
+
+	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
+)
+
+// ErrCapacityExceeded identifies an admission rejected because a backend's
+// configured run capacity is full.
+var ErrCapacityExceeded = errors.New("backend capacity exceeded")
+
+// Manager owns independent backend capacity pools with immutable limits.
+type Manager struct {
+	pools map[string]*pool
+}
+
+type pool struct {
+	mu               sync.Mutex
+	concurrencyLimit int
+	totalCapacity    int
+	admitted         int
+	active           int
+	waiters          list.List
+}
+
+type waiter struct {
+	ready   chan struct{}
+	element *list.Element
+	granted bool
+}
+
+// NewManager constructs independent pools from normalized backend policies.
+func NewManager(policies map[string]domain.BackendCapacityPolicy) (*Manager, error) {
+	manager := &Manager{
+		pools: make(map[string]*pool, len(policies)),
+	}
+	maxInt := int(^uint(0) >> 1)
+	for id, policy := range policies {
+		if strings.TrimSpace(id) == "" {
+			return nil, errors.New("backend capacity policy ID must not be blank")
+		}
+		if policy.ConcurrencyLimit <= 0 {
+			return nil, fmt.Errorf(
+				"backend %q concurrency limit must be positive",
+				id,
+			)
+		}
+		if policy.QueueCapacity < 0 {
+			return nil, fmt.Errorf(
+				"backend %q queue capacity must not be negative",
+				id,
+			)
+		}
+		if policy.QueueCapacity > maxInt-policy.ConcurrencyLimit {
+			return nil, fmt.Errorf("backend %q total capacity overflows int", id)
+		}
+		manager.pools[id] = &pool{
+			concurrencyLimit: policy.ConcurrencyLimit,
+			totalCapacity:    policy.ConcurrencyLimit + policy.QueueCapacity,
+		}
+	}
+	return manager, nil
+}
+
+// Admit immediately reserves one configured backend run slot. Backends without
+// a configured pool are unlimited.
+func (m *Manager) Admit(ctx context.Context, backendID string) (func(), error) {
+	pool := m.getPool(backendID)
+	if pool == nil {
+		return releaseNothing, nil
+	}
+
+	pool.mu.Lock()
+	defer pool.mu.Unlock()
+	if err := ctx.Err(); err != nil {
+		return nil, err
+	}
+	if pool.admitted >= pool.totalCapacity {
+		return nil, ErrCapacityExceeded
+	}
+	pool.admitted++
+
+	var once sync.Once
+	return func() {
+		once.Do(func() {
+			pool.mu.Lock()
+			pool.admitted--
+			pool.mu.Unlock()
+		})
+	}, nil
+}
+
+func releaseNothing() {}
+
+func (m *Manager) getPool(backendID string) *pool {
+	if m == nil || backendID == "" {
+		return nil
+	}
+	return m.pools[backendID]
+}
+
+func (p *pool) acquire(ctx context.Context) error {
+	p.mu.Lock()
+	if err := ctx.Err(); err != nil {
+		p.mu.Unlock()
+		return err
+	}
+	if p.active < p.concurrencyLimit && p.waiters.Len() == 0 {
+		p.active++
+		p.mu.Unlock()
+		return nil
+	}
+
+	waiter := &waiter{ready: make(chan struct{})}
+	waiter.element = p.waiters.PushBack(waiter)
+	p.mu.Unlock()
+
+	select {
+	case <-waiter.ready:
+		return nil
+	case <-ctx.Done():
+		p.mu.Lock()
+		if !waiter.granted {
+			p.waiters.Remove(waiter.element)
+			waiter.element = nil
+			p.mu.Unlock()
+			return ctx.Err()
+		}
+		p.mu.Unlock()
+		return nil
+	}
+}
+
+func (p *pool) releaseActive() {
+	var ready chan struct{}
+
+	p.mu.Lock()
+	if element := p.waiters.Front(); element != nil {
+		waiter := element.Value.(*waiter)
+		p.waiters.Remove(element)
+		waiter.element = nil
+		waiter.granted = true
+		ready = waiter.ready
+	} else {
+		p.active--
+	}
+	p.mu.Unlock()
+
+	if ready != nil {
+		close(ready)
+	}
+}

internal/capacity/manager_test.go

@@ -0,0 +1,163 @@
+package capacity
+
+import (
+	"context"
+	"errors"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
+)
+
+func TestNewManagerRejectsInvalidPolicies(t *testing.T) {
+	maxInt := int(^uint(0) >> 1)
+	tests := []struct {
+		name   string
+		id     string
+		policy domain.BackendCapacityPolicy
+	}{
+		{
+			name:   "blank ID",
+			id:     " \t ",
+			policy: domain.BackendCapacityPolicy{ConcurrencyLimit: 1},
+		},
+		{
+			name:   "zero concurrency",
+			id:     "backend",
+			policy: domain.BackendCapacityPolicy{},
+		},
+		{
+			name:   "negative concurrency",
+			id:     "backend",
+			policy: domain.BackendCapacityPolicy{ConcurrencyLimit: -1},
+		},
+		{
+			name: "negative queue",
+			id:   "backend",
+			policy: domain.BackendCapacityPolicy{
+				ConcurrencyLimit: 1,
+				QueueCapacity:    -1,
+			},
+		},
+		{
+			name: "total overflow",
+			id:   "backend",
+			policy: domain.BackendCapacityPolicy{
+				ConcurrencyLimit: maxInt,
+				QueueCapacity:    1,
+			},
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			_, err := NewManager(map[string]domain.BackendCapacityPolicy{
+				tc.id: tc.policy,
+			})
+			if err == nil {
+				t.Fatal("expected invalid policy error")
+			}
+		})
+	}
+}
+
+func TestManagerAdmissionIsBoundedAndReleaseIsIdempotent(t *testing.T) {
+	policies := map[string]domain.BackendCapacityPolicy{
+		"limited": {
+			ConcurrencyLimit: 2,
+			QueueCapacity:    1,
+		},
+		"independent": {
+			ConcurrencyLimit: 1,
+		},
+	}
+	manager, err := NewManager(policies)
+	if err != nil {
+		t.Fatalf("construct manager: %v", err)
+	}
+	policies["limited"] = domain.BackendCapacityPolicy{
+		ConcurrencyLimit: 100,
+		QueueCapacity:    100,
+	}
+
+	releases := make([]func(), 0, 3)
+	for range 3 {
+		release, err := manager.Admit(context.Background(), "limited")
+		if err != nil {
+			t.Fatalf("admit within configured capacity: %v", err)
+		}
+		releases = append(releases, release)
+	}
+	if release, err := manager.Admit(context.Background(), "limited"); release != nil ||
+		!errors.Is(err, ErrCapacityExceeded) {
+		t.Fatalf("admission beyond capacity=(release=%t, err=%v), want ErrCapacityExceeded",
+			release != nil, err)
+	}
+	independentRelease, err := manager.Admit(context.Background(), "independent")
+	if err != nil {
+		t.Fatalf("admit independent backend while first is full: %v", err)
+	}
+	independentRelease()
+
+	releases[0]()
+	releases[0]()
+	replacement, err := manager.Admit(context.Background(), "limited")
+	if err != nil {
+		t.Fatalf("admit after release: %v", err)
+	}
+	replacement()
+	releases[1]()
+	releases[2]()
+
+	pool := manager.pools["limited"]
+	pool.mu.Lock()
+	admitted := pool.admitted
+	pool.mu.Unlock()
+	if admitted != 0 {
+		t.Fatalf("admitted runs after releases=%d, want 0", admitted)
+	}
+}
+
+func TestManagerAdmissionHonorsContextAndUnlimitedBackends(t *testing.T) {
+	manager, err := NewManager(map[string]domain.BackendCapacityPolicy{
+		"limited": {ConcurrencyLimit: 1},
+	})
+	if err != nil {
+		t.Fatalf("construct manager: %v", err)
+	}
+	release, err := manager.Admit(context.Background(), "limited")
+	if err != nil {
+		t.Fatalf("fill limited pool: %v", err)
+	}
+	defer release()
+
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+	if release, err := manager.Admit(ctx, "limited"); release != nil ||
+		!errors.Is(err, context.Canceled) {
+		t.Fatalf("canceled limited admission=(release=%t, err=%v), want context cancellation",
+			release != nil, err)
+	}
+
+	var nilManager *Manager
+	for _, tc := range []struct {
+		name      string
+		manager   *Manager
+		backendID string
+	}{
+		{name: "nil manager", manager: nilManager, backendID: "limited"},
+		{name: "blank ID", manager: manager},
+		{name: "unknown ID", manager: manager, backendID: "unknown"},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			release, err := tc.manager.Admit(ctx, tc.backendID)
+			if err != nil {
+				t.Fatalf("unlimited admission: %v", err)
+			}
+			if release == nil {
+				t.Fatal("unlimited admission returned nil release")
+			}
+			release()
+			release()
+		})
+	}
+}

internal/defaults/defaults.go

@@ -0,0 +1,34 @@
+package defaults
+
+import (
+	"time"
+
+	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
+)
+
+const (
+	SchemaDirDefault           = "."
+	OutputArtifactName         = "output"
+	ContentTypeTextPlain       = "text/plain"
+	ContentTypeTextMarkdown    = "text/markdown"
+	ContentTypeApplicationJSON = "application/json"
+	OpenAIChatCompletionsPath  = "/chat/completions"
+
+	ExecutionDefaultTemperature    = 0.0
+	ExecutionDefaultMaxTokens      = 0
+	ExecutionDefaultTopP           = 1.0
+	ExecutionDefaultTimeoutSeconds = 600
+)
+
+var (
+	LLMRequestTimeoutDefault = 10 * time.Minute
+)
+
+func ExecutionTargetDefault() domain.ExecutionTarget {
+	return domain.ExecutionTarget{
+		Temperature:    ExecutionDefaultTemperature,
+		MaxTokens:      ExecutionDefaultMaxTokens,
+		TopP:           ExecutionDefaultTopP,
+		TimeoutSeconds: ExecutionDefaultTimeoutSeconds,
+	}
+}

internal/domain/domain.go

@@ -0,0 +1,328 @@
+package domain
+
+import (
+	"time"
+)
+
+// ArtifactRefType defines how an artifact is referenced.
+type ArtifactRefType string
+
+const (
+	ArtifactRefInline ArtifactRefType = "inline"
+	ArtifactRefFile   ArtifactRefType = "file"
+)
+
+// OutputFormat defines the desired format of the generated artifact.
+type OutputFormat string
+
+const (
+	FormatText     OutputFormat = "text"
+	FormatMarkdown OutputFormat = "markdown"
+	FormatJSON     OutputFormat = "json"
+)
+
+// ValidationMode defines how the output should be validated.
+type ValidationMode string
+
+const (
+	ValidationNone       ValidationMode = "none"
+	ValidationBasic      ValidationMode = "basic"
+	ValidationJSON       ValidationMode = "json"
+	ValidationJSONSchema ValidationMode = "json_schema"
+)
+
+// ValidationStatus defines the result of a validation check.
+type ValidationStatus string
+
+const (
+	ValidationPassed  ValidationStatus = "passed"
+	ValidationFailed  ValidationStatus = "failed"
+	ValidationSkipped ValidationStatus = "skipped"
+)
+
+// CacheControlType defines provider cache behavior for prompt content.
+type CacheControlType string
+
+const (
+	CacheControlEphemeral CacheControlType = "ephemeral"
+)
+
+const (
+	// SessionIDMaxLength is OpenRouter's documented maximum session_id length.
+	SessionIDMaxLength = 256
+)
+
+// CacheControl describes provider cache metadata attached to prompt content.
+type CacheControl struct {
+	Type CacheControlType `yaml:"type" json:"type"`
+	TTL  string           `yaml:"ttl,omitempty" json:"ttl,omitempty"`
+}
+
+// RunRequest represents a request to generate a single artifact.
+type RunRequest struct {
+	PromptID      string
+	PromptVersion string
+	ProfileID     string
+	SessionID     string
+	APIKey        string `json:"-" yaml:"-"`
+	Inputs        map[string]ArtifactRef
+	Vars          map[string]string
+	Execution     *ExecutionTargetOverride
+	Validation    *OutputContract
+}
+
+// RunResult represents the complete result of a prompt execution run.
+type RunResult struct {
+	RunID                string
+	Artifact             Artifact
+	RawOutput            string
+	Validation           ValidationResult
+	PromptID             string
+	PromptVersion        string
+	PromptHash           string
+	SessionID            string
+	RenderedPromptHash   string
+	SelectedProfileID    string
+	SelectedBackendID    string
+	ModelName            string
+	Endpoint             string
+	EffectiveModelParams ExecutionTarget
+	InputHashes          map[string]string
+	Usage                TokenUsage
+	StartTime            time.Time
+	EndTime              time.Time
+	Duration             time.Duration
+}
+
+// PreparedRun contains pre-LLM execution state from the prepare/render phase.
+// It must never include resolved API key values, model output, or validation data.
+type PreparedRun struct {
+	PromptID             string                  `json:"prompt_id"`
+	PromptVersion        string                  `json:"prompt_version,omitempty"`
+	PromptHash           string                  `json:"prompt_hash,omitempty"`
+	SelectedProfileID    string                  `json:"selected_profile_id"`
+	SelectedBackendID    string                  `json:"selected_backend_id,omitempty"`
+	EffectiveModelParams ExecutionTarget         `json:"effective_model_params"`
+	TargetPresence       ExecutionTargetPresence `json:"-"`
+	OutputContract       OutputContract          `json:"output_contract"`
+	StructuredOutput     *StructuredOutputSpec   `json:"structured_output,omitempty"`
+	InputHashes          map[string]string       `json:"input_hashes,omitempty"`
+	SessionID            string                  `json:"session_id,omitempty"`
+	RenderedPromptHash   string                  `json:"rendered_prompt_hash"`
+	Messages             []RenderedMessage       `json:"messages"`
+	StartTime            time.Time               `json:"start_time,omitempty"`
+	EndTime              time.Time               `json:"end_time,omitempty"`
+	DurationMS           int64                   `json:"duration_ms,omitempty"`
+}
+
+// ArtifactRef represents a reference to an input artifact.
+type ArtifactRef struct {
+	Type ArtifactRefType
+	URI  string
+	Body string // Used for inline
+}
+
+// Artifact represents the actual loaded content of a reference.
+type Artifact struct {
+	Name        string
+	ContentType string
+	Body        []byte
+	URI         string
+	Size        int64
+	Hash        string
+}
+
+// PromptDefinition represents a configured prompt execution definition.
+type PromptDefinition struct {
+	ID             string                  `yaml:"id"`
+	Version        string                  `yaml:"version"`
+	DefaultProfile string                  `yaml:"default_profile"`
+	Description    string                  `yaml:"description"`
+	SessionID      string                  `yaml:"session_id" json:"session_id,omitempty"`
+	Inputs         []PromptInput           `yaml:"inputs"`
+	Templates      []PromptMessageTemplate `yaml:"templates"`
+	OutputFormat   OutputFormat            `yaml:"output_format"`
+	Validation     OutputContract          `yaml:"validation"`
+}
+
+// PromptInspection is the resolved result of exact prompt inspection.
+type PromptInspection struct {
+	PromptID         string
+	PromptVersion    string
+	PromptHash       string
+	DefaultProfileID string
+	Inputs           []PromptInput
+	OutputContract   OutputContract
+}
+
+// PromptInput describes one named input expected by a prompt definition.
+type PromptInput struct {
+	Name        string `yaml:"name"`
+	Required    bool   `yaml:"required"`
+	ContentType string `yaml:"content_type"`
+	Description string `yaml:"description"`
+}
+
+// PromptMessageTemplate defines a template for a chat message.
+type PromptMessageTemplate struct {
+	Role         string        `yaml:"role"`
+	Content      string        `yaml:"content"`
+	ContentFile  string        `yaml:"content_file"`
+	CacheControl *CacheControl `yaml:"cache_control,omitempty" json:"cache_control,omitempty"`
+}
+
+// Backend describes reusable OpenAI-compatible connection defaults.
+type Backend struct {
+	ID               string
+	Endpoint         string
+	APIKeyEnv        string
+	ExtraParams      map[string]any
+	ConcurrencyLimit int
+	QueueCapacity    int
+	QueueCapacitySet bool
+}
+
+// BackendCapacityPolicy describes normalized run and generation capacity for
+// one limited backend.
+type BackendCapacityPolicy struct {
+	ConcurrencyLimit int
+	QueueCapacity    int
+}
+
+// ExecutionProfile describes how and where to execute a model.
+type ExecutionProfile struct {
+	ID              string         `yaml:"id"`
+	BackendID       string         `yaml:"backend"`
+	Endpoint        string         `yaml:"endpoint"`
+	Model           string         `yaml:"model"`
+	Temperature     float64        `yaml:"temperature"`
+	MaxTokens       int            `yaml:"max_tokens"`
+	TopP            float64        `yaml:"top_p"`
+	TimeoutSeconds  int            `yaml:"timeout_seconds"`
+	ServiceTier     string         `yaml:"service_tier"`
+	ReasoningEffort string         `yaml:"reasoning_effort"`
+	APIKeyEnv       string         `yaml:"api_key_env"`
+	APIKeyRequired  bool           `yaml:"-" json:"-"`
+	ExtraParams     map[string]any `yaml:"extra_params"`
+}
+
+// ExecutionTargetOverride represents per-request runtime setting overrides.
+type ExecutionTargetOverride struct {
+	Endpoint        string         `json:"endpoint,omitempty"`
+	Model           string         `json:"model,omitempty"`
+	Temperature     *float64       `json:"temperature,omitempty"`
+	MaxTokens       *int           `json:"max_tokens,omitempty"`
+	TopP            *float64       `json:"top_p,omitempty"`
+	TimeoutSeconds  *int           `json:"timeout_seconds,omitempty"`
+	ServiceTier     string         `json:"service_tier,omitempty"`
+	ReasoningEffort *string        `json:"reasoning_effort,omitempty"`
+	APIKeyEnv       string         `json:"api_key_env,omitempty"`
+	ExtraParams     map[string]any `json:"extra_params,omitempty"`
+}
+
+// ExecutionTargetPresence tracks which effective runtime fields came from an
+// explicit request override even when the resolved value is a zero value.
+type ExecutionTargetPresence struct {
+	Temperature    bool
+	MaxTokens      bool
+	TopP           bool
+	TimeoutSeconds bool
+}
+
+// ExecutionTarget represents effective model runtime settings for a run.
+type ExecutionTarget struct {
+	BackendID       string         `yaml:"backend" json:"backend_id,omitempty"`
+	Endpoint        string         `yaml:"endpoint" json:"endpoint"`
+	Model           string         `yaml:"model" json:"model"`
+	Temperature     float64        `yaml:"temperature" json:"temperature"`
+	MaxTokens       int            `yaml:"max_tokens" json:"max_tokens"`
+	TopP            float64        `yaml:"top_p" json:"top_p"`
+	TimeoutSeconds  int            `yaml:"timeout_seconds" json:"timeout_seconds"`
+	ServiceTier     string         `yaml:"service_tier" json:"service_tier"`
+	ReasoningEffort string         `yaml:"reasoning_effort" json:"reasoning_effort"`
+	APIKeyEnv       string         `yaml:"api_key_env" json:"api_key_env"`
+	APIKey          string         `yaml:"-" json:"-"`
+	APIKeyRequired  bool           `yaml:"-" json:"-"`
+	ExtraParams     map[string]any `yaml:"extra_params" json:"extra_params"`
+}
+
+// ProfileInspection is the resolved result of exact profile inspection.
+type ProfileInspection struct {
+	ProfileID            string
+	EffectiveModelParams ExecutionTarget
+	APIKeyRequired       bool
+}
+
+// OutputContract defines the requirements for the output artifact.
+type OutputContract struct {
+	Format         OutputFormat   `yaml:"format"`
+	ValidationMode ValidationMode `yaml:"validation_mode"`
+	SchemaPath     string         `yaml:"schema_path"`
+	RepairAttempts int            `yaml:"repair_attempts"`
+}
+
+// RenderedPrompt represents the prompt after template application.
+type RenderedPrompt struct {
+	SessionID string            `json:"session_id,omitempty"`
+	Messages  []RenderedMessage `json:"messages"`
+}
+
+// RenderedMessage is a single message in a rendered prompt.
+type RenderedMessage struct {
+	Role         string        `json:"role"`
+	Content      string        `json:"content"`
+	CacheControl *CacheControl `json:"cache_control,omitempty"`
+}
+
+// GenerateRequest is the internal request passed to the LLM client.
+type GenerateRequest struct {
+	Prompt           RenderedPrompt
+	Target           ExecutionTarget
+	TargetPresence   ExecutionTargetPresence
+	StructuredOutput *StructuredOutputSpec
+}
+
+// StructuredOutputType indicates which provider-level output mode is requested.
+type StructuredOutputType string
+
+const (
+	StructuredOutputJSONSchema StructuredOutputType = "json_schema"
+)
+
+// StructuredOutputSpec describes provider-level structured output requirements.
+type StructuredOutputSpec struct {
+	Type       StructuredOutputType      `json:"type"`
+	JSONSchema *StructuredOutputJSONSpec `json:"json_schema,omitempty"`
+}
+
+// StructuredOutputJSONSpec contains json_schema output constraints.
+type StructuredOutputJSONSpec struct {
+	Name   string `json:"name"`
+	Strict bool   `json:"strict"`
+	Schema any    `json:"schema"`
+}
+
+// GenerateResponse is the response received from the LLM client.
+type GenerateResponse struct {
+	Content string
+	Usage   TokenUsage
+}
+
+// TokenUsage tracks token consumption.
+type TokenUsage struct {
+	PromptTokens     int
+	CompletionTokens int
+	TotalTokens      int
+	CachedTokens     int
+	CacheWriteTokens int
+}
+
+// ValidationResult represents the outcome of an output validation.
+type ValidationResult struct {
+	Status         ValidationStatus
+	Mode           ValidationMode
+	Errors         []string
+	SchemaPath     string
+	RepairAttempts int
+	IsValid        bool
+}

internal/domain/prepared_run_test.go

@@ -0,0 +1,141 @@
+package domain
+
+import (
+	"encoding/json"
+	"strings"
+	"testing"
+)
+
+func TestPreparedRunJSONDoesNotIncludeSecretValues(t *testing.T) {
+	const envName = "PROMPTKIT_TEST_API_KEY"
+	const secret = "super-secret-value"
+	t.Setenv(envName, secret)
+
+	prepared := PreparedRun{
+		PromptID:          "prompt.id",
+		PromptVersion:     "v1",
+		PromptHash:        "prompt-hash",
+		SelectedProfileID: "local-fast",
+		EffectiveModelParams: ExecutionTarget{
+			Endpoint:  "http://llm/v1",
+			Model:     "gpt-test",
+			APIKeyEnv: envName,
+			APIKey:    secret,
+		},
+		InputHashes:        map[string]string{"transcript": "hash-1"},
+		RenderedPromptHash: "rendered-hash",
+		Messages: []RenderedMessage{
+			{Role: "system", Content: "You are helpful."},
+			{Role: "user", Content: "Summarize this."},
+		},
+	}
+
+	b, err := json.Marshal(prepared)
+	if err != nil {
+		t.Fatalf("marshal failed: %v", err)
+	}
+
+	out := string(b)
+	if strings.Contains(out, secret) {
+		t.Fatalf("prepared run JSON unexpectedly contains secret value: %s", out)
+	}
+	if !strings.Contains(out, `"api_key_env":"`+envName+`"`) {
+		t.Fatalf("prepared run JSON should include api_key_env name: %s", out)
+	}
+
+	var top map[string]any
+	if err := json.Unmarshal(b, &top); err != nil {
+		t.Fatalf("unmarshal failed: %v", err)
+	}
+
+	for _, forbidden := range []string{"raw_output", "validation", "artifact"} {
+		if _, ok := top[forbidden]; ok {
+			t.Fatalf("prepared run JSON should not include %q", forbidden)
+		}
+	}
+}
+
+func TestPreparedRunJSONIncludesMessageCacheControlOnlyWhenPresent(t *testing.T) {
+	prepared := PreparedRun{
+		PromptID:          "prompt.id",
+		SelectedProfileID: "local-fast",
+		EffectiveModelParams: ExecutionTarget{
+			Endpoint: "http://llm/v1",
+			Model:    "gpt-test",
+		},
+		RenderedPromptHash: "rendered-hash",
+		Messages: []RenderedMessage{
+			{
+				Role:    "system",
+				Content: "You are helpful.",
+				CacheControl: &CacheControl{
+					Type: CacheControlEphemeral,
+					TTL:  "1h",
+				},
+			},
+			{Role: "user", Content: "Summarize this."},
+		},
+	}
+
+	b, err := json.Marshal(prepared)
+	if err != nil {
+		t.Fatalf("marshal failed: %v", err)
+	}
+
+	var decoded struct {
+		Messages []map[string]any `json:"messages"`
+	}
+	if err := json.Unmarshal(b, &decoded); err != nil {
+		t.Fatalf("unmarshal failed: %v", err)
+	}
+	if len(decoded.Messages) != 2 {
+		t.Fatalf("expected 2 messages, got %d", len(decoded.Messages))
+	}
+
+	cacheControl, ok := decoded.Messages[0]["cache_control"].(map[string]any)
+	if !ok {
+		t.Fatalf("expected cache_control on first message, got %#v", decoded.Messages[0])
+	}
+	if cacheControl["type"] != string(CacheControlEphemeral) || cacheControl["ttl"] != "1h" {
+		t.Fatalf("unexpected cache_control payload: %#v", cacheControl)
+	}
+	if _, ok := decoded.Messages[1]["cache_control"]; ok {
+		t.Fatalf("expected second message to omit cache_control, got %#v", decoded.Messages[1])
+	}
+}
+
+func TestPreparedRunJSONIncludesSessionIDOnlyWhenPresent(t *testing.T) {
+	prepared := PreparedRun{
+		PromptID:          "prompt.id",
+		SelectedProfileID: "local-fast",
+		EffectiveModelParams: ExecutionTarget{
+			Endpoint: "http://llm/v1",
+			Model:    "gpt-test",
+		},
+		SessionID:          "session-123",
+		RenderedPromptHash: "rendered-hash",
+		Messages:           []RenderedMessage{{Role: "user", Content: "Summarize this."}},
+	}
+
+	b, err := json.Marshal(prepared)
+	if err != nil {
+		t.Fatalf("marshal failed: %v", err)
+	}
+
+	var decoded map[string]any
+	if err := json.Unmarshal(b, &decoded); err != nil {
+		t.Fatalf("unmarshal failed: %v", err)
+	}
+	if decoded["session_id"] != "session-123" {
+		t.Fatalf("expected session_id in prepared run JSON, got %#v", decoded["session_id"])
+	}
+
+	prepared.SessionID = ""
+	b, err = json.Marshal(prepared)
+	if err != nil {
+		t.Fatalf("marshal failed: %v", err)
+	}
+	if strings.Contains(string(b), "session_id") {
+		t.Fatalf("expected empty session_id to be omitted, got %s", b)
+	}
+}

internal/domain/session.go

@@ -0,0 +1,19 @@
+package domain
+
+import (
+	"fmt"
+	"strings"
+	"unicode/utf8"
+)
+
+// NormalizeSessionID applies the shared session identifier rule.
+func NormalizeSessionID(raw string) (string, error) {
+	normalized := strings.TrimSpace(raw)
+	if normalized == "" {
+		return "", nil
+	}
+	if length := utf8.RuneCountInString(normalized); length > SessionIDMaxLength {
+		return "", fmt.Errorf("session_id length %d exceeds maximum %d", length, SessionIDMaxLength)
+	}
+	return normalized, nil
+}

internal/domain/session_test.go

@@ -0,0 +1,57 @@
+package domain
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestNormalizeSessionID(t *testing.T) {
+	tests := []struct {
+		name    string
+		raw     string
+		want    string
+		wantErr bool
+	}{
+		{
+			name: "trims surrounding Unicode whitespace",
+			raw:  "\u2003 session-123 \u2003",
+			want: "session-123",
+		},
+		{
+			name: "blank input is omitted",
+			raw:  " \t\u2003 ",
+			want: "",
+		},
+		{
+			name: "maximum Unicode length is accepted",
+			raw:  strings.Repeat("界", SessionIDMaxLength),
+			want: strings.Repeat("界", SessionIDMaxLength),
+		},
+		{
+			name:    "one Unicode code point over maximum is rejected",
+			raw:     strings.Repeat("界", SessionIDMaxLength+1),
+			wantErr: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := NormalizeSessionID(tt.raw)
+			if tt.wantErr {
+				if err == nil {
+					t.Fatal("expected normalization error")
+				}
+				if !strings.Contains(err.Error(), "exceeds maximum") {
+					t.Fatalf("expected useful length diagnostic, got %v", err)
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("normalize session id: %v", err)
+			}
+			if got != tt.want {
+				t.Fatalf("normalized session id = %q, want %q", got, tt.want)
+			}
+		})
+	}
+}

internal/filecatalog/catalog.go

@@ -0,0 +1,142 @@
+package filecatalog
+
+import (
+	"context"
+	"fmt"
+	"io/fs"
+	"os"
+	"path"
+	"path/filepath"
+	"sort"
+	"strings"
+)
+
+// FindYAMLFiles returns sorted full paths for .yaml and .yml files under root.
+func FindYAMLFiles(ctx context.Context, root string) ([]string, error) {
+	var files []string
+	err := filepath.WalkDir(root, func(path string, d os.DirEntry, err error) error {
+		if err != nil {
+			return err
+		}
+		select {
+		case <-ctx.Done():
+			return ctx.Err()
+		default:
+		}
+		if d.IsDir() {
+			return nil
+		}
+		if !IsYAMLFile(d.Name()) {
+			return nil
+		}
+		files = append(files, path)
+		return nil
+	})
+	sort.Strings(files)
+	return files, err
+}
+
+// FindFSYAMLFiles returns sorted paths for .yaml and .yml files under root in fsys.
+func FindFSYAMLFiles(ctx context.Context, fsys fs.FS, root string) ([]string, error) {
+	cleanRoot := CleanFSRoot(root)
+	var files []string
+	err := fs.WalkDir(fsys, cleanRoot, func(name string, d fs.DirEntry, err error) error {
+		if err != nil {
+			return err
+		}
+		select {
+		case <-ctx.Done():
+			return ctx.Err()
+		default:
+		}
+		if d.IsDir() {
+			return nil
+		}
+		if !IsYAMLFile(d.Name()) {
+			return nil
+		}
+		files = append(files, name)
+		return nil
+	})
+	sort.Strings(files)
+	return files, err
+}
+
+// RelativePath computes a clean relative path from root to path.
+func RelativePath(root string, filePath string) string {
+	rel, err := filepath.Rel(root, filePath)
+	if err != nil {
+		return filepath.Clean(filePath)
+	}
+	return filepath.Clean(rel)
+}
+
+// CleanFSRoot normalizes a root path for use with fs.FS.
+func CleanFSRoot(root string) string {
+	root = strings.TrimSpace(root)
+	if root == "" || root == "." {
+		return "."
+	}
+	return path.Clean(root)
+}
+
+// DisplayPath returns name relative to root for messages about fs.FS paths.
+func DisplayPath(root string, name string) string {
+	cleanRoot := CleanFSRoot(root)
+	cleanName := path.Clean(name)
+	if cleanRoot == "." {
+		return cleanName
+	}
+	prefix := strings.TrimSuffix(cleanRoot, "/") + "/"
+	if strings.HasPrefix(cleanName, prefix) {
+		return strings.TrimPrefix(cleanName, prefix)
+	}
+	return cleanName
+}
+
+// ResolveFSPath resolves userPath from baseDir and keeps it inside root.
+func ResolveFSPath(root string, baseDir string, userPath string) (string, string, error) {
+	cleanRoot := CleanFSRoot(root)
+	cleanBase := path.Clean(strings.TrimSpace(baseDir))
+	if cleanBase == "" {
+		cleanBase = cleanRoot
+	}
+	if !containsFSPath(cleanRoot, cleanBase) {
+		return "", "", fmt.Errorf("base path %q is outside source root %q", cleanBase, cleanRoot)
+	}
+
+	cleanUserPath := strings.TrimSpace(userPath)
+	if cleanUserPath == "" {
+		return "", "", fmt.Errorf("path is required")
+	}
+	cleanUserPath = path.Clean(cleanUserPath)
+	if path.IsAbs(cleanUserPath) {
+		return "", "", fmt.Errorf("path %q must be relative", userPath)
+	}
+
+	resolved := path.Clean(path.Join(cleanBase, cleanUserPath))
+	if !containsFSPath(cleanRoot, resolved) {
+		return "", "", fmt.Errorf("path %q escapes source root %q", userPath, cleanRoot)
+	}
+	return resolved, DisplayPath(cleanRoot, resolved), nil
+}
+
+func containsFSPath(root string, name string) bool {
+	root = CleanFSRoot(root)
+	name = path.Clean(name)
+	if root == "." {
+		return name == "." || (name != ".." && !strings.HasPrefix(name, "../"))
+	}
+	return name == root || strings.HasPrefix(name, strings.TrimSuffix(root, "/")+"/")
+}
+
+// Stem strips .yaml or .yml from a file name.
+func Stem(name string) string {
+	name = strings.TrimSuffix(name, ".yaml")
+	name = strings.TrimSuffix(name, ".yml")
+	return name
+}
+
+func IsYAMLFile(name string) bool {
+	return strings.HasSuffix(name, ".yaml") || strings.HasSuffix(name, ".yml")
+}

internal/filecatalog/catalog_test.go

@@ -0,0 +1,270 @@
+package filecatalog
+
+import (
+	"context"
+	"errors"
+	"os"
+	"path/filepath"
+	"reflect"
+	"strings"
+	"testing"
+	"testing/fstest"
+)
+
+func TestFindYAMLFilesNestedSortedAndFiltered(t *testing.T) {
+	root := t.TempDir()
+	mustWriteFile(t, filepath.Join(root, "z", "prompt.yml"), "id: z")
+	mustWriteFile(t, filepath.Join(root, "a", "profile.yaml"), "id: a")
+	mustWriteFile(t, filepath.Join(root, "a", "ignore.txt"), "not yaml")
+	mustWriteFile(t, filepath.Join(root, "b", "ignore.yaml.bak"), "not yaml")
+
+	got, err := FindYAMLFiles(context.Background(), root)
+	if err != nil {
+		t.Fatalf("expected no error, got %v", err)
+	}
+
+	want := []string{
+		filepath.Join(root, "a", "profile.yaml"),
+		filepath.Join(root, "z", "prompt.yml"),
+	}
+	if !reflect.DeepEqual(got, want) {
+		t.Fatalf("expected sorted YAML files %v, got %v", want, got)
+	}
+}
+
+func TestFindYAMLFilesHonorsContextCancellation(t *testing.T) {
+	root := t.TempDir()
+	mustWriteFile(t, filepath.Join(root, "one.yaml"), "id: one")
+
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+
+	_, err := FindYAMLFiles(ctx, root)
+	if !errors.Is(err, context.Canceled) {
+		t.Fatalf("expected context.Canceled, got %v", err)
+	}
+}
+
+func TestFindFSYAMLFilesNestedSortedAndFiltered(t *testing.T) {
+	fsys := fstest.MapFS{
+		"prompts/z/prompt.yml":      &fstest.MapFile{Data: []byte("id: z")},
+		"prompts/a/profile.yaml":    &fstest.MapFile{Data: []byte("id: a")},
+		"prompts/a/ignore.txt":      &fstest.MapFile{Data: []byte("not yaml")},
+		"prompts/b/ignore.yaml.bak": &fstest.MapFile{Data: []byte("not yaml")},
+		"other/ignored.yaml":        &fstest.MapFile{Data: []byte("id: ignored")},
+	}
+
+	got, err := FindFSYAMLFiles(context.Background(), fsys, " prompts ")
+	if err != nil {
+		t.Fatalf("expected no error, got %v", err)
+	}
+
+	want := []string{
+		"prompts/a/profile.yaml",
+		"prompts/z/prompt.yml",
+	}
+	if !reflect.DeepEqual(got, want) {
+		t.Fatalf("expected sorted YAML files %v, got %v", want, got)
+	}
+}
+
+func TestFindFSYAMLFilesHonorsContextCancellation(t *testing.T) {
+	fsys := fstest.MapFS{
+		"one.yaml": &fstest.MapFile{Data: []byte("id: one")},
+	}
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+
+	_, err := FindFSYAMLFiles(ctx, fsys, ".")
+	if !errors.Is(err, context.Canceled) {
+		t.Fatalf("expected context.Canceled, got %v", err)
+	}
+}
+
+func TestRelativePathNested(t *testing.T) {
+	root := t.TempDir()
+	path := filepath.Join(root, "nested", "profiles", "local.yaml")
+	got := RelativePath(root, path)
+	want := filepath.Join("nested", "profiles", "local.yaml")
+	if got != want {
+		t.Fatalf("expected relative path %q, got %q", want, got)
+	}
+}
+
+func TestCleanFSRoot(t *testing.T) {
+	tests := []struct {
+		name string
+		root string
+		want string
+	}{
+		{name: "empty", root: "", want: "."},
+		{name: "dot", root: ".", want: "."},
+		{name: "trimmed", root: " prompts/../profiles ", want: "profiles"},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			if got := CleanFSRoot(tc.root); got != tc.want {
+				t.Fatalf("expected %q, got %q", tc.want, got)
+			}
+		})
+	}
+}
+
+func TestDisplayPath(t *testing.T) {
+	tests := []struct {
+		name string
+		root string
+		path string
+		want string
+	}{
+		{name: "root dot", root: ".", path: "profiles/local.yaml", want: "profiles/local.yaml"},
+		{name: "nested root", root: "profiles", path: "profiles/local.yaml", want: "local.yaml"},
+		{name: "outside root", root: "profiles", path: "other/local.yaml", want: "other/local.yaml"},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			if got := DisplayPath(tc.root, tc.path); got != tc.want {
+				t.Fatalf("expected %q, got %q", tc.want, got)
+			}
+		})
+	}
+}
+
+func TestResolveFSPath(t *testing.T) {
+	tests := []struct {
+		name        string
+		root        string
+		baseDir     string
+		userPath    string
+		wantPath    string
+		wantDisplay string
+		wantErr     string
+	}{
+		{
+			name:        "sibling inside root",
+			root:        "prompts",
+			baseDir:     "prompts/nested",
+			userPath:    "./messages/user.tmpl",
+			wantPath:    "prompts/nested/messages/user.tmpl",
+			wantDisplay: "nested/messages/user.tmpl",
+		},
+		{
+			name:        "parent inside root",
+			root:        "prompts",
+			baseDir:     "prompts/nested",
+			userPath:    "../shared/user.tmpl",
+			wantPath:    "prompts/shared/user.tmpl",
+			wantDisplay: "shared/user.tmpl",
+		},
+		{
+			name:     "escape rejected",
+			root:     "prompts",
+			baseDir:  "prompts/nested",
+			userPath: "../../outside.tmpl",
+			wantErr:  "escapes source root",
+		},
+		{
+			name:     "absolute path rejected",
+			root:     "prompts",
+			baseDir:  "prompts/nested",
+			userPath: "/outside.tmpl",
+			wantErr:  "must be relative",
+		},
+		{
+			name:     "empty path rejected",
+			root:     "prompts",
+			baseDir:  "prompts/nested",
+			userPath: " ",
+			wantErr:  "path is required",
+		},
+		{
+			name:        "dot root allows normal relative path",
+			root:        ".",
+			baseDir:     ".",
+			userPath:    "schemas/events.schema.json",
+			wantPath:    "schemas/events.schema.json",
+			wantDisplay: "schemas/events.schema.json",
+		},
+		{
+			name:     "dot root rejects parent escape",
+			root:     ".",
+			baseDir:  ".",
+			userPath: "../outside.tmpl",
+			wantErr:  "escapes source root",
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			gotPath, gotDisplay, err := ResolveFSPath(tc.root, tc.baseDir, tc.userPath)
+			if tc.wantErr != "" {
+				if err == nil {
+					t.Fatalf("expected error containing %q", tc.wantErr)
+				}
+				if !strings.Contains(err.Error(), tc.wantErr) {
+					t.Fatalf("expected error to contain %q, got %v", tc.wantErr, err)
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("expected no error, got %v", err)
+			}
+			if gotPath != tc.wantPath || gotDisplay != tc.wantDisplay {
+				t.Fatalf("expected path/display %q/%q, got %q/%q", tc.wantPath, tc.wantDisplay, gotPath, gotDisplay)
+			}
+		})
+	}
+}
+
+func TestStemStripsYAMLExtensions(t *testing.T) {
+	tests := []struct {
+		name string
+		in   string
+		want string
+	}{
+		{name: "yaml", in: "prompt.yaml", want: "prompt"},
+		{name: "yml", in: "profile.yml", want: "profile"},
+		{name: "other", in: "file.txt", want: "file.txt"},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			if got := Stem(tc.in); got != tc.want {
+				t.Fatalf("expected %q, got %q", tc.want, got)
+			}
+		})
+	}
+}
+
+func TestIsYAMLFile(t *testing.T) {
+	tests := []struct {
+		name string
+		in   string
+		want bool
+	}{
+		{name: "yaml", in: "prompt.yaml", want: true},
+		{name: "yml", in: "profile.yml", want: true},
+		{name: "backup", in: "profile.yaml.bak", want: false},
+		{name: "uppercase", in: "profile.YAML", want: false},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			if got := IsYAMLFile(tc.in); got != tc.want {
+				t.Fatalf("expected %v, got %v", tc.want, got)
+			}
+		})
+	}
+}
+
+func mustWriteFile(t *testing.T, path string, content string) {
+	t.Helper()
+	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
+		t.Fatalf("failed to create directory: %v", err)
+	}
+	if err := os.WriteFile(path, []byte(content), 0o644); err != nil {
+		t.Fatalf("failed to write file %q: %v", path, err)
+	}
+}

internal/jsonvalue/jsonvalue.go

@@ -0,0 +1,258 @@
+// Package jsonvalue validates and defensively copies JSON-compatible value
+// trees used by configuration, request, and prepared-state boundaries.
+package jsonvalue
+
+import (
+	"encoding/json"
+	"fmt"
+	"math"
+	"reflect"
+	"sort"
+	"strconv"
+)
+
+const maxSafeJSONInteger = 1<<53 - 1
+
+type visit struct {
+	typ reflect.Type
+	ptr uintptr
+}
+
+// Copy validates and deeply copies a JSON-compatible value while preserving
+// compatible concrete map, slice, array, scalar, and number types.
+func Copy(src any) (any, error) {
+	return copyValue(reflect.ValueOf(src), "value", make(map[visit]struct{}), true)
+}
+
+// CopyMap validates and deeply copies an extra-parameter map while preserving
+// compatible concrete map, slice, array, scalar, and number types.
+func CopyMap(src map[string]any) (map[string]any, error) {
+	if src == nil {
+		return nil, nil
+	}
+	copied, err := copyValue(reflect.ValueOf(src), "extra_params", make(map[visit]struct{}), false)
+	if err != nil {
+		return nil, err
+	}
+	out, ok := copied.(map[string]any)
+	if !ok {
+		return nil, fmt.Errorf("extra_params: expected object")
+	}
+	return out, nil
+}
+
+func copyValue(
+	value reflect.Value,
+	path string,
+	seen map[visit]struct{},
+	allowEmptyMapKeys bool,
+) (any, error) {
+	if !value.IsValid() {
+		return nil, nil
+	}
+	if value.Kind() == reflect.Interface {
+		if value.IsNil() {
+			return nil, nil
+		}
+		return copyValue(value.Elem(), path, seen, allowEmptyMapKeys)
+	}
+	if !value.CanInterface() {
+		return nil, fmt.Errorf("%s: value cannot be copied", path)
+	}
+	if number, ok := value.Interface().(json.Number); ok {
+		if _, err := json.Marshal(number); err != nil {
+			return nil, fmt.Errorf("%s: invalid JSON number", path)
+		}
+		f, err := strconv.ParseFloat(number.String(), 64)
+		if err != nil || math.IsNaN(f) || math.IsInf(f, 0) {
+			return nil, fmt.Errorf("%s: invalid JSON number", path)
+		}
+		return number, nil
+	}
+
+	switch value.Kind() {
+	case reflect.Bool, reflect.String:
+		return value.Interface(), nil
+	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+		if value.Int() < -maxSafeJSONInteger || value.Int() > maxSafeJSONInteger {
+			return nil, fmt.Errorf("%s: integer is outside the JSON-safe range", path)
+		}
+		return value.Interface(), nil
+	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
+		if value.Uint() > maxSafeJSONInteger {
+			return nil, fmt.Errorf("%s: integer is outside the JSON-safe range", path)
+		}
+		return value.Interface(), nil
+	case reflect.Float32, reflect.Float64:
+		number := value.Convert(reflect.TypeOf(float64(0))).Float()
+		if math.IsNaN(number) || math.IsInf(number, 0) {
+			return nil, fmt.Errorf("%s: floating-point value must be finite", path)
+		}
+		return value.Interface(), nil
+	case reflect.Pointer:
+		if value.IsNil() {
+			return nil, nil
+		}
+		current := visit{typ: value.Type(), ptr: value.Pointer()}
+		if _, ok := seen[current]; ok {
+			return nil, fmt.Errorf("%s: cyclic value is not supported", path)
+		}
+		seen[current] = struct{}{}
+		defer delete(seen, current)
+		return copyValue(value.Elem(), path, seen, allowEmptyMapKeys)
+	case reflect.Map:
+		return copyMapValue(value, path, seen, allowEmptyMapKeys)
+	case reflect.Slice:
+		if value.IsNil() {
+			return nil, nil
+		}
+		return copySequenceValue(value, path, seen, allowEmptyMapKeys)
+	case reflect.Array:
+		return copySequenceValue(value, path, seen, allowEmptyMapKeys)
+	default:
+		return nil, fmt.Errorf("%s: unsupported JSON value type %s", path, value.Type())
+	}
+}
+
+func copyMapValue(
+	value reflect.Value,
+	path string,
+	seen map[visit]struct{},
+	allowEmptyMapKeys bool,
+) (any, error) {
+	if value.IsNil() {
+		return nil, nil
+	}
+	if value.Type().Key().Kind() != reflect.String {
+		return nil, fmt.Errorf("%s: map key type %s is not supported", path, value.Type().Key())
+	}
+
+	current := visit{typ: value.Type(), ptr: value.Pointer()}
+	if _, ok := seen[current]; ok {
+		return nil, fmt.Errorf("%s: cyclic value is not supported", path)
+	}
+	seen[current] = struct{}{}
+	defer delete(seen, current)
+
+	keys := value.MapKeys()
+	sort.Slice(keys, func(i, j int) bool {
+		return keys[i].String() < keys[j].String()
+	})
+
+	type entry struct {
+		key   reflect.Value
+		name  string
+		value any
+	}
+	entries := make([]entry, 0, len(keys))
+	preserveType := true
+	elementType := value.Type().Elem()
+	for _, key := range keys {
+		name := key.String()
+		if name == "" && !allowEmptyMapKeys {
+			return nil, fmt.Errorf("%s: map key must not be empty", path)
+		}
+		copied, err := copyValue(value.MapIndex(key), path+"."+name, seen, allowEmptyMapKeys)
+		if err != nil {
+			return nil, err
+		}
+		entries = append(entries, entry{key: key, name: name, value: copied})
+		if copied == nil {
+			if !canAssignNil(elementType) {
+				preserveType = false
+			}
+			continue
+		}
+		if !reflect.TypeOf(copied).AssignableTo(elementType) {
+			preserveType = false
+		}
+	}
+
+	if preserveType {
+		out := reflect.MakeMapWithSize(value.Type(), len(entries))
+		for _, entry := range entries {
+			if entry.value == nil {
+				out.SetMapIndex(entry.key, reflect.Zero(elementType))
+				continue
+			}
+			out.SetMapIndex(entry.key, reflect.ValueOf(entry.value))
+		}
+		return out.Interface(), nil
+	}
+
+	out := make(map[string]any, len(entries))
+	for _, entry := range entries {
+		out[entry.name] = entry.value
+	}
+	return out, nil
+}
+
+func copySequenceValue(
+	value reflect.Value,
+	path string,
+	seen map[visit]struct{},
+	allowEmptyMapKeys bool,
+) (any, error) {
+	var current visit
+	if value.Kind() == reflect.Slice {
+		current = visit{typ: value.Type(), ptr: value.Pointer()}
+		if _, ok := seen[current]; ok {
+			return nil, fmt.Errorf("%s: cyclic value is not supported", path)
+		}
+		seen[current] = struct{}{}
+		defer delete(seen, current)
+	}
+
+	values := make([]any, value.Len())
+	preserveType := true
+	elementType := value.Type().Elem()
+	for i := 0; i < value.Len(); i++ {
+		copied, err := copyValue(
+			value.Index(i),
+			fmt.Sprintf("%s[%d]", path, i),
+			seen,
+			allowEmptyMapKeys,
+		)
+		if err != nil {
+			return nil, err
+		}
+		values[i] = copied
+		if copied == nil {
+			if !canAssignNil(elementType) {
+				preserveType = false
+			}
+			continue
+		}
+		if !reflect.TypeOf(copied).AssignableTo(elementType) {
+			preserveType = false
+		}
+	}
+
+	if preserveType {
+		out := reflect.New(value.Type()).Elem()
+		if value.Kind() == reflect.Slice {
+			out = reflect.MakeSlice(value.Type(), value.Len(), value.Len())
+		}
+		for i, copied := range values {
+			if copied == nil {
+				out.Index(i).Set(reflect.Zero(elementType))
+				continue
+			}
+			out.Index(i).Set(reflect.ValueOf(copied))
+		}
+		return out.Interface(), nil
+	}
+
+	out := make([]any, len(values))
+	copy(out, values)
+	return out, nil
+}
+
+func canAssignNil(typ reflect.Type) bool {
+	switch typ.Kind() {
+	case reflect.Chan, reflect.Func, reflect.Interface, reflect.Map, reflect.Pointer, reflect.Slice:
+		return true
+	default:
+		return false
+	}
+}

internal/jsonvalue/jsonvalue_test.go

@@ -0,0 +1,112 @@
+package jsonvalue_test
+
+import (
+	"encoding/json"
+	"math"
+	"reflect"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/promptkit/internal/jsonvalue"
+)
+
+func TestCopyMapPreservesTypesAndIsolatesMutations(t *testing.T) {
+	nested := map[string]int{"limit": 2}
+	sequence := []string{"one", "two"}
+	input := map[string]any{
+		"count":    int64(7),
+		"number":   json.Number("-1.25e+2"),
+		"nested":   nested,
+		"sequence": sequence,
+	}
+
+	copied, err := jsonvalue.CopyMap(input)
+	if err != nil {
+		t.Fatalf("copy map: %v", err)
+	}
+	nested["limit"] = 99
+	sequence[0] = "changed"
+	input["added"] = true
+
+	if got, ok := copied["count"].(int64); !ok || got != 7 {
+		t.Fatalf("integer type or value changed: %#v", copied["count"])
+	}
+	if got, ok := copied["number"].(json.Number); !ok || got != "-1.25e+2" {
+		t.Fatalf("JSON number type or value changed: %#v", copied["number"])
+	}
+	if got := copied["nested"].(map[string]int)["limit"]; got != 2 {
+		t.Fatalf("nested map was not isolated: %d", got)
+	}
+	if got := copied["sequence"].([]string)[0]; got != "one" {
+		t.Fatalf("sequence was not isolated: %q", got)
+	}
+	if _, ok := copied["added"]; ok {
+		t.Fatalf("top-level map was not isolated: %#v", copied)
+	}
+}
+
+func TestCopyAllowsEmptyObjectKeysAndIsolatesMutations(t *testing.T) {
+	nested := map[string]any{"": []any{"original"}}
+
+	copiedValue, err := jsonvalue.Copy(nested)
+	if err != nil {
+		t.Fatalf("copy value: %v", err)
+	}
+	nested[""].([]any)[0] = "changed"
+
+	copied := copiedValue.(map[string]any)
+	if got := copied[""].([]any)[0]; got != "original" {
+		t.Fatalf("copied value was not isolated: %v", got)
+	}
+}
+
+func TestCopyMapRejectsInvalidValues(t *testing.T) {
+	cyclicMap := map[string]any{}
+	cyclicMap["self"] = cyclicMap
+	cyclicSlice := []any{nil}
+	cyclicSlice[0] = cyclicSlice
+
+	tests := []struct {
+		name  string
+		value any
+	}{
+		{name: "empty nested key", value: map[string]int{"": 1}},
+		{name: "non-string map key", value: map[int]string{1: "one"}},
+		{name: "unsupported value", value: make(chan int)},
+		{name: "cyclic map", value: cyclicMap},
+		{name: "cyclic slice", value: cyclicSlice},
+		{name: "NaN", value: math.NaN()},
+		{name: "positive infinity", value: math.Inf(1)},
+		{name: "unsafe signed integer", value: int64(1 << 53)},
+		{name: "unsafe unsigned integer", value: uint64(1 << 53)},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			if _, err := jsonvalue.CopyMap(map[string]any{"value": tc.value}); err == nil {
+				t.Fatal("expected validation error")
+			}
+		})
+	}
+}
+
+func TestCopyMapValidatesJSONNumberSyntaxAndRange(t *testing.T) {
+	for _, number := range []json.Number{"0", "-1", "1.25", "-1.25e+2"} {
+		t.Run("valid "+number.String(), func(t *testing.T) {
+			got, err := jsonvalue.CopyMap(map[string]any{"value": number})
+			if err != nil {
+				t.Fatalf("copy valid JSON number: %v", err)
+			}
+			if !reflect.DeepEqual(got["value"], number) {
+				t.Fatalf("JSON number changed: got %#v want %#v", got["value"], number)
+			}
+		})
+	}
+
+	for _, number := range []json.Number{"", "01", "+1", "1.", ".1", "1e9999", "not-a-number"} {
+		t.Run("invalid "+number.String(), func(t *testing.T) {
+			if _, err := jsonvalue.CopyMap(map[string]any{"value": number}); err == nil {
+				t.Fatal("expected invalid JSON number error")
+			}
+		})
+	}
+}

internal/llm/client.go

@@ -0,0 +1,11 @@
+package llm
+
+import (
+	"context"
+	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
+)
+
+// Client executes a rendered prompt against an LLM endpoint.
+type Client interface {
+	Generate(ctx context.Context, req domain.GenerateRequest) (*domain.GenerateResponse, error)
+}

internal/llm/openai_compatible_client.go

@@ -0,0 +1,390 @@
+package llm
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"net/http"
+	"net/url"
+	"os"
+	"strings"
+	"time"
+
+	"gitea.maximumdirect.net/eric/promptkit/internal/defaults"
+	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
+)
+
+var (
+	ErrInvalidConfig     = errors.New("invalid llm client configuration")
+	ErrInvalidRequest    = errors.New("invalid generate request")
+	ErrRequestFailed     = errors.New("llm request failed")
+	ErrUnexpectedStatus  = errors.New("llm returned non-success status")
+	ErrMalformedResponse = errors.New("malformed llm response")
+)
+
+type OpenAICompatibleConfig struct {
+	BaseURL    string
+	Model      string
+	Timeout    time.Duration
+	HTTPClient *http.Client
+}
+
+type OpenAICompatibleClient struct {
+	baseURL      string
+	defaultModel string
+	httpClient   *http.Client
+}
+
+func NewOpenAICompatibleClient(cfg OpenAICompatibleConfig) (*OpenAICompatibleClient, error) {
+	baseURL := strings.TrimSpace(cfg.BaseURL)
+	if baseURL != "" {
+		if _, err := url.ParseRequestURI(baseURL); err != nil {
+			return nil, fmt.Errorf("%w: invalid base URL: %v", ErrInvalidConfig, err)
+		}
+	}
+
+	timeout := cfg.Timeout
+	if timeout <= 0 {
+		timeout = defaults.LLMRequestTimeoutDefault
+	}
+
+	var client *http.Client
+	if cfg.HTTPClient != nil {
+		cloned := *cfg.HTTPClient
+		if cloned.Timeout <= 0 {
+			cloned.Timeout = timeout
+		}
+		client = &cloned
+	} else {
+		client = &http.Client{Timeout: timeout}
+	}
+
+	return &OpenAICompatibleClient{
+		baseURL:      strings.TrimRight(baseURL, "/"),
+		defaultModel: cfg.Model,
+		httpClient:   client,
+	}, nil
+}
+
+func (c *OpenAICompatibleClient) Generate(ctx context.Context, req domain.GenerateRequest) (*domain.GenerateResponse, error) {
+	if req.Target.TimeoutSeconds < 0 {
+		return nil, fmt.Errorf("%w: timeout_seconds must be greater than or equal to 0", ErrInvalidRequest)
+	}
+
+	endpoint := strings.TrimSpace(req.Target.Endpoint)
+	if endpoint == "" {
+		endpoint = c.baseURL
+	}
+	if endpoint == "" {
+		return nil, fmt.Errorf("%w: endpoint is required", ErrInvalidRequest)
+	}
+	endpoint = strings.TrimRight(endpoint, "/") + defaults.OpenAIChatCompletionsPath
+
+	wireReq, err := openAIChatRequestFromGenerateRequest(req, c.defaultModel)
+	if err != nil {
+		return nil, fmt.Errorf("%w: %v", ErrInvalidRequest, err)
+	}
+
+	wirePayload, err := openAIChatRequestPayload(wireReq)
+	if err != nil {
+		return nil, fmt.Errorf("%w: %v", ErrInvalidRequest, err)
+	}
+
+	payload, err := json.Marshal(wirePayload)
+	if err != nil {
+		return nil, fmt.Errorf("%w: failed to encode request: %v", ErrRequestFailed, err)
+	}
+
+	requestContext := ctx
+	if req.Target.TimeoutSeconds > 0 {
+		var cancel context.CancelFunc
+		requestContext, cancel = context.WithTimeout(
+			ctx,
+			time.Duration(req.Target.TimeoutSeconds)*time.Second,
+		)
+		defer cancel()
+	}
+
+	httpReq, err := http.NewRequestWithContext(requestContext, http.MethodPost, endpoint, bytes.NewReader(payload))
+	if err != nil {
+		return nil, fmt.Errorf("%w: failed to create request: %v", ErrRequestFailed, err)
+	}
+	httpReq.Header.Set("Content-Type", "application/json")
+	if apiKey := strings.TrimSpace(req.Target.APIKey); apiKey != "" {
+		httpReq.Header.Set("Authorization", "Bearer "+apiKey)
+	} else if envName := strings.TrimSpace(req.Target.APIKeyEnv); envName != "" {
+		apiKey := strings.TrimSpace(os.Getenv(envName))
+		if apiKey == "" {
+			return nil, fmt.Errorf("%w: api key environment variable %q is not set", ErrInvalidRequest, envName)
+		}
+		httpReq.Header.Set("Authorization", "Bearer "+apiKey)
+	}
+
+	httpClient := c.httpClient
+	if httpClient == nil {
+		httpClient = &http.Client{Timeout: defaults.LLMRequestTimeoutDefault}
+	}
+
+	httpResp, err := httpClient.Do(httpReq)
+	if err != nil {
+		return nil, fmt.Errorf("%w: %v", ErrRequestFailed, err)
+	}
+	defer httpResp.Body.Close()
+
+	if httpResp.StatusCode < 200 || httpResp.StatusCode >= 300 {
+		_, _ = io.Copy(io.Discard, io.LimitReader(httpResp.Body, 4096))
+		return nil, fmt.Errorf("%w: status=%d", ErrUnexpectedStatus, httpResp.StatusCode)
+	}
+
+	var wireResp openAIChatResponse
+	if err := json.NewDecoder(httpResp.Body).Decode(&wireResp); err != nil {
+		return nil, fmt.Errorf("%w: failed to decode response: %v", ErrMalformedResponse, err)
+	}
+
+	if len(wireResp.Choices) == 0 {
+		return nil, fmt.Errorf("%w: no choices returned", ErrMalformedResponse)
+	}
+	content := wireResp.Choices[0].Message.Content
+	if content == "" {
+		return nil, fmt.Errorf("%w: first choice has empty message content", ErrMalformedResponse)
+	}
+
+	return &domain.GenerateResponse{
+		Content: content,
+		Usage: domain.TokenUsage{
+			PromptTokens:     wireResp.Usage.PromptTokens,
+			CompletionTokens: wireResp.Usage.CompletionTokens,
+			TotalTokens:      wireResp.Usage.TotalTokens,
+			CachedTokens:     wireResp.Usage.PromptTokensDetails.CachedTokens,
+			CacheWriteTokens: wireResp.Usage.CacheWriteTokens,
+		},
+	}, nil
+}
+
+func openAIChatRequestFromGenerateRequest(req domain.GenerateRequest, defaultModel string) (openAIChatRequest, error) {
+	model := strings.TrimSpace(req.Target.Model)
+	if model == "" {
+		model = strings.TrimSpace(defaultModel)
+	}
+	if model == "" {
+		return openAIChatRequest{}, errors.New("model is required")
+	}
+
+	wireReq := openAIChatRequest{
+		Model: model,
+	}
+	sessionID, err := domain.NormalizeSessionID(req.Prompt.SessionID)
+	if err != nil {
+		return openAIChatRequest{}, err
+	}
+	wireReq.SessionID = sessionID
+
+	wireReq.Messages = make([]openAIChatRequestMessage, 0, len(req.Prompt.Messages))
+	for _, msg := range req.Prompt.Messages {
+		wireReq.Messages = append(wireReq.Messages, openAIChatRequestMessageFromRenderedMessage(msg))
+	}
+
+	if req.Target.Temperature != 0 || req.TargetPresence.Temperature {
+		wireReq.Temperature = &req.Target.Temperature
+	}
+	if req.Target.MaxTokens != 0 || req.TargetPresence.MaxTokens {
+		wireReq.MaxTokens = &req.Target.MaxTokens
+	}
+	if req.Target.TopP != 0 || req.TargetPresence.TopP {
+		wireReq.TopP = &req.Target.TopP
+	}
+	if strings.TrimSpace(req.Target.ServiceTier) != "" {
+		wireReq.ServiceTier = req.Target.ServiceTier
+	}
+	if strings.TrimSpace(req.Target.ReasoningEffort) != "" {
+		wireReq.ReasoningEffort = req.Target.ReasoningEffort
+	}
+	if len(req.Target.ExtraParams) > 0 {
+		wireReq.ExtraParams = req.Target.ExtraParams
+	}
+	if req.StructuredOutput != nil {
+		responseFormat, err := toOpenAIResponseFormat(req.StructuredOutput)
+		if err != nil {
+			return openAIChatRequest{}, err
+		}
+		wireReq.ResponseFormat = responseFormat
+	}
+
+	return wireReq, nil
+}
+
+type openAIChatRequest struct {
+	Model           string                     `json:"model"`
+	SessionID       string                     `json:"session_id,omitempty"`
+	Messages        []openAIChatRequestMessage `json:"messages"`
+	Temperature     *float64                   `json:"temperature,omitempty"`
+	MaxTokens       *int                       `json:"max_tokens,omitempty"`
+	TopP            *float64                   `json:"top_p,omitempty"`
+	ServiceTier     string                     `json:"service_tier,omitempty"`
+	ReasoningEffort string                     `json:"reasoning_effort,omitempty"`
+	ResponseFormat  *openAIResponseFormat      `json:"response_format,omitempty"`
+	ExtraParams     map[string]any             `json:"-"`
+}
+
+func openAIChatRequestPayload(req openAIChatRequest) (map[string]any, error) {
+	out := map[string]any{
+		"model":    req.Model,
+		"messages": req.Messages,
+	}
+	if req.SessionID != "" {
+		out["session_id"] = req.SessionID
+	}
+	if req.Temperature != nil {
+		out["temperature"] = *req.Temperature
+	}
+	if req.MaxTokens != nil {
+		out["max_tokens"] = *req.MaxTokens
+	}
+	if req.TopP != nil {
+		out["top_p"] = *req.TopP
+	}
+	if req.ServiceTier != "" {
+		out["service_tier"] = req.ServiceTier
+	}
+	if req.ReasoningEffort != "" {
+		out["reasoning_effort"] = req.ReasoningEffort
+	}
+	if req.ResponseFormat != nil {
+		out["response_format"] = req.ResponseFormat
+	}
+
+	for key, value := range req.ExtraParams {
+		if key == "" {
+			return nil, errors.New("extra_params key must not be empty")
+		}
+		if IsReservedOpenAIChatRequestField(key) {
+			return nil, fmt.Errorf("extra_params key %q collides with reserved request field", key)
+		}
+		if _, err := json.Marshal(value); err != nil {
+			return nil, fmt.Errorf("extra_params.%s must be JSON-serializable: %w", key, err)
+		}
+		out[key] = value
+	}
+
+	return out, nil
+}
+
+// IsReservedOpenAIChatRequestField reports whether name is owned by the
+// standard OpenAI-compatible chat request rather than extra parameters.
+func IsReservedOpenAIChatRequestField(name string) bool {
+	switch name {
+	case "model",
+		"session_id",
+		"messages",
+		"temperature",
+		"max_tokens",
+		"top_p",
+		"service_tier",
+		"reasoning_effort",
+		"response_format":
+		return true
+	default:
+		return false
+	}
+}
+
+type openAIChatRequestMessage struct {
+	Role    string `json:"role"`
+	Content any    `json:"content"`
+}
+
+type openAIChatTextContentBlock struct {
+	Type         string              `json:"type"`
+	Text         string              `json:"text"`
+	CacheControl *openAICacheControl `json:"cache_control,omitempty"`
+}
+
+type openAICacheControl struct {
+	Type string `json:"type"`
+	TTL  string `json:"ttl,omitempty"`
+}
+
+type openAIChatResponseMessage struct {
+	Role    string `json:"role"`
+	Content string `json:"content"`
+}
+
+type openAIChatResponse struct {
+	Choices []struct {
+		Message openAIChatResponseMessage `json:"message"`
+	} `json:"choices"`
+	Usage struct {
+		PromptTokens        int `json:"prompt_tokens"`
+		CompletionTokens    int `json:"completion_tokens"`
+		TotalTokens         int `json:"total_tokens"`
+		PromptTokensDetails struct {
+			CachedTokens int `json:"cached_tokens"`
+		} `json:"prompt_tokens_details"`
+		CacheWriteTokens int `json:"cache_write_tokens"`
+	} `json:"usage"`
+}
+
+type openAIResponseFormat struct {
+	Type       string                    `json:"type"`
+	JSONSchema *openAIJSONSchemaEnvelope `json:"json_schema,omitempty"`
+}
+
+type openAIJSONSchemaEnvelope struct {
+	Name   string `json:"name"`
+	Strict bool   `json:"strict"`
+	Schema any    `json:"schema"`
+}
+
+func openAIChatRequestMessageFromRenderedMessage(msg domain.RenderedMessage) openAIChatRequestMessage {
+	wireMsg := openAIChatRequestMessage{
+		Role:    msg.Role,
+		Content: msg.Content,
+	}
+	if msg.CacheControl == nil {
+		return wireMsg
+	}
+
+	wireMsg.Content = []openAIChatTextContentBlock{
+		{
+			Type: "text",
+			Text: msg.Content,
+			CacheControl: &openAICacheControl{
+				Type: string(msg.CacheControl.Type),
+				TTL:  msg.CacheControl.TTL,
+			},
+		},
+	}
+	return wireMsg
+}
+
+func toOpenAIResponseFormat(spec *domain.StructuredOutputSpec) (*openAIResponseFormat, error) {
+	if spec == nil {
+		return nil, nil
+	}
+
+	switch spec.Type {
+	case domain.StructuredOutputJSONSchema:
+		if spec.JSONSchema == nil {
+			return nil, errors.New("json_schema structured output requires schema payload")
+		}
+		if strings.TrimSpace(spec.JSONSchema.Name) == "" {
+			return nil, errors.New("json_schema structured output requires non-empty schema name")
+		}
+		if spec.JSONSchema.Schema == nil {
+			return nil, errors.New("json_schema structured output requires schema document")
+		}
+		return &openAIResponseFormat{
+			Type: "json_schema",
+			JSONSchema: &openAIJSONSchemaEnvelope{
+				Name:   spec.JSONSchema.Name,
+				Strict: spec.JSONSchema.Strict,
+				Schema: spec.JSONSchema.Schema,
+			},
+		}, nil
+	default:
+		return nil, fmt.Errorf("unsupported structured output type %q", spec.Type)
+	}
+}

internal/profile/builtin/assets/aion-labs/aion-2.yml

@@ -0,0 +1,8 @@
+id: aion-2
+backend: openrouter
+model: aion-labs/aion-2.0
+temperature: 0.72
+reasoning_effort: high
+top_p: 0.95
+timeout_seconds: 180
+service_tier: flex

internal/profile/builtin/assets/anthropic/claude-fable-latest.yml

@@ -0,0 +1,6 @@
+id: claude-fable-latest
+backend: openrouter
+model: "~anthropic/claude-fable-latest"
+reasoning_effort: high
+timeout_seconds: 600
+service_tier: flex

internal/profile/builtin/assets/anthropic/claude-haiku-latest.yml

@@ -0,0 +1,6 @@
+id: claude-haiku-latest
+backend: openrouter
+model: "~anthropic/claude-haiku-latest"
+reasoning_effort: medium
+timeout_seconds: 240
+service_tier: flex

internal/profile/builtin/assets/anthropic/claude-opus-latest.yml

@@ -0,0 +1,6 @@
+id: claude-opus-latest
+backend: openrouter
+model: "~anthropic/claude-opus-latest"
+reasoning_effort: high
+timeout_seconds: 240
+service_tier: flex

internal/profile/builtin/assets/anthropic/claude-sonnet-latest.yml

@@ -0,0 +1,6 @@
+id: claude-sonnet-latest
+backend: openrouter
+model: "~anthropic/claude-sonnet-latest"
+reasoning_effort: high
+timeout_seconds: 240
+service_tier: flex

internal/profile/builtin/assets/deepseek/deepseek-3-2.yml

@@ -0,0 +1,6 @@
+id: deepseek-3-2
+backend: openrouter
+model: deepseek/deepseek-v3.2
+reasoning_effort: high
+timeout_seconds: 180
+service_tier: flex

internal/profile/builtin/assets/deepseek/deepseek-4-flash.yml

@@ -0,0 +1,6 @@
+id: deepseek-4-flash
+backend: openrouter
+model: deepseek/deepseek-v4-flash
+#reasoning_effort: medium
+timeout_seconds: 180
+service_tier: flex

internal/profile/builtin/assets/deepseek/deepseek-4-pro.yml

@@ -0,0 +1,6 @@
+id: deepseek-4-pro
+backend: openrouter
+model: deepseek/deepseek-v4-pro
+reasoning_effort: high
+timeout_seconds: 180
+service_tier: flex

internal/profile/builtin/assets/google/gemini-2-flash-lite.yml

@@ -0,0 +1,8 @@
+id: gemini-2-flash-lite
+backend: openrouter
+model: "google/gemini-2.5-flash-lite"
+#temperature: 0.15
+reasoning_effort: high
+#top_p: 0.98
+timeout_seconds: 240
+service_tier: flex

internal/profile/builtin/assets/google/gemini-2-flash.yml

@@ -0,0 +1,8 @@
+id: gemini-2-flash
+backend: openrouter
+model: "google/gemini-2.5-flash"
+#temperature: 0.15
+reasoning_effort: high
+#top_p: 0.98
+timeout_seconds: 240
+service_tier: flex

internal/profile/builtin/assets/google/gemini-2-pro.yml

@@ -0,0 +1,8 @@
+id: gemini-2-pro
+backend: openrouter
+model: "google/gemini-2.5-pro"
+#temperature: 0.15
+reasoning_effort: high
+#top_p: 0.98
+timeout_seconds: 240
+service_tier: flex

internal/profile/builtin/assets/google/gemini-3-flash-lite.yml

@@ -0,0 +1,8 @@
+id: gemini-3-flash-lite
+backend: openrouter
+model: "google/gemini-3.1-flash-lite"
+#temperature: 0.15
+reasoning_effort: high
+#top_p: 0.98
+timeout_seconds: 240
+service_tier: flex

internal/profile/builtin/assets/google/gemini-flash-latest.yml

@@ -0,0 +1,8 @@
+id: gemini-flash-latest
+backend: openrouter
+model: "~google/gemini-flash-latest"
+#temperature: 0.15
+reasoning_effort: high
+#top_p: 0.98
+timeout_seconds: 240
+service_tier: flex

internal/profile/builtin/assets/google/gemini-pro-latest.yml

@@ -0,0 +1,8 @@
+id: gemini-pro-latest
+backend: openrouter
+model: "~google/gemini-pro-latest"
+#temperature: 0.15
+reasoning_effort: high
+#top_p: 0.98
+timeout_seconds: 240
+service_tier: flex

internal/profile/builtin/assets/google/gemma-4-31b.yml

@@ -0,0 +1,8 @@
+id: gemma-4-31b
+backend: openrouter
+model: google/gemma-4-31b-it:exacto
+temperature: 0.15
+reasoning_effort: high
+top_p: 0.98
+timeout_seconds: 240
+service_tier: flex

internal/profile/builtin/assets/minimax/minimax-m2.yml

@@ -0,0 +1,8 @@
+id: minimax-m2
+backend: openrouter
+model: minimax/minimax-m2.5
+temperature: 0.5
+reasoning_effort: high
+top_p: 0.95
+timeout_seconds: 180
+service_tier: flex

internal/profile/builtin/assets/minimax/minimax-m3.yml

@@ -0,0 +1,8 @@
+id: minimax-m3
+backend: openrouter
+model: minimax/minimax-m3
+#temperature: 0.5
+reasoning_effort: high
+#top_p: 0.95
+timeout_seconds: 180
+service_tier: flex

internal/profile/builtin/assets/mistral/mistral-large-2512.yml

@@ -0,0 +1,6 @@
+id: mistral-large-2512
+backend: openrouter
+model: mistralai/mistral-large-2512
+temperature: 0.15
+top_p: 0.98
+timeout_seconds: 180

internal/profile/builtin/assets/mistral/mistral-medium-3-5.yml

@@ -0,0 +1,7 @@
+id: mistral-medium-3-5
+backend: openrouter
+model: mistralai/mistral-medium-3-5
+temperature: 0.15
+reasoning_effort: high
+top_p: 0.98
+timeout_seconds: 180

internal/profile/builtin/assets/mistral/mistral-small-3.yml

@@ -0,0 +1,6 @@
+id: mistral-small-3
+backend: openrouter
+model: mistralai/mistral-small-3.2-24b-instruct
+temperature: 0.05
+top_p: 1.0
+timeout_seconds: 180

internal/profile/builtin/assets/mistral/mistral-small-4.yml

@@ -0,0 +1,7 @@
+id: mistral-small-4
+backend: openrouter
+model: mistralai/mistral-small-2603
+temperature: 0.1
+reasoning_effort: high
+top_p: 0.98
+timeout_seconds: 180

internal/profile/builtin/assets/nvidia/nemotron-3-ultra.yml

@@ -0,0 +1,6 @@
+id: nemotron-3-ultra
+backend: openrouter
+model: nvidia/nemotron-3-ultra-550b-a55b
+reasoning_effort: high
+timeout_seconds: 180
+service_tier: flex

internal/profile/builtin/assets/openai/gpt-5-mini.yml

@@ -0,0 +1,6 @@
+id: gpt-5-mini
+backend: openrouter
+model: "openai/gpt-5.4-mini"
+reasoning_effort: high
+timeout_seconds: 240
+service_tier: flex

internal/profile/builtin/assets/openai/gpt-5-nano.yml

@@ -0,0 +1,6 @@
+id: gpt-5-nano
+backend: openrouter
+model: "openai/gpt-5.4-nano"
+reasoning_effort: high
+timeout_seconds: 240
+service_tier: flex

internal/profile/builtin/repository.go

@@ -0,0 +1,31 @@
+package builtin
+
+import (
+	"embed"
+	"strings"
+
+	"gitea.maximumdirect.net/eric/promptkit/internal/profile"
+)
+
+const assetRoot = "assets"
+
+//go:embed assets/**/*.yml
+var assets embed.FS
+
+func NewRepository() profile.Repository {
+	return profile.NewFSRepository(assets, assetRoot)
+}
+
+func NewRepositoryWithPrimary(primary profile.Repository) profile.Repository {
+	if primary == nil {
+		return NewRepository()
+	}
+	return profile.NewOverlayRepository(primary, NewRepository())
+}
+
+func NewRepositoryWithDirectory(dir string) profile.Repository {
+	if strings.TrimSpace(dir) == "" {
+		return NewRepository()
+	}
+	return NewRepositoryWithPrimary(profile.NewFilesystemRepository(dir))
+}

internal/profile/builtin/repository_test.go

@@ -0,0 +1,143 @@
+package builtin
+
+import (
+	"context"
+	"errors"
+	"io/fs"
+	"strings"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/promptkit/internal/backend"
+	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
+	"gitea.maximumdirect.net/eric/promptkit/internal/profile"
+	"gopkg.in/yaml.v3"
+)
+
+func TestBuiltInProfilesValidateThroughRepository(t *testing.T) {
+	repo := NewRepository()
+	ids := loadBuiltInProfileIDs(t)
+	if len(ids) == 0 {
+		t.Fatal("expected built-in profiles")
+	}
+
+	for id := range ids {
+		t.Run(id, func(t *testing.T) {
+			p, err := repo.GetProfile(context.Background(), id)
+			if err != nil {
+				t.Fatalf("expected built-in profile %q to load, got %v", id, err)
+			}
+			if p.ID != id {
+				t.Fatalf("expected profile id %q, got %q", id, p.ID)
+			}
+			if p.BackendID != backend.OpenRouterID {
+				t.Fatalf("expected profile %q to select %q, got %q", id, backend.OpenRouterID, p.BackendID)
+			}
+			if p.Endpoint != "" || p.APIKeyEnv != "" {
+				t.Fatalf("expected profile %q to inherit backend connection settings, got endpoint=%q api_key_env=%q", id, p.Endpoint, p.APIKeyEnv)
+			}
+		})
+	}
+}
+
+func TestBuiltInProfilesDoNotContainDuplicateIDsOrRawAPIKeys(t *testing.T) {
+	loadBuiltInProfileIDs(t)
+}
+
+func loadBuiltInProfileIDs(t *testing.T) map[string]string {
+	t.Helper()
+
+	ids := map[string]string{}
+	err := fs.WalkDir(assets, assetRoot, func(name string, d fs.DirEntry, err error) error {
+		if err != nil {
+			return err
+		}
+		if d.IsDir() || !strings.HasSuffix(name, ".yml") {
+			return nil
+		}
+
+		data, err := assets.ReadFile(name)
+		if err != nil {
+			t.Fatalf("failed to read built-in profile %s: %v", name, err)
+		}
+
+		var raw map[string]any
+		if err := yaml.Unmarshal(data, &raw); err != nil {
+			t.Fatalf("failed to decode built-in profile %s: %v", name, err)
+		}
+		if _, ok := raw["api_key"]; ok {
+			t.Fatalf("built-in profile %s contains raw api_key", name)
+		}
+		if raw["backend"] != backend.OpenRouterID {
+			t.Fatalf("built-in profile %s does not select %q", name, backend.OpenRouterID)
+		}
+		if _, ok := raw["endpoint"]; ok {
+			t.Fatalf("built-in profile %s repeats endpoint", name)
+		}
+		if _, ok := raw["api_key_env"]; ok {
+			t.Fatalf("built-in profile %s repeats api_key_env", name)
+		}
+		id, ok := raw["id"].(string)
+		if !ok || strings.TrimSpace(id) == "" {
+			t.Fatalf("built-in profile %s has missing id", name)
+		}
+		if previous, ok := ids[id]; ok {
+			t.Fatalf("duplicate built-in profile id %q in %s and %s", id, previous, name)
+		}
+		ids[id] = name
+		return nil
+	})
+	if err != nil {
+		t.Fatalf("failed to walk built-in profiles: %v", err)
+	}
+	return ids
+}
+
+func TestRepositoryWithPrimaryUsesPrimaryBeforeBuiltIns(t *testing.T) {
+	repo := NewRepositoryWithPrimary(staticProfileRepo{
+		profiles: map[string]string{"mistral-small-3": "custom-model"},
+	})
+
+	p, err := repo.GetProfile(context.Background(), "mistral-small-3")
+	if err != nil {
+		t.Fatalf("expected profile to load, got %v", err)
+	}
+	if p.Model != "custom-model" {
+		t.Fatalf("expected primary profile to override built-in, got %+v", p)
+	}
+}
+
+func TestRepositoryWithPrimaryFallsBackToBuiltIns(t *testing.T) {
+	repo := NewRepositoryWithPrimary(staticProfileRepo{})
+
+	p, err := repo.GetProfile(context.Background(), "mistral-small-3")
+	if err != nil {
+		t.Fatalf("expected built-in profile to load, got %v", err)
+	}
+	if p.ID != "mistral-small-3" {
+		t.Fatalf("unexpected profile: %+v", p)
+	}
+}
+
+func TestRepositoryWithPrimaryDoesNotFallBackAfterPrimaryError(t *testing.T) {
+	repo := NewRepositoryWithPrimary(staticProfileRepo{err: profile.ErrInvalidProfile})
+
+	_, err := repo.GetProfile(context.Background(), "mistral-small-3")
+	if !errors.Is(err, profile.ErrInvalidProfile) {
+		t.Fatalf("expected primary error, got %v", err)
+	}
+}
+
+type staticProfileRepo struct {
+	profiles map[string]string
+	err      error
+}
+
+func (r staticProfileRepo) GetProfile(_ context.Context, id string) (*domain.ExecutionProfile, error) {
+	if r.err != nil {
+		return nil, r.err
+	}
+	if model, ok := r.profiles[id]; ok {
+		return &domain.ExecutionProfile{ID: id, Endpoint: "http://primary/v1", Model: model}, nil
+	}
+	return nil, profile.ErrProfileNotFound
+}

internal/profile/filesystem_repository.go

@@ -0,0 +1,214 @@
+package profile
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"fmt"
+	"io/fs"
+	"os"
+	"path"
+	"strings"
+
+	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
+	"gitea.maximumdirect.net/eric/promptkit/internal/filecatalog"
+	"gopkg.in/yaml.v3"
+)
+
+var (
+	ErrProfileNotFound     = errors.New("execution profile not found")
+	ErrInvalidYAML         = errors.New("invalid YAML format")
+	ErrInvalidProfile      = errors.New("invalid execution profile configuration")
+	ErrRawAPIKeyNotAllowed = errors.New("raw api_key is not allowed; use api_key_env")
+)
+
+type filesystemRepository struct {
+	dir string
+}
+
+func NewFilesystemRepository(dir string) Repository {
+	return &filesystemRepository{dir: dir}
+}
+
+func (r *filesystemRepository) GetProfile(ctx context.Context, id string) (*domain.ExecutionProfile, error) {
+	return loadProfile(ctx, os.DirFS(r.dir), ".", id)
+}
+
+type fsRepository struct {
+	fsys fs.FS
+	root string
+}
+
+func NewFSRepository(fsys fs.FS, root string) Repository {
+	return &fsRepository{fsys: fsys, root: root}
+}
+
+func (r *fsRepository) GetProfile(ctx context.Context, id string) (*domain.ExecutionProfile, error) {
+	return loadProfile(ctx, r.fsys, r.root, id)
+}
+
+type overlayRepository struct {
+	primary  Repository
+	fallback Repository
+}
+
+func NewOverlayRepository(primary, fallback Repository) Repository {
+	return &overlayRepository{primary: primary, fallback: fallback}
+}
+
+func (r *overlayRepository) GetProfile(ctx context.Context, id string) (*domain.ExecutionProfile, error) {
+	if r.primary != nil {
+		prof, err := r.primary.GetProfile(ctx, id)
+		if err == nil {
+			return prof, nil
+		}
+		if !errors.Is(err, ErrProfileNotFound) {
+			return nil, err
+		}
+	}
+	if r.fallback == nil {
+		return nil, ErrProfileNotFound
+	}
+	return r.fallback.GetProfile(ctx, id)
+}
+
+func loadProfile(ctx context.Context, fsys fs.FS, root string, id string) (*domain.ExecutionProfile, error) {
+	if strings.TrimSpace(id) == "" {
+		return nil, fmt.Errorf("%w: profile id is required", ErrInvalidProfile)
+	}
+	if fsys == nil {
+		return nil, fmt.Errorf("failed to read profile directory: filesystem is nil")
+	}
+
+	files, err := filecatalog.FindFSYAMLFiles(ctx, fsys, root)
+	if err != nil {
+		return nil, fmt.Errorf("failed to read profile directory: %w", err)
+	}
+
+	var matches []profileMatch
+	for _, fullPath := range files {
+		select {
+		case <-ctx.Done():
+			return nil, ctx.Err()
+		default:
+		}
+
+		relPath := filecatalog.DisplayPath(root, fullPath)
+		fileMatch := filecatalog.Stem(path.Base(fullPath)) == id
+		data, err := fs.ReadFile(fsys, fullPath)
+		if err != nil {
+			return nil, fmt.Errorf("failed to read profile file %s: %w", relPath, err)
+		}
+		metadata := readProfileFileMetadata(data)
+		idMatch := fileMatch || metadata.id == id
+		if metadata.hasRawAPIKey {
+			if idMatch {
+				return nil, fmt.Errorf("%w: %s", ErrRawAPIKeyNotAllowed, relPath)
+			}
+			continue
+		}
+
+		var prof domain.ExecutionProfile
+		decoder := yaml.NewDecoder(bytes.NewReader(data))
+		decoder.KnownFields(true)
+		if err := decoder.Decode(&prof); err != nil {
+			if idMatch {
+				return nil, fmt.Errorf("%w: %s: %v", ErrInvalidYAML, relPath, err)
+			}
+			continue
+		}
+
+		if prof.ID != id {
+			continue
+		}
+		prof.BackendID = strings.TrimSpace(prof.BackendID)
+		if err := validateProfile(&prof); err != nil {
+			if errors.Is(err, ErrRawAPIKeyNotAllowed) {
+				return nil, fmt.Errorf("%w: %s", err, relPath)
+			}
+			return nil, fmt.Errorf("%w: %s: %v", ErrInvalidProfile, relPath, err)
+		}
+		matches = append(matches, profileMatch{
+			profile: &prof,
+			path:    relPath,
+		})
+	}
+
+	if len(matches) > 1 {
+		paths := make([]string, 0, len(matches))
+		for _, match := range matches {
+			paths = append(paths, match.path)
+		}
+		return nil, fmt.Errorf("%w: duplicate execution profile id %q found in: %s", ErrInvalidProfile, id, strings.Join(paths, ", "))
+	}
+
+	if len(matches) == 1 {
+		return matches[0].profile, nil
+	}
+
+	return nil, ErrProfileNotFound
+}
+
+type profileMatch struct {
+	profile *domain.ExecutionProfile
+	path    string
+}
+
+type profileFileMetadata struct {
+	id           string
+	hasRawAPIKey bool
+}
+
+func readProfileFileMetadata(data []byte) profileFileMetadata {
+	var node yaml.Node
+	if err := yaml.NewDecoder(bytes.NewReader(data)).Decode(&node); err != nil {
+		return profileFileMetadata{}
+	}
+	if node.Kind != yaml.DocumentNode || len(node.Content) == 0 {
+		return profileFileMetadata{}
+	}
+	mapping := node.Content[0]
+	if mapping.Kind != yaml.MappingNode {
+		return profileFileMetadata{}
+	}
+
+	var metadata profileFileMetadata
+	for i := 0; i+1 < len(mapping.Content); i += 2 {
+		key := mapping.Content[i]
+		value := mapping.Content[i+1]
+		switch key.Value {
+		case "id":
+			metadata.id = strings.TrimSpace(value.Value)
+		case "api_key":
+			metadata.hasRawAPIKey = true
+		}
+	}
+	return metadata
+}
+
+func validateProfile(p *domain.ExecutionProfile) error {
+	if strings.TrimSpace(p.ID) == "" {
+		return errors.New("id is required")
+	}
+	if strings.TrimSpace(p.BackendID) == "" && strings.TrimSpace(p.Endpoint) == "" {
+		return errors.New("backend or endpoint is required")
+	}
+	if strings.TrimSpace(p.Model) == "" {
+		return errors.New("model is required")
+	}
+
+	if p.Temperature < 0 || p.Temperature > 2 {
+		return errors.New("temperature must be between 0 and 2")
+	}
+	if p.MaxTokens < 0 {
+		return errors.New("max_tokens must be greater than or equal to 0")
+	}
+	if p.TopP < 0 || p.TopP > 1 {
+		return errors.New("top_p must be between 0 and 1")
+	}
+	if p.TimeoutSeconds < 0 {
+		return errors.New("timeout_seconds must be greater than or equal to 0")
+	}
+
+	return nil
+}

internal/profile/repository.go

@@ -0,0 +1,12 @@
+package profile
+
+import (
+	"context"
+
+	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
+)
+
+// Repository loads execution profiles.
+type Repository interface {
+	GetProfile(ctx context.Context, id string) (*domain.ExecutionProfile, error)
+}

internal/profile/repository_test.go

@@ -0,0 +1,516 @@
+package profile
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+	"testing/fstest"
+
+	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
+)
+
+func TestFilesystemRepository_GetProfile(t *testing.T) {
+	tmpDir, err := os.MkdirTemp("", "execution_profile_test")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tmpDir)
+
+	files, err := os.ReadDir("testdata")
+	if err != nil {
+		t.Fatalf("failed to read testdata: %v", err)
+	}
+	for _, f := range files {
+		src := filepath.Join("testdata", f.Name())
+		dst := filepath.Join(tmpDir, f.Name())
+		data, err := os.ReadFile(src)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if err := os.WriteFile(dst, data, 0644); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	repo := NewFilesystemRepository(tmpDir)
+	ctx := context.Background()
+
+	t.Run("valid local profile", func(t *testing.T) {
+		p, err := repo.GetProfile(ctx, "local-default")
+		if err != nil {
+			t.Fatalf("expected no error, got %v", err)
+		}
+		if p.ID != "local-default" {
+			t.Fatalf("unexpected id: %q", p.ID)
+		}
+		if p.Endpoint == "" || p.Model == "" {
+			t.Fatalf("expected endpoint/model to be set: %+v", p)
+		}
+	})
+
+	t.Run("backend and endpoint connection matrix", func(t *testing.T) {
+		tests := []struct {
+			name         string
+			connection   string
+			wantBackend  string
+			wantEndpoint string
+			wantErr      bool
+		}{
+			{name: "backend only", connection: "backend: '  openrouter  '", wantBackend: "openrouter"},
+			{name: "endpoint only", connection: "endpoint: http://localhost:8000/v1", wantEndpoint: "http://localhost:8000/v1"},
+			{name: "both", connection: "backend: openrouter\nendpoint: http://localhost:8000/v1", wantBackend: "openrouter", wantEndpoint: "http://localhost:8000/v1"},
+			{name: "neither", wantErr: true},
+			{name: "blank backend", connection: "backend: '   '", wantErr: true},
+		}
+
+		for _, tt := range tests {
+			t.Run(tt.name, func(t *testing.T) {
+				id := "connection-" + strings.ReplaceAll(tt.name, " ", "-")
+				writeProfileTestFile(t, filepath.Join(tmpDir, id+".yaml"), "id: "+id+"\nmodel: model\n"+tt.connection+"\n")
+
+				p, err := repo.GetProfile(ctx, id)
+				if tt.wantErr {
+					if !errors.Is(err, ErrInvalidProfile) {
+						t.Fatalf("expected ErrInvalidProfile, got %v", err)
+					}
+					return
+				}
+				if err != nil {
+					t.Fatalf("expected profile to load, got %v", err)
+				}
+				if p.BackendID != tt.wantBackend || p.Endpoint != tt.wantEndpoint {
+					t.Fatalf("unexpected connection values: backend=%q endpoint=%q", p.BackendID, p.Endpoint)
+				}
+			})
+		}
+	})
+
+	t.Run("valid profile with api_key_env", func(t *testing.T) {
+		p, err := repo.GetProfile(ctx, "local-secure")
+		if err != nil {
+			t.Fatalf("expected no error, got %v", err)
+		}
+		if p.APIKeyEnv != "PROMPTKIT_API_KEY" {
+			t.Fatalf("unexpected api_key_env: %q", p.APIKeyEnv)
+		}
+		if p.ReasoningEffort != "medium" {
+			t.Fatalf("unexpected reasoning_effort: %q", p.ReasoningEffort)
+		}
+		if p.ServiceTier != "priority" {
+			t.Fatalf("unexpected service_tier: %q", p.ServiceTier)
+		}
+	})
+
+	t.Run("valid nested profile", func(t *testing.T) {
+		nestedDir := filepath.Join(tmpDir, "local")
+		if err := os.MkdirAll(nestedDir, 0o755); err != nil {
+			t.Fatal(err)
+		}
+		writeProfileTestFile(t, filepath.Join(nestedDir, "nested-local.yaml"), `
+id: nested-local
+endpoint: http://localhost:8000/v1
+model: nested-model
+temperature: 0.1
+`)
+
+		p, err := repo.GetProfile(ctx, "nested-local")
+		if err != nil {
+			t.Fatalf("expected no error, got %v", err)
+		}
+		if p.Model != "nested-model" {
+			t.Fatalf("unexpected model: %q", p.Model)
+		}
+	})
+
+	t.Run("valid profile with JSON-compatible extra params", func(t *testing.T) {
+		writeProfileTestFile(t, filepath.Join(tmpDir, "json-extra-params.yaml"), `
+id: json-extra-params
+endpoint: http://localhost:8000/v1
+model: nested-model
+extra_params:
+  string_value: enabled
+  number_value: 42
+  boolean_value: true
+  object_value:
+    nested: value
+    count: 2
+  array_value:
+    - first
+    - 3
+    - false
+`)
+
+		p, err := repo.GetProfile(ctx, "json-extra-params")
+		if err != nil {
+			t.Fatalf("expected no error, got %v", err)
+		}
+
+		var got map[string]any
+		encoded, err := json.Marshal(p.ExtraParams)
+		if err != nil {
+			t.Fatalf("expected extra_params to marshal as JSON, got %v", err)
+		}
+		if err := json.Unmarshal(encoded, &got); err != nil {
+			t.Fatalf("expected extra_params JSON to decode, got %v", err)
+		}
+
+		if got["string_value"] != "enabled" {
+			t.Fatalf("unexpected string extra param: %#v", got["string_value"])
+		}
+		if got["number_value"] != float64(42) {
+			t.Fatalf("unexpected number extra param: %#v", got["number_value"])
+		}
+		if got["boolean_value"] != true {
+			t.Fatalf("unexpected boolean extra param: %#v", got["boolean_value"])
+		}
+		objectValue, ok := got["object_value"].(map[string]any)
+		if !ok {
+			t.Fatalf("expected object extra param, got %#v", got["object_value"])
+		}
+		if objectValue["nested"] != "value" || objectValue["count"] != float64(2) {
+			t.Fatalf("unexpected object extra param: %#v", objectValue)
+		}
+		arrayValue, ok := got["array_value"].([]any)
+		if !ok {
+			t.Fatalf("expected array extra param, got %#v", got["array_value"])
+		}
+		if len(arrayValue) != 3 || arrayValue[0] != "first" || arrayValue[1] != float64(3) || arrayValue[2] != false {
+			t.Fatalf("unexpected array extra param: %#v", arrayValue)
+		}
+	})
+
+	t.Run("duplicate profile IDs fail as ambiguous", func(t *testing.T) {
+		writeProfileTestFile(t, filepath.Join(tmpDir, "duplicate-profile-a.yaml"), `
+id: duplicate-profile
+endpoint: http://localhost:8000/v1
+model: first-model
+`)
+		nestedDir := filepath.Join(tmpDir, "duplicates")
+		if err := os.MkdirAll(nestedDir, 0o755); err != nil {
+			t.Fatal(err)
+		}
+		writeProfileTestFile(t, filepath.Join(nestedDir, "duplicate-profile-b.yaml"), `
+id: duplicate-profile
+endpoint: http://localhost:8000/v1
+model: second-model
+`)
+
+		_, err := repo.GetProfile(ctx, "duplicate-profile")
+		if !errors.Is(err, ErrInvalidProfile) {
+			t.Fatalf("expected duplicate profile to return ErrInvalidProfile, got %v", err)
+		}
+		for _, want := range []string{"duplicate execution profile id", "duplicate-profile-a.yaml", filepath.Join("duplicates", "duplicate-profile-b.yaml")} {
+			if !strings.Contains(err.Error(), want) {
+				t.Fatalf("expected error to contain %q, got %v", want, err)
+			}
+		}
+	})
+
+	t.Run("nested raw api_key rejected for likely target file", func(t *testing.T) {
+		nestedDir := filepath.Join(tmpDir, "secure")
+		if err := os.MkdirAll(nestedDir, 0o755); err != nil {
+			t.Fatal(err)
+		}
+		writeProfileTestFile(t, filepath.Join(nestedDir, "not_named_like_id.yaml"), `
+id: nested_raw_api_key
+endpoint: http://localhost:8000/v1
+model: m
+api_key: secret
+`)
+
+		_, err := repo.GetProfile(ctx, "nested_raw_api_key")
+		if !errors.Is(err, ErrRawAPIKeyNotAllowed) {
+			t.Fatalf("expected ErrRawAPIKeyNotAllowed, got %v", err)
+		}
+		if !strings.Contains(err.Error(), filepath.Join("secure", "not_named_like_id.yaml")) {
+			t.Fatalf("expected nested path in error, got %v", err)
+		}
+	})
+
+	t.Run("raw api_key in non-target profile is ignored", func(t *testing.T) {
+		writeProfileTestFile(t, filepath.Join(tmpDir, "raw-api-key-non-target.yaml"), `
+id: raw-api-key-non-target
+endpoint: http://localhost:8000/v1
+model: m
+api_key: secret
+`)
+
+		_, err := repo.GetProfile(ctx, "does-not-exist-with-raw-key-nearby")
+		if !errors.Is(err, ErrProfileNotFound) {
+			t.Fatalf("expected ErrProfileNotFound for non-target raw api_key file, got %v", err)
+		}
+	})
+
+	t.Run("invalid yaml", func(t *testing.T) {
+		_, err := repo.GetProfile(ctx, "invalid_yaml")
+		if !errors.Is(err, ErrInvalidYAML) {
+			t.Fatalf("expected ErrInvalidYAML, got %v", err)
+		}
+	})
+
+	t.Run("missing id", func(t *testing.T) {
+		_, err := repo.GetProfile(ctx, "missing_id")
+		if !errors.Is(err, ErrProfileNotFound) {
+			t.Fatalf("expected ErrProfileNotFound, got %v", err)
+		}
+	})
+
+	t.Run("missing endpoint", func(t *testing.T) {
+		_, err := repo.GetProfile(ctx, "missing-endpoint")
+		if !errors.Is(err, ErrInvalidProfile) {
+			t.Fatalf("expected ErrInvalidProfile, got %v", err)
+		}
+	})
+
+	t.Run("missing model", func(t *testing.T) {
+		_, err := repo.GetProfile(ctx, "missing-model")
+		if !errors.Is(err, ErrInvalidProfile) {
+			t.Fatalf("expected ErrInvalidProfile, got %v", err)
+		}
+	})
+
+	t.Run("unknown field", func(t *testing.T) {
+		_, err := repo.GetProfile(ctx, "unknown_field")
+		if !errors.Is(err, ErrInvalidYAML) {
+			t.Fatalf("expected ErrInvalidYAML for strict decode unknown field, got %v", err)
+		}
+	})
+
+	t.Run("raw api_key rejected", func(t *testing.T) {
+		_, err := repo.GetProfile(ctx, "raw_api_key")
+		if !errors.Is(err, ErrRawAPIKeyNotAllowed) {
+			t.Fatalf("expected ErrRawAPIKeyNotAllowed, got %v", err)
+		}
+	})
+
+	t.Run("profile not found", func(t *testing.T) {
+		_, err := repo.GetProfile(ctx, "does-not-exist")
+		if !errors.Is(err, ErrProfileNotFound) {
+			t.Fatalf("expected ErrProfileNotFound, got %v", err)
+		}
+	})
+}
+
+func writeProfileTestFile(t *testing.T, path string, content string) {
+	t.Helper()
+	if err := os.WriteFile(path, []byte(strings.TrimLeft(content, "\n")), 0o644); err != nil {
+		t.Fatalf("failed to write profile test file %q: %v", path, err)
+	}
+}
+
+func TestFSRepository(t *testing.T) {
+	ctx := context.Background()
+
+	t.Run("loads valid profiles from nested directories", func(t *testing.T) {
+		repo := NewFSRepository(fstest.MapFS{
+			"profiles/provider/nested.yaml": profileMapFile(`
+id: nested-profile
+endpoint: http://localhost:8000/v1
+model: nested-model
+temperature: 0.1
+`),
+		}, "profiles")
+
+		p, err := repo.GetProfile(ctx, "nested-profile")
+		if err != nil {
+			t.Fatalf("expected no error, got %v", err)
+		}
+		if p.ID != "nested-profile" || p.Model != "nested-model" {
+			t.Fatalf("unexpected profile: %+v", p)
+		}
+	})
+
+	t.Run("rejects unknown YAML fields", func(t *testing.T) {
+		repo := NewFSRepository(fstest.MapFS{
+			"profiles/unknown.yaml": profileMapFile(`
+id: unknown-profile
+endpoint: http://localhost:8000/v1
+model: model
+unknown: value
+`),
+		}, "profiles")
+
+		_, err := repo.GetProfile(ctx, "unknown-profile")
+		if !errors.Is(err, ErrInvalidYAML) {
+			t.Fatalf("expected ErrInvalidYAML, got %v", err)
+		}
+	})
+
+	t.Run("rejects raw api_key in selected profile", func(t *testing.T) {
+		repo := NewFSRepository(fstest.MapFS{
+			"profiles/raw.yaml": profileMapFile(`
+id: raw-profile
+endpoint: http://localhost:8000/v1
+model: model
+api_key: secret
+`),
+		}, "profiles")
+
+		_, err := repo.GetProfile(ctx, "raw-profile")
+		if !errors.Is(err, ErrRawAPIKeyNotAllowed) {
+			t.Fatalf("expected ErrRawAPIKeyNotAllowed, got %v", err)
+		}
+	})
+
+	t.Run("ignores raw api_key in non-selected profiles", func(t *testing.T) {
+		repo := NewFSRepository(fstest.MapFS{
+			"profiles/raw.yaml": profileMapFile(`
+id: raw-profile
+endpoint: http://localhost:8000/v1
+model: model
+api_key: secret
+`),
+			"profiles/valid.yaml": profileMapFile(`
+id: valid-profile
+endpoint: http://localhost:8000/v1
+model: model
+`),
+		}, "profiles")
+
+		p, err := repo.GetProfile(ctx, "valid-profile")
+		if err != nil {
+			t.Fatalf("expected no error, got %v", err)
+		}
+		if p.ID != "valid-profile" {
+			t.Fatalf("unexpected profile: %+v", p)
+		}
+	})
+
+	t.Run("rejects duplicate IDs within one source", func(t *testing.T) {
+		repo := NewFSRepository(fstest.MapFS{
+			"profiles/a.yaml": profileMapFile(`
+id: duplicate-profile
+endpoint: http://localhost:8000/v1
+model: first
+`),
+			"profiles/nested/b.yaml": profileMapFile(`
+id: duplicate-profile
+endpoint: http://localhost:8000/v1
+model: second
+`),
+		}, "profiles")
+
+		_, err := repo.GetProfile(ctx, "duplicate-profile")
+		if !errors.Is(err, ErrInvalidProfile) {
+			t.Fatalf("expected ErrInvalidProfile, got %v", err)
+		}
+		for _, want := range []string{"duplicate execution profile id", "a.yaml", "nested/b.yaml"} {
+			if !strings.Contains(err.Error(), want) {
+				t.Fatalf("expected error to contain %q, got %v", want, err)
+			}
+		}
+	})
+}
+
+func TestOverlayRepository(t *testing.T) {
+	ctx := context.Background()
+	primaryProfile := &domain.ExecutionProfile{ID: "shared", Endpoint: "http://primary", Model: "primary"}
+	fallbackProfile := &domain.ExecutionProfile{ID: "shared", Endpoint: "http://fallback", Model: "fallback"}
+
+	t.Run("returns primary matches before fallback matches", func(t *testing.T) {
+		repo := NewOverlayRepository(
+			staticProfileRepo{profiles: map[string]*domain.ExecutionProfile{"shared": primaryProfile}},
+			staticProfileRepo{profiles: map[string]*domain.ExecutionProfile{"shared": fallbackProfile}},
+		)
+
+		p, err := repo.GetProfile(ctx, "shared")
+		if err != nil {
+			t.Fatalf("expected no error, got %v", err)
+		}
+		if p.Model != "primary" {
+			t.Fatalf("expected primary profile, got %+v", p)
+		}
+	})
+
+	t.Run("falls back on primary not found", func(t *testing.T) {
+		repo := NewOverlayRepository(
+			staticProfileRepo{},
+			staticProfileRepo{profiles: map[string]*domain.ExecutionProfile{"shared": fallbackProfile}},
+		)
+
+		p, err := repo.GetProfile(ctx, "shared")
+		if err != nil {
+			t.Fatalf("expected no error, got %v", err)
+		}
+		if p.Model != "fallback" {
+			t.Fatalf("expected fallback profile, got %+v", p)
+		}
+	})
+
+	t.Run("does not fall back after primary load errors", func(t *testing.T) {
+		for _, tc := range []struct {
+			name string
+			err  error
+		}{
+			{name: "invalid yaml", err: ErrInvalidYAML},
+			{name: "invalid profile", err: ErrInvalidProfile},
+			{name: "raw api key", err: ErrRawAPIKeyNotAllowed},
+		} {
+			t.Run(tc.name, func(t *testing.T) {
+				repo := NewOverlayRepository(
+					staticProfileRepo{err: tc.err},
+					staticProfileRepo{profiles: map[string]*domain.ExecutionProfile{"shared": fallbackProfile}},
+				)
+
+				_, err := repo.GetProfile(ctx, "shared")
+				if !errors.Is(err, tc.err) {
+					t.Fatalf("expected %v, got %v", tc.err, err)
+				}
+			})
+		}
+	})
+
+	t.Run("returns not found when both sources miss", func(t *testing.T) {
+		repo := NewOverlayRepository(staticProfileRepo{}, staticProfileRepo{})
+
+		_, err := repo.GetProfile(ctx, "missing")
+		if !errors.Is(err, ErrProfileNotFound) {
+			t.Fatalf("expected ErrProfileNotFound, got %v", err)
+		}
+	})
+
+	t.Run("nil primary uses fallback", func(t *testing.T) {
+		repo := NewOverlayRepository(nil, staticProfileRepo{profiles: map[string]*domain.ExecutionProfile{"shared": fallbackProfile}})
+
+		p, err := repo.GetProfile(ctx, "shared")
+		if err != nil {
+			t.Fatalf("expected no error, got %v", err)
+		}
+		if p.Model != "fallback" {
+			t.Fatalf("expected fallback profile, got %+v", p)
+		}
+	})
+
+	t.Run("nil fallback returns not found after primary miss", func(t *testing.T) {
+		repo := NewOverlayRepository(staticProfileRepo{}, nil)
+
+		_, err := repo.GetProfile(ctx, "missing")
+		if !errors.Is(err, ErrProfileNotFound) {
+			t.Fatalf("expected ErrProfileNotFound, got %v", err)
+		}
+	})
+}
+
+func profileMapFile(content string) *fstest.MapFile {
+	return &fstest.MapFile{Data: []byte(strings.TrimLeft(content, "\n"))}
+}
+
+type staticProfileRepo struct {
+	profiles map[string]*domain.ExecutionProfile
+	err      error
+}
+
+func (r staticProfileRepo) GetProfile(_ context.Context, id string) (*domain.ExecutionProfile, error) {
+	if r.err != nil {
+		return nil, r.err
+	}
+	if p, ok := r.profiles[id]; ok {
+		cp := *p
+		return &cp, nil
+	}
+	return nil, ErrProfileNotFound
+}

internal/profile/testdata/invalid_yaml.yaml

@@ -0,0 +1,3 @@
+id: invalid_yaml
+endpoint: http://localhost:8000/v1
+model: [broken

internal/profile/testdata/missing_endpoint.yaml

@@ -0,0 +1,2 @@
+id: missing-endpoint
+model: gpt-4o-mini

internal/profile/testdata/missing_id.yaml

@@ -0,0 +1,2 @@
+endpoint: http://localhost:8000/v1
+model: gpt-4o-mini

internal/profile/testdata/missing_model.yaml

@@ -0,0 +1,2 @@
+id: missing-model
+endpoint: http://localhost:8000/v1

internal/profile/testdata/raw_api_key.yaml

@@ -0,0 +1,4 @@
+id: raw-api-key
+endpoint: http://localhost:8000/v1
+model: gpt-4o-mini
+api_key: super-secret-should-not-be-here

internal/profile/testdata/unknown_field.yaml

@@ -0,0 +1,4 @@
+id: unknown-field
+endpoint: http://localhost:8000/v1
+model: gpt-4o-mini
+foo: bar

internal/profile/testdata/valid_local_profile.yaml

@@ -0,0 +1,7 @@
+id: local-default
+endpoint: http://localhost:8000/v1
+model: gpt-4o-mini
+temperature: 0.2
+max_tokens: 700
+top_p: 1.0
+timeout_seconds: 120

internal/profile/testdata/valid_with_api_key_env.yaml

@@ -0,0 +1,8 @@
+id: local-secure
+endpoint: http://localhost:8000/v1
+model: gpt-4o-mini
+api_key_env: PROMPTKIT_API_KEY
+service_tier: priority
+reasoning_effort: medium
+extra_params:
+  provider: local

internal/prompt/go_renderer.go

@@ -0,0 +1,120 @@
+package prompt
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"fmt"
+	"text/template"
+
+	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
+)
+
+var (
+	ErrMissingRequiredInput = errors.New("missing required input artifact")
+	ErrUnknownInput         = errors.New("referenced unknown input artifact")
+	ErrInvalidTemplate      = errors.New("invalid prompt template")
+	ErrRenderFailure        = errors.New("prompt render failure")
+	ErrInvalidMessageRole   = errors.New("invalid or empty message role")
+)
+
+type goRenderer struct{}
+
+func NewGoRenderer() Renderer {
+	return &goRenderer{}
+}
+
+func (r *goRenderer) Render(ctx context.Context, definition *domain.PromptDefinition, inputs map[string]*domain.Artifact, vars map[string]string) (*domain.RenderedPrompt, error) {
+	if definition == nil {
+		return nil, fmt.Errorf("%w: nil prompt definition", ErrRenderFailure)
+	}
+
+	// 1. Verify required inputs
+	for _, in := range definition.Inputs {
+		if !in.Required {
+			continue
+		}
+		art, ok := inputs[in.Name]
+		if !ok || art == nil {
+			return nil, fmt.Errorf("%w: %s", ErrMissingRequiredInput, in.Name)
+		}
+	}
+
+	// 2. Setup template functions
+	funcs := template.FuncMap{
+		"input": func(name string) (string, error) {
+			art, ok := inputs[name]
+			if !ok || art == nil {
+				return "", fmt.Errorf("%w: %s", ErrUnknownInput, name)
+			}
+			return string(art.Body), nil
+		},
+	}
+
+	sessionID, err := renderSessionID(definition.SessionID, funcs, vars)
+	if err != nil {
+		return nil, err
+	}
+
+	var renderedMessages []domain.RenderedMessage
+
+	for i, tmplMsg := range definition.Templates {
+		select {
+		case <-ctx.Done():
+			return nil, ctx.Err()
+		default:
+		}
+
+		if tmplMsg.Role == "" {
+			return nil, fmt.Errorf("%w: message %d", ErrInvalidMessageRole, i)
+		}
+
+		// Parse and execute template
+		tmpl, err := template.New(fmt.Sprintf("msg_%d", i)).Funcs(funcs).Option("missingkey=error").Parse(tmplMsg.Content)
+		if err != nil {
+			return nil, fmt.Errorf("%w: message %d: %v", ErrInvalidTemplate, i, err)
+		}
+
+		var buf bytes.Buffer
+		if err := tmpl.Execute(&buf, vars); err != nil {
+			return nil, fmt.Errorf("%w: message %d: %w", ErrRenderFailure, i, err)
+		}
+
+		renderedMessages = append(renderedMessages, domain.RenderedMessage{
+			Role:         tmplMsg.Role,
+			Content:      buf.String(),
+			CacheControl: cloneCacheControl(tmplMsg.CacheControl),
+		})
+	}
+
+	return &domain.RenderedPrompt{
+		SessionID: sessionID,
+		Messages:  renderedMessages,
+	}, nil
+}
+
+func renderSessionID(raw string, funcs template.FuncMap, vars map[string]string) (string, error) {
+	tmpl, err := template.New("session_id").Funcs(funcs).Option("missingkey=error").Parse(raw)
+	if err != nil {
+		return "", fmt.Errorf("%w: session_id: %v", ErrInvalidTemplate, err)
+	}
+
+	var buf bytes.Buffer
+	if err := tmpl.Execute(&buf, vars); err != nil {
+		return "", fmt.Errorf("%w: session_id: %w", ErrRenderFailure, err)
+	}
+
+	sessionID, err := domain.NormalizeSessionID(buf.String())
+	if err != nil {
+		return "", fmt.Errorf("%w: session_id: %v", ErrRenderFailure, err)
+	}
+	return sessionID, nil
+}
+
+func cloneCacheControl(in *domain.CacheControl) *domain.CacheControl {
+	if in == nil {
+		return nil
+	}
+	out := *in
+	return &out
+}

internal/prompt/renderer.go

@@ -0,0 +1,11 @@
+package prompt
+
+import (
+	"context"
+	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
+)
+
+// Renderer renders prompt templates using named artifacts and variables.
+type Renderer interface {
+	Render(ctx context.Context, definition *domain.PromptDefinition, inputs map[string]*domain.Artifact, vars map[string]string) (*domain.RenderedPrompt, error)
+}

internal/prompt/renderer_test.go

@@ -0,0 +1,345 @@
+package prompt
+
+import (
+	"context"
+	"errors"
+	"strings"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
+)
+
+func TestGoRenderer_Render(t *testing.T) {
+	renderer := NewGoRenderer()
+	ctx := context.Background()
+
+	inputs := map[string]*domain.Artifact{
+		"transcript": {Body: []byte("The quick brown fox.")},
+	}
+	vars := map[string]string{
+		"role": "helpful assistant",
+		"tone": "concise",
+	}
+
+	t.Run("rendering inline message content", func(t *testing.T) {
+		def := &domain.PromptDefinition{
+			Inputs: []domain.PromptInput{{Name: "transcript", Required: true}},
+			Templates: []domain.PromptMessageTemplate{
+				{Role: "user", Content: "Analyze this: {{input \"transcript\"}}"},
+			},
+		}
+
+		res, err := renderer.Render(ctx, def, inputs, vars)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if len(res.Messages) != 1 {
+			t.Fatalf("expected 1 message, got %d", len(res.Messages))
+		}
+		if res.Messages[0].Content != "Analyze this: The quick brown fox." {
+			t.Fatalf("unexpected rendered content: %q", res.Messages[0].Content)
+		}
+	})
+
+	t.Run("rendering file-backed message content loaded into prompt definition", func(t *testing.T) {
+		def := &domain.PromptDefinition{
+			Inputs: []domain.PromptInput{{Name: "transcript", Required: true}},
+			Templates: []domain.PromptMessageTemplate{
+				{Role: "user", Content: "From file: {{input \"transcript\"}}", ContentFile: "/tmp/user.tmpl"},
+			},
+		}
+
+		res, err := renderer.Render(ctx, def, inputs, vars)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if got := res.Messages[0].Content; got != "From file: The quick brown fox." {
+			t.Fatalf("unexpected file-backed render result: %q", got)
+		}
+	})
+
+	t.Run("rendering system and user messages", func(t *testing.T) {
+		def := &domain.PromptDefinition{
+			Inputs: []domain.PromptInput{{Name: "transcript", Required: true}},
+			Templates: []domain.PromptMessageTemplate{
+				{Role: "system", Content: "You are a {{.role}}."},
+				{Role: "user", Content: "Analyze this: {{input \"transcript\"}}"},
+			},
+		}
+
+		res, err := renderer.Render(ctx, def, inputs, vars)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if len(res.Messages) != 2 {
+			t.Fatalf("expected 2 messages, got %d", len(res.Messages))
+		}
+		if res.Messages[0].Role != "system" || res.Messages[1].Role != "user" {
+			t.Fatalf("unexpected roles: %#v", res.Messages)
+		}
+	})
+
+	t.Run("copying cache control to rendered messages", func(t *testing.T) {
+		def := &domain.PromptDefinition{
+			Inputs: []domain.PromptInput{{Name: "transcript", Required: true}},
+			Templates: []domain.PromptMessageTemplate{
+				{
+					Role:    "system",
+					Content: "You are concise.",
+					CacheControl: &domain.CacheControl{
+						Type: domain.CacheControlEphemeral,
+						TTL:  "1h",
+					},
+				},
+				{Role: "user", Content: "Analyze this: {{input \"transcript\"}}"},
+			},
+		}
+
+		res, err := renderer.Render(ctx, def, inputs, vars)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if len(res.Messages) != 2 {
+			t.Fatalf("expected 2 messages, got %d", len(res.Messages))
+		}
+		if res.Messages[0].CacheControl == nil {
+			t.Fatal("expected rendered cache control")
+		}
+		if res.Messages[0].CacheControl.Type != domain.CacheControlEphemeral {
+			t.Fatalf("unexpected cache control type: %q", res.Messages[0].CacheControl.Type)
+		}
+		if res.Messages[0].CacheControl.TTL != "1h" {
+			t.Fatalf("unexpected cache control ttl: %q", res.Messages[0].CacheControl.TTL)
+		}
+		if res.Messages[1].CacheControl != nil {
+			t.Fatalf("expected no cache control on second message, got %#v", res.Messages[1].CacheControl)
+		}
+	})
+
+	t.Run("rendered cache control does not alias source template", func(t *testing.T) {
+		source := &domain.CacheControl{Type: domain.CacheControlEphemeral, TTL: "1h"}
+		def := &domain.PromptDefinition{
+			Inputs: []domain.PromptInput{{Name: "transcript", Required: true}},
+			Templates: []domain.PromptMessageTemplate{
+				{Role: "system", Content: "You are concise.", CacheControl: source},
+			},
+		}
+
+		res, err := renderer.Render(ctx, def, inputs, vars)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if res.Messages[0].CacheControl == source {
+			t.Fatal("expected rendered cache control to be cloned")
+		}
+
+		res.Messages[0].CacheControl.TTL = ""
+		if source.TTL != "1h" {
+			t.Fatalf("source cache control was mutated, ttl=%q", source.TTL)
+		}
+	})
+
+	t.Run("accessing vars", func(t *testing.T) {
+		def := &domain.PromptDefinition{
+			Inputs: []domain.PromptInput{{Name: "transcript", Required: true}},
+			Templates: []domain.PromptMessageTemplate{
+				{Role: "system", Content: "Speak in a {{.tone}} tone."},
+			},
+		}
+
+		res, err := renderer.Render(ctx, def, inputs, vars)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if res.Messages[0].Content != "Speak in a concise tone." {
+			t.Fatalf("unexpected vars rendering: %q", res.Messages[0].Content)
+		}
+	})
+
+	t.Run("rendering session id from vars", func(t *testing.T) {
+		def := &domain.PromptDefinition{
+			SessionID: "  {{ .session_id }}  ",
+			Inputs:    []domain.PromptInput{{Name: "transcript", Required: true}},
+			Templates: []domain.PromptMessageTemplate{
+				{Role: "system", Content: "Speak in a {{.tone}} tone."},
+			},
+		}
+
+		res, err := renderer.Render(ctx, def, inputs, map[string]string{
+			"tone":       "concise",
+			"session_id": "agent-session-123",
+		})
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if res.SessionID != "agent-session-123" {
+			t.Fatalf("unexpected session id: %q", res.SessionID)
+		}
+	})
+
+	t.Run("empty rendered session id is omitted", func(t *testing.T) {
+		def := &domain.PromptDefinition{
+			SessionID: "   ",
+			Inputs:    []domain.PromptInput{{Name: "transcript", Required: true}},
+			Templates: []domain.PromptMessageTemplate{
+				{Role: "system", Content: "Speak in a {{.tone}} tone."},
+			},
+		}
+
+		res, err := renderer.Render(ctx, def, inputs, vars)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if res.SessionID != "" {
+			t.Fatalf("expected empty session id, got %q", res.SessionID)
+		}
+	})
+
+	t.Run("missing session id var fails rendering", func(t *testing.T) {
+		def := &domain.PromptDefinition{
+			SessionID: "{{ .session_id }}",
+			Inputs:    []domain.PromptInput{{Name: "transcript", Required: true}},
+			Templates: []domain.PromptMessageTemplate{
+				{Role: "system", Content: "Speak in a {{.tone}} tone."},
+			},
+		}
+
+		_, err := renderer.Render(ctx, def, inputs, vars)
+		if !errors.Is(err, ErrRenderFailure) {
+			t.Fatalf("expected ErrRenderFailure, got %v", err)
+		}
+	})
+
+	t.Run("too long rendered session id fails rendering", func(t *testing.T) {
+		def := &domain.PromptDefinition{
+			SessionID: "{{ .session_id }}",
+			Inputs:    []domain.PromptInput{{Name: "transcript", Required: true}},
+			Templates: []domain.PromptMessageTemplate{
+				{Role: "system", Content: "Speak in a {{.tone}} tone."},
+			},
+		}
+
+		_, err := renderer.Render(ctx, def, inputs, map[string]string{
+			"tone":       "concise",
+			"session_id": strings.Repeat("x", domain.SessionIDMaxLength+1),
+		})
+		if !errors.Is(err, ErrRenderFailure) {
+			t.Fatalf("expected ErrRenderFailure, got %v", err)
+		}
+	})
+
+	t.Run("inserting required input artifact", func(t *testing.T) {
+		def := &domain.PromptDefinition{
+			Inputs: []domain.PromptInput{{Name: "transcript", Required: true}},
+			Templates: []domain.PromptMessageTemplate{
+				{Role: "user", Content: "{{input \"transcript\"}}"},
+			},
+		}
+
+		res, err := renderer.Render(ctx, def, inputs, vars)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if res.Messages[0].Content != "The quick brown fox." {
+			t.Fatalf("unexpected required input rendering: %q", res.Messages[0].Content)
+		}
+	})
+
+	t.Run("optional input absent and not referenced", func(t *testing.T) {
+		def := &domain.PromptDefinition{
+			Inputs: []domain.PromptInput{
+				{Name: "transcript", Required: true},
+				{Name: "glossary", Required: false},
+			},
+			Templates: []domain.PromptMessageTemplate{
+				{Role: "user", Content: "Transcript: {{input \"transcript\"}}"},
+			},
+		}
+
+		res, err := renderer.Render(ctx, def, inputs, vars)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if len(res.Messages) != 1 {
+			t.Fatalf("expected one rendered message, got %d", len(res.Messages))
+		}
+	})
+
+	t.Run("optional input absent but referenced, expecting failure", func(t *testing.T) {
+		def := &domain.PromptDefinition{
+			Inputs: []domain.PromptInput{
+				{Name: "transcript", Required: true},
+				{Name: "glossary", Required: false},
+			},
+			Templates: []domain.PromptMessageTemplate{
+				{Role: "user", Content: "Glossary: {{input \"glossary\"}}"},
+			},
+		}
+
+		_, err := renderer.Render(ctx, def, inputs, vars)
+		if !errors.Is(err, ErrRenderFailure) {
+			t.Fatalf("expected ErrRenderFailure, got %v", err)
+		}
+		if !errors.Is(err, ErrUnknownInput) {
+			t.Fatalf("expected ErrUnknownInput, got %v", err)
+		}
+	})
+
+	t.Run("required input missing, expecting failure", func(t *testing.T) {
+		def := &domain.PromptDefinition{
+			Inputs: []domain.PromptInput{{Name: "transcript", Required: true}},
+			Templates: []domain.PromptMessageTemplate{
+				{Role: "user", Content: "Analyze this: {{input \"transcript\"}}"},
+			},
+		}
+
+		_, err := renderer.Render(ctx, def, map[string]*domain.Artifact{}, vars)
+		if !errors.Is(err, ErrMissingRequiredInput) {
+			t.Fatalf("expected ErrMissingRequiredInput, got %v", err)
+		}
+	})
+
+	t.Run("invalid template syntax", func(t *testing.T) {
+		def := &domain.PromptDefinition{
+			Inputs: []domain.PromptInput{{Name: "transcript", Required: true}},
+			Templates: []domain.PromptMessageTemplate{
+				{Role: "user", Content: "Hello {{.unclosed"},
+			},
+		}
+
+		_, err := renderer.Render(ctx, def, inputs, vars)
+		if !errors.Is(err, ErrInvalidTemplate) {
+			t.Fatalf("expected ErrInvalidTemplate, got %v", err)
+		}
+	})
+
+	t.Run("unknown input reference", func(t *testing.T) {
+		def := &domain.PromptDefinition{
+			Inputs: []domain.PromptInput{{Name: "transcript", Required: true}},
+			Templates: []domain.PromptMessageTemplate{
+				{Role: "user", Content: "Hello {{input \"ghost\"}}"},
+			},
+		}
+
+		_, err := renderer.Render(ctx, def, inputs, vars)
+		if !errors.Is(err, ErrRenderFailure) {
+			t.Fatalf("expected ErrRenderFailure, got %v", err)
+		}
+		if !errors.Is(err, ErrUnknownInput) {
+			t.Fatalf("expected ErrUnknownInput, got %v", err)
+		}
+	})
+
+	t.Run("empty message role", func(t *testing.T) {
+		def := &domain.PromptDefinition{
+			Inputs: []domain.PromptInput{{Name: "transcript", Required: true}},
+			Templates: []domain.PromptMessageTemplate{
+				{Role: "", Content: "Hello"},
+			},
+		}
+		_, err := renderer.Render(ctx, def, inputs, vars)
+		if !errors.Is(err, ErrInvalidMessageRole) {
+			t.Fatalf("expected ErrInvalidMessageRole, got %v", err)
+		}
+	})
+}

internal/promptdef/filesystem_repository.go

@@ -0,0 +1,484 @@
+package promptdef
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"fmt"
+	"io/fs"
+	"os"
+	"path"
+	"path/filepath"
+	"strings"
+
+	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
+	"gitea.maximumdirect.net/eric/promptkit/internal/filecatalog"
+	"gopkg.in/yaml.v3"
+)
+
+var (
+	ErrPromptDefinitionNotFound = errors.New("prompt definition not found")
+	ErrInvalidYAML              = errors.New("invalid YAML format")
+	ErrInvalidPromptDefinition  = errors.New("invalid prompt definition configuration")
+)
+
+type filesystemRepository struct {
+	dir string
+}
+
+type fsRepository struct {
+	fsys fs.FS
+	root string
+}
+
+type promptDefinitionFile struct {
+	ID             string                   `yaml:"id"`
+	Version        string                   `yaml:"version"`
+	DefaultProfile *string                  `yaml:"default_profile"`
+	Description    string                   `yaml:"description"`
+	SessionID      string                   `yaml:"session_id"`
+	Inputs         []promptInputFile        `yaml:"inputs"`
+	Messages       []promptMessageFile      `yaml:"messages"`
+	Output         promptOutputContractFile `yaml:"output"`
+}
+
+type promptInputFile struct {
+	Name        string `yaml:"name"`
+	Required    bool   `yaml:"required"`
+	ContentType string `yaml:"content_type"`
+	Description string `yaml:"description"`
+}
+
+type promptMessageFile struct {
+	Role         string            `yaml:"role"`
+	Content      string            `yaml:"content"`
+	ContentFile  string            `yaml:"content_file"`
+	CacheControl *cacheControlFile `yaml:"cache_control"`
+}
+
+type cacheControlFile struct {
+	Type string `yaml:"type"`
+	TTL  string `yaml:"ttl"`
+}
+
+type promptOutputContractFile struct {
+	Format         domain.OutputFormat   `yaml:"format"`
+	ValidationMode domain.ValidationMode `yaml:"validation_mode"`
+	SchemaPath     string                `yaml:"schema_path"`
+	RepairAttempts int                   `yaml:"repair_attempts"`
+}
+
+func NewFilesystemRepository(dir string) Repository {
+	return &filesystemRepository{dir: dir}
+}
+
+func NewFSRepository(fsys fs.FS, root string) Repository {
+	return &fsRepository{fsys: fsys, root: root}
+}
+
+func (r *filesystemRepository) GetPromptDefinition(ctx context.Context, id string, version string) (*domain.PromptDefinition, error) {
+	if strings.TrimSpace(id) == "" {
+		return nil, fmt.Errorf("%w: prompt id is required", ErrInvalidPromptDefinition)
+	}
+
+	files, err := filecatalog.FindYAMLFiles(ctx, r.dir)
+	if err != nil {
+		return nil, fmt.Errorf("failed to read prompt definition directory: %w", err)
+	}
+
+	var matches []promptDefinitionMatch
+	for _, fullPath := range files {
+		select {
+		case <-ctx.Done():
+			return nil, ctx.Err()
+		default:
+		}
+
+		relPath := filecatalog.RelativePath(r.dir, fullPath)
+		fileMatch := filecatalog.Stem(filepath.Base(fullPath)) == id
+
+		raw, err := loadPromptDefinitionFile(fullPath)
+		if err != nil {
+			if fileMatch || promptDefinitionFileHasID(fullPath, id) {
+				return nil, fmt.Errorf("%w: %s: %v", ErrInvalidYAML, relPath, err)
+			}
+			continue
+		}
+
+		def, err := normalizePromptDefinition(raw, fullPath)
+		if err != nil {
+			if fileMatch || strings.TrimSpace(raw.ID) == id {
+				return nil, fmt.Errorf("%w: %s: %v", ErrInvalidPromptDefinition, relPath, err)
+			}
+			continue
+		}
+
+		if def.ID != id {
+			continue
+		}
+		if version != "" && def.Version != version {
+			continue
+		}
+		matches = append(matches, promptDefinitionMatch{
+			def:  def,
+			path: relPath,
+		})
+	}
+
+	if len(matches) > 1 {
+		paths := make([]string, 0, len(matches))
+		for _, match := range matches {
+			paths = append(paths, match.path)
+		}
+		if version != "" {
+			return nil, fmt.Errorf("%w: duplicate prompt definition id %q version %q found in: %s", ErrInvalidPromptDefinition, id, version, strings.Join(paths, ", "))
+		}
+		return nil, fmt.Errorf("%w: duplicate prompt definition id %q found in: %s", ErrInvalidPromptDefinition, id, strings.Join(paths, ", "))
+	}
+
+	if len(matches) == 1 {
+		return matches[0].def, nil
+	}
+
+	return nil, ErrPromptDefinitionNotFound
+}
+
+func (r *fsRepository) GetPromptDefinition(ctx context.Context, id string, version string) (*domain.PromptDefinition, error) {
+	return loadPromptDefinition(ctx, r.fsys, r.root, id, version)
+}
+
+type promptDefinitionMatch struct {
+	def  *domain.PromptDefinition
+	path string
+}
+
+func loadPromptDefinitionFile(path string) (*promptDefinitionFile, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return nil, fmt.Errorf("failed to read prompt definition file: %w", err)
+	}
+
+	var raw promptDefinitionFile
+	decoder := yaml.NewDecoder(bytes.NewReader(data))
+	decoder.KnownFields(true)
+	if err := decoder.Decode(&raw); err != nil {
+		return nil, err
+	}
+	return &raw, nil
+}
+
+func promptDefinitionFileHasID(path string, id string) bool {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return false
+	}
+	var raw struct {
+		ID string `yaml:"id"`
+	}
+	if err := yaml.NewDecoder(bytes.NewReader(data)).Decode(&raw); err != nil {
+		return false
+	}
+	return strings.TrimSpace(raw.ID) == id
+}
+
+func loadPromptDefinition(ctx context.Context, fsys fs.FS, root string, id string, version string) (*domain.PromptDefinition, error) {
+	if strings.TrimSpace(id) == "" {
+		return nil, fmt.Errorf("%w: prompt id is required", ErrInvalidPromptDefinition)
+	}
+	if fsys == nil {
+		return nil, fmt.Errorf("failed to read prompt definition directory: filesystem is nil")
+	}
+
+	files, err := filecatalog.FindFSYAMLFiles(ctx, fsys, root)
+	if err != nil {
+		return nil, fmt.Errorf("failed to read prompt definition directory: %w", err)
+	}
+	cleanRoot := filecatalog.CleanFSRoot(root)
+	rootInfo, err := fs.Stat(fsys, cleanRoot)
+	if err != nil {
+		return nil, fmt.Errorf("failed to read prompt definition directory: %w", err)
+	}
+
+	var matches []promptDefinitionMatch
+	for _, fullPath := range files {
+		select {
+		case <-ctx.Done():
+			return nil, ctx.Err()
+		default:
+		}
+
+		relPath := filecatalog.DisplayPath(root, fullPath)
+		fileMatch := filecatalog.Stem(path.Base(fullPath)) == id
+		data, err := fs.ReadFile(fsys, fullPath)
+		if err != nil {
+			if fileMatch {
+				return nil, fmt.Errorf("%w: %s: failed to read prompt definition file: %v", ErrInvalidYAML, relPath, err)
+			}
+			continue
+		}
+
+		raw, err := decodePromptDefinition(data)
+		if err != nil {
+			if fileMatch || promptDefinitionDataHasID(data, id) {
+				return nil, fmt.Errorf("%w: %s: %v", ErrInvalidYAML, relPath, err)
+			}
+			continue
+		}
+
+		def, err := normalizePromptDefinitionFromFS(raw, fsys, root, fullPath, rootInfo.IsDir())
+		if err != nil {
+			if fileMatch || strings.TrimSpace(raw.ID) == id {
+				return nil, fmt.Errorf("%w: %s: %v", ErrInvalidPromptDefinition, relPath, err)
+			}
+			continue
+		}
+
+		if def.ID != id {
+			continue
+		}
+		if version != "" && def.Version != version {
+			continue
+		}
+		matches = append(matches, promptDefinitionMatch{
+			def:  def,
+			path: relPath,
+		})
+	}
+
+	if len(matches) > 1 {
+		paths := make([]string, 0, len(matches))
+		for _, match := range matches {
+			paths = append(paths, match.path)
+		}
+		if version != "" {
+			return nil, fmt.Errorf("%w: duplicate prompt definition id %q version %q found in: %s", ErrInvalidPromptDefinition, id, version, strings.Join(paths, ", "))
+		}
+		return nil, fmt.Errorf("%w: duplicate prompt definition id %q found in: %s", ErrInvalidPromptDefinition, id, strings.Join(paths, ", "))
+	}
+
+	if len(matches) == 1 {
+		return matches[0].def, nil
+	}
+
+	return nil, ErrPromptDefinitionNotFound
+}
+
+func decodePromptDefinition(data []byte) (*promptDefinitionFile, error) {
+	var raw promptDefinitionFile
+	decoder := yaml.NewDecoder(bytes.NewReader(data))
+	decoder.KnownFields(true)
+	if err := decoder.Decode(&raw); err != nil {
+		return nil, err
+	}
+	return &raw, nil
+}
+
+func promptDefinitionDataHasID(data []byte, id string) bool {
+	var raw struct {
+		ID string `yaml:"id"`
+	}
+	if err := yaml.NewDecoder(bytes.NewReader(data)).Decode(&raw); err != nil {
+		return false
+	}
+	return strings.TrimSpace(raw.ID) == id
+}
+
+func normalizePromptDefinition(raw *promptDefinitionFile, sourcePath string) (*domain.PromptDefinition, error) {
+	promptDir := filepath.Dir(sourcePath)
+	return normalizePromptDefinitionWithContent(raw, func(contentFile string) (string, string, error) {
+		resolvedPath := strings.TrimSpace(contentFile)
+		if !filepath.IsAbs(resolvedPath) {
+			resolvedPath = filepath.Join(promptDir, resolvedPath)
+		}
+		resolvedPath = filepath.Clean(resolvedPath)
+
+		body, err := os.ReadFile(resolvedPath)
+		if err != nil {
+			return "", "", err
+		}
+		return string(body), resolvedPath, nil
+	})
+}
+
+func normalizePromptDefinitionFromFS(raw *promptDefinitionFile, fsys fs.FS, root string, sourcePath string, rootIsDir bool) (*domain.PromptDefinition, error) {
+	promptDir := path.Dir(sourcePath)
+	return normalizePromptDefinitionWithContent(raw, func(contentFile string) (string, string, error) {
+		var resolvedPath string
+		if rootIsDir {
+			var err error
+			resolvedPath, _, err = filecatalog.ResolveFSPath(root, promptDir, contentFile)
+			if err != nil {
+				return "", "", err
+			}
+		} else {
+			resolvedPath = strings.TrimSpace(contentFile)
+			if !path.IsAbs(resolvedPath) {
+				resolvedPath = path.Join(promptDir, resolvedPath)
+			}
+			resolvedPath = strings.TrimPrefix(path.Clean(resolvedPath), "/")
+		}
+
+		body, err := fs.ReadFile(fsys, resolvedPath)
+		if err != nil {
+			return "", "", err
+		}
+		return string(body), resolvedPath, nil
+	})
+}
+
+func normalizePromptDefinitionWithContent(raw *promptDefinitionFile, readContentFile func(string) (string, string, error)) (*domain.PromptDefinition, error) {
+	if raw == nil {
+		return nil, errors.New("prompt definition is nil")
+	}
+
+	id := strings.TrimSpace(raw.ID)
+	if id == "" {
+		return nil, errors.New("id is required")
+	}
+
+	version := strings.TrimSpace(raw.Version)
+	if version == "" {
+		return nil, errors.New("version is required")
+	}
+
+	if len(raw.Messages) == 0 {
+		return nil, errors.New("at least one message is required")
+	}
+
+	inputs := make([]domain.PromptInput, 0, len(raw.Inputs))
+	seenInputNames := make(map[string]struct{}, len(raw.Inputs))
+	for i, in := range raw.Inputs {
+		name := strings.TrimSpace(in.Name)
+		if name == "" {
+			return nil, fmt.Errorf("input %d has empty name", i)
+		}
+		if _, exists := seenInputNames[name]; exists {
+			return nil, fmt.Errorf("duplicate input name %q", name)
+		}
+		seenInputNames[name] = struct{}{}
+
+		inputs = append(inputs, domain.PromptInput{
+			Name:        name,
+			Required:    in.Required,
+			ContentType: strings.TrimSpace(in.ContentType),
+			Description: strings.TrimSpace(in.Description),
+		})
+	}
+
+	templates := make([]domain.PromptMessageTemplate, 0, len(raw.Messages))
+	for i, msg := range raw.Messages {
+		role := strings.TrimSpace(msg.Role)
+		if role == "" {
+			return nil, fmt.Errorf("message %d role is required", i)
+		}
+
+		hasContent := strings.TrimSpace(msg.Content) != ""
+		hasContentFile := strings.TrimSpace(msg.ContentFile) != ""
+		if hasContent == hasContentFile {
+			return nil, fmt.Errorf("message %d (%s) must set exactly one of content or content_file", i, role)
+		}
+
+		cacheControl, err := normalizeCacheControl(msg.CacheControl)
+		if err != nil {
+			return nil, fmt.Errorf("message %d (%s) cache_control: %w", i, role, err)
+		}
+
+		templateContent := msg.Content
+		resolvedContentFile := ""
+		if hasContentFile {
+			body, resolvedPath, err := readContentFile(msg.ContentFile)
+			if err != nil {
+				return nil, fmt.Errorf("prompt %q message %d (%s): failed to read content_file %q: %w", id, i, role, msg.ContentFile, err)
+			}
+			templateContent = body
+			resolvedContentFile = resolvedPath
+		}
+
+		templates = append(templates, domain.PromptMessageTemplate{
+			Role:         role,
+			Content:      templateContent,
+			ContentFile:  resolvedContentFile,
+			CacheControl: cacheControl,
+		})
+	}
+
+	if !isValidOutputFormat(raw.Output.Format) {
+		return nil, fmt.Errorf("invalid output format: %q", raw.Output.Format)
+	}
+	if !isValidValidationMode(raw.Output.ValidationMode) {
+		return nil, fmt.Errorf("invalid validation mode: %q", raw.Output.ValidationMode)
+	}
+	if raw.Output.ValidationMode == domain.ValidationJSONSchema && strings.TrimSpace(raw.Output.SchemaPath) == "" {
+		return nil, errors.New("output.schema_path is required when output.validation_mode is json_schema")
+	}
+	if raw.Output.RepairAttempts < 0 {
+		return nil, errors.New("output.repair_attempts must be greater than or equal to 0")
+	}
+
+	defaultProfile := ""
+	if raw.DefaultProfile != nil {
+		defaultProfile = strings.TrimSpace(*raw.DefaultProfile)
+		if defaultProfile == "" {
+			return nil, errors.New("default_profile must be a non-empty string when set")
+		}
+	}
+
+	return &domain.PromptDefinition{
+		ID:             id,
+		Version:        version,
+		DefaultProfile: defaultProfile,
+		Description:    strings.TrimSpace(raw.Description),
+		SessionID:      strings.TrimSpace(raw.SessionID),
+		Inputs:         inputs,
+		Templates:      templates,
+		OutputFormat:   raw.Output.Format,
+		Validation: domain.OutputContract{
+			Format:         raw.Output.Format,
+			ValidationMode: raw.Output.ValidationMode,
+			SchemaPath:     strings.TrimSpace(raw.Output.SchemaPath),
+			RepairAttempts: raw.Output.RepairAttempts,
+		},
+	}, nil
+}
+
+func normalizeCacheControl(raw *cacheControlFile) (*domain.CacheControl, error) {
+	if raw == nil {
+		return nil, nil
+	}
+
+	cacheType := strings.TrimSpace(raw.Type)
+	if cacheType == "" {
+		return nil, errors.New("type is required")
+	}
+	if domain.CacheControlType(cacheType) != domain.CacheControlEphemeral {
+		return nil, fmt.Errorf("unsupported type %q", cacheType)
+	}
+
+	ttl := strings.TrimSpace(raw.TTL)
+	if ttl != "" && ttl != "1h" {
+		return nil, fmt.Errorf("unsupported ttl %q", ttl)
+	}
+
+	return &domain.CacheControl{
+		Type: domain.CacheControlType(cacheType),
+		TTL:  ttl,
+	}, nil
+}
+
+func isValidOutputFormat(f domain.OutputFormat) bool {
+	switch f {
+	case domain.FormatText, domain.FormatMarkdown, domain.FormatJSON:
+		return true
+	default:
+		return false
+	}
+}
+
+func isValidValidationMode(m domain.ValidationMode) bool {
+	switch m {
+	case domain.ValidationNone, domain.ValidationBasic, domain.ValidationJSON, domain.ValidationJSONSchema:
+		return true
+	default:
+		return false
+	}
+}