Prepare documentation for Promptkit v0.4.0

README.md

@@ -33,6 +33,9 @@ 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).
 

backends.go

@@ -40,12 +40,12 @@ type Backend struct {
 	// 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 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 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
 }
 

capacity_contract_test.go

@@ -60,7 +60,18 @@ func TestEngineRejectsRunBeforeCompletionWhenAdmissionIsFull(t *testing.T) {
 
 	awaitCapacitySignal(t, reader.entered, "first artifact read")
 
-	result, err := engine.Run(context.Background(), capacityInputRequest("http://second.example/v1"))
+	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)
 	}
@@ -70,6 +81,21 @@ func TestEngineRejectsRunBeforeCompletionWhenAdmissionIsFull(t *testing.T) {
 	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)
 	}
@@ -174,6 +200,19 @@ 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,
@@ -181,7 +220,8 @@ func TestCapacityExceededSentinelContract(t *testing.T) {
 		promptkit.ErrValidation,
 	} {
 		if errors.Is(promptkit.ErrCapacityExceeded, unrelated) ||
-			errors.Is(unrelated, promptkit.ErrCapacityExceeded) {
+			errors.Is(unrelated, promptkit.ErrCapacityExceeded) ||
+			errors.Is(populatedCapacityErr, unrelated) {
 			t.Fatalf("ErrCapacityExceeded aliases unrelated sentinel %v", unrelated)
 		}
 	}

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

@@ -170,6 +170,40 @@ func fromDomainExecutionTarget(target domain.ExecutionTarget) ExecutionTarget {
 	}
 }
 
+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,

doc.go

@@ -3,23 +3,28 @@
 //
 // Applications construct an [Engine] with [NewEngine], select filesystem or
 // in-memory sources and optional engine-scoped [Backend] registrations, and
-// call [Engine.Prepare] or [Engine.Run]. Concrete registries, repositories,
-// validators, and the built-in OpenAI-compatible client remain internal
-// implementation details.
+// 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 Prepare and Run calls. Engine-local backend
-// policies bound admitted Run 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.
+// 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 and Run
-// copy request maps, slices, pointer values, and JSON-compatible extra
-// parameters before using them. 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.
+// 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
 //
@@ -45,10 +50,12 @@
 // [GenerateResponse], [ExecutionTargetPresence], and the string value types
 // used by those values.
 //
-// Construction values, including [Config], [Backend], [RunRequest],
-// [ArtifactRef], [ExecutionTargetOverride], [Profile], and
-// [OpenAICompatibleProfileConfig], do not have stable JSON representations.
-// Direct API keys are nevertheless excluded from JSON for every public value.
+// 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

docs/consumers/pkg-promptkit.md

@@ -40,11 +40,37 @@ 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:
+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{
@@ -61,12 +87,48 @@ 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. A completed content check may return `ValidationFailed` in the result;
-an operational inability to validate returns an error.
+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
@@ -97,6 +159,34 @@ 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
@@ -287,6 +377,11 @@ When a limited backend has admitted all active and waiting calls, handle
 ```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.
 }
 ```
@@ -294,8 +389,9 @@ if errors.Is(err, promptkit.ErrCapacityExceeded) {
 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
-[`Engine.Run` and error GoDoc](../../engine.go) owns exact error and
-cancellation identities.
+[`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
 

docs/formats.md

@@ -58,6 +58,11 @@ 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:
@@ -152,7 +157,7 @@ extra_params:
 | 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. |
+| `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. |
@@ -219,7 +224,9 @@ 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.
+`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:
 
@@ -231,6 +238,7 @@ 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
 

docs/internal/capacity.md

@@ -29,21 +29,28 @@ 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 Run Admission
+## Bounded Execution Admission
 
-The runner asks the manager to admit a run after resolving the prompt, profile,
-selected backend, effective execution target, credentials, and output contract,
-but before schema loading, artifact loading, or rendering. Admission is
-immediate: a limited pool either reserves a slot or returns the internal
-`ErrCapacityExceeded` identity. The root facade maps that identity to the
-public error without treating it as an invalid request or generation failure.
+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 and holds the lease across remaining
-preparation, initial generation, validation, every repair attempt, and all
-failure or cancellation exits. A repair is part of its original admission and
-does not reserve another bounded slot.
+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
 
@@ -90,11 +97,17 @@ unlimited admission. The
 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 early admission,
-lease lifetime, failure release, and shared initial/repair scheduling. The
+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

@@ -43,6 +43,21 @@ 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

docs/internal/overview.md

@@ -11,23 +11,23 @@ contributor workflow and validation.
 
 | Component | Implemented responsibility | References |
 | --- | --- | --- |
-| Root `promptkit` package | Provides the supported engine facade, source, backend-registration, and injection options, public request and result values, profile construction, extension interfaces, value conversion, redacted formatting, public error mapping, and engine-local assembly. | [Package GoDoc](../../doc.go), [backend API](../../backends.go), [engine assembly](../../engine.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 run admission and FIFO model-generation permits for limited backend IDs, including cancellation-safe waiter removal and client wrapping. | [Internal capacity management](capacity.md) |
+| `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 trees while preserving supported concrete value types. | [JSON values](../../internal/jsonvalue/jsonvalue.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. | [Framework formats](../formats.md#schemas), [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 backend, profile, and request settings and coordinates preparation and execution across internal sources, rendering, artifact loading, generation, validation, and optional repair. | [Internal runner](runner.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 package assembles these internal components without exposing their
 representations. Consumers depend only on the root facade.

docs/internal/runner.md

@@ -28,6 +28,34 @@ 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
@@ -124,15 +152,17 @@ 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 and adds
-the selected backend ID as context. 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.
+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.

docs/internal/sources.md

@@ -16,6 +16,11 @@ validation modes, built-in catalog, and source precedence.
 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).
@@ -28,7 +33,12 @@ 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.
+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
@@ -68,6 +78,22 @@ 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, and
-content-failure behavior.
+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/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/concurrency.md

@@ -1,237 +0,0 @@
-# Backend-Specific Concurrency Management
-
-**Status:** Complete.
-
-## Purpose
-
-This roadmap defines the scope and target end state for engine-local,
-backend-specific concurrency management. It records the intended capability,
-consumer value, and important policy choices.
-
-This document is planning material, not a description of current behavior.
-Current exported contracts remain owned by Go declarations and GoDoc, backend
-registration guidance by the
-[consumer guide](../consumers/pkg-promptkit.md#configure-a-local-openai-compatible-endpoint),
-and
-implemented orchestration by the
-[internal runner document](../internal/runner.md).
-
-## Motivation
-
-Different model backends can sustain very different request loads. A local
-network endpoint may need a small concurrency limit, while OpenRouter can
-usually accept substantially more simultaneous work. Requiring every consumer
-to build its own semaphores and queues would duplicate routing knowledge,
-create inconsistent cancellation behavior, and make it easy for one caller to
-bypass the intended backend limit.
-
-Promptkit should own this coordination because it already resolves each run to
-an engine-scoped backend identity and owns every model-generation call made by
-the runner. Consumers should continue submitting ready-to-run requests through
-the synchronous API, including concurrently from multiple goroutines, without
-implementing their own backend scheduler.
-
-The buffered queue is a safety boundary, not an ordinary throughput
-restriction. Its primary purpose is to prevent a bug or unintended submission
-loop from creating an unbounded in-memory backlog.
-
-## Scope
-
-The feature will add optional concurrency policy to registered backends and
-coordinate `Run` calls against independent per-backend capacity pools.
-
-Each policy has two distinct controls:
-
-- an active-generation limit, which protects the backend from too many
-  simultaneous model requests; and
-- a bounded waiting capacity, which protects the process from admitting an
-  unbounded backlog.
-
-Concurrency policy belongs to a backend registration. It is not a profile
-model parameter and cannot be overridden per run. Profiles select the policy
-through their backend ID, while a profile or request endpoint override remains
-in the selected backend's pool.
-
-`Prepare` does not call a model and will remain outside concurrency admission.
-
-## Defaults And Configuration
-
-The built-in OpenRouter backend will use:
-
-- an active-generation limit of 16; and
-- a waiting capacity of 1024.
-
-The waiting default is intentionally generous. Reaching it should indicate
-abnormal submission pressure rather than normal application behavior.
-
-Consumer-registered backends will remain unlimited unless the consumer
-configures an active-generation limit. When a consumer enables a limit and
-does not specify waiting capacity, the waiting capacity will default to 1024.
-Consumers may configure a different bounded capacity, including zero when
-they want no admitted backlog beyond the active-limit-sized run set.
-
-The public representation must distinguish an omitted waiting capacity from
-an explicit zero.
-
-Endpoint-only profiles have no backend registration from which to obtain
-policy and will remain unlimited. A future engine-wide or endpoint-keyed
-policy can be considered separately if consumers demonstrate that need.
-
-Invalid limits or capacities will fail engine construction as invalid
-configuration. Policy values will be copied into engine-owned immutable state
-along with the rest of the backend registration.
-
-## Admission And Execution Behavior
-
-`Run` remains a synchronous, wait-for-result operation. Concurrent callers may
-block inside `Run` while waiting for their selected backend, then receive the
-ordinary result or error from that invocation.
-
-For a configured pool, the active-generation limit plus the waiting capacity
-defines the maximum number of concurrent `Run` invocations that Promptkit will
-accept for that backend. A waiting capacity of zero therefore accepts no more
-runs than the active limit. Admission is immediate: a call either reserves one
-of those bounded slots or receives the capacity error. An accepted run may
-then wait internally for active-generation capacity.
-
-For a limited backend, Promptkit will bound the number of accepted runs before
-expensive artifact loading, prompt rendering, and large defensive copies where
-practical. Lightweight prompt, profile, and backend resolution may occur first
-when it is required to identify the correct capacity pool. This pre-admission
-resolution must not become a second execution-precedence path with behavior
-that can drift from `Prepare`.
-
-An accepted run retains its admission until it completes or fails. Every
-actual model-generation call for that run must separately observe the
-backend's active-generation limit. This includes:
-
-- the initial generation;
-- every output-repair generation; and
-- calls made through either the built-in or an injected model client.
-
-Preparation and output validation should not hold an active-generation permit.
-A repair remains part of its already-admitted run, but reacquires active
-generation capacity so repairs cannot exceed the backend limit. It must not be
-rejected merely because new runs filled the waiting queue after its initial
-generation.
-
-Within one backend pool, waiting generation calls should be served in FIFO
-order, subject to canceled calls being removed. Different backend pools make
-progress independently; a saturated local backend must not consume
-OpenRouter's active or waiting capacity.
-
-The feature will not promise ordering across backend pools or completion order
-among admitted runs.
-
-## Capacity Failure And Cancellation
-
-When a backend's bounded waiting capacity is full, a new `Run` call will fail
-promptly rather than waiting outside the bounded admission system. The public
-API will expose a recognizable capacity-exhaustion error identity distinct
-from invalid configuration, invalid requests, and model-client failures.
-Rejected calls return no partial result and do not invoke the model client.
-
-Waiting within the admitted backlog or for active-generation capacity must
-honor the caller's context. Cancellation or deadline expiry will:
-
-- stop waiting promptly;
-- release any admission or generation capacity held by that invocation;
-- preserve the applicable context error identity; and
-- avoid invoking the model client if cancellation wins before generation
-  starts.
-
-Capacity must also be released after preparation, generation, validation,
-repair, or collaborator failure. One failed or canceled run must not reduce
-the backend's future usable capacity.
-
-Elapsed `Run` timing will include time spent waiting after the call is
-accepted. `PreparedRun` timing will continue to describe preparation rather
-than queue waiting.
-
-## Engine And Client Boundaries
-
-All pools and queued state belong to one `Engine`. Separate engines do not
-share capacity, even when they register the same backend ID or endpoint. The
-feature introduces no process-global scheduler.
-
-The engine will apply policy consistently to the built-in model client and an
-injected `LLMClient`. Consumers calling their own client outside Promptkit are
-outside this boundary. Injected clients remain responsible for their internal
-thread safety and cancellation behavior.
-
-Backend policy is keyed by the resolved backend ID rather than endpoint text.
-This preserves stable routing when a selected backend's endpoint is overridden
-and avoids accidentally combining unrelated registrations that happen to use
-the same URL.
-
-## Queue Lifetime And Observability
-
-Admission state is buffered, ephemeral, and in-process. It is not persisted
-and has no survival guarantee across engine disposal or process termination.
-Promptkit will not introduce background job ownership or require consumers to
-start or stop workers.
-
-The initial feature does not require public queue-depth metrics, callbacks, or
-inspection APIs. Capacity errors and ordinary call timing provide the
-consumer-visible behavior. Operational observability can be added later
-without coupling the scheduling mechanism to an application logging or
-metrics system.
-
-## Compatibility
-
-Consumer-registered backends and endpoint-only profiles remain unlimited
-unless concurrency is explicitly configured, preserving their existing
-behavior.
-
-The built-in OpenRouter backend will change from unlimited concurrency to a
-limit of 16 with a bounded waiting capacity of 1024. Ordinary synchronous
-calls remain unchanged, while unusually high concurrent use may now wait or
-return the capacity error. This behavioral change must be identified in the
-release notes for the version that publishes it.
-
-Adding backend policy fields and a public capacity error is otherwise
-additive. The change will use a pre-`v1` minor release under Promptkit's
-[release policy](../release.md#release-model).
-
-## Non-Goals
-
-This scope does not include:
-
-- asynchronous job handles, polling, or detached result delivery;
-- durable or cross-process queues;
-- persistence or recovery across engine or process shutdown;
-- priorities, scheduling weights, or consumer-defined fairness classes;
-- automatic retries, backoff, rate-limit interpretation, or provider quota
-  discovery;
-- token-per-minute or request-per-minute rate limiting;
-- dynamic reconfiguration after engine construction;
-- per-profile or per-run concurrency overrides;
-- endpoint-keyed pooling for profiles without a backend ID;
-- process-global coordination across engines;
-- application worker lifecycle, logging, tracing, or metrics policy; or
-- changes to prompt, profile, schema, or model-provider wire formats.
-
-## Target End State
-
-This roadmap reaches its target end state when:
-
-- each engine independently coordinates configured backend capacity;
-- the built-in OpenRouter backend allows 16 active generations and up to 1024
-  waiting runs;
-- consumer backends can opt into their own active and waiting limits while
-  remaining unlimited by default;
-- endpoint overrides retain the selected backend's capacity pool and
-  endpoint-only profiles remain unlimited;
-- synchronous `Run` callers wait for and receive their ordinary result;
-- admission is bounded before expensive preparation work where practical;
-- every initial and repair generation observes the backend's active limit
-  without serializing preparation or validation;
-- a full waiting queue returns a recognizable capacity error without invoking
-  the model client;
-- cancellation and all failure paths promptly release capacity and preserve
-  context error identity;
-- built-in and injected model clients receive the same scheduling behavior;
-- pools remain ephemeral, engine-scoped, and independent across backend IDs;
-  and
-- current-state GoDoc, consumer, internal, and release documentation describe
-  the implemented behavior once it lands.

docs/roadmap/future.md

@@ -33,9 +33,7 @@ consumers.
 
 ## Ideas
 
-No ideas are currently cataloged. Backend-specific concurrency management has
-been selected for active planning in the
-[focused concurrency roadmap](concurrency.md).
+No ideas currently await selection.
 
 ## Entry Format
 

docs/roadmap/implementation.md

@@ -1,386 +0,0 @@
-# Local Backend Convenience Implementation Plan
-
-**Status:** Complete.
-
-## Purpose
-
-This document is the decision-complete implementation plan for
-[local backend convenience](local-backend.md). It is written for a coding
-agent that will implement each stage in order.
-
-The feature roadmap owns the motivation, consumer paths, policy choices,
-compatibility requirements, non-goals, and target end state. This document
-owns the exact proposed API, file-level changes, implementation sequence, test
-ownership, documentation work, validation commands, and completion gates.
-
-## Implementation Rules
-
-- Complete the stages in order. Keep the root package compiling and its
-  focused tests passing at every stage boundary.
-- Preserve unrelated working-tree changes. The feature roadmap may already be
-  uncommitted when implementation begins; retain it.
-- Follow every policy under `docs/policy/`, the task-specific reading guide in
-  `docs/development.md`, and the accepted behavior in
-  `local-backend.md`.
-- Keep the API in the root `promptkit` package. Do not add a public or internal
-  package for this feature.
-- Implement the helper as a transparent constructor for the existing
-  `Backend` type. Do not add a second backend representation or bypass
-  `WithBackend`.
-- Leave normalization, validation, copying, registry construction, capacity
-  defaulting, and duplicate detection in their existing owners.
-- Do not add environment-variable discovery, package-global registration,
-  implicit local defaults, model selection, profile construction, or support
-  for non-OpenAI-compatible transports.
-- Keep tests lean and behavior-focused. Do not duplicate the existing
-  registry and concurrency test matrices merely because the constructor
-  reaches those mechanisms.
-- Update exact GoDoc with the exported declarations. Update current-state
-  consumer guidance only after the corresponding API is implemented.
-- Do not update release notes, create a release, change a module version, or
-  tag a commit as part of this work.
-
-## Fixed Design
-
-### Exported API
-
-Add these declarations to `backends.go` in package `promptkit`:
-
-```go
-// BackendLocal is the conventional ID used by LocalBackend. It is not a
-// built-in or reserved backend and must be registered with WithBackend.
-const BackendLocal = "local"
-
-// LocalBackend returns a Backend for a conventional local OpenAI-compatible
-// endpoint.
-func LocalBackend(endpoint string, concurrencyLimit int) Backend {
-	return Backend{
-		ID:               BackendLocal,
-		Endpoint:         endpoint,
-		ConcurrencyLimit: concurrencyLimit,
-	}
-}
-```
-
-The exact GoDoc may be wrapped or expanded for clarity, but it must own and
-communicate all of these contract points:
-
-- `BackendLocal` is the case-sensitive conventional ID `"local"`;
-- it is neither built in nor reserved;
-- calling `LocalBackend` does not register anything;
-- the returned value must be supplied through `WithBackend`;
-- `endpoint` and `concurrencyLimit` are copied into the corresponding fields
-  without normalization or validation;
-- `APIKeyEnv`, `ExtraParams`, and `QueueCapacity` retain their zero values; and
-- normal `NewEngine` backend validation and concurrency semantics apply after
-  registration.
-
-Keep `BackendOpenRouter` unchanged. `BackendLocal` must not alias an internal
-registry constant because the internal registry has no special local-backend
-identity or behavior.
-
-Place `BackendLocal` near `BackendOpenRouter`, and place `LocalBackend` after
-the `Backend` declaration and before `WithBackend`. This keeps the conventional
-IDs, configured value, convenience constructor, and registration option
-discoverable in one file.
-
-### Constructor Semantics
-
-`LocalBackend` is a pure struct constructor. Its complete behavior is
-equivalent to the keyed literal shown above.
-
-In particular, the constructor must not:
-
-- trim or parse the endpoint;
-- reject blank endpoints or negative limits;
-- choose a default limit;
-- assign an API-key environment variable;
-- allocate an empty `ExtraParams` map;
-- assign a queue-capacity pointer;
-- read process environment;
-- mutate package-global or engine state; or
-- call `WithBackend` itself.
-
-Deferred validation is intentional. It keeps one validation path for all
-`Backend` values: `WithBackend` copies the public value into engine options,
-and `NewEngine` constructs the validated immutable registry. A positive limit
-with a nil queue continues to select the existing default queue capacity of
-1024; zero continues to mean unlimited; a negative value continues to make
-`NewEngine` fail with `ErrInvalidConfig`.
-
-The returned `Backend` remains an ordinary caller-owned value. Consumers can
-modify it before passing it to `WithBackend`, although task-oriented
-documentation should direct materially customized configurations to an
-explicit keyed `Backend` literal.
-
-### Identity And Compatibility
-
-Do not add `"local"` to the internal built-in or reserved ID set. A consumer
-must be able to register either:
-
-```go
-promptkit.LocalBackend(endpoint, limit)
-```
-
-or:
-
-```go
-promptkit.Backend{
-	ID:       promptkit.BackendLocal,
-	Endpoint: endpoint,
-}
-```
-
-through the existing `WithBackend` option. Both forms participate in ordinary
-duplicate-ID detection. Existing consumers that already use the literal ID
-`"local"` remain source- and behavior-compatible.
-
-No existing `Backend`, `WithBackend`, profile, registry, execution-target, or
-capacity semantics change. Do not modify `internal/backend`,
-`internal/capacity`, `internal/domain`, `engine.go`, `profiles.go`, or
-`types.go` for this feature.
-
-### Test Ownership
-
-The root external-package contract suite in `public_contract_test.go` owns the
-new public behavior. Add one focused test named:
-
-```go
-func TestLocalBackendConstructsAndRegistersConventionalBackend(t *testing.T)
-```
-
-The test must:
-
-1. call `promptkit.LocalBackend` with a test endpoint and a positive,
-   test-owned concurrency limit;
-2. compare the returned value with this complete expected value:
-
-   ```go
-   promptkit.Backend{
-       ID:               promptkit.BackendLocal,
-       Endpoint:         endpoint,
-       ConcurrencyLimit: limit,
-   }
-   ```
-
-   A whole-value comparison is appropriate here because the exact zero-value
-   fields are part of this small public constructor's contract.
-3. register that returned value with `WithBackend`;
-4. add an in-memory profile whose `BackendID` is
-   `promptkit.BackendLocal`;
-5. construct an engine through the existing contract-test prompt fixture;
-6. call `Prepare`; and
-7. assert that the prepared result exposes `BackendLocal` as the selected
-   backend and the supplied endpoint as the effective endpoint.
-
-This single test protects the realistic compatibility risks: accidental field
-defaults, a changed conventional ID, failure to compose with `WithBackend`,
-and accidental treatment of `"local"` as reserved. It also demonstrates that
-the helper uses the existing backend/profile path.
-
-Do not add separate tests for blank endpoints, malformed endpoints, negative
-limits, queue defaulting, duplicate IDs, engine isolation, runtime capacity,
-or caller mutation. Those mechanisms are unchanged and already have tests at
-their owning boundaries. Do not add internal-package tests for this root
-facade constructor.
-
-### Consumer Documentation
-
-Update `docs/consumers/pkg-promptkit.md` after the API exists. Keep Go
-declarations and GoDoc as the exact API owner; the guide should help consumers
-choose a workflow and link to `backends.go` for precise semantics.
-
-Restructure the backend guidance to present these paths in increasing order of
-configuration:
-
-1. **Endpoint-only profile.** Show a small in-memory `Profile` with
-   `Endpoint` and `Model`. Explain that this is the simplest choice when only
-   one profile needs the endpoint and shared backend identity or capacity
-   policy is unnecessary.
-2. **Local convenience constructor.** Show
-   `WithBackend(promptkit.LocalBackend("http://localhost:8000/v1", 2))`
-   together with a profile using
-   `BackendID: promptkit.BackendLocal`. Explain briefly that the helper is
-   explicit, is not pre-registered, does not read environment variables, and
-   leaves the queue capacity at the existing default for a positive limit.
-3. **Complete backend value.** Preserve an advanced example using a keyed
-   `Backend` literal for needs such as `APIKeyEnv`, an explicit
-   `QueueCapacity`, extra parameters, a custom ID, or multiple local
-   endpoints. Use a custom ID other than `"local"` in that example so the
-   distinction from the conventional helper is clear.
-
-Keep the existing backend-routing, selected-backend identity, concurrency,
-credential, and error guidance unless a small wording adjustment is required
-to make the new decision path coherent. Avoid repeating the complete field
-contract or registry validation rules from GoDoc.
-
-Do not add a new maintained example, README section, format-reference entry,
-integration-contract change, internal-document change, or release note. The
-consumer-guide snippets are sufficient for this small convenience API, and
-none of those other documents owns the affected task or contract.
-
-## Stage 1: Add The Public Constructor And Contract Test
-
-**Status:** Complete.
-
-### Objective
-
-Add the smallest public API that expresses the accepted local-backend
-convention and protect its compatibility through the root public boundary.
-
-### Implementation Prompt
-
-1. Re-read `docs/development.md`, all files under `docs/policy/`,
-   `local-backend.md`, `backends.go`, the backend-related portion of
-   `public_contract_test.go`, and the existing backend/concurrency GoDoc before
-   editing.
-2. Confirm the working tree and preserve the uncommitted roadmaps and any
-   unrelated consumer changes.
-3. Add the untyped exported string constant `BackendLocal = "local"` to
-   `backends.go` without changing `BackendOpenRouter`.
-4. Add `LocalBackend(endpoint string, concurrencyLimit int) Backend` to
-   `backends.go` using the exact keyed-literal implementation in the fixed
-   design.
-5. Write complete GoDoc for both declarations. Make their conventional,
-   explicit, non-built-in, non-reserved, and deferred-validation semantics
-   unambiguous.
-6. Add
-   `TestLocalBackendConstructsAndRegistersConventionalBackend` to
-   `public_contract_test.go` exactly as specified under Test Ownership. Reuse
-   the existing contract prompt fixture rather than adding a fixture or test
-   helper.
-7. Do not edit internal packages. If the constructor appears to require an
-   internal change, stop and reconcile the implementation with the fixed
-   transparent-constructor design instead.
-
-### Focused Validation
-
-Run:
-
-```sh
-gofmt -w backends.go public_contract_test.go
-go test . -run '^TestLocalBackendConstructsAndRegistersConventionalBackend$'
-go test .
-go vet .
-go build .
-git diff --check
-```
-
-Inspect the diff and confirm that this stage changes only `backends.go`,
-`public_contract_test.go`, and the already-present roadmap files.
-
-### Completion Gate
-
-Stage 1 is complete when:
-
-- the exported constant and constructor match the fixed API;
-- the constructor returns only the three specified non-zero fields;
-- the helper registers through the ordinary `WithBackend` path;
-- `"local"` remains a valid consumer registration rather than a reserved
-  built-in;
-- the focused public-contract test passes; and
-- the root package test, vet, build, formatting, and whitespace checks pass.
-
-## Stage 2: Publish Consumer Guidance And Validate The Repository
-
-**Status:** Complete.
-
-### Objective
-
-Make the simplest suitable local-endpoint configuration easy to discover,
-confirm the complete change across the repository, and close the temporary
-roadmaps.
-
-### Implementation Prompt
-
-1. Re-read the implemented declarations and GoDoc before describing them.
-2. Update `docs/consumers/pkg-promptkit.md` according to the three-path
-   structure under Consumer Documentation.
-3. Keep examples illustrative, minimal, secret-free, and consistent with the
-   implemented declarations. Link precise semantics to `backends.go` rather
-   than duplicating its field-by-field contract.
-4. Follow every added or changed Markdown link and confirm its target exists.
-   Confirm that all local repository-relative links in
-   `local-backend.md`, this plan, and the changed consumer guide resolve.
-5. Run the complete maintainer validation sequence below.
-6. Inspect the final diff for accidental internal behavior changes,
-   generated artifacts, credentials, local workspace files, or module
-   replacements.
-7. After every completion gate passes, set both stage statuses, this plan's
-   status, and the status in `local-backend.md` to `Complete`. Do not retire or
-   remove the roadmaps in the implementation change; roadmap retirement
-   follows implementation review.
-
-### Full Validation
-
-Run the complete sequence from `docs/development.md`:
-
-```sh
-go test ./...
-go test -race ./...
-go vet ./...
-go build ./...
-go run ./examples/go-library/prepare
-gofmt -l $(git ls-files '*.go')
-git diff --check
-```
-
-The formatting command must produce no paths. Follow every added or changed
-Markdown link and confirm its target and heading exist.
-
-Also inspect:
-
-```sh
-git status --short
-git diff --stat
-git diff
-```
-
-Confirm that:
-
-- production changes are limited to the root convenience API;
-- no internal backend, registry, capacity, profile, or engine behavior
-  changed;
-- `BackendLocal` is conventional and consumer-registerable, not built in or
-  reserved;
-- `LocalBackend` performs no validation, normalization, environment lookup,
-  registration, allocation, or hidden default selection;
-- the returned `Backend` leaves `QueueCapacity` nil so existing positive-limit
-  defaulting remains owned by the registry;
-- existing endpoint-only profiles and complete custom backends remain
-  documented and supported;
-- current-state documentation describes only the now-implemented API and
-  links to the canonical GoDoc for exact semantics;
-- no `go.work`, `go.work.sum`, local module replacement, credential,
-  generated binary, or unrelated change was introduced; and
-- the feature and implementation roadmaps contain no unresolved work marked
-  complete.
-
-### Completion Gate
-
-Stage 2 is complete when every target-end-state item in
-`local-backend.md` is implemented, the consumer guide clearly presents all
-three configuration paths, all complete validation commands pass, all changed
-links resolve, and the roadmap statuses accurately report completion.
-
-## Implementation Handoff
-
-The implementation handoff should report:
-
-- the new `BackendLocal` and `LocalBackend` public API;
-- that the helper remains explicit and composes with the ordinary backend
-  registry;
-- the focused public-contract coverage added;
-- the consumer-guide decision path added;
-- the complete validation commands and results; and
-- any unrelated working-tree changes that were preserved.
-
-Do not claim that a local backend is built in, pre-registered, configured from
-environment variables, or assigned a default model or concurrency limit.
-
-## Open Questions
-
-None. The feature roadmap and this plan fix the exported API, constructor
-semantics, identity treatment, compatibility behavior, test boundary,
-documentation ownership, implementation sequence, validation gates, and
-non-goals required for implementation.

docs/roadmap/local-backend.md

@@ -1,163 +0,0 @@
-# Local Backend Convenience
-
-**Status:** Complete.
-
-## Purpose
-
-Make the common case of using a local OpenAI-compatible endpoint concise and
-easy to discover without introducing implicit configuration or a separate
-backend abstraction.
-
-The existing `Backend` type and registry remain the canonical, fully
-configurable interface. A small convenience constructor will cover the usual
-local-network case, while improved consumer documentation will make it clear
-when an endpoint-only profile, the convenience constructor, or a complete
-`Backend` value is appropriate.
-
-## Motivation
-
-Consumers can already use a local endpoint by setting `Profile.Endpoint`, or
-register one as a backend with `WithBackend`. The first option is concise but
-does not provide shared backend-level concurrency control. The second supports
-the complete backend feature set but requires consumers to understand and
-populate several fields for a common configuration.
-
-Most consumers adding a local backend need only:
-
-- a stable backend ID;
-- an OpenAI-compatible endpoint; and
-- a concurrency limit appropriate for the local server.
-
-Promptkit should provide a direct path for that case while keeping all
-configuration explicit and preserving the full registry interface for
-advanced needs.
-
-## Consumer Paths
-
-Documentation should present three progressively more configurable paths:
-
-1. Set `Profile.Endpoint` when a profile only needs to target a local endpoint
-   and does not need shared backend policy.
-2. Use the local-backend convenience constructor when profiles should share a
-   named local endpoint and its concurrency limit.
-3. Construct a complete `Backend` value when the consumer needs a custom
-   backend ID, authentication, extra request parameters, an explicit queue
-   capacity, or multiple local backends.
-
-These are complementary interfaces. The convenience constructor must return an
-ordinary `Backend`, so it does not create a second configuration model.
-
-## Public Convenience API
-
-The public package should expose:
-
-```go
-const BackendLocal = "local"
-
-func LocalBackend(endpoint string, concurrencyLimit int) Backend
-```
-
-`LocalBackend` should return a `Backend` with:
-
-- `ID` set to `BackendLocal`;
-- `Endpoint` set to the supplied endpoint;
-- `ConcurrencyLimit` set to the supplied limit; and
-- all other fields left at their zero values.
-
-The returned value is passed to `WithBackend` and follows the same copying,
-normalization, validation, and registration rules as any consumer-constructed
-`Backend`.
-
-The constructor should be a transparent value constructor. It should not read
-environment variables, mutate global state, register the backend, validate
-arguments independently, or create profiles. Consumers may inspect or modify
-the returned value before registration, although documentation should direct
-substantially customized configurations to the full `Backend` form.
-
-## Identity and Registration
-
-`BackendLocal` is a conventional ID used by the convenience constructor. It is
-not pre-registered and should not become a specially reserved registry ID.
-Consumers remain responsible for registering the returned backend with
-`WithBackend` and naming it from profiles through `BackendID`.
-
-This distinction preserves compatibility with consumers that may already
-register their own backend using the ID `"local"`. Normal duplicate-ID rules
-still apply if a consumer attempts to register more than one backend with that
-ID.
-
-Consumers that need multiple local endpoints should choose distinct IDs and
-use complete `Backend` values rather than the single conventional helper ID.
-
-## Concurrency and Queue Semantics
-
-The constructor must preserve the existing backend concurrency contract:
-
-- a positive concurrency limit bounds simultaneous requests and uses the
-  existing default queue capacity because `QueueCapacity` remains `nil`;
-- a zero concurrency limit leaves the backend unconstrained; and
-- a negative concurrency limit is rejected through the existing engine
-  configuration validation path.
-
-The constructor should not select a hidden default concurrency limit. Local
-servers vary substantially in capacity, so the consumer should make this
-choice explicitly.
-
-## Documentation
-
-The final documentation state has two canonical surfaces:
-
-- Public Go documentation describes the exact contract of
-  `BackendLocal` and `LocalBackend`, including their conventional,
-  non-pre-registered nature.
-- The [promptkit consumer guide](../consumers/pkg-promptkit.md) includes
-  a task-oriented local-endpoint section that shows the three consumer paths,
-  explains the decision between them, and provides concise examples of the
-  endpoint-only and convenience-constructor forms.
-
-The consumer guide continues to document the full `Backend` interface as
-the advanced path rather than attempting to reproduce every configuration
-variation through convenience APIs.
-
-## Compatibility
-
-This feature is additive:
-
-- existing endpoint-only profiles continue to work unchanged;
-- existing `Backend` values and `WithBackend` registrations remain the
-  canonical general-purpose interface;
-- existing registrations using the literal ID `"local"` remain valid; and
-- OpenRouter defaults and all other backend behavior remain unchanged.
-
-No consumer is required to adopt the convenience constructor.
-
-## Non-Goals
-
-This work does not include:
-
-- pre-registering or implicitly enabling a local backend;
-- discovering a local endpoint, API key, or concurrency limit from environment
-  variables;
-- adding local-backend fields to `Config`;
-- selecting a default local model or creating a profile automatically;
-- adding a combined backend-and-profile constructor;
-- adding convenience parameters for API keys, extra request parameters, or
-  queue capacity;
-- replacing or redesigning the backend registry;
-- adding support for non-OpenAI-compatible local APIs; or
-- changing backend routing, scheduling, or queue behavior.
-
-## Target End State
-
-After this work:
-
-- consumers with a simple one-profile local endpoint can continue to configure
-  it directly on the profile;
-- consumers needing a shared local endpoint and concurrency policy can express
-  it with one `LocalBackend` call and register the returned value normally;
-- consumers with advanced or multiple-local-backend requirements have a clear
-  path to the complete `Backend` interface;
-- all local configuration remains explicit, inspectable, and compatible with
-  dependency injection; and
-- canonical documentation makes the simplest suitable interface easy to find
-  without obscuring the underlying registry model.

docs/roadmap/notarius-promptkit-wishlist.md

@@ -6,17 +6,20 @@ 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 v0.3.0 provides the capabilities Notarius currently needs. None of
-the ideas below blocks current Notarius development. They are opportunities to
-reduce downstream workarounds, improve integration correctness, and make
-PromptKit more ergonomic for applications with configuration validation,
-debugging, checkpointing, and operational-observability requirements.
+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:
@@ -28,61 +31,25 @@ Notarius needs both:
 Notarius uses the prepared details to construct redaction-aware debug bundles
 and retain enough information to diagnose model behavior.
 
-### Current integration
+### Implemented behavior
 
-Notarius currently calls `Engine.Prepare` and then `Engine.Run` with the same
-request. Because `Run` performs preparation internally, a successful request
-resolves and prepares the same work twice.
-
-This duplicates profile resolution, input hashing, schema loading, and prompt
-rendering. It also creates a theoretical consistency window in which a
-filesystem-backed prompt, profile, schema, or input could change between the
-explicit preparation and the preparation performed by `Run`.
-
-### Requested capability
-
-Add an opt-in execution method that prepares exactly once and returns both the
-prepared details and completed result:
-
-```go
-type RunReport struct {
-	Prepared PreparedRun
-	Result   RunResult
-}
-
-func (e *Engine) RunDetailed(
-	ctx context.Context,
-	req RunRequest,
-) (*RunReport, error)
-```
-
-The exact names are flexible. The important contract is that preparation
-occurs once and that the returned prepared state describes the execution that
-produced the returned result.
-
-Existing `Prepare` and `Run` behavior should remain available for consumers
-that need only one side of the operation.
-
-### Design considerations
-
-- Keep this API additive and preserve the existing simple `Run` workflow.
-- Return caller-owned copies under PromptKit's existing ownership rules.
-- Define whether any prepared details are available after an operational
-  generation or validation error. Notarius does not require partial results
-  for the initial use case, but an explicit contract would be valuable.
-- Preserve cancellation and backend-admission semantics.
-- Do not add all prepared content directly to `RunResult`. Rendered prompt
-  content can be large and sensitive, and consumers should opt in to receiving
-  it.
+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 is the highest-value wishlist item. It would remove duplicate work from
-every successful PromptKit-backed call and ensure that retained debug material
-corresponds atomically to the actual execution.
+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
@@ -96,60 +63,23 @@ needs to determine whether:
 
 This validation should not require model generation.
 
-### Current integration
+### Previous integration
 
-Notarius constructs a synthetic prompt using `testing/fstest.MapFS`, supplies a
-dummy transcript, and calls `Engine.Prepare` solely to exercise profile and
-backend resolution. This works, but prompt preparation is serving as a
-substitute for a profile-inspection interface.
-
-### Requested capability
-
-Add a prompt-independent profile-resolution API, for example:
-
-```go
-type ResolvedProfile struct {
-	ProfileID       string
-	BackendID       string
-	EffectiveTarget ExecutionTarget
-	APIKeyEnv       string
-}
-
-func (e *Engine) ResolveProfile(
-	ctx context.Context,
-	profileID string,
-) (ResolvedProfile, error)
-```
-
-The returned shape may differ, but it should provide enough information for a
-consumer to validate an explicit profile selection without inventing a prompt
-or supplying placeholder inputs.
-
-### Design considerations
-
-- Resolve built-in, file-backed, and programmatic profiles using normal
-  PromptKit precedence.
-- Validate that a referenced backend registration exists.
-- Do not resolve, retain, or expose credential values.
-- Report credential requirements, such as an environment-variable name, so
-  the consuming application can decide whether availability is required at
-  configuration-validation time or only at execution time.
-- Return caller-owned values.
-- Preserve typed or sentinel error classification for missing and invalid
-  profiles.
-- Consider accepting an `ExecutionTargetOverride` if consumers need to inspect
-  the same effective target that a run-level override would produce.
-- Enumeration of all profiles is not required for the Notarius use case; exact
-  lookup by ID is sufficient.
+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
 
-This would eliminate a synthetic production-only prompt fixture and establish
-a direct, supported contract for configuration-time profile and backend
-validation.
+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
@@ -233,6 +163,9 @@ 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
@@ -240,46 +173,16 @@ 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.
 
-### Current integration
+### Implemented behavior
 
-PromptKit provides the useful `ErrCapacityExceeded` sentinel. Notarius can
-classify the failure reliably, but it retains only a sanitized diagnostic
-string as additional context.
-
-### Requested capability
-
-Add a typed error that continues to match `ErrCapacityExceeded`:
-
-```go
-type CapacityError struct {
-	BackendID string
-}
-
-func (e *CapacityError) Is(target error) bool {
-	return target == ErrCapacityExceeded
-}
-```
-
-The exact implementation may use `Unwrap` or another idiomatic mechanism. The
-important properties are compatibility with `errors.Is` and discoverability
-through `errors.As`.
-
-### Design considerations
-
-- Include the stable backend ID.
-- Do not expose the backend endpoint, credential environment, credential
-  value, request content, or other sensitive configuration.
-- Consider including the configured concurrency and queue limits if they are
-  useful and safe, but backend identity alone provides most of the downstream
-  value.
-- Add a retry delay only if PromptKit can provide a meaningful value. A full
-  queue does not necessarily imply a reliable `Retry-After` duration.
-- Keep retry and backoff policy with the consuming application. PromptKit
-  should classify the admission failure rather than silently retry it.
+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 would improve operational diagnostics and future metrics while preserving
+This improves operational diagnostics and future metrics while preserving
 the provider-neutral error boundary used by Notarius.
 
 ## Capabilities PromptKit Already Provides Well
@@ -330,16 +233,13 @@ application-policy decision, not an upstream PromptKit gap.
 
 ## Suggested Upstream Sequence
 
-If the PromptKit team chooses to pursue these ideas, the most useful order for
-Notarius would be:
+For downstream adoption and any remaining upstream work, the useful order is:
 
-1. Add atomic execution that returns prepared details and the completed result.
-2. Add prompt-independent profile inspection.
-3. Add a semantic execution-target digest, preferably as part of profile
+1. Adopt prepared execution for atomic details and results.
+2. Add a semantic execution-target digest, preferably alongside profile
    inspection.
-4. Add a typed capacity error carrying backend identity.
-
-The first two address concrete workarounds in current Notarius code. The third
-would improve checkpoint correctness and reduce coupling. The fourth is
-operational polish.
+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

@@ -7,10 +7,9 @@ additions to PromptKit from the perspective of the maintainers of
 Weatherreporter, a downstream application planning to replace its Scriptorium
 CLI integration with PromptKit.
 
-PromptKit v0.3.0 provides the capabilities Weatherreporter needs for the
-migration. None of the ideas below is a hard adoption requirement. They are
-opportunities to avoid duplicate preparation, validate configuration earlier,
-improve durable failure diagnostics, and make the integration more direct.
+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
@@ -19,6 +18,10 @@ 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
@@ -32,77 +35,27 @@ needs to:
 Persisting preflight before generation leaves useful evidence when a provider
 call fails or the process is interrupted during generation.
 
-### Current integration option
+### Implemented behavior
 
-With PromptKit v0.3.0, Weatherreporter can call `Engine.Prepare`, save selected
-fields from the returned `PreparedRun`, and then call `Engine.Run` with the
-same request. Because `Run` performs preparation internally, the work is
-repeated.
-
-Weatherreporter plans to use embedded prompt and schema files plus immutable
-inline input bytes, which removes most of the consistency risk. An external
-profile file or directory can still change between the two calls, and the
-second preparation remains unnecessary work.
-
-The atomic `RunDetailed` operation proposed by the
-[Notarius wishlist](notarius-promptkit-wishlist.md#priority-1-atomic-execution-with-prepared-details)
-would guarantee that returned preparation details describe the completed
-execution. However, returning those details only after generation would not
-preserve Weatherreporter's preflight-before-generation persistence boundary.
-
-### Requested capability
-
-Add an opt-in two-phase API that returns a prepared execution handle:
-
-```go
-prepared, err := engine.PrepareExecution(ctx, request)
-if err != nil {
-	// Handle preparation failure.
-}
-
-details := prepared.Details()
-// Persist a consumer-selected safe preparation record.
-
-result, err := engine.RunPrepared(ctx, prepared)
-```
-
-The exact names and shapes are flexible. The important contract is that
-`RunPrepared` executes the already prepared prompt and does not reload or
-rerender its prompt, profile, schema, or input sources.
-
-`Details` should return the same caller-owned public preparation information
-currently represented by `PreparedRun`. The execution handle may retain opaque
-engine-owned state needed to invoke the model and validate the response.
-
-### Design considerations
-
-- Keep `Prepare` and `Run` available for consumers that do not need a
-  two-phase execution boundary.
-- Bind a prepared handle to the engine that constructed it.
-- Define whether a handle is one-shot, reusable, or safe for concurrent use.
-  A one-shot contract may be the safest initial design.
-- Do not give the opaque handle a stable JSON representation.
-- Do not expose or serialize resolved credential values through `Details`.
-- Define how a direct request API key is retained and released when an opaque
-  handle must carry it until execution.
-- Preserve caller-owned copies for all public details.
-- Make context cancellation and backend admission timing explicit.
-- Document whether profile credential environment values are resolved during
-  preparation or execution.
-- Ensure an execution error does not invalidate the public details already
-  returned to the consumer.
-- Consider whether an atomic `RunDetailed` can share the same internal
-  prepared-execution implementation.
+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 is the highest-value upstream addition. It would preserve
-Weatherreporter's durable preflight behavior, remove duplicate work, eliminate
-the remaining source-consistency window, and ensure that persisted provenance
-describes the actual execution.
+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
@@ -117,113 +70,49 @@ 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.
 
-### Current integration option
+### Previous integration option
 
-Weatherreporter can maintain synthetic data-package fixtures and call
-`Engine.Prepare` for every report prompt during tests. Runtime validation can
-also occur through the ordinary per-report preparation stage.
+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 works, but it requires complete placeholder inputs and profile resolution
-when the application primarily wants to inspect prompt identity and declared
-contracts.
-
-### Requested capability
-
-Add exact prompt-definition lookup without rendering or generation:
-
-```go
-type PromptInfo struct {
-	PromptID         string
-	PromptVersion    string
-	PromptHash       string
-	DefaultProfileID string
-	Inputs           []InputDefinition
-	OutputContract   OutputContract
-}
-
-func (e *Engine) ResolvePrompt(
-	ctx context.Context,
-	promptID string,
-	promptVersion string,
-) (PromptInfo, error)
-```
-
-The exact returned shape may differ. Weatherreporter needs enough information
-to verify prompt existence, version selection, declared inputs, default
-profile identity, output format, validation mode, and schema selection without
-supplying synthetic prompt input.
-
-### Design considerations
-
-- Use ordinary PromptKit prompt-source precedence and exact ID/version
-  selection.
-- Fully load and structurally validate the selected prompt definition.
-- Validate referenced prompt content files without rendering their templates.
-- Resolve and validate the selected output contract and schema reference where
-  practical.
-- Return an opaque prompt-definition equality value rather than raw source
-  bytes.
-- Do not return rendered messages, schema bodies, profile credentials, or
-  another source of sensitive content.
-- Preserve typed or sentinel errors for missing and invalid prompts.
-- Return caller-owned values.
-- Enumeration of all known prompts is not required for Weatherreporter; exact
-  lookup is sufficient.
+This required complete placeholder inputs and profile resolution when the
+application primarily wanted to inspect prompt identity and declared contracts.
 
 ### Value to Weatherreporter
 
-This would let 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 would reduce synthetic test setup
-and move failures ahead of weather collection.
+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 should reject a missing
-profile, unknown backend, malformed execution target, or unsatisfied credential
-requirement before collecting weather data or writing report artifacts.
-
-### Current integration option
-
-Weatherreporter can validate an explicit profile by preparing one embedded
-prompt with fixture input. Prompts that use their own default profiles can be
-validated during their normal preparation stage.
-
-This couples configuration validation to one prompt and requires placeholder
-input even when only profile and backend resolution are relevant.
-
-### Requested capability
-
-The prompt-independent `ResolveProfile` API proposed by the
-[Notarius wishlist](notarius-promptkit-wishlist.md#priority-2-prompt-independent-profile-inspection)
-would satisfy this need. It should resolve built-in, file-backed, and
-programmatic profiles, validate backend membership, report credential
-requirements without resolving credential values, and preserve typed error
-classification.
-
-### Additional Weatherreporter considerations
-
-- An explicit application profile override should be inspectable without
-  selecting a report prompt.
-- A prompt-definition inspection result may expose its default profile ID so
-  Weatherreporter can inspect that profile separately.
-- Inspection should distinguish structural profile validity from current
-  credential availability so configuration validation can apply explicit
-  application policy.
-- An optional execution-target override should be considered only if it
-  describes the same target that a later run will use.
+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
 
-This would improve fail-fast configuration validation and give operator-facing
-errors direct profile and backend context. It is valuable but not required for
-the initial migration.
+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`
@@ -279,6 +168,10 @@ 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
@@ -335,19 +228,23 @@ the existing sentinel remain available.
 
 ### Structured Capacity Errors
 
-The typed capacity error proposed by the
-[Notarius wishlist](notarius-promptkit-wishlist.md#priority-4-structured-capacity-errors)
-would improve Weatherreporter diagnostics by exposing the stable backend ID
-without parsing error text.
+**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 feature would become more valuable if report
-generation later becomes concurrent or PromptKit engines become longer-lived.
+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.
@@ -361,12 +258,13 @@ migration priority.
 
 ## Capabilities PromptKit Already Provides Well
 
-PromptKit v0.3.0 already provides the essential Weatherreporter integration
-surface:
+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
@@ -378,7 +276,8 @@ surface:
 - 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.
+  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.
@@ -403,43 +302,30 @@ PromptKit:
 
 ## Suggested Upstream Sequence
 
-If the PromptKit team chooses to pursue these ideas, the most useful order for
-Weatherreporter would be:
+For downstream adoption and any remaining upstream work, the useful order is:
 
-1. Add executable preparation handles, ideally sharing implementation with an
-   atomic detailed-run API.
-2. Add prompt-definition inspection.
-3. Add prompt-independent profile inspection.
-4. Consider eager source validation after evaluating whether the two exact
+1. Adopt the implemented executable preparation handles.
+2. Consider eager source validation after evaluating whether the two exact
    inspection APIs are sufficient.
-5. Add structured generation errors.
-6. Add structured capacity errors and semantic execution-target fingerprints
-   as lower-priority operational improvements.
+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 only material integration workaround. Prompt and
-profile inspection improve fail-fast validation. The remaining items improve
-ergonomics and diagnostics.
+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 complete wishlist. PromptKit v0.3.0 is
-already sufficient when Weatherreporter:
+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;
-- calls `Prepare` and `Run` with the same request; and
+- prepares an execution, persists selected `Details`, and calls
+  `RunPrepared`; and
 - keeps PromptKit behind a weatherreporter-owned adapter contract.
 
-If executable preparation handles are scheduled for a near-term PromptKit
-release, Weatherreporter may defer only its final adapter implementation to
-avoid implementing and then removing duplicate preparation. Prompt corpus
-retrieval, application-contract design, configuration work, embedded assets,
-state contracts, and offline fixtures can proceed independently.
-
-If the feature is not scheduled, Weatherreporter can adopt v0.3.0 and keep the
-duplicate `Prepare` and `Run` sequence inside its adapter. A later PromptKit
-upgrade would remain localized behind that neutral boundary.
-
 Prompt inspection, profile inspection, source validation, structured errors,
 capacity details, and semantic fingerprints should not gate adoption.

engine.go

@@ -63,9 +63,10 @@ var (
 	// 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 rejected because the selected backend
-	// already admitted ConcurrencyLimit + QueueCapacity calls. It is not an
-	// invalid request, an LLM or provider rate-limit response, or ErrLLMGenerate.
+	// 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
@@ -77,12 +78,15 @@ var (
 	ErrValidation = errors.New("failed to validate output")
 )
 
-// Engine prepares and runs Promptkit prompt requests.
+// Engine inspects prompts and profiles and prepares and runs Promptkit prompt
+// requests.
 //
-// An Engine is safe for concurrent calls to [Engine.Prepare] and [Engine.Run].
-// Each Engine owns independent backend-capacity pools that coordinate Run
-// admission and model generation. Injected collaborators may still be invoked
-// concurrently across different backend pools or for unlimited backends.
+// 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
 }
@@ -146,12 +150,14 @@ type engineOptions struct {
 	artifactSource  bool
 }
 
-// WithLLMClient replaces the built-in model client used by [Engine.Run].
+// 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].
+// 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 {
@@ -423,6 +429,93 @@ func fileSource(name string) (fs.FS, string, error) {
 	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
@@ -457,6 +550,38 @@ func (e *Engine) Prepare(ctx context.Context, req RunRequest) (*PreparedRun, 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.
 //
@@ -467,9 +592,10 @@ func (e *Engine) Prepare(ctx context.Context, req RunRequest) (*PreparedRun, err
 // single-pass even when OutputContract.RepairAttempts is positive.
 //
 // Run can return every error category documented by [Engine.Prepare], plus
-// ErrCapacityExceeded and ErrLLMGenerate. ErrCapacityExceeded identifies
-// rejection before artifacts, schemas, rendering, or model generation because
-// the selected backend's admission capacity is full; it does not match
+// 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
@@ -491,3 +617,42 @@ func (e *Engine) Run(ctx context.Context, req RunRequest) (*RunResult, error) {
 	}
 	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

@@ -3,6 +3,7 @@ package promptkit
 import (
 	"errors"
 	"fmt"
+	"strings"
 
 	"gitea.maximumdirect.net/eric/promptkit/internal/capacity"
 	"gitea.maximumdirect.net/eric/promptkit/internal/profile"
@@ -14,6 +15,11 @@ 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

errors_internal_test.go

@@ -20,3 +20,31 @@ func TestMapPublicErrorPreservesGenerationCancellation(t *testing.T) {
 		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)
+	}
+}

internal/capacity/manager_test.go

@@ -124,6 +124,11 @@ func TestManagerAdmissionHonorsContextAndUnlimitedBackends(t *testing.T) {
 	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()

internal/domain/domain.go

@@ -145,6 +145,16 @@ type PromptDefinition struct {
 	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"`
@@ -236,6 +246,13 @@ type ExecutionTarget struct {
 	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"`

internal/jsonvalue/jsonvalue.go

@@ -1,5 +1,5 @@
 // Package jsonvalue validates and defensively copies JSON-compatible value
-// trees used by public configuration and request boundaries.
+// trees used by configuration, request, and prepared-state boundaries.
 package jsonvalue
 
 import (
@@ -18,13 +18,19 @@ type visit struct {
 	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{}))
+	copied, err := copyValue(reflect.ValueOf(src), "extra_params", make(map[visit]struct{}), false)
 	if err != nil {
 		return nil, err
 	}
@@ -35,7 +41,12 @@ func CopyMap(src map[string]any) (map[string]any, error) {
 	return out, nil
 }
 
-func copyValue(value reflect.Value, path string, seen map[visit]struct{}) (any, error) {
+func copyValue(
+	value reflect.Value,
+	path string,
+	seen map[visit]struct{},
+	allowEmptyMapKeys bool,
+) (any, error) {
 	if !value.IsValid() {
 		return nil, nil
 	}
@@ -43,7 +54,7 @@ func copyValue(value reflect.Value, path string, seen map[visit]struct{}) (any,
 		if value.IsNil() {
 			return nil, nil
 		}
-		return copyValue(value.Elem(), path, seen)
+		return copyValue(value.Elem(), path, seen, allowEmptyMapKeys)
 	}
 	if !value.CanInterface() {
 		return nil, fmt.Errorf("%s: value cannot be copied", path)
@@ -88,22 +99,27 @@ func copyValue(value reflect.Value, path string, seen map[visit]struct{}) (any,
 		}
 		seen[current] = struct{}{}
 		defer delete(seen, current)
-		return copyValue(value.Elem(), path, seen)
+		return copyValue(value.Elem(), path, seen, allowEmptyMapKeys)
 	case reflect.Map:
-		return copyMapValue(value, path, seen)
+		return copyMapValue(value, path, seen, allowEmptyMapKeys)
 	case reflect.Slice:
 		if value.IsNil() {
 			return nil, nil
 		}
-		return copySequenceValue(value, path, seen)
+		return copySequenceValue(value, path, seen, allowEmptyMapKeys)
 	case reflect.Array:
-		return copySequenceValue(value, path, seen)
+		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{}) (any, error) {
+func copyMapValue(
+	value reflect.Value,
+	path string,
+	seen map[visit]struct{},
+	allowEmptyMapKeys bool,
+) (any, error) {
 	if value.IsNil() {
 		return nil, nil
 	}
@@ -133,10 +149,10 @@ func copyMapValue(value reflect.Value, path string, seen map[visit]struct{}) (an
 	elementType := value.Type().Elem()
 	for _, key := range keys {
 		name := key.String()
-		if name == "" {
+		if name == "" && !allowEmptyMapKeys {
 			return nil, fmt.Errorf("%s: map key must not be empty", path)
 		}
-		copied, err := copyValue(value.MapIndex(key), path+"."+name, seen)
+		copied, err := copyValue(value.MapIndex(key), path+"."+name, seen, allowEmptyMapKeys)
 		if err != nil {
 			return nil, err
 		}
@@ -171,7 +187,12 @@ func copyMapValue(value reflect.Value, path string, seen map[visit]struct{}) (an
 	return out, nil
 }
 
-func copySequenceValue(value reflect.Value, path string, seen map[visit]struct{}) (any, error) {
+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()}
@@ -186,7 +207,12 @@ func copySequenceValue(value reflect.Value, path string, seen map[visit]struct{}
 	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)
+		copied, err := copyValue(
+			value.Index(i),
+			fmt.Sprintf("%s[%d]", path, i),
+			seen,
+			allowEmptyMapKeys,
+		)
 		if err != nil {
 			return nil, err
 		}

internal/jsonvalue/jsonvalue_test.go

@@ -44,6 +44,21 @@ func TestCopyMapPreservesTypesAndIsolatesMutations(t *testing.T) {
 	}
 }
 
+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

internal/usecase/capacity_error.go

@@ -0,0 +1,24 @@
+package usecase
+
+import (
+	"fmt"
+	"strings"
+
+	"gitea.maximumdirect.net/eric/promptkit/internal/capacity"
+)
+
+// CapacityError identifies bounded admission rejected for one selected backend.
+type CapacityError struct {
+	BackendID string
+}
+
+func (e *CapacityError) Error() string {
+	if e == nil || strings.TrimSpace(e.BackendID) == "" {
+		return capacity.ErrCapacityExceeded.Error()
+	}
+	return fmt.Sprintf("backend %q admission: %v", e.BackendID, capacity.ErrCapacityExceeded)
+}
+
+func (e *CapacityError) Unwrap() error {
+	return capacity.ErrCapacityExceeded
+}

internal/usecase/prepared_execution.go

@@ -0,0 +1,298 @@
+package usecase
+
+import (
+	"context"
+	"fmt"
+	"sync"
+	"time"
+
+	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
+	"gitea.maximumdirect.net/eric/promptkit/internal/jsonvalue"
+	"gitea.maximumdirect.net/eric/promptkit/internal/validate"
+)
+
+type preparedExecutionState uint8
+
+const (
+	preparedExecutionReady preparedExecutionState = iota
+	preparedExecutionClaimed
+	preparedExecutionDiscarded
+)
+
+// PreparedExecution owns one frozen, single-use runner execution.
+type PreparedExecution struct {
+	owner   *Runner
+	mu      sync.Mutex
+	state   preparedExecutionState
+	details *domain.PreparedRun
+	payload *preparedExecutionPayload
+}
+
+type preparedExecutionPayload struct {
+	prepared   *domain.PreparedRun
+	validation validate.PreparedValidation
+	directKey  string
+}
+
+// PrepareExecution completes preparation without generation or admission and
+// returns a runner-bound, single-use execution.
+func (r *Runner) PrepareExecution(ctx context.Context, req domain.RunRequest) (*PreparedExecution, error) {
+	state, err := r.resolvePreparation(ctx, req, time.Now().UTC())
+	if err != nil {
+		return nil, err
+	}
+
+	validationPlan, err := r.prepareValidation(ctx, state.effectiveContract)
+	if err != nil {
+		return nil, err
+	}
+
+	structuredOutput, err := r.structuredOutputFromValidationPlan(
+		state.definition,
+		state.effectiveContract,
+		validationPlan,
+	)
+	if err != nil {
+		return nil, err
+	}
+	prepared, err := r.completePreparationWithStructuredOutput(ctx, req, state, structuredOutput)
+	if err != nil {
+		return nil, err
+	}
+
+	executionSnapshot, err := clonePreparedRun(prepared)
+	if err != nil {
+		return nil, fmt.Errorf("%w: failed to copy prepared execution: %v", ErrInvalidRequest, err)
+	}
+
+	details, err := clonePreparedRun(executionSnapshot)
+	if err != nil {
+		return nil, fmt.Errorf("%w: failed to copy prepared execution details: %v", ErrInvalidRequest, err)
+	}
+
+	return &PreparedExecution{
+		owner:   r,
+		state:   preparedExecutionReady,
+		details: details,
+		payload: &preparedExecutionPayload{
+			prepared:   executionSnapshot,
+			validation: validationPlan,
+			directKey:  state.effectiveModel.APIKey,
+		},
+	}, nil
+}
+
+func (r *Runner) prepareValidation(
+	ctx context.Context,
+	contract domain.OutputContract,
+) (validate.PreparedValidation, error) {
+	if r.validator == nil {
+		return noOpPreparedValidation{contract: contract}, nil
+	}
+	preparer, ok := r.validator.(validate.ValidationPreparer)
+	if !ok {
+		return nil, fmt.Errorf("%w: validator does not support prepared validation", ErrValidation)
+	}
+	plan, err := preparer.PrepareValidation(ctx, contract)
+	if err != nil {
+		return nil, fmt.Errorf("%w: %w", ErrValidation, err)
+	}
+	if plan == nil {
+		return nil, fmt.Errorf("%w: validator returned nil prepared validation", ErrValidation)
+	}
+	return plan, nil
+}
+
+func (r *Runner) structuredOutputFromValidationPlan(
+	def *domain.PromptDefinition,
+	contract domain.OutputContract,
+	plan validate.PreparedValidation,
+) (*domain.StructuredOutputSpec, error) {
+	if contract.ValidationMode != domain.ValidationJSONSchema {
+		return nil, nil
+	}
+
+	schemaDocument := plan.SchemaDocument()
+	if schemaDocument == nil {
+		if r.validator == nil {
+			return nil, nil
+		}
+		return nil, fmt.Errorf("%w: prepared json_schema validation has no schema document", ErrValidation)
+	}
+	return structuredOutputSpec(def, schemaDocument), nil
+}
+
+// Details returns a fresh credential-redacted copy of the prepared run.
+func (p *PreparedExecution) Details() *domain.PreparedRun {
+	if p == nil {
+		return nil
+	}
+	p.mu.Lock()
+	detailsSnapshot := p.details
+	p.mu.Unlock()
+
+	details, err := clonePreparedRun(detailsSnapshot)
+	if err != nil {
+		return nil
+	}
+	return details
+}
+
+// Discard invalidates an unclaimed execution and drops its private payload.
+func (p *PreparedExecution) Discard() {
+	if p == nil {
+		return
+	}
+
+	p.mu.Lock()
+	if p.state != preparedExecutionReady {
+		p.mu.Unlock()
+		return
+	}
+	p.state = preparedExecutionDiscarded
+	payload := p.payload
+	p.payload = nil
+	p.mu.Unlock()
+
+	payload.clear()
+}
+
+// RunPrepared claims and executes one prepared execution owned by this runner.
+func (r *Runner) RunPrepared(ctx context.Context, prepared *PreparedExecution) (*domain.RunResult, error) {
+	payload, err := prepared.claim(r)
+	if err != nil {
+		return nil, err
+	}
+	defer payload.clear()
+
+	runID, err := newRunID()
+	if err != nil {
+		return nil, fmt.Errorf("failed to create run id: %w", err)
+	}
+	start := time.Now().UTC()
+
+	target := payload.prepared.EffectiveModelParams
+	if err := validateAPIKey(target.APIKeyEnv, payload.directKey, target.APIKeyRequired); err != nil {
+		return nil, fmt.Errorf("%w: %w", ErrInvalidRequest, err)
+	}
+
+	release, err := r.admitRun(ctx, target.BackendID)
+	if err != nil {
+		return nil, err
+	}
+	defer release()
+
+	return r.executePreparedRun(ctx, payload.prepared, payload.directKey, runID, start, func(
+		ctx context.Context,
+		artifact *domain.Artifact,
+		attemptsUsed int,
+	) (domain.ValidationResult, error) {
+		result, validationErr := payload.validation.Validate(ctx, artifact)
+		if validationErr != nil {
+			return domain.ValidationResult{}, validationErr
+		}
+		result.RepairAttempts = attemptsUsed
+		return result, nil
+	})
+}
+
+func (p *PreparedExecution) claim(owner *Runner) (*preparedExecutionPayload, error) {
+	if p == nil || owner == nil || p.owner != owner {
+		return nil, fmt.Errorf("%w: prepared execution does not belong to this runner", ErrInvalidRequest)
+	}
+
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	if p.state != preparedExecutionReady || p.payload == nil {
+		return nil, fmt.Errorf("%w: prepared execution is not ready", ErrInvalidRequest)
+	}
+	p.state = preparedExecutionClaimed
+	payload := p.payload
+	p.payload = nil
+	return payload, nil
+}
+
+func (p *preparedExecutionPayload) clear() {
+	if p == nil {
+		return
+	}
+	if p.prepared != nil {
+		p.prepared.EffectiveModelParams.APIKey = ""
+	}
+	p.prepared = nil
+	p.validation = nil
+	p.directKey = ""
+}
+
+type noOpPreparedValidation struct {
+	contract domain.OutputContract
+}
+
+func (p noOpPreparedValidation) Validate(
+	ctx context.Context,
+	_ *domain.Artifact,
+) (domain.ValidationResult, error) {
+	if err := ctx.Err(); err != nil {
+		return domain.ValidationResult{}, err
+	}
+	return domain.ValidationResult{
+		Status:         domain.ValidationSkipped,
+		Mode:           p.contract.ValidationMode,
+		SchemaPath:     p.contract.SchemaPath,
+		RepairAttempts: p.contract.RepairAttempts,
+		IsValid:        true,
+	}, nil
+}
+
+func (noOpPreparedValidation) SchemaDocument() any {
+	return nil
+}
+
+func clonePreparedRun(source *domain.PreparedRun) (*domain.PreparedRun, error) {
+	if source == nil {
+		return nil, nil
+	}
+
+	copied := *source
+	extraParams, err := jsonvalue.CopyMap(source.EffectiveModelParams.ExtraParams)
+	if err != nil {
+		return nil, err
+	}
+	copied.EffectiveModelParams.ExtraParams = extraParams
+
+	if source.InputHashes != nil {
+		copied.InputHashes = make(map[string]string, len(source.InputHashes))
+		for name, hash := range source.InputHashes {
+			copied.InputHashes[name] = hash
+		}
+	}
+	if source.Messages != nil {
+		copied.Messages = make([]domain.RenderedMessage, len(source.Messages))
+		for i, message := range source.Messages {
+			copied.Messages[i] = message
+			if message.CacheControl != nil {
+				cacheControl := *message.CacheControl
+				copied.Messages[i].CacheControl = &cacheControl
+			}
+		}
+	}
+
+	if source.StructuredOutput != nil {
+		structuredOutput := *source.StructuredOutput
+		copied.StructuredOutput = &structuredOutput
+		if source.StructuredOutput.JSONSchema != nil {
+			jsonSchema := *source.StructuredOutput.JSONSchema
+			copied.StructuredOutput.JSONSchema = &jsonSchema
+			schema, copyErr := cloneJSONValue(source.StructuredOutput.JSONSchema.Schema)
+			if copyErr != nil {
+				return nil, copyErr
+			}
+			copied.StructuredOutput.JSONSchema.Schema = schema
+		}
+	}
+	return &copied, nil
+}
+
+func cloneJSONValue(source any) (any, error) {
+	return jsonvalue.Copy(source)
+}

internal/usecase/prepared_execution_test.go

@@ -0,0 +1,510 @@
+package usecase
+
+import (
+	"context"
+	"errors"
+	"os"
+	"reflect"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
+	"gitea.maximumdirect.net/eric/promptkit/internal/validate"
+)
+
+type recordingPreparedValidation struct {
+	contract       domain.OutputContract
+	schemaDocument any
+	results        []domain.ValidationResult
+	errs           []error
+	artifacts      []string
+}
+
+func (p *recordingPreparedValidation) Validate(
+	_ context.Context,
+	artifact *domain.Artifact,
+) (domain.ValidationResult, error) {
+	p.artifacts = append(p.artifacts, string(artifact.Body))
+	index := len(p.artifacts) - 1
+	if index < len(p.errs) && p.errs[index] != nil {
+		return domain.ValidationResult{}, p.errs[index]
+	}
+	if len(p.results) == 0 {
+		return domain.ValidationResult{
+			Status:  domain.ValidationPassed,
+			Mode:    p.contract.ValidationMode,
+			IsValid: true,
+		}, nil
+	}
+	if index >= len(p.results) {
+		index = len(p.results) - 1
+	}
+	return p.results[index], nil
+}
+
+func (p *recordingPreparedValidation) SchemaDocument() any {
+	return p.schemaDocument
+}
+
+type recordingValidationPreparer struct {
+	plan                *recordingPreparedValidation
+	prepareErr          error
+	prepareCalls        int
+	directValidateCalls int
+}
+
+func (v *recordingValidationPreparer) Validate(
+	context.Context,
+	*domain.Artifact,
+	domain.OutputContract,
+) (domain.ValidationResult, error) {
+	v.directValidateCalls++
+	return domain.ValidationResult{}, errors.New("live validation must not be used")
+}
+
+func (v *recordingValidationPreparer) PrepareValidation(
+	_ context.Context,
+	contract domain.OutputContract,
+) (validate.PreparedValidation, error) {
+	v.prepareCalls++
+	if v.prepareErr != nil {
+		return nil, v.prepareErr
+	}
+	v.plan.contract = contract
+	return v.plan, nil
+}
+
+func TestRunnerPrepareExecutionCompletesWithoutAdmissionOrGeneration(t *testing.T) {
+	schemaDocument := map[string]any{
+		"type": "object",
+		"properties": map[string]any{
+			"value": map[string]any{"type": "string"},
+			"":      map[string]any{"type": "boolean"},
+		},
+	}
+	def := promptDef(domain.FormatJSON, domain.ValidationJSONSchema, 0)
+	def.Validation.SchemaPath = "schema.json"
+	reader := defaultArtifactReader()
+	renderer := &fakeRenderer{rendered: &domain.RenderedPrompt{
+		SessionID: "prepared-session",
+		Messages: []domain.RenderedMessage{{
+			Role:    "user",
+			Content: "original message",
+		}},
+	}}
+	llmClient := &fakeLLM{forbid: true}
+	validator := &recordingValidationPreparer{
+		plan: &recordingPreparedValidation{schemaDocument: schemaDocument},
+	}
+	admitter := &fakeRunAdmitter{}
+	profile := defaultExecutionProfile()
+	profile.ExtraParams = map[string]any{
+		"metadata": map[string]any{"source": "original"},
+	}
+	runner := NewRunner(
+		&fakePromptRepo{def: def},
+		&fakeExecutionProfileRepo{profiles: map[string]*domain.ExecutionProfile{"exec": profile}},
+		nil,
+		reader,
+		renderer,
+		llmClient,
+		validator,
+		admitter,
+	)
+
+	prepared, err := runner.PrepareExecution(context.Background(), domain.RunRequest{
+		PromptID:  "p",
+		ProfileID: "exec",
+		APIKey:    "direct-test-key",
+		Inputs:    singleInputRef(),
+	})
+	if err != nil {
+		t.Fatalf("prepare execution: %v", err)
+	}
+	defer prepared.Discard()
+
+	if validator.prepareCalls != 1 || validator.directValidateCalls != 0 {
+		t.Fatalf(
+			"validation calls=(prepare=%d direct=%d), want (1, 0)",
+			validator.prepareCalls,
+			validator.directValidateCalls,
+		)
+	}
+	if reader.calls != 1 || renderer.calls != 1 {
+		t.Fatalf("completion calls=(artifact=%d render=%d), want (1, 1)", reader.calls, renderer.calls)
+	}
+	if len(admitter.backendIDs) != 0 || llmClient.calls != 0 {
+		t.Fatalf("prepare invoked execution collaborators: admission=%v generation=%d", admitter.backendIDs, llmClient.calls)
+	}
+
+	first := prepared.Details()
+	if first == nil {
+		t.Fatal("prepared details are nil")
+	}
+	if first.EffectiveModelParams.APIKey != "" {
+		t.Fatal("prepared details retained the direct API key")
+	}
+	if first.StructuredOutput == nil ||
+		first.StructuredOutput.JSONSchema == nil ||
+		!reflect.DeepEqual(first.StructuredOutput.JSONSchema.Schema, schemaDocument) {
+		t.Fatalf("prepared details have unexpected structured output: %#v", first.StructuredOutput)
+	}
+
+	first.Messages[0].Content = "caller mutation"
+	first.InputHashes["input"] = "caller mutation"
+	first.EffectiveModelParams.ExtraParams["metadata"].(map[string]any)["source"] = "caller mutation"
+	first.StructuredOutput.JSONSchema.Schema.(map[string]any)["type"] = "string"
+	renderer.rendered.Messages[0].Content = "source mutation"
+
+	second := prepared.Details()
+	if second.Messages[0].Content != "original message" ||
+		second.InputHashes["input"] == "caller mutation" ||
+		second.EffectiveModelParams.ExtraParams["metadata"].(map[string]any)["source"] != "original" ||
+		second.StructuredOutput.JSONSchema.Schema.(map[string]any)["type"] != "object" {
+		t.Fatalf("details did not preserve an independent snapshot: %#v", second)
+	}
+}
+
+func TestRunnerRunPreparedRechecksEnvironmentCredentialBeforeAdmission(t *testing.T) {
+	const environmentName = "PROMPTKIT_PREPARED_EXECUTION_TEST_KEY"
+	t.Setenv(environmentName, "available-during-preparation")
+
+	profile := defaultExecutionProfile()
+	profile.APIKeyEnv = environmentName
+	validator := &recordingValidationPreparer{plan: &recordingPreparedValidation{}}
+	admitter := &fakeRunAdmitter{}
+	llmClient := &fakeLLM{resp: &domain.GenerateResponse{Content: "unexpected"}}
+	runner := NewRunner(
+		&fakePromptRepo{def: promptDef(domain.FormatText, domain.ValidationNone, 0)},
+		&fakeExecutionProfileRepo{profiles: map[string]*domain.ExecutionProfile{"exec": profile}},
+		nil,
+		defaultArtifactReader(),
+		defaultRenderer(),
+		llmClient,
+		validator,
+		admitter,
+	)
+	prepared, err := runner.PrepareExecution(context.Background(), domain.RunRequest{
+		PromptID:  "p",
+		ProfileID: "exec",
+		Inputs:    singleInputRef(),
+	})
+	if err != nil {
+		t.Fatalf("prepare execution: %v", err)
+	}
+	if err := os.Unsetenv(environmentName); err != nil {
+		t.Fatalf("unset credential environment: %v", err)
+	}
+
+	result, err := runner.RunPrepared(context.Background(), prepared)
+	if result != nil {
+		t.Fatalf("credential failure returned partial result: %+v", result)
+	}
+	if !errors.Is(err, ErrInvalidRequest) || !errors.Is(err, ErrAPIKeyEnvMissing) {
+		t.Fatalf("credential error identities are missing: %v", err)
+	}
+	if len(admitter.backendIDs) != 0 || llmClient.calls != 0 {
+		t.Fatalf("credential failure reached admission or generation: admission=%v generation=%d", admitter.backendIDs, llmClient.calls)
+	}
+	if _, err := runner.RunPrepared(context.Background(), prepared); !errors.Is(err, ErrInvalidRequest) {
+		t.Fatalf("credential failure did not consume execution: %v", err)
+	}
+}
+
+func TestRunnerRunPreparedKeepsDirectCredentialOutOfMetadata(t *testing.T) {
+	const directKey = "direct-prepared-test-key"
+
+	profile := defaultExecutionProfile()
+	profile.APIKeyRequired = true
+	client := &fakeLLM{resp: &domain.GenerateResponse{Content: "ok"}}
+	runner := NewRunner(
+		&fakePromptRepo{def: promptDef(domain.FormatText, domain.ValidationNone, 0)},
+		&fakeExecutionProfileRepo{profiles: map[string]*domain.ExecutionProfile{"exec": profile}},
+		nil,
+		defaultArtifactReader(),
+		defaultRenderer(),
+		client,
+		&recordingValidationPreparer{plan: &recordingPreparedValidation{}},
+		nil,
+	)
+
+	prepared, err := runner.PrepareExecution(context.Background(), domain.RunRequest{
+		PromptID:  "p",
+		ProfileID: "exec",
+		APIKey:    directKey,
+		Inputs:    singleInputRef(),
+	})
+	if err != nil {
+		t.Fatalf("prepare execution: %v", err)
+	}
+	if details := prepared.Details(); details.EffectiveModelParams.APIKey != "" {
+		t.Fatal("prepared details retained direct credential")
+	}
+
+	result, err := runner.RunPrepared(context.Background(), prepared)
+	if err != nil {
+		t.Fatalf("run prepared: %v", err)
+	}
+	if client.lastReq.Target.APIKey != directKey {
+		t.Fatal("generation did not receive direct credential")
+	}
+	if result.EffectiveModelParams.APIKey != "" {
+		t.Fatal("run result retained direct credential")
+	}
+}
+
+func TestRunnerRunPreparedUsesFrozenValidationForInitialAndRepairOutputs(t *testing.T) {
+	validator := &recordingValidationPreparer{
+		plan: &recordingPreparedValidation{
+			results: []domain.ValidationResult{
+				{
+					Status:  domain.ValidationFailed,
+					Mode:    domain.ValidationJSON,
+					Errors:  []string{"invalid"},
+					IsValid: false,
+				},
+				{
+					Status:  domain.ValidationPassed,
+					Mode:    domain.ValidationJSON,
+					IsValid: true,
+				},
+			},
+		},
+	}
+	repairer := &fakeRepairer{
+		responses: []*domain.GenerateResponse{{Content: `{"repaired":true}`}},
+	}
+	admitter := &fakeRunAdmitter{}
+	reader := defaultArtifactReader()
+	renderer := defaultRenderer()
+	runner := NewRunnerWithRepairer(
+		&fakePromptRepo{def: promptDef(domain.FormatJSON, domain.ValidationJSON, 1)},
+		&fakeExecutionProfileRepo{profiles: map[string]*domain.ExecutionProfile{"exec": defaultExecutionProfile()}},
+		nil,
+		reader,
+		renderer,
+		&fakeLLM{resp: &domain.GenerateResponse{Content: `{"broken":true}`}},
+		validator,
+		repairer,
+		admitter,
+	)
+	prepared, err := runner.PrepareExecution(context.Background(), domain.RunRequest{
+		PromptID:  "p",
+		ProfileID: "exec",
+		Inputs:    singleInputRef(),
+	})
+	if err != nil {
+		t.Fatalf("prepare execution: %v", err)
+	}
+
+	result, err := runner.RunPrepared(context.Background(), prepared)
+	if err != nil {
+		t.Fatalf("run prepared: %v", err)
+	}
+	if !reflect.DeepEqual(validator.plan.artifacts, []string{`{"broken":true}`, `{"repaired":true}`}) {
+		t.Fatalf("prepared validation artifacts=%#v", validator.plan.artifacts)
+	}
+	if validator.directValidateCalls != 0 || repairer.calls != 1 {
+		t.Fatalf("validation/repair calls=(direct=%d repair=%d), want (0, 1)", validator.directValidateCalls, repairer.calls)
+	}
+	if result.Validation.Status != domain.ValidationPassed || result.Validation.RepairAttempts != 1 {
+		t.Fatalf("unexpected repaired validation result: %+v", result.Validation)
+	}
+	if admitter.releaseCalls != 1 {
+		t.Fatalf("admission releases=%d, want 1", admitter.releaseCalls)
+	}
+	if reader.calls != 1 || renderer.calls != 1 {
+		t.Fatalf("execution reopened preparation sources: artifact=%d render=%d", reader.calls, renderer.calls)
+	}
+}
+
+func TestRunnerRunPreparedReleasesAdmissionAcrossExecutionErrors(t *testing.T) {
+	generationFailure := errors.New("generation failed")
+	validationFailure := errors.New("validation failed")
+	repairFailure := errors.New("repair failed")
+
+	tests := []struct {
+		name          string
+		generationErr error
+		validation    *recordingPreparedValidation
+		repairer      *fakeRepairer
+		wantError     error
+	}{
+		{
+			name:          "generation failure",
+			generationErr: generationFailure,
+			validation:    &recordingPreparedValidation{},
+			wantError:     ErrLLMGenerate,
+		},
+		{
+			name: "validation failure",
+			validation: &recordingPreparedValidation{
+				errs: []error{validationFailure},
+			},
+			wantError: ErrValidation,
+		},
+		{
+			name: "repair failure",
+			validation: &recordingPreparedValidation{
+				results: []domain.ValidationResult{{
+					Status:  domain.ValidationFailed,
+					Mode:    domain.ValidationJSON,
+					Errors:  []string{"invalid"},
+					IsValid: false,
+				}},
+			},
+			repairer:  &fakeRepairer{err: repairFailure},
+			wantError: ErrValidation,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			def := promptDef(domain.FormatJSON, domain.ValidationJSON, 1)
+			if test.repairer == nil {
+				def.Validation.RepairAttempts = 0
+			}
+			validator := &recordingValidationPreparer{plan: test.validation}
+			admitter := &fakeRunAdmitter{}
+			runner := NewRunnerWithRepairer(
+				&fakePromptRepo{def: def},
+				&fakeExecutionProfileRepo{profiles: map[string]*domain.ExecutionProfile{"exec": defaultExecutionProfile()}},
+				nil,
+				defaultArtifactReader(),
+				defaultRenderer(),
+				&fakeLLM{
+					resp: &domain.GenerateResponse{Content: `{"value":true}`},
+					err:  test.generationErr,
+				},
+				validator,
+				test.repairer,
+				admitter,
+			)
+			prepared, err := runner.PrepareExecution(context.Background(), domain.RunRequest{
+				PromptID:  "p",
+				ProfileID: "exec",
+				Inputs:    singleInputRef(),
+			})
+			if err != nil {
+				t.Fatalf("prepare execution: %v", err)
+			}
+
+			result, err := runner.RunPrepared(context.Background(), prepared)
+			if result != nil || !errors.Is(err, test.wantError) {
+				t.Fatalf("run prepared=(%+v, %v), want %v", result, err, test.wantError)
+			}
+			if len(admitter.backendIDs) != 1 || admitter.releaseCalls != 1 {
+				t.Fatalf(
+					"admission calls=%#v releases=%d, want one each",
+					admitter.backendIDs,
+					admitter.releaseCalls,
+				)
+			}
+		})
+	}
+}
+
+func TestRunnerPreparedExecutionOwnershipUseAndDiscard(t *testing.T) {
+	newRunner := func() *Runner {
+		return NewRunner(
+			&fakePromptRepo{def: promptDef(domain.FormatText, domain.ValidationNone, 0)},
+			&fakeExecutionProfileRepo{profiles: map[string]*domain.ExecutionProfile{"exec": defaultExecutionProfile()}},
+			nil,
+			defaultArtifactReader(),
+			defaultRenderer(),
+			&fakeLLM{resp: &domain.GenerateResponse{Content: "ok"}},
+			&recordingValidationPreparer{plan: &recordingPreparedValidation{}},
+			nil,
+		)
+	}
+	request := domain.RunRequest{
+		PromptID:  "p",
+		ProfileID: "exec",
+		Inputs:    singleInputRef(),
+	}
+
+	owner := newRunner()
+	prepared, err := owner.PrepareExecution(context.Background(), request)
+	if err != nil {
+		t.Fatalf("prepare execution: %v", err)
+	}
+	if _, err := newRunner().RunPrepared(context.Background(), prepared); !errors.Is(err, ErrInvalidRequest) {
+		t.Fatalf("foreign runner error=%v, want ErrInvalidRequest", err)
+	}
+	if _, err := owner.RunPrepared(context.Background(), prepared); err != nil {
+		t.Fatalf("owner run prepared: %v", err)
+	}
+	if _, err := owner.RunPrepared(context.Background(), prepared); !errors.Is(err, ErrInvalidRequest) {
+		t.Fatalf("second owner run error=%v, want ErrInvalidRequest", err)
+	}
+	if prepared.Details() == nil {
+		t.Fatal("details unavailable after execution")
+	}
+
+	discarded, err := owner.PrepareExecution(context.Background(), request)
+	if err != nil {
+		t.Fatalf("prepare discarded execution: %v", err)
+	}
+	discarded.Discard()
+	discarded.Discard()
+	if _, err := owner.RunPrepared(context.Background(), discarded); !errors.Is(err, ErrInvalidRequest) {
+		t.Fatalf("discarded execution error=%v, want ErrInvalidRequest", err)
+	}
+	if discarded.Details() == nil {
+		t.Fatal("details unavailable after discard")
+	}
+}
+
+func TestRunnerPreparedExecutionWithoutValidatorSkipsValidation(t *testing.T) {
+	runner := NewRunner(
+		&fakePromptRepo{def: promptDef(domain.FormatJSON, domain.ValidationJSON, 0)},
+		&fakeExecutionProfileRepo{profiles: map[string]*domain.ExecutionProfile{"exec": defaultExecutionProfile()}},
+		nil,
+		defaultArtifactReader(),
+		defaultRenderer(),
+		&fakeLLM{resp: &domain.GenerateResponse{Content: `{}`}},
+		nil,
+		nil,
+	)
+	prepared, err := runner.PrepareExecution(context.Background(), domain.RunRequest{
+		PromptID:  "p",
+		ProfileID: "exec",
+		Inputs:    singleInputRef(),
+	})
+	if err != nil {
+		t.Fatalf("prepare execution: %v", err)
+	}
+
+	result, err := runner.RunPrepared(context.Background(), prepared)
+	if err != nil {
+		t.Fatalf("run prepared: %v", err)
+	}
+	if result.Validation.Status != domain.ValidationSkipped || !result.Validation.IsValid {
+		t.Fatalf("unexpected no-validator result: %+v", result.Validation)
+	}
+}
+
+func TestRunnerPrepareExecutionRequiresValidationPreparer(t *testing.T) {
+	reader := defaultArtifactReader()
+	runner := NewRunner(
+		&fakePromptRepo{def: promptDef(domain.FormatText, domain.ValidationBasic, 0)},
+		&fakeExecutionProfileRepo{profiles: map[string]*domain.ExecutionProfile{"exec": defaultExecutionProfile()}},
+		nil,
+		reader,
+		defaultRenderer(),
+		&fakeLLM{forbid: true},
+		&fakeValidator{},
+		nil,
+	)
+
+	prepared, err := runner.PrepareExecution(context.Background(), domain.RunRequest{
+		PromptID:  "p",
+		ProfileID: "exec",
+		Inputs:    singleInputRef(),
+	})
+	if prepared != nil || !errors.Is(err, ErrValidation) {
+		t.Fatalf("prepare execution=(%+v, %v), want ErrValidation", prepared, err)
+	}
+	if reader.calls != 0 {
+		t.Fatalf("unsupported validator allowed completion, artifact calls=%d", reader.calls)
+	}
+}

internal/usecase/profile_inspection.go

@@ -0,0 +1,103 @@
+package usecase
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"strings"
+
+	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
+)
+
+type resolvedProfileSelection struct {
+	id      string
+	profile *domain.ExecutionProfile
+	backend *domain.Backend
+}
+
+func (r *Runner) resolveProfileSelection(
+	ctx context.Context,
+	profileID string,
+) (*resolvedProfileSelection, error) {
+	normalizedID := strings.TrimSpace(profileID)
+	if normalizedID == "" {
+		return nil, fmt.Errorf("%w: profile id is required", ErrInvalidRequest)
+	}
+	if r == nil || r.profiles == nil {
+		return nil, fmt.Errorf("%w: profile repository is not configured", ErrProfileLoad)
+	}
+
+	selectedProfile, err := r.profiles.GetProfile(ctx, normalizedID)
+	if err != nil {
+		return nil, fmt.Errorf("%w: %w", ErrProfileLoad, err)
+	}
+	if selectedProfile == nil {
+		return nil, fmt.Errorf("%w: profile repository returned nil profile", ErrProfileLoad)
+	}
+
+	profileValue := *selectedProfile
+	profileValue.BackendID = strings.TrimSpace(profileValue.BackendID)
+
+	var selectedBackend *domain.Backend
+	if profileValue.BackendID != "" {
+		if r.backends == nil {
+			return nil, fmt.Errorf("%w: backend %q cannot be resolved", ErrProfileLoad, profileValue.BackendID)
+		}
+		backendValue, err := r.backends.GetBackend(profileValue.BackendID)
+		if err != nil {
+			return nil, fmt.Errorf("%w: backend %q: %w", ErrProfileLoad, profileValue.BackendID, err)
+		}
+		selectedBackend = &backendValue
+	}
+
+	return &resolvedProfileSelection{
+		id:      normalizedID,
+		profile: &profileValue,
+		backend: selectedBackend,
+	}, nil
+}
+
+func validateResolvedExecutionTarget(target domain.ExecutionTarget) error {
+	if strings.TrimSpace(target.Endpoint) == "" {
+		return errors.New("execution endpoint is required")
+	}
+	if strings.TrimSpace(target.Model) == "" {
+		return errors.New("execution model is required")
+	}
+	return nil
+}
+
+// InspectProfile resolves one explicit profile without prompt or execution work.
+func (r *Runner) InspectProfile(
+	ctx context.Context,
+	profileID string,
+) (*domain.ProfileInspection, error) {
+	normalizedID := strings.TrimSpace(profileID)
+	if normalizedID == "" {
+		return nil, fmt.Errorf("%w: profile id is required", ErrInvalidRequest)
+	}
+	select {
+	case <-ctx.Done():
+		return nil, fmt.Errorf("%w: %w", ErrProfileLoad, ctx.Err())
+	default:
+	}
+
+	selection, err := r.resolveProfileSelection(ctx, normalizedID)
+	if err != nil {
+		return nil, err
+	}
+	target, _, err := resolveExecutionTarget(selection.backend, selection.profile, nil)
+	if err != nil {
+		return nil, fmt.Errorf("%w: %w", ErrProfileLoad, err)
+	}
+	if err := validateResolvedExecutionTarget(target); err != nil {
+		return nil, fmt.Errorf("%w: %w", ErrProfileLoad, err)
+	}
+	target.APIKey = ""
+
+	return &domain.ProfileInspection{
+		ProfileID:            selection.id,
+		EffectiveModelParams: target,
+		APIKeyRequired:       target.APIKeyRequired,
+	}, nil
+}

internal/usecase/profile_inspection_test.go

@@ -0,0 +1,213 @@
+package usecase
+
+import (
+	"context"
+	"errors"
+	"reflect"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/promptkit/internal/defaults"
+	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
+	"gitea.maximumdirect.net/eric/promptkit/internal/profile"
+)
+
+type inspectionProfileRepository struct {
+	profile *domain.ExecutionProfile
+	err     error
+	calls   int
+	id      string
+}
+
+func (r *inspectionProfileRepository) GetProfile(
+	_ context.Context,
+	id string,
+) (*domain.ExecutionProfile, error) {
+	r.calls++
+	r.id = id
+	if r.err != nil {
+		return nil, r.err
+	}
+	return r.profile, nil
+}
+
+type inspectionBackendResolver struct {
+	backend domain.Backend
+	err     error
+	calls   int
+	id      string
+}
+
+func (r *inspectionBackendResolver) GetBackend(id string) (domain.Backend, error) {
+	r.calls++
+	r.id = id
+	if r.err != nil {
+		return domain.Backend{}, r.err
+	}
+	return r.backend, nil
+}
+
+func TestRunnerInspectProfileResolvesProfileAndBackendOnce(t *testing.T) {
+	profiles := &inspectionProfileRepository{profile: &domain.ExecutionProfile{
+		ID:              "profile",
+		BackendID:       "  backend  ",
+		Model:           "profile-model",
+		Temperature:     0.4,
+		MaxTokens:       32,
+		TimeoutSeconds:  45,
+		ServiceTier:     "priority",
+		ReasoningEffort: "high",
+		ExtraParams: map[string]any{
+			"profile": "value",
+		},
+	}}
+	backends := &inspectionBackendResolver{backend: domain.Backend{
+		ID:        "backend",
+		Endpoint:  "https://backend.example/v1",
+		APIKeyEnv: "BACKEND_KEY",
+		ExtraParams: map[string]any{
+			"backend": "value",
+		},
+	}}
+	runner := &Runner{profiles: profiles, backends: backends}
+
+	inspection, err := runner.InspectProfile(context.Background(), " profile ")
+	if err != nil {
+		t.Fatalf("inspect profile: %v", err)
+	}
+	if profiles.calls != 1 || profiles.id != "profile" {
+		t.Fatalf("profile lookup=(calls=%d id=%q), want one exact lookup", profiles.calls, profiles.id)
+	}
+	if backends.calls != 1 || backends.id != "backend" {
+		t.Fatalf("backend lookup=(calls=%d id=%q), want one exact lookup", backends.calls, backends.id)
+	}
+	if profiles.profile.BackendID != "  backend  " {
+		t.Fatalf("inspection mutated repository profile backend: %q", profiles.profile.BackendID)
+	}
+
+	wantTarget := domain.ExecutionTarget{
+		BackendID:       "backend",
+		Endpoint:        "https://backend.example/v1",
+		Model:           "profile-model",
+		Temperature:     0.4,
+		MaxTokens:       32,
+		TopP:            defaults.ExecutionTargetDefault().TopP,
+		TimeoutSeconds:  45,
+		ServiceTier:     "priority",
+		ReasoningEffort: "high",
+		APIKeyEnv:       "BACKEND_KEY",
+		ExtraParams: map[string]any{
+			"profile": "value",
+		},
+	}
+	if inspection.ProfileID != "profile" || inspection.APIKeyRequired ||
+		!reflect.DeepEqual(inspection.EffectiveModelParams, wantTarget) {
+		t.Fatalf("inspection=%#v, want profile=%q target=%#v", inspection, "profile", wantTarget)
+	}
+}
+
+func TestRunnerInspectProfileDoesNotNeedExecutionCollaboratorsOrCredentials(t *testing.T) {
+	t.Setenv("PROMPTKIT_INSPECTION_TEST_KEY", "")
+	profiles := &inspectionProfileRepository{profile: &domain.ExecutionProfile{
+		ID:        "endpoint-only",
+		Endpoint:  "https://profile.example/v1",
+		Model:     "profile-model",
+		APIKeyEnv: "PROMPTKIT_INSPECTION_TEST_KEY",
+	}}
+	runner := &Runner{profiles: profiles}
+
+	inspection, err := runner.InspectProfile(context.Background(), "endpoint-only")
+	if err != nil {
+		t.Fatalf("inspect endpoint-only profile: %v", err)
+	}
+	if inspection.EffectiveModelParams.BackendID != "" ||
+		inspection.EffectiveModelParams.APIKeyEnv != "PROMPTKIT_INSPECTION_TEST_KEY" ||
+		inspection.APIKeyRequired {
+		t.Fatalf("unexpected endpoint-only inspection: %#v", inspection)
+	}
+}
+
+func TestRunnerInspectProfileDirectCredentialRequirementClearsBackendEnvironment(t *testing.T) {
+	profiles := &inspectionProfileRepository{profile: &domain.ExecutionProfile{
+		ID:             "direct-key",
+		BackendID:      "backend",
+		Model:          "profile-model",
+		APIKeyRequired: true,
+	}}
+	backends := &inspectionBackendResolver{backend: domain.Backend{
+		ID:        "backend",
+		Endpoint:  "https://backend.example/v1",
+		APIKeyEnv: "BACKEND_KEY",
+	}}
+
+	inspection, err := (&Runner{profiles: profiles, backends: backends}).InspectProfile(
+		context.Background(),
+		"direct-key",
+	)
+	if err != nil {
+		t.Fatalf("inspect direct-key profile: %v", err)
+	}
+	if !inspection.APIKeyRequired || inspection.EffectiveModelParams.APIKeyEnv != "" {
+		t.Fatalf("credential requirement was not resolved exclusively: %#v", inspection)
+	}
+}
+
+func TestRunnerInspectProfileClassifiesFailuresWithoutRepositoryWorkAfterCancellation(t *testing.T) {
+	t.Run("blank ID", func(t *testing.T) {
+		profiles := &inspectionProfileRepository{}
+		_, err := (&Runner{profiles: profiles}).InspectProfile(context.Background(), " \t ")
+		if !errors.Is(err, ErrInvalidRequest) || profiles.calls != 0 {
+			t.Fatalf("blank inspection=(%v, calls=%d), want invalid request without lookup", err, profiles.calls)
+		}
+	})
+
+	t.Run("canceled context", func(t *testing.T) {
+		ctx, cancel := context.WithCancel(context.Background())
+		cancel()
+		profiles := &inspectionProfileRepository{}
+		_, err := (&Runner{profiles: profiles}).InspectProfile(ctx, "profile")
+		if !errors.Is(err, ErrProfileLoad) || !errors.Is(err, context.Canceled) || profiles.calls != 0 {
+			t.Fatalf("canceled inspection=(%v, calls=%d), want profile load and context identities without lookup", err, profiles.calls)
+		}
+	})
+
+	t.Run("missing profile", func(t *testing.T) {
+		profiles := &inspectionProfileRepository{err: profile.ErrProfileNotFound}
+		_, err := (&Runner{profiles: profiles}).InspectProfile(context.Background(), "missing")
+		if !errors.Is(err, ErrProfileLoad) || !errors.Is(err, profile.ErrProfileNotFound) {
+			t.Fatalf("missing profile error=%v, want profile load and not-found identities", err)
+		}
+	})
+
+	t.Run("unknown backend", func(t *testing.T) {
+		backendErr := errors.New("unknown backend")
+		profiles := &inspectionProfileRepository{profile: &domain.ExecutionProfile{
+			ID: "profile", BackendID: "backend", Model: "profile-model",
+		}}
+		backends := &inspectionBackendResolver{err: backendErr}
+		_, err := (&Runner{profiles: profiles, backends: backends}).InspectProfile(context.Background(), "profile")
+		if !errors.Is(err, ErrProfileLoad) || !errors.Is(err, backendErr) {
+			t.Fatalf("unknown backend error=%v, want profile load and backend identities", err)
+		}
+	})
+
+	t.Run("defensive invalid dependencies", func(t *testing.T) {
+		cases := []struct {
+			name   string
+			runner *Runner
+		}{
+			{name: "nil repository", runner: &Runner{}},
+			{name: "nil profile", runner: &Runner{profiles: &inspectionProfileRepository{}}},
+			{name: "invalid target", runner: &Runner{profiles: &inspectionProfileRepository{
+				profile: &domain.ExecutionProfile{ID: "profile", Endpoint: "https://profile.example/v1"},
+			}}},
+		}
+		for _, tc := range cases {
+			t.Run(tc.name, func(t *testing.T) {
+				_, err := tc.runner.InspectProfile(context.Background(), "profile")
+				if !errors.Is(err, ErrProfileLoad) {
+					t.Fatalf("inspection error=%v, want profile load", err)
+				}
+			})
+		}
+	})
+}

internal/usecase/prompt_inspection.go

@@ -0,0 +1,73 @@
+package usecase
+
+import (
+	"context"
+	"fmt"
+	"strings"
+
+	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
+)
+
+type resolvedPromptDefinition struct {
+	definition *domain.PromptDefinition
+	hash       string
+}
+
+func (r *Runner) resolvePromptDefinition(
+	ctx context.Context,
+	promptID string,
+	promptVersion string,
+) (*resolvedPromptDefinition, error) {
+	if strings.TrimSpace(promptID) == "" {
+		return nil, fmt.Errorf("%w: prompt id is required", ErrInvalidRequest)
+	}
+	if r == nil || r.promptDefs == nil {
+		return nil, fmt.Errorf("%w: prompt repository is not configured", ErrPromptLoad)
+	}
+
+	definition, err := r.promptDefs.GetPromptDefinition(ctx, promptID, promptVersion)
+	if err != nil {
+		return nil, fmt.Errorf("%w: %w", ErrPromptLoad, err)
+	}
+	if definition == nil {
+		return nil, fmt.Errorf("%w: prompt repository returned nil definition", ErrPromptLoad)
+	}
+
+	hash, err := hashPromptDefinition(definition)
+	if err != nil {
+		return nil, fmt.Errorf("%w: failed to hash prompt definition: %v", ErrPromptLoad, err)
+	}
+	return &resolvedPromptDefinition{definition: definition, hash: hash}, nil
+}
+
+// InspectPrompt resolves one explicit prompt without execution work.
+func (r *Runner) InspectPrompt(
+	ctx context.Context,
+	promptID string,
+	promptVersion string,
+) (*domain.PromptInspection, error) {
+	if strings.TrimSpace(promptID) == "" {
+		return nil, fmt.Errorf("%w: prompt id is required", ErrInvalidRequest)
+	}
+	select {
+	case <-ctx.Done():
+		return nil, fmt.Errorf("%w: %w", ErrPromptLoad, ctx.Err())
+	default:
+	}
+
+	selection, err := r.resolvePromptDefinition(ctx, promptID, promptVersion)
+	if err != nil {
+		return nil, err
+	}
+	inputs := make([]domain.PromptInput, len(selection.definition.Inputs))
+	copy(inputs, selection.definition.Inputs)
+
+	return &domain.PromptInspection{
+		PromptID:         selection.definition.ID,
+		PromptVersion:    selection.definition.Version,
+		PromptHash:       selection.hash,
+		DefaultProfileID: selection.definition.DefaultProfile,
+		Inputs:           inputs,
+		OutputContract:   selection.definition.Validation,
+	}, nil
+}

internal/usecase/prompt_inspection_test.go

@@ -0,0 +1,157 @@
+package usecase
+
+import (
+	"context"
+	"errors"
+	"reflect"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
+	"gitea.maximumdirect.net/eric/promptkit/internal/promptdef"
+)
+
+type inspectionPromptRepository struct {
+	definition *domain.PromptDefinition
+	err        error
+	calls      int
+	id         string
+	version    string
+}
+
+func (r *inspectionPromptRepository) GetPromptDefinition(
+	_ context.Context,
+	id string,
+	version string,
+) (*domain.PromptDefinition, error) {
+	r.calls++
+	r.id = id
+	r.version = version
+	if r.err != nil {
+		return nil, r.err
+	}
+	return r.definition, nil
+}
+
+func TestRunnerInspectPromptResolvesOneDefinitionWithoutExecutionCollaborators(t *testing.T) {
+	definition := &domain.PromptDefinition{
+		ID:             "normalized.prompt",
+		Version:        "1.2.3",
+		DefaultProfile: "not-resolved",
+		Inputs: []domain.PromptInput{
+			{Name: "document", Required: true, ContentType: "text/plain", Description: "Source document."},
+			{Name: "audience", ContentType: "text/plain", Description: "Intended reader."},
+		},
+		Validation: domain.OutputContract{
+			Format:         domain.FormatJSON,
+			ValidationMode: domain.ValidationJSONSchema,
+			SchemaPath:     "schemas/result.json",
+			RepairAttempts: 2,
+		},
+	}
+	repository := &inspectionPromptRepository{definition: definition}
+	runner := &Runner{promptDefs: repository}
+
+	inspection, err := runner.InspectPrompt(context.Background(), " prompt-id ", " version ")
+	if err != nil {
+		t.Fatalf("inspect prompt: %v", err)
+	}
+	wantHash, err := hashPromptDefinition(definition)
+	if err != nil {
+		t.Fatalf("hash prompt definition: %v", err)
+	}
+	if repository.calls != 1 || repository.id != " prompt-id " || repository.version != " version " {
+		t.Fatalf("prompt lookup=(calls=%d id=%q version=%q), want one unchanged lookup", repository.calls, repository.id, repository.version)
+	}
+	if inspection.PromptID != definition.ID ||
+		inspection.PromptVersion != definition.Version ||
+		inspection.PromptHash != wantHash ||
+		inspection.DefaultProfileID != definition.DefaultProfile ||
+		!reflect.DeepEqual(inspection.Inputs, definition.Inputs) ||
+		inspection.OutputContract != definition.Validation {
+		t.Fatalf("inspection=%#v, want definition metadata", inspection)
+	}
+
+	inspection.Inputs[0].Name = "changed"
+	second, err := runner.InspectPrompt(context.Background(), " prompt-id ", " version ")
+	if err != nil {
+		t.Fatalf("inspect prompt again: %v", err)
+	}
+	if definition.Inputs[0].Name != "document" || second.Inputs[0].Name != "document" {
+		t.Fatalf("inspection input mutation escaped caller result: definition=%#v next=%#v", definition.Inputs, second.Inputs)
+	}
+}
+
+func TestRunnerInspectPromptClassifiesFailuresWithoutRepositoryWorkAfterCancellation(t *testing.T) {
+	t.Run("blank ID", func(t *testing.T) {
+		repository := &inspectionPromptRepository{}
+		_, err := (&Runner{promptDefs: repository}).InspectPrompt(context.Background(), " \t ", "version")
+		if !errors.Is(err, ErrInvalidRequest) || repository.calls != 0 {
+			t.Fatalf("blank inspection=(%v, calls=%d), want invalid request without lookup", err, repository.calls)
+		}
+	})
+
+	t.Run("canceled context", func(t *testing.T) {
+		ctx, cancel := context.WithCancel(context.Background())
+		cancel()
+		repository := &inspectionPromptRepository{}
+		_, err := (&Runner{promptDefs: repository}).InspectPrompt(ctx, "prompt", "version")
+		if !errors.Is(err, ErrPromptLoad) || !errors.Is(err, context.Canceled) || repository.calls != 0 {
+			t.Fatalf("canceled inspection=(%v, calls=%d), want prompt load and context identities without lookup", err, repository.calls)
+		}
+	})
+
+	t.Run("missing prompt", func(t *testing.T) {
+		repository := &inspectionPromptRepository{err: promptdef.ErrPromptDefinitionNotFound}
+		_, err := (&Runner{promptDefs: repository}).InspectPrompt(context.Background(), "missing", "version")
+		if !errors.Is(err, ErrPromptLoad) || !errors.Is(err, promptdef.ErrPromptDefinitionNotFound) {
+			t.Fatalf("missing prompt error=%v, want prompt load and not-found identities", err)
+		}
+	})
+
+	t.Run("defensive prompt dependencies", func(t *testing.T) {
+		var nilRunner *Runner
+		cases := []struct {
+			name   string
+			runner *Runner
+		}{
+			{name: "nil runner", runner: nilRunner},
+			{name: "nil repository", runner: &Runner{}},
+			{name: "nil definition", runner: &Runner{promptDefs: &inspectionPromptRepository{}}},
+		}
+		for _, tc := range cases {
+			t.Run(tc.name, func(t *testing.T) {
+				_, err := tc.runner.InspectPrompt(context.Background(), "prompt", "version")
+				if !errors.Is(err, ErrPromptLoad) {
+					t.Fatalf("inspection error=%v, want prompt load", err)
+				}
+			})
+		}
+	})
+}
+
+func TestRunnerPrepareUsesThePromptInspectionSelectionAndHash(t *testing.T) {
+	definition := promptDef(domain.FormatMarkdown, domain.ValidationBasic, 0)
+	repository := &fakePromptRepo{def: definition}
+	runner := NewRunner(
+		repository,
+		&fakeExecutionProfileRepo{profiles: map[string]*domain.ExecutionProfile{"exec": defaultExecutionProfile()}},
+		nil,
+		&fakeArtifactReader{},
+		&fakeRenderer{rendered: &domain.RenderedPrompt{Messages: []domain.RenderedMessage{{Role: "user", Content: "hello"}}}},
+		nil,
+		nil,
+		nil,
+	)
+
+	inspection, err := runner.InspectPrompt(context.Background(), definition.ID, definition.Version)
+	if err != nil {
+		t.Fatalf("inspect prompt: %v", err)
+	}
+	prepared, err := runner.Prepare(context.Background(), domain.RunRequest{PromptID: definition.ID, PromptVersion: definition.Version, ProfileID: "exec"})
+	if err != nil {
+		t.Fatalf("prepare prompt: %v", err)
+	}
+	if inspection.PromptHash != prepared.PromptHash {
+		t.Fatalf("inspection hash=%q, preparation hash=%q", inspection.PromptHash, prepared.PromptHash)
+	}
+}

internal/usecase/runner.go

@@ -131,29 +131,46 @@ func (r *Runner) Run(ctx context.Context, req domain.RunRequest) (*domain.RunRes
 		return nil, err
 	}
 
-	if r.admitter != nil {
-		release, admitErr := r.admitter.Admit(ctx, state.effectiveModel.BackendID)
-		if admitErr != nil {
-			if errors.Is(admitErr, capacity.ErrCapacityExceeded) {
-				return nil, fmt.Errorf(
-					"backend %q admission: %w",
-					state.effectiveModel.BackendID,
-					admitErr,
-				)
-			}
-			return nil, admitErr
+	release, err := r.admitRun(ctx, state.effectiveModel.BackendID)
+	if err != nil {
+		return nil, err
 	}
 	defer release()
-	}
 
 	prepared, err := r.completePreparation(ctx, req, state)
 	if err != nil {
 		return nil, err
 	}
+	directAPIKey := state.effectiveModel.APIKey
 
+	return r.executePreparedRun(ctx, prepared, directAPIKey, runID, start, func(
+		ctx context.Context,
+		artifact *domain.Artifact,
+		attemptsUsed int,
+	) (domain.ValidationResult, error) {
+		return r.validateOutput(ctx, artifact, prepared.OutputContract, attemptsUsed)
+	})
+}
+
+type preparedValidationFunc func(
+	context.Context,
+	*domain.Artifact,
+	int,
+) (domain.ValidationResult, error)
+
+func (r *Runner) executePreparedRun(
+	ctx context.Context,
+	prepared *domain.PreparedRun,
+	directAPIKey string,
+	runID string,
+	start time.Time,
+	validateArtifact preparedValidationFunc,
+) (*domain.RunResult, error) {
+	executionTarget := prepared.EffectiveModelParams
+	executionTarget.APIKey = directAPIKey
 	genResp, err := r.llm.Generate(ctx, domain.GenerateRequest{
 		Prompt:           domain.RenderedPrompt{SessionID: prepared.SessionID, Messages: prepared.Messages},
-		Target:           prepared.EffectiveModelParams,
+		Target:           executionTarget,
 		TargetPresence:   prepared.TargetPresence,
 		StructuredOutput: prepared.StructuredOutput,
 	})
@@ -165,7 +182,7 @@ func (r *Runner) Run(ctx context.Context, req domain.RunRequest) (*domain.RunRes
 	}
 
 	outputArtifact := buildOutputArtifact(genResp.Content, prepared.OutputContract.Format)
-	validationResult, err := r.validateOutput(ctx, &outputArtifact, prepared.OutputContract, 0)
+	validationResult, err := validateArtifact(ctx, &outputArtifact, 0)
 	if err != nil {
 		return nil, fmt.Errorf("%w: %w", ErrValidation, err)
 	}
@@ -179,7 +196,7 @@ func (r *Runner) Run(ctx context.Context, req domain.RunRequest) (*domain.RunRes
 				PreviousOutput:   genResp.Content,
 				ValidationErrors: validationResult.Errors,
 				SessionID:        prepared.SessionID,
-				Target:           prepared.EffectiveModelParams,
+				Target:           executionTarget,
 				StructuredOutput: prepared.StructuredOutput,
 				Attempt:          attemptsUsed,
 				MaxAttempts:      prepared.OutputContract.RepairAttempts,
@@ -195,7 +212,7 @@ func (r *Runner) Run(ctx context.Context, req domain.RunRequest) (*domain.RunRes
 			genResp = repairResp
 			outputArtifact = buildOutputArtifact(genResp.Content, prepared.OutputContract.Format)
 
-			validationResult, err = r.validateOutput(ctx, &outputArtifact, prepared.OutputContract, attemptsUsed)
+			validationResult, err = validateArtifact(ctx, &outputArtifact, attemptsUsed)
 			if err != nil {
 				return nil, fmt.Errorf("%w: %w", ErrValidation, err)
 			}
@@ -203,6 +220,7 @@ func (r *Runner) Run(ctx context.Context, req domain.RunRequest) (*domain.RunRes
 	}
 
 	end := time.Now().UTC()
+	executionTarget.APIKey = ""
 
 	return &domain.RunResult{
 		RunID:                runID,
@@ -218,7 +236,7 @@ func (r *Runner) Run(ctx context.Context, req domain.RunRequest) (*domain.RunRes
 		SelectedBackendID:    prepared.SelectedBackendID,
 		ModelName:            prepared.EffectiveModelParams.Model,
 		Endpoint:             prepared.EffectiveModelParams.Endpoint,
-		EffectiveModelParams: prepared.EffectiveModelParams,
+		EffectiveModelParams: executionTarget,
 		InputHashes:          prepared.InputHashes,
 		Usage:                genResp.Usage,
 		StartTime:            start,
@@ -248,14 +266,12 @@ func (r *Runner) resolvePreparation(
 		return nil, fmt.Errorf("%w: session_id: %v", ErrInvalidRequest, err)
 	}
 
-	def, err := r.promptDefs.GetPromptDefinition(ctx, req.PromptID, req.PromptVersion)
+	promptSelection, err := r.resolvePromptDefinition(ctx, req.PromptID, req.PromptVersion)
 	if err != nil {
-		return nil, fmt.Errorf("%w: %w", ErrPromptLoad, err)
-	}
-	promptDefinitionHash, err := hashPromptDefinition(def)
-	if err != nil {
-		return nil, fmt.Errorf("%w: failed to hash prompt definition: %v", ErrPromptLoad, err)
+		return nil, err
 	}
+	def := promptSelection.definition
+	promptDefinitionHash := promptSelection.hash
 
 	selectedProfileID := strings.TrimSpace(req.ProfileID)
 	if selectedProfileID == "" {
@@ -265,34 +281,18 @@ func (r *Runner) resolvePreparation(
 		return nil, fmt.Errorf("%w: %w: profile id is required either in request or prompt default_profile", ErrInvalidRequest, ErrProfileRequired)
 	}
 
-	execProfile, err := r.profiles.GetProfile(ctx, selectedProfileID)
+	selection, err := r.resolveProfileSelection(ctx, selectedProfileID)
 	if err != nil {
-		return nil, fmt.Errorf("%w: %w", ErrProfileLoad, err)
+		return nil, err
 	}
 
-	var selectedBackend *domain.Backend
-	if backendID := strings.TrimSpace(execProfile.BackendID); backendID != "" {
-		execProfile.BackendID = backendID
-		if r.backends == nil {
-			return nil, fmt.Errorf("%w: backend %q cannot be resolved", ErrProfileLoad, backendID)
-		}
-		resolvedBackend, resolveErr := r.backends.GetBackend(backendID)
-		if resolveErr != nil {
-			return nil, fmt.Errorf("%w: backend %q: %w", ErrProfileLoad, backendID, resolveErr)
-		}
-		selectedBackend = &resolvedBackend
-	}
-
-	effectiveModel, targetPresence, err := resolveExecutionTarget(selectedBackend, execProfile, req.Execution)
+	effectiveModel, targetPresence, err := resolveExecutionTarget(selection.backend, selection.profile, req.Execution)
 	if err != nil {
 		return nil, fmt.Errorf("%w: %w", ErrInvalidRequest, err)
 	}
 	effectiveModel.APIKey = req.APIKey
-	if strings.TrimSpace(effectiveModel.Endpoint) == "" {
-		return nil, fmt.Errorf("%w: execution endpoint is required", ErrInvalidRequest)
-	}
-	if strings.TrimSpace(effectiveModel.Model) == "" {
-		return nil, fmt.Errorf("%w: execution model is required", ErrInvalidRequest)
+	if err := validateResolvedExecutionTarget(effectiveModel); err != nil {
+		return nil, fmt.Errorf("%w: %w", ErrInvalidRequest, err)
 	}
 	if err := validateAPIKey(effectiveModel.APIKeyEnv, effectiveModel.APIKey, effectiveModel.APIKeyRequired); err != nil {
 		return nil, fmt.Errorf("%w: %w", ErrInvalidRequest, err)
@@ -303,7 +303,7 @@ func (r *Runner) resolvePreparation(
 		definition:           def,
 		directSessionID:      directSessionID,
 		promptDefinitionHash: promptDefinitionHash,
-		selectedProfileID:    selectedProfileID,
+		selectedProfileID:    selection.id,
 		effectiveModel:       effectiveModel,
 		targetPresence:       targetPresence,
 		effectiveContract:    effectiveContract,
@@ -324,7 +324,15 @@ func (r *Runner) completePreparation(
 	if err != nil {
 		return nil, err
 	}
+	return r.completePreparationWithStructuredOutput(ctx, req, state, structuredOutput)
+}
 
+func (r *Runner) completePreparationWithStructuredOutput(
+	ctx context.Context,
+	req domain.RunRequest,
+	state *preparationState,
+	structuredOutput *domain.StructuredOutputSpec,
+) (*domain.PreparedRun, error) {
 	resolvedInputs := make(map[string]*domain.Artifact, len(req.Inputs))
 	inputHashes := make(map[string]string, len(req.Inputs))
 	for name, ref := range req.Inputs {
@@ -354,13 +362,15 @@ func (r *Runner) completePreparation(
 	}
 
 	end := time.Now().UTC()
+	effectiveModel := state.effectiveModel
+	effectiveModel.APIKey = ""
 	return &domain.PreparedRun{
 		PromptID:             state.definition.ID,
 		PromptVersion:        state.definition.Version,
 		PromptHash:           state.promptDefinitionHash,
 		SelectedProfileID:    state.selectedProfileID,
 		SelectedBackendID:    state.effectiveModel.BackendID,
-		EffectiveModelParams: state.effectiveModel,
+		EffectiveModelParams: effectiveModel,
 		TargetPresence:       state.targetPresence,
 		OutputContract:       state.effectiveContract,
 		StructuredOutput:     structuredOutput,
@@ -374,6 +384,20 @@ func (r *Runner) completePreparation(
 	}, nil
 }
 
+func (r *Runner) admitRun(ctx context.Context, backendID string) (func(), error) {
+	if r.admitter == nil {
+		return func() {}, nil
+	}
+	release, err := r.admitter.Admit(ctx, backendID)
+	if err != nil {
+		if errors.Is(err, capacity.ErrCapacityExceeded) && strings.TrimSpace(backendID) != "" {
+			return nil, &CapacityError{BackendID: backendID}
+		}
+		return nil, err
+	}
+	return release, nil
+}
+
 func (r *Runner) resolveStructuredOutput(ctx context.Context, def *domain.PromptDefinition, contract domain.OutputContract) (*domain.StructuredOutputSpec, error) {
 	if contract.ValidationMode != domain.ValidationJSONSchema {
 		return nil, nil
@@ -389,14 +413,18 @@ func (r *Runner) resolveStructuredOutput(ctx context.Context, def *domain.Prompt
 		return nil, fmt.Errorf("%w: failed to load json schema for structured output: %v", ErrValidation, err)
 	}
 
+	return structuredOutputSpec(def, schemaDoc), nil
+}
+
+func structuredOutputSpec(def *domain.PromptDefinition, schemaDocument any) *domain.StructuredOutputSpec {
 	return &domain.StructuredOutputSpec{
 		Type: domain.StructuredOutputJSONSchema,
 		JSONSchema: &domain.StructuredOutputJSONSpec{
 			Name:   deriveStructuredSchemaName(def.ID, def.Version),
 			Strict: true,
-			Schema: schemaDoc,
+			Schema: schemaDocument,
 		},
-	}, nil
+	}
 }
 
 func deriveStructuredSchemaName(promptID string, promptVersion string) string {

internal/usecase/runner_test.go

@@ -1359,12 +1359,12 @@ func TestRunnerAdmissionFailureSkipsCompletionCollaborators(t *testing.T) {
 	tests := []struct {
 		name             string
 		admissionError   error
-		wantBackendContext bool
+		wantCapacityType bool
 	}{
 		{
 			name:             "capacity exhausted",
 			admissionError:   capacity.ErrCapacityExceeded,
-			wantBackendContext: true,
+			wantCapacityType: true,
 		},
 		{
 			name:           "context canceled",
@@ -1414,8 +1414,16 @@ func TestRunnerAdmissionFailureSkipsCompletionCollaborators(t *testing.T) {
 			if errors.Is(err, ErrInvalidRequest) || errors.Is(err, ErrLLMGenerate) {
 				t.Fatalf("admission error was recategorized: %v", err)
 			}
-			if tc.wantBackendContext && !strings.Contains(err.Error(), "custom") {
-				t.Fatalf("capacity error lacks backend context: %v", err)
+			var capacityErr *CapacityError
+			if tc.wantCapacityType {
+				if !errors.As(err, &capacityErr) {
+					t.Fatalf("capacity error=%v, want internal typed identity", err)
+				}
+				if capacityErr.BackendID != "custom" {
+					t.Fatalf("capacity backend ID=%q, want custom", capacityErr.BackendID)
+				}
+			} else if errors.As(err, &capacityErr) {
+				t.Fatalf("non-capacity admission error exposed typed capacity identity: %v", err)
 			}
 			if !reflect.DeepEqual(admitter.backendIDs, []string{"custom"}) {
 				t.Fatalf("admitted backend IDs=%#v, want custom", admitter.backendIDs)
@@ -1757,7 +1765,7 @@ func TestRunnerRunDirectAPIKeyBypassesMissingEnvAndReachesLLM(t *testing.T) {
 	llmClient := &fakeLLM{resp: &domain.GenerateResponse{Content: "ok"}}
 	runner := NewRunner(promptRepo, execRepo, nil, defaultArtifactReader(), defaultRenderer(), llmClient, nil, nil)
 
-	_, err := runner.Run(context.Background(), domain.RunRequest{
+	result, err := runner.Run(context.Background(), domain.RunRequest{
 		PromptID:  "p",
 		ProfileID: "exec",
 		APIKey:    directKey,
@@ -1769,6 +1777,9 @@ func TestRunnerRunDirectAPIKeyBypassesMissingEnvAndReachesLLM(t *testing.T) {
 	if llmClient.lastReq.Target.APIKey != directKey {
 		t.Fatalf("expected direct API key to reach LLM request")
 	}
+	if result.EffectiveModelParams.APIKey != "" {
+		t.Fatal("run result retained direct API key")
+	}
 	if llmClient.lastReq.Target.APIKeyEnv != "PROMPTKIT_MISSING_KEY" {
 		t.Fatalf("expected api_key_env name to remain on target, got %q", llmClient.lastReq.Target.APIKeyEnv)
 	}

internal/validate/standard_validator.go

@@ -45,6 +45,101 @@ func (v *FSValidator) Validate(ctx context.Context, artifact *domain.Artifact, c
 	return validateArtifact(ctx, artifact, contract, v.validateJSONSchema)
 }
 
+type preparedValidation struct {
+	contract       domain.OutputContract
+	schemaDocument any
+	schema         *jsonschema.Schema
+}
+
+func (p *preparedValidation) Validate(ctx context.Context, artifact *domain.Artifact) (domain.ValidationResult, error) {
+	return validateArtifact(ctx, artifact, p.contract, p.validateJSONSchema)
+}
+
+func (p *preparedValidation) SchemaDocument() any {
+	return p.schemaDocument
+}
+
+func (p *preparedValidation) validateJSONSchema(instance any, _ string) ([]string, error) {
+	if p.schema == nil {
+		return nil, errors.New("prepared JSON schema is unavailable")
+	}
+	if err := p.schema.Validate(instance); err != nil {
+		return []string{fmt.Sprintf("json schema validation failed: %v", err)}, nil
+	}
+	return nil, nil
+}
+
+func (v *StandardValidator) PrepareValidation(ctx context.Context, contract domain.OutputContract) (PreparedValidation, error) {
+	if err := ctx.Err(); err != nil {
+		return nil, err
+	}
+
+	prepared := &preparedValidation{contract: contract}
+	if contract.ValidationMode != domain.ValidationJSONSchema {
+		return prepared, nil
+	}
+
+	resolvedSchemaPath, err := v.resolveSchemaPath(contract.SchemaPath)
+	if err != nil {
+		return nil, err
+	}
+	schemaDocument, err := loadJSONSchemaFile(resolvedSchemaPath)
+	if err != nil {
+		return nil, fmt.Errorf("failed to compile JSON schema %q: %w", resolvedSchemaPath, err)
+	}
+	schemaRoot, err := v.schemaRoot()
+	if err != nil {
+		return nil, err
+	}
+	compiler := newSchemaCompiler(standardSchemaLoader{root: schemaRoot})
+	if err := compiler.AddResource(resolvedSchemaPath, schemaDocument); err != nil {
+		return nil, fmt.Errorf("failed to register JSON schema %q: %w", resolvedSchemaPath, err)
+	}
+	schema, err := compiler.Compile(resolvedSchemaPath)
+	if err != nil {
+		return nil, fmt.Errorf("failed to compile JSON schema %q: %w", resolvedSchemaPath, err)
+	}
+	if err := ctx.Err(); err != nil {
+		return nil, err
+	}
+
+	prepared.schemaDocument = schemaDocument
+	prepared.schema = schema
+	return prepared, nil
+}
+
+func (v *FSValidator) PrepareValidation(ctx context.Context, contract domain.OutputContract) (PreparedValidation, error) {
+	if err := ctx.Err(); err != nil {
+		return nil, err
+	}
+
+	prepared := &preparedValidation{contract: contract}
+	if contract.ValidationMode != domain.ValidationJSONSchema {
+		return prepared, nil
+	}
+
+	schemaName, schemaDocument, err := v.loadSchemaDocument(contract.SchemaPath)
+	if err != nil {
+		return nil, err
+	}
+	resourceURL := fsSchemaResourceURL(schemaName)
+	compiler := newSchemaCompiler(fsSchemaLoader{fsys: v.fsys, root: filecatalog.CleanFSRoot(v.root)})
+	if err := compiler.AddResource(resourceURL, schemaDocument); err != nil {
+		return nil, fmt.Errorf("failed to register JSON schema %q: %w", schemaName, err)
+	}
+	schema, err := compiler.Compile(resourceURL)
+	if err != nil {
+		return nil, fmt.Errorf("failed to compile JSON schema %q: %w", schemaName, err)
+	}
+	if err := ctx.Err(); err != nil {
+		return nil, err
+	}
+
+	prepared.schemaDocument = schemaDocument
+	prepared.schema = schema
+	return prepared, nil
+}
+
 type schemaValidatorFunc func(instance any, schemaPath string) ([]string, error)
 
 func validateArtifact(ctx context.Context, artifact *domain.Artifact, contract domain.OutputContract, validateSchema schemaValidatorFunc) (domain.ValidationResult, error) {

internal/validate/standard_validator_test.go

@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"os"
 	"path/filepath"
+	"reflect"
 	"strconv"
 	"strings"
 	"testing"
@@ -120,6 +121,68 @@ func TestStandardValidatorJSONSchemaSuccess(t *testing.T) {
 	}
 }
 
+func TestStandardValidatorPreparedSchemaSurvivesSourceRemoval(t *testing.T) {
+	tmp := t.TempDir()
+	rootSchema := []byte(`{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "title": "original root",
+  "type": "object",
+  "required": ["value"],
+  "properties": {
+    "value": {"$ref": "value.json"}
+  }
+}`)
+	rootPath := filepath.Join(tmp, "schema.json")
+	referencePath := filepath.Join(tmp, "value.json")
+	if err := os.WriteFile(rootPath, rootSchema, 0o644); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(referencePath, []byte(`{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "type": "integer",
+  "minimum": 2
+}`), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	validator := NewStandardValidator(tmp)
+	preparer, ok := validator.(ValidationPreparer)
+	if !ok {
+		t.Fatal("standard validator does not support validation preparation")
+	}
+	prepared, err := preparer.PrepareValidation(context.Background(), domain.OutputContract{
+		ValidationMode: domain.ValidationJSONSchema,
+		SchemaPath:     "schema.json",
+	})
+	if err != nil {
+		t.Fatalf("prepare validation: %v", err)
+	}
+	assertSchemaDocument(t, prepared.SchemaDocument(), rootSchema)
+
+	if err := os.Remove(rootPath); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.Remove(referencePath); err != nil {
+		t.Fatal(err)
+	}
+
+	valid, err := prepared.Validate(context.Background(), &domain.Artifact{Body: []byte(`{"value":3}`)})
+	if err != nil {
+		t.Fatalf("validate prepared artifact: %v", err)
+	}
+	if valid.Status != domain.ValidationPassed || !valid.IsValid {
+		t.Fatalf("expected passed/valid, got status=%q valid=%v errors=%v", valid.Status, valid.IsValid, valid.Errors)
+	}
+
+	invalid, err := prepared.Validate(context.Background(), &domain.Artifact{Body: []byte(`{"value":"changed"}`)})
+	if err != nil {
+		t.Fatalf("validate prepared artifact: %v", err)
+	}
+	if invalid.Status != domain.ValidationFailed || invalid.IsValid {
+		t.Fatalf("expected failed/invalid, got status=%q valid=%v", invalid.Status, invalid.IsValid)
+	}
+}
+
 func TestStandardValidatorJSONSchemaNestedSchemaPathSuccess(t *testing.T) {
 	tmp := t.TempDir()
 	nestedDir := filepath.Join(tmp, "dnd")
@@ -295,6 +358,64 @@ func TestFSValidatorJSONSchemaSuccess(t *testing.T) {
 	}
 }
 
+func TestFSValidatorPreparedSchemaSurvivesSourceMutation(t *testing.T) {
+	rootSchema := []byte(`{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "title": "original root",
+  "type": "object",
+  "required": ["value"],
+  "properties": {
+    "value": {"$ref": "value.json"}
+  }
+}`)
+	fsys := fstest.MapFS{
+		"schema.json": &fstest.MapFile{Data: rootSchema},
+		"value.json": &fstest.MapFile{Data: []byte(`{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "type": "integer",
+  "minimum": 2
+}`)},
+	}
+	validator := NewFSValidator(fsys, ".")
+	preparer, ok := validator.(ValidationPreparer)
+	if !ok {
+		t.Fatal("filesystem validator does not support validation preparation")
+	}
+	prepared, err := preparer.PrepareValidation(context.Background(), domain.OutputContract{
+		ValidationMode: domain.ValidationJSONSchema,
+		SchemaPath:     "schema.json",
+	})
+	if err != nil {
+		t.Fatalf("prepare validation: %v", err)
+	}
+	assertSchemaDocument(t, prepared.SchemaDocument(), rootSchema)
+
+	fsys["schema.json"] = &fstest.MapFile{Data: []byte(`{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "type": "string"
+}`)}
+	fsys["value.json"] = &fstest.MapFile{Data: []byte(`{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "type": "string"
+}`)}
+
+	valid, err := prepared.Validate(context.Background(), &domain.Artifact{Body: []byte(`{"value":3}`)})
+	if err != nil {
+		t.Fatalf("validate prepared artifact: %v", err)
+	}
+	if valid.Status != domain.ValidationPassed || !valid.IsValid {
+		t.Fatalf("expected passed/valid, got status=%q valid=%v errors=%v", valid.Status, valid.IsValid, valid.Errors)
+	}
+
+	invalid, err := prepared.Validate(context.Background(), &domain.Artifact{Body: []byte(`{"value":"changed"}`)})
+	if err != nil {
+		t.Fatalf("validate prepared artifact: %v", err)
+	}
+	if invalid.Status != domain.ValidationFailed || invalid.IsValid {
+		t.Fatalf("expected failed/invalid, got status=%q valid=%v", invalid.Status, invalid.IsValid)
+	}
+}
+
 func TestFSValidatorJSONSchemaRegistrationError(t *testing.T) {
 	v := NewFSValidator(fstest.MapFS{
 		"schemas/%zz.json": &fstest.MapFile{Data: []byte(`{"type":"object"}`)},
@@ -548,3 +669,15 @@ func TestJSONSchemaDialectIsDraft2020(t *testing.T) {
 		})
 	}
 }
+
+func assertSchemaDocument(t *testing.T, got any, expectedJSON []byte) {
+	t.Helper()
+
+	var expected any
+	if err := json.Unmarshal(expectedJSON, &expected); err != nil {
+		t.Fatalf("decode expected schema document: %v", err)
+	}
+	if !reflect.DeepEqual(got, expected) {
+		t.Fatalf("schema document mismatch:\n got: %#v\nwant: %#v", got, expected)
+	}
+}

internal/validate/validator.go

@@ -2,6 +2,7 @@ package validate
 
 import (
 	"context"
+
 	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
 )
 
@@ -10,6 +11,20 @@ type Validator interface {
 	Validate(ctx context.Context, artifact *domain.Artifact, contract domain.OutputContract) (domain.ValidationResult, error)
 }
 
+// PreparedValidation validates artifacts against one frozen output contract.
+type PreparedValidation interface {
+	Validate(ctx context.Context, artifact *domain.Artifact) (domain.ValidationResult, error)
+	// SchemaDocument returns the root JSON Schema document used for provider
+	// structured output, or nil for non-schema modes. Returned internal
+	// immutable state must not be mutated.
+	SchemaDocument() any
+}
+
+// ValidationPreparer freezes validation resources for one output contract.
+type ValidationPreparer interface {
+	PrepareValidation(ctx context.Context, contract domain.OutputContract) (PreparedValidation, error)
+}
+
 // SchemaDocumentLoader loads JSON schema documents using validator path semantics.
 type SchemaDocumentLoader interface {
 	LoadSchemaDocument(ctx context.Context, schemaPath string) (any, error)

prepared_execution.go

@@ -0,0 +1,55 @@
+package promptkit
+
+import "gitea.maximumdirect.net/eric/promptkit/internal/usecase"
+
+const preparedExecutionString = "promptkit.PreparedExecution{opaque}"
+
+// PreparedExecution is an opaque, in-process handle for one completely
+// prepared execution. A handle is bound to the [Engine] that created it and
+// permits one [Engine.RunPrepared] invocation.
+//
+// PreparedExecution contains no supported serializable state and cannot be
+// used as a restartable job. Copying the value preserves the same shared
+// lifecycle; it does not create another execution attempt.
+type PreparedExecution struct {
+	internal *usecase.PreparedExecution
+}
+
+// Details returns a fresh caller-owned, credential-redacted copy of the
+// prepared request details. Mutating the result cannot affect execution or a
+// later Details call. Details remains available after execution or discard.
+//
+// A nil receiver or zero-value PreparedExecution returns a zero [PreparedRun].
+func (p *PreparedExecution) Details() PreparedRun {
+	if p == nil || p.internal == nil {
+		return PreparedRun{}
+	}
+	details := fromDomainPreparedRun(p.internal.Details())
+	if details == nil {
+		return PreparedRun{}
+	}
+	return *details
+}
+
+// Discard invalidates an unclaimed handle and drops Promptkit's references to
+// its execution-only state. Discard is nil-safe and idempotent. It does not
+// cancel an execution that has already claimed the handle; use the
+// [Engine.RunPrepared] context for cancellation.
+func (p *PreparedExecution) Discard() {
+	if p == nil || p.internal == nil {
+		return
+	}
+	p.internal.Discard()
+}
+
+// String returns a constant representation that exposes no retained request,
+// rendered content, or credential data.
+func (p *PreparedExecution) String() string {
+	return preparedExecutionString
+}
+
+// GoString returns a constant Go-syntax representation that exposes no
+// retained request, rendered content, or credential data.
+func (p *PreparedExecution) GoString() string {
+	return preparedExecutionString
+}

prepared_execution_contract_test.go

@@ -0,0 +1,773 @@
+package promptkit_test
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"os"
+	"reflect"
+	"strings"
+	"sync"
+	"testing"
+	"testing/fstest"
+	"time"
+
+	"gitea.maximumdirect.net/eric/promptkit"
+)
+
+func TestPreparedExecutionFreezesSourcesAndReturnsIndependentDetails(t *testing.T) {
+	promptSource := preparedPromptSource("original")
+	profileSource := preparedProfileSource("original-model")
+	schemaSource := preparedSchemaSource()
+	reader := &mutablePreparedArtifactReader{
+		body: "original artifact",
+		hash: "original-input-hash",
+	}
+	client := &preparedRecordingClient{
+		response: &promptkit.GenerateResponse{Content: `{"value":3}`},
+	}
+	engine, err := promptkit.NewEngine(
+		promptkit.Config{},
+		promptkit.WithPromptFS(promptSource, "."),
+		promptkit.WithProfileFS(profileSource, "."),
+		promptkit.WithSchemaFS(schemaSource, "."),
+		promptkit.WithArtifactReader(reader),
+		promptkit.WithLLMClient(client),
+	)
+	if err != nil {
+		t.Fatalf("construct engine: %v", err)
+	}
+
+	temperature := 0.25
+	extraParams := map[string]any{
+		"nested": map[string]any{"source": "original"},
+	}
+	request := promptkit.RunRequest{
+		PromptID: "prepared",
+		Inputs: map[string]promptkit.ArtifactRef{
+			"input": promptkit.Inline("original request input"),
+		},
+		Vars: map[string]string{"label": "original variable"},
+		Execution: &promptkit.ExecutionTargetOverride{
+			Temperature: &temperature,
+			ExtraParams: extraParams,
+		},
+	}
+	preparationContext, cancelPreparation := context.WithCancel(context.Background())
+	prepared, err := engine.PrepareExecution(preparationContext, request)
+	if err != nil {
+		t.Fatalf("prepare execution: %v", err)
+	}
+	cancelPreparation()
+
+	request.PromptID = "changed"
+	request.Inputs["input"] = promptkit.Inline("changed request input")
+	request.Vars["label"] = "changed variable"
+	temperature = 1.5
+	extraParams["nested"].(map[string]any)["source"] = "changed"
+	promptSource["prompt.yaml"] = &fstest.MapFile{Data: []byte(`id: changed`)}
+	profileSource["profile.yaml"] = &fstest.MapFile{Data: []byte(`id: changed`)}
+	schemaSource["schema.json"] = &fstest.MapFile{Data: []byte(`{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "title": "changed root",
+  "type": "string"
+}`)}
+	schemaSource["value.json"] = &fstest.MapFile{Data: []byte(`{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "type": "string"
+}`)}
+	reader.set("changed artifact", "changed-input-hash")
+
+	first := prepared.Details()
+	first.Messages[0].Content = "changed details"
+	first.InputHashes["input"] = "changed-details-hash"
+	first.EffectiveModelParams.ExtraParams["nested"].(map[string]any)["source"] = "changed details"
+	first.StructuredOutput.JSONSchema.Schema.(map[string]any)["title"] = "changed details"
+
+	second := prepared.Details()
+	if second.Messages[0].Content != "Input=original artifact Label=original variable" {
+		t.Fatalf("details message changed: %q", second.Messages[0].Content)
+	}
+	if second.InputHashes["input"] != "original-input-hash" {
+		t.Fatalf("details input hash changed: %q", second.InputHashes["input"])
+	}
+	if second.EffectiveModelParams.Model != "original-model" ||
+		second.EffectiveModelParams.Temperature != 0.25 ||
+		second.EffectiveModelParams.ExtraParams["nested"].(map[string]any)["source"] != "original" {
+		t.Fatalf("details target changed: %+v", second.EffectiveModelParams)
+	}
+	schema := second.StructuredOutput.JSONSchema.Schema.(map[string]any)
+	if schema["title"] != "original root" {
+		t.Fatalf("details schema changed: %#v", schema)
+	}
+
+	result, err := engine.RunPrepared(context.Background(), prepared)
+	if err != nil {
+		t.Fatalf("run prepared after preparation-context cancellation: %v", err)
+	}
+	if result.Validation.Status != promptkit.ValidationPassed || !result.Validation.IsValid {
+		t.Fatalf("frozen schema did not validate original output: %+v", result.Validation)
+	}
+	if reader.callCount() != 1 {
+		t.Fatalf("execution reopened artifact source: calls=%d", reader.callCount())
+	}
+
+	requests := client.snapshot()
+	if len(requests) != 1 {
+		t.Fatalf("generation calls=%d, want 1", len(requests))
+	}
+	generated := requests[0]
+	if generated.Prompt.Messages[0].Content != second.Messages[0].Content ||
+		generated.Target.Model != second.EffectiveModelParams.Model ||
+		!reflect.DeepEqual(generated.Target.ExtraParams, second.EffectiveModelParams.ExtraParams) ||
+		!reflect.DeepEqual(generated.StructuredOutput, second.StructuredOutput) {
+		t.Fatalf("generation did not use frozen details:\nrequest=%+v\ndetails=%+v", generated, second)
+	}
+	if result.PromptID != second.PromptID ||
+		result.PromptVersion != second.PromptVersion ||
+		result.PromptHash != second.PromptHash ||
+		result.SessionID != second.SessionID ||
+		result.RenderedPromptHash != second.RenderedPromptHash ||
+		result.SelectedProfileID != second.SelectedProfileID ||
+		result.SelectedBackendID != second.SelectedBackendID ||
+		!reflect.DeepEqual(result.EffectiveModelParams, second.EffectiveModelParams) ||
+		!reflect.DeepEqual(result.InputHashes, second.InputHashes) {
+		t.Fatalf("result provenance does not match details:\nresult=%+v\ndetails=%+v", result, second)
+	}
+}
+
+func TestPreparedExecutionLifecycleAndEngineBinding(t *testing.T) {
+	ownerClient := &preparedRecordingClient{
+		response: &promptkit.GenerateResponse{Content: "ok"},
+	}
+	owner := newPreparedContractEngine(t, ownerClient, "owner content")
+	foreign := newPreparedContractEngine(t, &preparedRecordingClient{
+		response: &promptkit.GenerateResponse{Content: "unexpected"},
+	}, "foreign content")
+
+	prepared, err := owner.PrepareExecution(context.Background(), promptkit.RunRequest{PromptID: "prepared"})
+	if err != nil {
+		t.Fatalf("prepare execution: %v", err)
+	}
+	copied := *prepared
+
+	var nilEngine *promptkit.Engine
+	if result, err := nilEngine.RunPrepared(context.Background(), prepared); result != nil ||
+		!errors.Is(err, promptkit.ErrInvalidConfig) {
+		t.Fatalf("nil engine result=(%+v, %v), want ErrInvalidConfig", result, err)
+	}
+	if result, err := foreign.RunPrepared(context.Background(), prepared); result != nil ||
+		!errors.Is(err, promptkit.ErrInvalidRequest) {
+		t.Fatalf("foreign engine result=(%+v, %v), want ErrInvalidRequest", result, err)
+	}
+	if result, err := owner.RunPrepared(context.Background(), nil); result != nil ||
+		!errors.Is(err, promptkit.ErrInvalidRequest) {
+		t.Fatalf("nil handle result=(%+v, %v), want ErrInvalidRequest", result, err)
+	}
+	if result, err := owner.RunPrepared(context.Background(), &promptkit.PreparedExecution{}); result != nil ||
+		!errors.Is(err, promptkit.ErrInvalidRequest) {
+		t.Fatalf("zero handle result=(%+v, %v), want ErrInvalidRequest", result, err)
+	}
+
+	result, err := owner.RunPrepared(context.Background(), &copied)
+	if err != nil || result == nil {
+		t.Fatalf("owner run prepared=(%+v, %v), want success", result, err)
+	}
+	for name, handle := range map[string]*promptkit.PreparedExecution{
+		"original": prepared,
+		"copy":     &copied,
+	} {
+		if result, err := owner.RunPrepared(context.Background(), handle); result != nil ||
+			!errors.Is(err, promptkit.ErrInvalidRequest) {
+			t.Fatalf("%s reused handle result=(%+v, %v), want ErrInvalidRequest", name, result, err)
+		}
+		if handle.Details().PromptID != "prepared" {
+			t.Fatalf("%s details unavailable after execution", name)
+		}
+	}
+	if len(ownerClient.snapshot()) != 1 {
+		t.Fatalf("owner generation calls=%d, want 1", len(ownerClient.snapshot()))
+	}
+
+	collaboratorFailure := errors.New("prepared collaborator failure")
+	failingClient := &preparedRecordingClient{err: collaboratorFailure}
+	failingEngine := newPreparedContractEngine(t, failingClient, "failure content")
+	failing, err := failingEngine.PrepareExecution(context.Background(), promptkit.RunRequest{PromptID: "prepared"})
+	if err != nil {
+		t.Fatalf("prepare failing execution: %v", err)
+	}
+	if result, err := failingEngine.RunPrepared(context.Background(), failing); result != nil ||
+		!errors.Is(err, promptkit.ErrLLMGenerate) ||
+		!errors.Is(err, collaboratorFailure) {
+		t.Fatalf("generation failure result=(%+v, %v), want public and collaborator identities", result, err)
+	}
+	if result, err := failingEngine.RunPrepared(context.Background(), failing); result != nil ||
+		!errors.Is(err, promptkit.ErrInvalidRequest) {
+		t.Fatalf("failed execution was reusable: result=(%+v, %v)", result, err)
+	}
+
+	cancellationRelease := make(chan struct{})
+	cancellationStarted := make(chan struct{}, 1)
+	cancelingEngine := newPreparedContractEngine(t, &preparedRecordingClient{
+		response: &promptkit.GenerateResponse{Content: "unexpected"},
+		started:  cancellationStarted,
+		release:  cancellationRelease,
+	}, "cancellation content")
+	canceling, err := cancelingEngine.PrepareExecution(
+		context.Background(),
+		promptkit.RunRequest{PromptID: "prepared"},
+	)
+	if err != nil {
+		t.Fatalf("prepare canceled execution: %v", err)
+	}
+	executionContext, cancelExecution := context.WithCancel(context.Background())
+	type canceledOutcome struct {
+		result *promptkit.RunResult
+		err    error
+	}
+	canceledResult := make(chan canceledOutcome, 1)
+	go func() {
+		result, runErr := cancelingEngine.RunPrepared(executionContext, canceling)
+		canceledResult <- canceledOutcome{result: result, err: runErr}
+	}()
+	select {
+	case <-cancellationStarted:
+	case <-time.After(5 * time.Second):
+		t.Fatal("timed out waiting for cancelable generation")
+	}
+	cancelExecution()
+	select {
+	case outcome := <-canceledResult:
+		if outcome.result != nil ||
+			!errors.Is(outcome.err, promptkit.ErrLLMGenerate) ||
+			!errors.Is(outcome.err, context.Canceled) {
+			t.Fatalf(
+				"canceled execution=(%+v, %v), want generation and context identities",
+				outcome.result,
+				outcome.err,
+			)
+		}
+	case <-time.After(5 * time.Second):
+		t.Fatal("timed out waiting for canceled execution")
+	}
+	if result, err := cancelingEngine.RunPrepared(context.Background(), canceling); result != nil ||
+		!errors.Is(err, promptkit.ErrInvalidRequest) {
+		t.Fatalf("canceled execution was reusable: result=(%+v, %v)", result, err)
+	}
+}
+
+func TestPreparedExecutionConcurrentClaimAllowsOneGeneration(t *testing.T) {
+	release := make(chan struct{})
+	client := &preparedRecordingClient{
+		response: &promptkit.GenerateResponse{Content: "ok"},
+		started:  make(chan struct{}, 1),
+		release:  release,
+	}
+	engine := newPreparedContractEngine(t, client, "concurrent content")
+	prepared, err := engine.PrepareExecution(context.Background(), promptkit.RunRequest{PromptID: "prepared"})
+	if err != nil {
+		t.Fatalf("prepare execution: %v", err)
+	}
+
+	type outcome struct {
+		result *promptkit.RunResult
+		err    error
+	}
+	outcomes := make(chan outcome, 2)
+	for i := 0; i < 2; i++ {
+		go func() {
+			result, runErr := engine.RunPrepared(context.Background(), prepared)
+			outcomes <- outcome{result: result, err: runErr}
+		}()
+	}
+
+	select {
+	case <-client.started:
+	case <-time.After(5 * time.Second):
+		t.Fatal("timed out waiting for generation")
+	}
+	select {
+	case loser := <-outcomes:
+		if loser.result != nil || !errors.Is(loser.err, promptkit.ErrInvalidRequest) {
+			t.Fatalf("concurrent loser=(%+v, %v), want ErrInvalidRequest", loser.result, loser.err)
+		}
+	case <-time.After(5 * time.Second):
+		t.Fatal("timed out waiting for rejected concurrent claim")
+	}
+
+	close(release)
+	select {
+	case winner := <-outcomes:
+		if winner.err != nil || winner.result == nil {
+			t.Fatalf("concurrent winner=(%+v, %v), want success", winner.result, winner.err)
+		}
+	case <-time.After(5 * time.Second):
+		t.Fatal("timed out waiting for successful concurrent claim")
+	}
+	if len(client.snapshot()) != 1 {
+		t.Fatalf("generation calls=%d, want 1", len(client.snapshot()))
+	}
+}
+
+func TestPreparedExecutionRunAndDiscardRaceHasOneWinner(t *testing.T) {
+	const attempts = 32
+
+	for i := 0; i < attempts; i++ {
+		client := &preparedRecordingClient{
+			response: &promptkit.GenerateResponse{Content: "ok"},
+		}
+		engine := newPreparedContractEngine(t, client, "race content")
+		prepared, err := engine.PrepareExecution(
+			context.Background(),
+			promptkit.RunRequest{PromptID: "prepared"},
+		)
+		if err != nil {
+			t.Fatalf("attempt %d prepare execution: %v", i, err)
+		}
+
+		start := make(chan struct{})
+		type outcome struct {
+			result *promptkit.RunResult
+			err    error
+		}
+		runOutcome := make(chan outcome, 1)
+		discardDone := make(chan struct{})
+
+		go func() {
+			<-start
+			result, runErr := engine.RunPrepared(context.Background(), prepared)
+			runOutcome <- outcome{result: result, err: runErr}
+		}()
+		go func() {
+			<-start
+			prepared.Discard()
+			close(discardDone)
+		}()
+
+		close(start)
+		runResult := <-runOutcome
+		<-discardDone
+		calls := len(client.snapshot())
+		switch {
+		case runResult.err == nil:
+			if runResult.result == nil || calls != 1 {
+				t.Fatalf(
+					"attempt %d run won with outcome=(%+v, %v), generation calls=%d",
+					i,
+					runResult.result,
+					runResult.err,
+					calls,
+				)
+			}
+		case errors.Is(runResult.err, promptkit.ErrInvalidRequest):
+			if runResult.result != nil || calls != 0 {
+				t.Fatalf(
+					"attempt %d discard won with outcome=(%+v, %v), generation calls=%d",
+					i,
+					runResult.result,
+					runResult.err,
+					calls,
+				)
+			}
+		default:
+			t.Fatalf("attempt %d unexpected run outcome=(%+v, %v)", i, runResult.result, runResult.err)
+		}
+		if prepared.Details().PromptID != "prepared" {
+			t.Fatalf("attempt %d details unavailable after race", i)
+		}
+	}
+}
+
+func TestPreparedExecutionDiscardAndFormattingDoNotExposePrivateState(t *testing.T) {
+	const (
+		directCredential = "pk-test-direct-credential-41f7"
+		renderedContent  = "rendered-content-sentinel-98d2"
+	)
+	client := &preparedRecordingClient{
+		response: &promptkit.GenerateResponse{Content: "generated output"},
+	}
+	engine, err := promptkit.NewEngine(
+		promptkit.Config{},
+		promptkit.WithPromptFS(contractPromptFS("prepared", "profile", renderedContent), "."),
+		promptkit.WithProfiles(promptkit.Profile{
+			ID:             "profile",
+			Endpoint:       "http://example.test/v1",
+			Model:          "model",
+			APIKeyRequired: true,
+		}),
+		promptkit.WithLLMClient(client),
+	)
+	if err != nil {
+		t.Fatalf("construct engine: %v", err)
+	}
+	prepared, err := engine.PrepareExecution(context.Background(), promptkit.RunRequest{
+		PromptID: "prepared",
+		APIKey:   directCredential,
+	})
+	if err != nil {
+		t.Fatalf("prepare execution: %v", err)
+	}
+
+	formattedValues := []string{
+		fmt.Sprint(prepared),
+		fmt.Sprintf("%+v", prepared),
+		fmt.Sprintf("%#v", prepared),
+	}
+	for _, formatted := range formattedValues {
+		if formatted != "promptkit.PreparedExecution{opaque}" {
+			t.Fatalf("unexpected opaque formatting: %q", formatted)
+		}
+		assertPreparedPrivateValuesAbsent(t, formatted, directCredential, renderedContent)
+	}
+	payload, err := json.Marshal(prepared)
+	if err != nil {
+		t.Fatalf("marshal opaque handle: %v", err)
+	}
+	assertPreparedPrivateValuesAbsent(t, string(payload), directCredential, renderedContent)
+
+	detailsBefore := prepared.Details()
+	detailsJSON, err := json.Marshal(detailsBefore)
+	if err != nil {
+		t.Fatalf("marshal prepared details: %v", err)
+	}
+	assertPreparedPrivateValuesAbsent(t, string(detailsJSON), directCredential)
+
+	prepared.Discard()
+	prepared.Discard()
+	result, lifecycleErr := engine.RunPrepared(context.Background(), prepared)
+	if result != nil || !errors.Is(lifecycleErr, promptkit.ErrInvalidRequest) {
+		t.Fatalf("discarded execution result=(%+v, %v), want ErrInvalidRequest", result, lifecycleErr)
+	}
+	assertPreparedPrivateValuesAbsent(t, lifecycleErr.Error(), directCredential, renderedContent)
+	if !reflect.DeepEqual(prepared.Details(), detailsBefore) {
+		t.Fatal("details changed after discard")
+	}
+
+	executed, err := engine.PrepareExecution(context.Background(), promptkit.RunRequest{
+		PromptID: "prepared",
+		APIKey:   directCredential,
+	})
+	if err != nil {
+		t.Fatalf("prepare execution for request inspection: %v", err)
+	}
+	executionResult, err := engine.RunPrepared(context.Background(), executed)
+	if err != nil {
+		t.Fatalf("run execution for request inspection: %v", err)
+	}
+	requests := client.snapshot()
+	if len(requests) != 1 || requests[0].APIKey != directCredential {
+		t.Fatalf("direct credential did not reach only the client credential field: %#v", requests)
+	}
+	requestJSON, err := json.Marshal(requests[0])
+	if err != nil {
+		t.Fatalf("marshal captured generate request: %v", err)
+	}
+	for _, value := range []string{
+		fmt.Sprint(requests[0]),
+		fmt.Sprintf("%+v", requests[0]),
+		fmt.Sprintf("%#v", requests[0]),
+		string(requestJSON),
+		fmt.Sprint(executionResult),
+	} {
+		assertPreparedPrivateValuesAbsent(t, value, directCredential)
+	}
+	resultJSON, err := json.Marshal(executionResult)
+	if err != nil {
+		t.Fatalf("marshal execution result: %v", err)
+	}
+	assertPreparedPrivateValuesAbsent(t, string(resultJSON), directCredential)
+
+	var nilHandle *promptkit.PreparedExecution
+	nilHandle.Discard()
+	if !reflect.DeepEqual(nilHandle.Details(), promptkit.PreparedRun{}) {
+		t.Fatalf("nil handle details=%+v, want zero value", nilHandle.Details())
+	}
+	zeroHandle := &promptkit.PreparedExecution{}
+	zeroHandle.Discard()
+	if !reflect.DeepEqual(zeroHandle.Details(), promptkit.PreparedRun{}) {
+		t.Fatalf("zero handle details=%+v, want zero value", zeroHandle.Details())
+	}
+}
+
+func TestPreparedExecutionCredentialCapacityAndTimingBoundaries(t *testing.T) {
+	t.Run("credential is rechecked before generation", func(t *testing.T) {
+		const (
+			environmentName = "PROMPTKIT_PREPARED_CONTRACT_KEY"
+			environmentKey  = "environment-credential-sentinel"
+		)
+		t.Setenv(environmentName, environmentKey)
+		client := &preparedRecordingClient{
+			response: &promptkit.GenerateResponse{Content: "unexpected"},
+		}
+		engine, err := promptkit.NewEngine(
+			promptkit.Config{},
+			promptkit.WithPromptFS(contractPromptFS("prepared", "profile", "content"), "."),
+			promptkit.WithProfileFS(preparedCredentialProfileSource(environmentName), "."),
+			promptkit.WithLLMClient(client),
+		)
+		if err != nil {
+			t.Fatalf("construct credential engine: %v", err)
+		}
+		prepared, err := engine.PrepareExecution(context.Background(), promptkit.RunRequest{PromptID: "prepared"})
+		if err != nil {
+			t.Fatalf("prepare credential execution: %v", err)
+		}
+		if err := os.Unsetenv(environmentName); err != nil {
+			t.Fatalf("unset credential environment: %v", err)
+		}
+
+		result, err := engine.RunPrepared(context.Background(), prepared)
+		if result != nil ||
+			!errors.Is(err, promptkit.ErrInvalidRequest) ||
+			!errors.Is(err, promptkit.ErrAPIKeyEnvMissing) {
+			t.Fatalf("credential execution=(%+v, %v), want credential identities", result, err)
+		}
+		if len(client.snapshot()) != 0 {
+			t.Fatalf("credential failure reached generation: %d calls", len(client.snapshot()))
+		}
+		assertPreparedPrivateValuesAbsent(t, err.Error(), environmentKey)
+		if result, err := engine.RunPrepared(context.Background(), prepared); result != nil ||
+			!errors.Is(err, promptkit.ErrInvalidRequest) {
+			t.Fatalf("credential failure did not consume handle: result=(%+v, %v)", result, err)
+		}
+	})
+
+	t.Run("preparation does not admit and execution timing starts after retention", func(t *testing.T) {
+		release := make(chan struct{})
+		client := newCapacityGateClient(release, 4)
+		engine := newBackendCapacityEngine(t, client, 1, capacityInt(0), nil)
+
+		activeRun := make(chan capacityRunResult, 1)
+		go runCapacityRequest(
+			engine,
+			context.Background(),
+			promptkit.RunRequest{PromptID: "prompt"},
+			activeRun,
+		)
+		awaitCapacityRequest(t, client.started)
+
+		prepared, err := engine.PrepareExecution(
+			context.Background(),
+			promptkit.RunRequest{PromptID: "prompt"},
+		)
+		if err != nil {
+			t.Fatalf("prepare while capacity is full: %v", err)
+		}
+		if _, _, calls := client.snapshot(); calls != 1 {
+			t.Fatalf("preparation invoked generation: calls=%d", calls)
+		}
+		if result, err := engine.RunPrepared(context.Background(), prepared); result != nil ||
+			!errors.Is(err, promptkit.ErrCapacityExceeded) {
+			t.Fatalf("capacity execution=(%+v, %v), want ErrCapacityExceeded", result, err)
+		} else {
+			var capacityErr *promptkit.CapacityError
+			if !errors.As(err, &capacityErr) || capacityErr == nil || capacityErr.BackendID != "limited" {
+				t.Fatalf("capacity execution=%v, want limited CapacityError", err)
+			}
+		}
+		if result, err := engine.RunPrepared(context.Background(), prepared); result != nil ||
+			!errors.Is(err, promptkit.ErrInvalidRequest) {
+			t.Fatalf("capacity rejection did not consume handle: result=(%+v, %v)", result, err)
+		}
+		if prepared.Details().PromptID != "prompt" {
+			t.Fatal("details unavailable after capacity rejection")
+		}
+
+		close(release)
+		activeOutcome := awaitCapacityRun(t, activeRun)
+		if activeOutcome.err != nil || activeOutcome.result == nil {
+			t.Fatalf("active run outcome=(%+v, %v), want success", activeOutcome.result, activeOutcome.err)
+		}
+
+		timed, err := engine.PrepareExecution(
+			context.Background(),
+			promptkit.RunRequest{PromptID: "prompt"},
+		)
+		if err != nil {
+			t.Fatalf("prepare timed execution: %v", err)
+		}
+		details := timed.Details()
+		time.Sleep(25 * time.Millisecond)
+		executionFloor := time.Now().UTC()
+		result, err := engine.RunPrepared(context.Background(), timed)
+		if err != nil {
+			t.Fatalf("run timed execution: %v", err)
+		}
+		if result.StartTime.Before(executionFloor) ||
+			!result.StartTime.After(details.EndTime) ||
+			result.EndTime.Before(result.StartTime) ||
+			result.Duration != result.EndTime.Sub(result.StartTime) {
+			t.Fatalf(
+				"execution timing includes preparation or retention: details_end=%s floor=%s result=%+v",
+				details.EndTime,
+				executionFloor,
+				result,
+			)
+		}
+		if _, _, calls := client.snapshot(); calls != 2 {
+			t.Fatalf("generation calls=%d, want active and timed executions only", calls)
+		}
+	})
+}
+
+type mutablePreparedArtifactReader struct {
+	mu    sync.Mutex
+	body  string
+	hash  string
+	calls int
+}
+
+func (r *mutablePreparedArtifactReader) Read(
+	_ context.Context,
+	_ promptkit.ArtifactRef,
+) (*promptkit.Artifact, error) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	r.calls++
+	return &promptkit.Artifact{
+		Body: []byte(r.body),
+		Hash: r.hash,
+	}, nil
+}
+
+func (r *mutablePreparedArtifactReader) set(body, hash string) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	r.body = body
+	r.hash = hash
+}
+
+func (r *mutablePreparedArtifactReader) callCount() int {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	return r.calls
+}
+
+type preparedRecordingClient struct {
+	mu       sync.Mutex
+	response *promptkit.GenerateResponse
+	err      error
+	requests []promptkit.GenerateRequest
+	started  chan struct{}
+	release  <-chan struct{}
+}
+
+func (c *preparedRecordingClient) Generate(
+	ctx context.Context,
+	request promptkit.GenerateRequest,
+) (*promptkit.GenerateResponse, error) {
+	c.mu.Lock()
+	c.requests = append(c.requests, request)
+	c.mu.Unlock()
+
+	if c.started != nil {
+		c.started <- struct{}{}
+	}
+	if c.release != nil {
+		select {
+		case <-c.release:
+		case <-ctx.Done():
+			return nil, ctx.Err()
+		}
+	}
+	if c.err != nil {
+		return nil, c.err
+	}
+	return c.response, nil
+}
+
+func (c *preparedRecordingClient) snapshot() []promptkit.GenerateRequest {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	return append([]promptkit.GenerateRequest(nil), c.requests...)
+}
+
+func newPreparedContractEngine(
+	t *testing.T,
+	client promptkit.LLMClient,
+	message string,
+) *promptkit.Engine {
+	t.Helper()
+	engine, err := promptkit.NewEngine(
+		promptkit.Config{},
+		promptkit.WithPromptFS(contractPromptFS("prepared", "profile", message), "."),
+		promptkit.WithProfiles(promptkit.Profile{
+			ID:       "profile",
+			Endpoint: "http://example.test/v1",
+			Model:    "model",
+		}),
+		promptkit.WithLLMClient(client),
+	)
+	if err != nil {
+		t.Fatalf("construct prepared execution engine: %v", err)
+	}
+	return engine
+}
+
+func preparedPromptSource(label string) fstest.MapFS {
+	return fstest.MapFS{
+		"prompt.yaml": &fstest.MapFile{Data: []byte(`id: prepared
+version: "1"
+default_profile: profile
+inputs:
+  - name: input
+    required: true
+messages:
+  - role: user
+    content: 'Input={{input "input"}} Label={{.label}}'
+description: ` + label + `
+output:
+  format: json
+  validation_mode: json_schema
+  schema_path: schema.json
+`)},
+	}
+}
+
+func preparedProfileSource(model string) fstest.MapFS {
+	return fstest.MapFS{
+		"profile.yaml": &fstest.MapFile{Data: []byte(`id: profile
+endpoint: http://example.test/v1
+model: ` + model + `
+`)},
+	}
+}
+
+func preparedCredentialProfileSource(environmentName string) fstest.MapFS {
+	return fstest.MapFS{
+		"profile.yaml": &fstest.MapFile{Data: []byte(`id: profile
+endpoint: http://example.test/v1
+model: model
+api_key_env: ` + environmentName + `
+`)},
+	}
+}
+
+func preparedSchemaSource() fstest.MapFS {
+	return fstest.MapFS{
+		"schema.json": &fstest.MapFile{Data: []byte(`{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "title": "original root",
+  "type": "object",
+  "required": ["value"],
+  "properties": {
+    "value": {"$ref": "value.json"}
+  }
+}`)},
+		"value.json": &fstest.MapFile{Data: []byte(`{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "type": "integer",
+  "minimum": 2
+}`)},
+	}
+}
+
+func assertPreparedPrivateValuesAbsent(t *testing.T, value string, privateValues ...string) {
+	t.Helper()
+	for _, privateValue := range privateValues {
+		if strings.Contains(value, privateValue) {
+			t.Fatalf("value exposed private data %q: %s", privateValue, value)
+		}
+	}
+}

public_contract_test.go

@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"io/fs"
 	"reflect"
 	"strings"
 	"sync"
@@ -16,6 +17,380 @@ import (
 	"gitea.maximumdirect.net/eric/promptkit"
 )
 
+type inspectionCountingFS struct {
+	opens atomic.Int64
+}
+
+func (f *inspectionCountingFS) Open(string) (fs.File, error) {
+	f.opens.Add(1)
+	return nil, fs.ErrNotExist
+}
+
+func TestInspectProfileResolvesCredentialStatesWithoutPromptOrGeneration(t *testing.T) {
+	const environmentName = "PROMPTKIT_INSPECTION_ABSENT_KEY"
+	t.Setenv(environmentName, "")
+	client := &fakeLLMClient{response: &promptkit.GenerateResponse{Content: "unexpected"}}
+	engine, err := promptkit.NewEngine(promptkit.Config{},
+		promptkit.WithPromptFS(fstest.MapFS{}, "."),
+		promptkit.WithProfileFS(fstest.MapFS{
+			"environment.yaml": &fstest.MapFile{Data: []byte(`id: environment
+endpoint: http://environment.example/v1
+model: environment-model
+api_key_env: PROMPTKIT_INSPECTION_ABSENT_KEY
+`)},
+		}, "."),
+		promptkit.WithProfiles(
+			promptkit.Profile{ID: "direct", Endpoint: "http://direct.example/v1", Model: "direct-model", APIKeyRequired: true},
+			promptkit.Profile{ID: "none", Endpoint: "http://none.example/v1", Model: "none-model"},
+		),
+		promptkit.WithLLMClient(client),
+	)
+	if err != nil {
+		t.Fatalf("construct inspection engine: %v", err)
+	}
+
+	for _, tc := range []struct {
+		profileID     string
+		wantEnv       string
+		wantDirectKey bool
+		wantEndpoint  string
+	}{
+		{profileID: " environment ", wantEnv: environmentName, wantEndpoint: "http://environment.example/v1"},
+		{profileID: "direct", wantDirectKey: true, wantEndpoint: "http://direct.example/v1"},
+		{profileID: "none", wantEndpoint: "http://none.example/v1"},
+	} {
+		t.Run(tc.profileID, func(t *testing.T) {
+			inspection, err := engine.InspectProfile(context.Background(), tc.profileID)
+			if err != nil {
+				t.Fatalf("inspect profile: %v", err)
+			}
+			if inspection.ProfileID != strings.TrimSpace(tc.profileID) ||
+				inspection.EffectiveModelParams.Endpoint != tc.wantEndpoint ||
+				inspection.EffectiveModelParams.BackendID != "" ||
+				inspection.EffectiveModelParams.APIKeyEnv != tc.wantEnv ||
+				inspection.APIKeyRequired != tc.wantDirectKey {
+				t.Fatalf("unexpected inspection: %#v", inspection)
+			}
+		})
+	}
+	if len(client.requests) != 0 {
+		t.Fatalf("inspection invoked the model client %d times", len(client.requests))
+	}
+}
+
+func TestInspectProfilePreservesPublicErrorIdentities(t *testing.T) {
+	newEngine := func(t *testing.T, options ...promptkit.Option) *promptkit.Engine {
+		t.Helper()
+		engine, err := promptkit.NewEngine(
+			promptkit.Config{},
+			append([]promptkit.Option{promptkit.WithPromptFS(fstest.MapFS{}, ".")}, options...)...,
+		)
+		if err != nil {
+			t.Fatalf("construct inspection engine: %v", err)
+		}
+		return engine
+	}
+
+	var nilEngine *promptkit.Engine
+	if result, err := nilEngine.InspectProfile(context.Background(), "profile"); result != nil ||
+		!errors.Is(err, promptkit.ErrInvalidConfig) {
+		t.Fatalf("nil engine result=(%#v, %v), want ErrInvalidConfig", result, err)
+	}
+
+	valid := newEngine(t, promptkit.WithProfiles(promptkit.Profile{
+		ID: "profile", Endpoint: "http://profile.example/v1", Model: "model",
+	}))
+	if result, err := valid.InspectProfile(context.Background(), " \t "); result != nil ||
+		!errors.Is(err, promptkit.ErrInvalidRequest) {
+		t.Fatalf("blank profile result=(%#v, %v), want ErrInvalidRequest", result, err)
+	}
+	if result, err := valid.InspectProfile(context.Background(), "missing"); result != nil ||
+		!errors.Is(err, promptkit.ErrProfileNotFound) || errors.Is(err, promptkit.ErrProfileLoad) {
+		t.Fatalf("missing profile result=(%#v, %v), want only ErrProfileNotFound", result, err)
+	}
+
+	malformed := newEngine(t, promptkit.WithProfileFS(fstest.MapFS{
+		"broken.yaml": &fstest.MapFile{Data: []byte("id: broken\nendpoint: http://broken.example/v1\nmodel: model\nunknown: value\n")},
+	}, "."))
+	if result, err := malformed.InspectProfile(context.Background(), "broken"); result != nil ||
+		!errors.Is(err, promptkit.ErrProfileLoad) {
+		t.Fatalf("malformed profile result=(%#v, %v), want ErrProfileLoad", result, err)
+	}
+
+	unknownBackend := newEngine(t, promptkit.WithProfiles(promptkit.Profile{
+		ID: "unknown-backend", BackendID: "unknown", Model: "model",
+	}))
+	if result, err := unknownBackend.InspectProfile(context.Background(), "unknown-backend"); result != nil ||
+		!errors.Is(err, promptkit.ErrProfileLoad) {
+		t.Fatalf("unknown backend result=(%#v, %v), want ErrProfileLoad", result, err)
+	}
+
+	countingFS := &inspectionCountingFS{}
+	canceled := newEngine(t, promptkit.WithProfileFS(countingFS, "."))
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+	if result, err := canceled.InspectProfile(ctx, "profile"); result != nil ||
+		!errors.Is(err, promptkit.ErrProfileLoad) || !errors.Is(err, context.Canceled) || countingFS.opens.Load() != 0 {
+		t.Fatalf("canceled inspection result=(%#v, %v), opens=%d", result, err, countingFS.opens.Load())
+	}
+}
+
+func TestInspectProfileReturnsIndependentTargetMatchingPreparation(t *testing.T) {
+	extraParams := map[string]any{
+		"nested": map[string]any{"value": "original"},
+	}
+	engine, err := promptkit.NewEngine(promptkit.Config{},
+		promptkit.WithPromptFS(contractPromptFS("prompt", "profile", "message"), "."),
+		promptkit.WithProfiles(promptkit.Profile{
+			ID: "profile", Endpoint: "http://profile.example/v1", Model: "model", ExtraParams: extraParams,
+		}),
+	)
+	if err != nil {
+		t.Fatalf("construct inspection engine: %v", err)
+	}
+
+	first, err := engine.InspectProfile(context.Background(), "profile")
+	if err != nil {
+		t.Fatalf("first inspection: %v", err)
+	}
+	first.EffectiveModelParams.ExtraParams["nested"].(map[string]any)["value"] = "changed"
+	first.EffectiveModelParams.ExtraParams["later"] = true
+
+	second, err := engine.InspectProfile(context.Background(), "profile")
+	if err != nil {
+		t.Fatalf("second inspection: %v", err)
+	}
+	prepared, err := engine.Prepare(context.Background(), promptkit.RunRequest{PromptID: "prompt"})
+	if err != nil {
+		t.Fatalf("prepare after inspection mutation: %v", err)
+	}
+	for _, target := range []promptkit.ExecutionTarget{second.EffectiveModelParams, prepared.EffectiveModelParams} {
+		if target.ExtraParams["nested"].(map[string]any)["value"] != "original" || target.ExtraParams["later"] != nil {
+			t.Fatalf("inspection mutation reached engine-owned target: %#v", target.ExtraParams)
+		}
+	}
+	if !reflect.DeepEqual(second.EffectiveModelParams, prepared.EffectiveModelParams) {
+		t.Fatalf("inspection target=%#v, preparation target=%#v", second.EffectiveModelParams, prepared.EffectiveModelParams)
+	}
+}
+
+func TestInspectPromptReturnsDeclaredMetadataWithoutExecutionWork(t *testing.T) {
+	client := &fakeLLMClient{response: &promptkit.GenerateResponse{Content: "unexpected"}}
+	engine, err := promptkit.NewEngine(promptkit.Config{},
+		promptkit.WithPromptFS(fstest.MapFS{
+			"report-v1.yaml": &fstest.MapFile{Data: []byte(`id: report
+version: "1.0.0"
+messages:
+  - role: user
+    content: old report
+output:
+  format: text
+  validation_mode: none
+`)},
+			"report-v2.yaml": &fstest.MapFile{Data: []byte(`id: report
+version: "2.0.0"
+default_profile: missing-profile
+inputs:
+  - name: location
+    required: true
+    content_type: text/plain
+    description: Forecast location.
+  - name: units
+    content_type: text/plain
+    description: Unit preference.
+messages:
+  - role: user
+    content_file: messages/report.md
+output:
+  format: json
+  validation_mode: json_schema
+  schema_path: schemas/report.json
+`)},
+			"messages/report.md": &fstest.MapFile{Data: []byte("rendered report body is not returned")},
+		}, "."),
+		promptkit.WithLLMClient(client),
+	)
+	if err != nil {
+		t.Fatalf("construct prompt-inspection engine: %v", err)
+	}
+
+	inspection, err := engine.InspectPrompt(context.Background(), "report", "2.0.0")
+	if err != nil {
+		t.Fatalf("inspect prompt: %v", err)
+	}
+	if inspection.PromptID != "report" ||
+		inspection.PromptVersion != "2.0.0" ||
+		inspection.PromptHash == "" ||
+		inspection.DefaultProfileID != "missing-profile" ||
+		inspection.OutputContract != (promptkit.OutputContract{
+			Format:         promptkit.FormatJSON,
+			ValidationMode: promptkit.ValidationJSONSchema,
+			SchemaPath:     "schemas/report.json",
+		}) {
+		t.Fatalf("unexpected inspection metadata: %#v", inspection)
+	}
+	wantInputs := []promptkit.PromptInputDefinition{
+		{Name: "location", Required: true, ContentType: "text/plain", Description: "Forecast location."},
+		{Name: "units", ContentType: "text/plain", Description: "Unit preference."},
+	}
+	if !reflect.DeepEqual(inspection.Inputs, wantInputs) {
+		t.Fatalf("inspection inputs=%#v, want %#v", inspection.Inputs, wantInputs)
+	}
+	if len(client.requests) != 0 {
+		t.Fatalf("inspection invoked the model client %d times", len(client.requests))
+	}
+}
+
+func TestInspectPromptPreservesPublicErrorIdentities(t *testing.T) {
+	newEngine := func(t *testing.T, source fstest.MapFS) *promptkit.Engine {
+		t.Helper()
+		engine, err := promptkit.NewEngine(promptkit.Config{}, promptkit.WithPromptFS(source, "."))
+		if err != nil {
+			t.Fatalf("construct prompt-inspection engine: %v", err)
+		}
+		return engine
+	}
+	validSource := fstest.MapFS{
+		"prompt.yaml": &fstest.MapFile{Data: []byte(`id: prompt
+version: "1"
+messages:
+  - role: user
+    content: body
+output:
+  format: text
+  validation_mode: none
+`)},
+	}
+
+	var nilEngine *promptkit.Engine
+	if result, err := nilEngine.InspectPrompt(context.Background(), "prompt", "1"); result != nil ||
+		!errors.Is(err, promptkit.ErrInvalidConfig) {
+		t.Fatalf("nil engine result=(%#v, %v), want ErrInvalidConfig", result, err)
+	}
+
+	valid := newEngine(t, validSource)
+	if result, err := valid.InspectPrompt(context.Background(), " \t ", "1"); result != nil ||
+		!errors.Is(err, promptkit.ErrInvalidRequest) {
+		t.Fatalf("blank prompt result=(%#v, %v), want ErrInvalidRequest", result, err)
+	}
+	if result, err := valid.InspectPrompt(context.Background(), "missing", "1"); result != nil ||
+		!errors.Is(err, promptkit.ErrPromptNotFound) || errors.Is(err, promptkit.ErrPromptLoad) {
+		t.Fatalf("missing prompt result=(%#v, %v), want only ErrPromptNotFound", result, err)
+	}
+	if result, err := valid.InspectPrompt(context.Background(), "prompt", "missing"); result != nil ||
+		!errors.Is(err, promptkit.ErrPromptNotFound) || errors.Is(err, promptkit.ErrPromptLoad) {
+		t.Fatalf("missing version result=(%#v, %v), want only ErrPromptNotFound", result, err)
+	}
+
+	ambiguous := newEngine(t, fstest.MapFS{
+		"one.yaml": &fstest.MapFile{Data: []byte(`id: prompt
+version: "1"
+messages:
+  - role: user
+    content: first
+output:
+  format: text
+  validation_mode: none
+`)},
+		"two.yaml": &fstest.MapFile{Data: []byte(`id: prompt
+version: "2"
+messages:
+  - role: user
+    content: second
+output:
+  format: text
+  validation_mode: none
+`)},
+	})
+	if result, err := ambiguous.InspectPrompt(context.Background(), "prompt", ""); result != nil ||
+		!errors.Is(err, promptkit.ErrPromptLoad) {
+		t.Fatalf("ambiguous prompt result=(%#v, %v), want ErrPromptLoad", result, err)
+	}
+
+	for name, source := range map[string]fstest.MapFS{
+		"malformed definition": {
+			"broken.yaml": &fstest.MapFile{Data: []byte("id: broken\nversion: \"1\"\nunknown: value\n")},
+		},
+		"missing content file": {
+			"broken.yaml": &fstest.MapFile{Data: []byte(`id: broken
+version: "1"
+messages:
+  - role: user
+    content_file: missing.md
+output:
+  format: text
+  validation_mode: none
+`)},
+		},
+	} {
+		t.Run(name, func(t *testing.T) {
+			if result, err := newEngine(t, source).InspectPrompt(context.Background(), "broken", "1"); result != nil ||
+				!errors.Is(err, promptkit.ErrPromptLoad) {
+				t.Fatalf("broken prompt result=(%#v, %v), want ErrPromptLoad", result, err)
+			}
+		})
+	}
+
+	countingFS := &inspectionCountingFS{}
+	canceled, err := promptkit.NewEngine(promptkit.Config{}, promptkit.WithPromptFS(countingFS, "."))
+	if err != nil {
+		t.Fatalf("construct canceled prompt-inspection engine: %v", err)
+	}
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+	if result, err := canceled.InspectPrompt(ctx, "prompt", "1"); result != nil ||
+		!errors.Is(err, promptkit.ErrPromptLoad) || !errors.Is(err, context.Canceled) || countingFS.opens.Load() != 0 {
+		t.Fatalf("canceled inspection result=(%#v, %v), opens=%d", result, err, countingFS.opens.Load())
+	}
+}
+
+func TestInspectPromptReturnsIndependentMetadataMatchingPreparation(t *testing.T) {
+	engine, err := promptkit.NewEngine(promptkit.Config{},
+		promptkit.WithPromptFS(fstest.MapFS{
+			"prompt.yaml": &fstest.MapFile{Data: []byte(`id: prompt
+version: "1"
+default_profile: profile
+inputs:
+  - name: subject
+    content_type: text/plain
+    description: Summary subject.
+messages:
+  - role: user
+    content: summarize
+output:
+  format: markdown
+  validation_mode: basic
+`)},
+		}, "."),
+		promptkit.WithProfiles(promptkit.Profile{
+			ID: "profile", Endpoint: "http://profile.example/v1", Model: "model",
+		}),
+	)
+	if err != nil {
+		t.Fatalf("construct prompt-inspection engine: %v", err)
+	}
+
+	first, err := engine.InspectPrompt(context.Background(), "prompt", "1")
+	if err != nil {
+		t.Fatalf("first inspection: %v", err)
+	}
+	first.Inputs[0].Name = "changed"
+	first.OutputContract.SchemaPath = "changed.json"
+
+	second, err := engine.InspectPrompt(context.Background(), "prompt", "1")
+	if err != nil {
+		t.Fatalf("second inspection: %v", err)
+	}
+	prepared, err := engine.Prepare(context.Background(), promptkit.RunRequest{PromptID: "prompt", PromptVersion: "1"})
+	if err != nil {
+		t.Fatalf("prepare after inspection mutation: %v", err)
+	}
+	if second.Inputs[0].Name != "subject" || second.OutputContract.SchemaPath != "" ||
+		prepared.OutputContract.SchemaPath != "" || second.PromptHash != prepared.PromptHash {
+		t.Fatalf("inspection mutation reached engine-owned prompt metadata: inspection=%#v prepared=%#v", second, prepared)
+	}
+}
+
 func TestPreparedRunJSONOmitsZeroTimingValues(t *testing.T) {
 	payload, err := json.Marshal(promptkit.PreparedRun{})
 	if err != nil {

types.go

@@ -51,7 +51,8 @@ const (
 	// ValidationPassed means the generated output satisfied its contract.
 	ValidationPassed ValidationStatus = "passed"
 	// ValidationFailed means validation completed and rejected the generated
-	// output. Engine.Run returns this status in a result, not as an error.
+	// output. Engine.Run and Engine.RunPrepared return this status in a result,
+	// not as an error.
 	ValidationFailed ValidationStatus = "failed"
 	// ValidationSkipped means ValidationNone selected no content check.
 	ValidationSkipped ValidationStatus = "skipped"
@@ -78,9 +79,10 @@ const (
 // RunRequest selects one prompt execution. It has no stable JSON
 // representation.
 //
-// Prepare and Run copy the request's maps, pointers, and nested
-// JSON-compatible values before using them. The caller may mutate the request
-// after either method returns.
+// Prepare, PrepareExecution, and Run copy the request's maps, pointers, and
+// nested JSON-compatible values before using them. The caller may mutate the
+// request after any method returns. A successful PrepareExecution retains its
+// own private execution snapshot for RunPrepared.
 type RunRequest struct {
 	// PromptID is the required non-empty prompt identifier.
 	PromptID string
@@ -98,12 +100,14 @@ type RunRequest struct {
 	// opaque consumer metadata, not a credential, and may be exposed in
 	// prepared values, results, collaborator requests, provider requests, and
 	// provider observability. Callers should use stable, non-sensitive
-	// identifiers. An overlong direct value makes Prepare or Run return an
-	// error matching ErrInvalidRequest.
+	// identifiers. An overlong direct value makes Prepare, PrepareExecution, or
+	// Run return an error matching ErrInvalidRequest.
 	SessionID string
 	// APIKey is a request-scoped direct credential. It takes precedence over
 	// APIKeyEnv, is passed to the selected LLMClient, and is never included in
-	// prepared values, results, hashes, JSON, String, or GoString output.
+	// prepared values, results, hashes, JSON, String, or GoString output. A
+	// successful PrepareExecution retains it only in the opaque handle until
+	// RunPrepared claims the handle or Discard invalidates it.
 	APIKey string `json:"-"`
 	// Inputs maps prompt input names to references. A nil or empty map is valid
 	// only when the selected prompt and its templates require no inputs.
@@ -119,9 +123,10 @@ type RunRequest struct {
 	Validation *OutputContract
 }
 
-// PreparedRun contains prepared prompt execution state. It does not include
-// resolved API key values, model output, validation results, or internal target
-// presence metadata. PreparedRun has a stable JSON representation.
+// PreparedRun contains prepared prompt execution state returned by
+// [Engine.Prepare] or [PreparedExecution.Details]. It does not include resolved
+// API key values, model output, validation results, or internal target presence
+// metadata. PreparedRun has a stable JSON representation.
 //
 // All maps, slices, pointers, and schema values are caller-owned copies. JSON
 // timestamps use RFC 3339 and zero timing values are omitted. Hash formats are
@@ -154,7 +159,8 @@ type PreparedRun struct {
 	SessionID string `json:"session_id,omitempty"`
 	// RenderedPromptHash is an opaque equality value for SessionID and Messages.
 	RenderedPromptHash string `json:"rendered_prompt_hash"`
-	// Messages are the rendered messages that Run passes to the LLM client.
+	// Messages are the rendered messages that Run or RunPrepared passes to the
+	// LLM client.
 	Messages []RenderedMessage `json:"messages"`
 	// StartTime is the UTC time at which preparation began.
 	StartTime time.Time `json:"start_time,omitempty"`
@@ -214,12 +220,15 @@ type RunResult struct {
 	InputHashes map[string]string `json:"input_hashes,omitempty"`
 	// Usage is the token accounting reported by the LLM client.
 	Usage TokenUsage `json:"usage"`
-	// StartTime is the UTC time immediately before preparation begins.
+	// StartTime is the UTC time immediately before ordinary Run preparation or
+	// after RunPrepared claims its handle.
 	StartTime time.Time `json:"start_time,omitempty"`
 	// EndTime is the UTC time after generation and validation complete.
 	EndTime time.Time `json:"end_time,omitempty"`
-	// Duration covers preparation, generation, and validation. JSON represents
-	// it as integer milliseconds in duration_ms and omits a zero value.
+	// Duration covers preparation, generation, and validation for Run. For
+	// RunPrepared it covers only the execution attempt after claim and excludes
+	// preparation and consumer-held delay. JSON represents it as integer
+	// milliseconds in duration_ms and omits a zero value.
 	Duration time.Duration `json:"-"`
 }
 
@@ -259,10 +268,11 @@ type Artifact struct {
 // ArtifactReader resolves a prompt input reference into its content.
 //
 // Read may be called concurrently. It must honor ctx cancellation to make
-// Prepare and Run responsive to cancellation. The engine passes a copied ref
-// and immediately copies the returned Artifact.Body; it does not retain either
-// value. Readers supply artifact metadata, and the engine assigns an input-map
-// name only when the returned artifact name is empty.
+// Prepare, PrepareExecution, and Run responsive to cancellation. The engine
+// passes a copied ref and immediately copies the returned Artifact.Body; it
+// does not retain either value. Readers supply artifact metadata, and the
+// engine assigns an input-map name only when the returned artifact name is
+// empty.
 //
 // An injected reader owns any application-specific path containment,
 // authorization, content-size, and content-type policy. It must protect
@@ -310,6 +320,61 @@ type ExecutionTarget struct {
 	ExtraParams map[string]any `json:"extra_params"`
 }
 
+// ProfileInspection is the caller-owned result of [Engine.InspectProfile].
+// It has no stable JSON representation.
+//
+// EffectiveModelParams contains a copied effective target. APIKeyRequired is
+// separate from that target to preserve ExecutionTarget's general execution
+// and stable JSON contracts.
+type ProfileInspection struct {
+	// ProfileID is the trimmed, exact profile ID inspected by the engine.
+	ProfileID string
+	// EffectiveModelParams contains framework defaults overlaid by the selected
+	// backend and then the profile, without a request override. APIKeyEnv is an
+	// environment-variable name, never its credential value.
+	EffectiveModelParams ExecutionTarget
+	// APIKeyRequired reports that a later execution must supply a direct API
+	// key or an explicit request environment override. It is mutually exclusive
+	// with a nonblank EffectiveModelParams.APIKeyEnv.
+	APIKeyRequired bool
+}
+
+// PromptInputDefinition describes one declared prompt input.
+// It has no stable JSON representation.
+type PromptInputDefinition struct {
+	// Name is the normalized prompt input name.
+	Name string
+	// Required is the prompt definition's declared required flag. When true,
+	// preparation fails if the input is omitted. A false value does not account
+	// for input references in message or session-ID templates.
+	Required bool
+	// ContentType is the declared input media-type metadata.
+	ContentType string
+	// Description is the declared human-readable input description.
+	Description string
+}
+
+// PromptInspection is the caller-owned result of [Engine.InspectPrompt].
+// It has no stable JSON representation.
+//
+// Inputs contains copied declared input metadata in definition order.
+// OutputContract is the normalized contract declared by the prompt definition,
+// rather than a request-level effective override. PromptHash is opaque.
+type PromptInspection struct {
+	// PromptID is the normalized ID of the selected prompt definition.
+	PromptID string
+	// PromptVersion is the normalized version of the selected prompt definition.
+	PromptVersion string
+	// PromptHash is the opaque equality value for the selected definition.
+	PromptHash string
+	// DefaultProfileID is declared metadata and is not resolved by inspection.
+	DefaultProfileID string
+	// Inputs contains caller-owned declared input metadata in definition order.
+	Inputs []PromptInputDefinition
+	// OutputContract is the normalized contract declared by the definition.
+	OutputContract OutputContract
+}
+
 // ExecutionTargetOverride represents per-request runtime setting overrides and
 // has no stable JSON representation.
 //
@@ -559,25 +624,26 @@ type StructuredOutputJSONSpec struct {
 	Schema any `json:"schema"`
 }
 
-// LLMClient executes rendered prompts for [Engine.Run].
+// LLMClient executes rendered prompts for [Engine.Run] and
+// [Engine.RunPrepared].
 //
 // Generate is scheduled according to the resolved backend's capacity policy.
 // It may still be called concurrently for different backend pools or unlimited
 // backends. Cancellation while waiting for capacity can prevent Generate from
 // being called. Once invoked, it must honor context cancellation to make Run
-// responsive to cancellation. The request and all nested maps, slices, and
-// pointers are client-owned copies and may be mutated or retained without
-// affecting engine state.
+// and RunPrepared responsive to cancellation. The request and all nested maps,
+// slices, and pointers are client-owned copies and may be mutated or retained
+// without affecting engine state.
 //
 // Generate receives rendered messages and may receive a direct API key. A
 // client must protect those values and any raw output in its logging, storage,
 // and retained copies. It is responsible for the cancellation behavior of any
 // work it starts and for synchronizing access to retained or shared data.
 //
-// A returned error makes Run return ErrLLMGenerate while preserving the client
-// error through errors.Is. A nil response with a nil error also produces
-// ErrLLMGenerate. Promptkit copies the non-nil response before returning from
-// Run.
+// A returned error makes Run or RunPrepared return ErrLLMGenerate while
+// preserving the client error through errors.Is. A nil response with a nil
+// error also produces ErrLLMGenerate. Promptkit copies the non-nil response
+// before returning from either method.
 type LLMClient interface {
 	Generate(context.Context, GenerateRequest) (*GenerateResponse, error)
 }