Prepare v0.23.0 release notes and retire completed roadmaps

README.md

@@ -45,6 +45,17 @@ boundary and constraints that framework work must preserve.
 
 ## Release Guidance
 
+The [v0.23.0 release notes](docs/releases/v0.23.0.md) cover per-operation
+timeouts and opt-in prepared-request notifications for ordinary and coordinated
+execution.
+
+The [v0.22.0 release notes](docs/releases/v0.22.0.md) cover captured conversation
+exchanges, application-directed turn coordination, and explicit execution groups.
+
+The [v0.21.0 release notes](docs/releases/v0.21.0.md) cover ConfigKit v1.2.0
+and main-relative companion imports through the existing configuration-source
+API.
+
 The [v0.20.0 release notes](docs/releases/v0.20.0.md) cover resolved resource
 inspection, generic JSON/YAML reports, typed prompt sources, and backend
 inheritance. Upgrading consumers should review the changed inspection result

configuration_sources_test.go

@@ -63,6 +63,47 @@ func TestCollectionInputLoadsNativeImportsAndLaterOverride(t *testing.T) {
 	}
 }
 
+func TestCollectionInputUsesMainRelativeContext(t *testing.T) {
+	filesystem := configkit.NewFilesystem("application", fstest.MapFS{
+		"site/main.yaml": {Data: []byte("version: '2'\n")},
+		"site/prompts/greeting.yaml": {Data: []byte(`id: greeting
+messages:
+  - role: user
+    content_file: greeting.txt
+output: {format: text, validation_mode: none}
+`)},
+		"site/prompts/greeting.txt": {Data: []byte("Hello from the imported prompt's directory.")},
+	})
+	companion, err := promptkit.CollectionInput("support", configkit.Bytes("companion prompts", []byte(`
+config:
+  imports:
+    - dir: prompts
+      pattern: ["*.yaml", "*.yml"]
+      destination: [prompts]
+      placement: sequence-item
+      required: true
+`)))
+	if err != nil {
+		t.Fatal(err)
+	}
+	companion.MainRelative = configkit.MainRelative()
+	main := configkit.Input{Source: filesystem.File("site/main.yaml")}
+	engine, err := promptkit.NewEngine(promptkit.Config{Sources: promptkit.ConfigurationSources{
+		Main:      &main,
+		Underlays: []configkit.Underlay{{Inputs: []configkit.Input{companion}}},
+	}}, promptkit.WithBundledOpenRouter(false))
+	if err != nil {
+		t.Fatalf("construct engine with main-relative imports: %v", err)
+	}
+	inspection, err := engine.InspectPrompt(t.Context(), "support/greeting")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(inspection.Messages) != 1 || inspection.Messages[0].Content != "Hello from the imported prompt's directory." {
+		t.Fatalf("unexpected imported messages: %+v", inspection.Messages)
+	}
+}
+
 func TestCollectionInputLoadsEmbeddedImportsAndLaterOverride(t *testing.T) {
 	files := fstest.MapFS{
 		"collection.yaml": {Data: []byte("config:\n  imports:\n    - path: defaults.yaml\n      required: true\n")},

continuation_contract_test.go

@@ -0,0 +1,141 @@
+package promptkit_test
+
+import (
+	"context"
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"reflect"
+	"sync"
+	"testing"
+
+	"go.maximumdirect.net/promptkit"
+)
+
+func TestContinuationBranchesAcrossEngines(t *testing.T) {
+	ctx := context.Background()
+	firstClient := &fakeLLMClient{response: &promptkit.GenerateResponse{Content: "first answer"}}
+	firstEngine := newSimpleEngine(t, "first question", promptkit.WithLLMClient(firstClient))
+	initial, err := firstEngine.Run(ctx, promptkit.RunRequest{PromptID: "prompt", CaptureExchange: true, APIKey: "synthetic-first-key"})
+	if err != nil {
+		t.Fatal(err)
+	}
+	original := initial.Exchange.Details()
+	secondClient := &fakeLLMClient{response: &promptkit.GenerateResponse{Content: "follow-up answer"}}
+	secondEngine := newNamespacedContractEngine(t, secondClient)
+	scoped, err := secondEngine.WithNamespaces(promptkit.Namespaces{Prompt: "alpha", Profile: "alpha"})
+	if err != nil {
+		t.Fatal(err)
+	}
+	request := promptkit.RunRequest{PromptID: "shared", ProfileID: "shared", ContinueFrom: initial.Exchange, SessionID: "new-session", Validation: &promptkit.OutputContract{Format: promptkit.FormatJSON, ValidationMode: promptkit.ValidationJSON}, AppendedMessages: []promptkit.RenderedMessage{{Role: promptkit.RoleUser, Content: "branch one"}}}
+	branch, err := scoped.Run(ctx, request)
+	if err != nil {
+		t.Fatal(err)
+	}
+	request.AppendedMessages[0].Content = "branch two"
+	sibling, err := scoped.Run(ctx, request)
+	if err != nil {
+		t.Fatal(err)
+	}
+	next, err := scoped.Run(ctx, promptkit.RunRequest{PromptID: "shared", ProfileID: "shared", ContinueFrom: branch.Exchange})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if branch.ParentRunID != initial.RunID || sibling.ParentRunID != initial.RunID || next.ParentRunID != branch.RunID || branch.PromptID != "alpha/shared" || branch.SelectedProfileID != "alpha/shared" || branch.SessionID != "new-session" {
+		t.Fatalf("ancestry or selections incorrect: %#v %#v %#v", branch, sibling, next)
+	}
+	if !reflect.DeepEqual(initial.Exchange.Details(), original) {
+		t.Fatal("branching changed source")
+	}
+	if branch.Validation.Status != promptkit.ValidationFailed || branch.ModelName != "alpha-model" {
+		t.Fatalf("follow-up settings: %#v", branch)
+	}
+	branchMessages := branch.Exchange.Details().Messages
+	siblingMessages := sibling.Exchange.Details().Messages
+	if len(branchMessages) != 5 || len(siblingMessages) != 5 || branchMessages[3].Content != "branch one" || siblingMessages[3].Content != "branch two" || len(next.Exchange.Details().Messages) != 7 {
+		t.Fatalf("branch histories = %#v / %#v", branchMessages, siblingMessages)
+	}
+	encoded, err := json.Marshal(branch.Exchange.Details())
+	if err != nil || !json.Valid(encoded) {
+		t.Fatalf("details JSON = %s, %v", encoded, err)
+	}
+	branchMessages[0].Content = "changed details"
+	branch.RawOutput = "changed result"
+	if initial.Exchange.Details().Messages[0].Content != "first question" || branch.Exchange.Details().Messages[4].Content != "follow-up answer" {
+		t.Fatal("caller mutation changed captured history")
+	}
+	for _, request := range secondClient.requests {
+		if request.APIKey == "synthetic-first-key" || request.Prompt.Messages[0].Content != "first question" || request.Prompt.Messages[1].Content != "first answer" {
+			t.Fatalf("unexpected follow-up request: %#v", request)
+		}
+	}
+}
+
+func TestContinuationDoesNotReloadCapturedInputs(t *testing.T) {
+	ctx := context.Background()
+	path := filepath.Join(t.TempDir(), "input.txt")
+	if err := os.WriteFile(path, []byte("original input"), 0600); err != nil {
+		t.Fatal(err)
+	}
+	client := &fakeLLMClient{response: &promptkit.GenerateResponse{Content: ""}}
+	engine := newSimpleEngine(t, `{{input "source"}}`, promptkit.WithLLMClient(client))
+	initial, err := engine.Run(ctx, promptkit.RunRequest{PromptID: "prompt", Inputs: map[string]promptkit.ArtifactRef{"source": promptkit.File(path)}, CaptureExchange: true})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(path, []byte("new input"), 0600); err != nil {
+		t.Fatal(err)
+	}
+	request := promptkit.RunRequest{PromptID: "prompt", Inputs: map[string]promptkit.ArtifactRef{"source": promptkit.File(path)}, ContinueFrom: initial.Exchange}
+	prepared, err := engine.PrepareExecution(ctx, request)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := os.Remove(path); err != nil {
+		t.Fatal(err)
+	}
+	result, err := engine.RunPrepared(ctx, prepared)
+	if err != nil {
+		t.Fatal(err)
+	}
+	want := []promptkit.RenderedMessage{{Role: promptkit.RoleUser, Content: "original input"}, {Role: promptkit.RoleAssistant, Content: ""}, {Role: promptkit.RoleUser, Content: "new input"}, {Role: promptkit.RoleAssistant, Content: ""}}
+	if !reflect.DeepEqual(result.Exchange.Details().Messages, want) {
+		t.Fatalf("history = %#v", result.Exchange.Details().Messages)
+	}
+	if result, err := engine.Run(ctx, request); err == nil || result != nil {
+		t.Fatalf("missing new input = %#v, %v", result, err)
+	}
+}
+
+func TestContinuationSourceSupportsConcurrentRuns(t *testing.T) {
+	client := &preparedRecordingClient{response: &promptkit.GenerateResponse{Content: "answer"}}
+	engine := newSimpleEngine(t, "question", promptkit.WithLLMClient(client))
+	initial, err := engine.Run(context.Background(), promptkit.RunRequest{PromptID: "prompt", CaptureExchange: true})
+	if err != nil {
+		t.Fatal(err)
+	}
+	const count = 8
+	var wg sync.WaitGroup
+	errors := make(chan error, count)
+	for i := 0; i < count; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			result, err := engine.Run(context.Background(), promptkit.RunRequest{PromptID: "prompt", ContinueFrom: initial.Exchange})
+			if err == nil && (result.Exchange == nil || result.ParentRunID != initial.RunID) {
+				t.Error("missing continuation ancestry")
+			}
+			errors <- err
+		}()
+	}
+	wg.Wait()
+	close(errors)
+	for err := range errors {
+		if err != nil {
+			t.Fatal(err)
+		}
+	}
+	if len(initial.Exchange.Details().Messages) != 2 {
+		t.Fatal("source changed")
+	}
+}

convert.go

@@ -1,6 +1,7 @@
 package promptkit
 
 import (
+	"context"
 	"fmt"
 	"unicode/utf8"
 
@@ -10,6 +11,13 @@ import (
 )
 
 func toDomainRunRequest(req RunRequest) (domain.RunRequest, error) {
+	var history *domain.Exchange
+	if req.ContinueFrom != nil && req.ContinueFrom.internal == nil {
+		return domain.RunRequest{}, fmt.Errorf("continuation exchange is empty")
+	}
+	if req.ContinueFrom != nil {
+		history = req.ContinueFrom.internal
+	}
 	execution, err := toDomainExecutionTargetOverride(req.Execution)
 	if err != nil {
 		return domain.RunRequest{}, err
@@ -23,7 +31,16 @@ func toDomainRunRequest(req RunRequest) (domain.RunRequest, error) {
 		callback := req.Observer
 		observer = func(event domain.ExecutionEvent) { callback(fromDomainExecutionEvent(event)) }
 	}
+	var onPrepared func(context.Context, *domain.PreparedRun)
+	if callback := req.OnPrepared; callback != nil {
+		onPrepared = func(ctx context.Context, prepared *domain.PreparedRun) {
+			callback(ctx, *fromDomainPreparedRun(prepared))
+		}
+	}
 	return domain.RunRequest{
+		OperationTimeout: req.OperationTimeout,
+		OnPrepared:       onPrepared,
+		CaptureExchange:  req.CaptureExchange || req.ContinueFrom != nil,
 		PromptID:         req.PromptID,
 		ProfileID:        req.ProfileID,
 		SessionID:        req.SessionID,
@@ -35,6 +52,7 @@ func toDomainRunRequest(req RunRequest) (domain.RunRequest, error) {
 		AppendedMessages: appendedMessages,
 		TransientRetries: copyIntPtr(req.TransientRetries),
 		Observer:         observer,
+		ContinueFrom:     history,
 	}, nil
 }
 
@@ -71,6 +89,7 @@ func fromDomainPreparedRun(prepared *domain.PreparedRun) *PreparedRun {
 		return nil
 	}
 	return &PreparedRun{
+		ParentRunID:          prepared.ParentRunID,
 		PromptID:             prepared.PromptID,
 		PromptHash:           prepared.PromptHash,
 		SelectedProfileID:    prepared.SelectedProfileID,
@@ -92,7 +111,13 @@ func fromDomainRunResult(result *domain.RunResult) *RunResult {
 	if result == nil {
 		return nil
 	}
+	var exchange *Exchange
+	if result.Exchange != nil {
+		exchange = &Exchange{internal: result.Exchange}
+	}
 	return &RunResult{
+		Exchange:             exchange,
+		ParentRunID:          result.ParentRunID,
 		RunID:                result.RunID,
 		Artifact:             fromDomainArtifact(result.Artifact),
 		RawOutput:            result.RawOutput,

coordinate.go

@@ -0,0 +1,82 @@
+package promptkit
+
+import (
+	"context"
+	"fmt"
+
+	"go.maximumdirect.net/promptkit/internal/domain"
+	"go.maximumdirect.net/promptkit/internal/usecase"
+)
+
+// ConversationTurn is delivered before each requested continuation. Result is
+// nil on the initial callback; RemainingTurns counts additional runs allowed.
+type ConversationTurn struct {
+	Exchange       *Exchange
+	Result         *RunResult
+	RemainingTurns int
+}
+
+// CoordinateRequest selects a starting exchange, turn budget and callback.
+// Observer selects continuation runs only, independently of callback calls.
+type CoordinateRequest struct {
+	Start           *Exchange
+	AdditionalTurns int
+	Next            func(context.Context, ConversationTurn) (*RunRequest, error)
+	Observer        ExecutionObserver
+}
+
+// CoordinationOutcome distinguishes caller-directed stopping from exhaustion.
+type CoordinationOutcome string
+
+const (
+	// CoordinationStopped means the callback chose not to continue.
+	CoordinationStopped CoordinationOutcome = "stopped"
+	// CoordinationExhausted means another run was requested without budget.
+	CoordinationExhausted CoordinationOutcome = "exhausted"
+)
+
+// CoordinationResult contains the normal outcome or a response withheld by
+// cancellation before delivery to the callback.
+type CoordinationResult struct {
+	Outcome     CoordinationOutcome `json:"outcome,omitempty"`
+	Undelivered *RunResult          `json:"undelivered,omitempty"`
+}
+
+// Coordinate advances one branch from Start using ordinary runs. Next is called
+// initially even for a zero budget and receives each completed result once,
+// including the final permitted response. The initial exchange is not itself a
+// newly delivered result. Each started continuation consumes one turn; retries
+// and structural repairs do not. Callback work runs after admission is released.
+// A nil request stops; requesting a run without remaining budget exhausts it.
+// Callback and execution errors retain their identities. Cancellation observed
+// before delivery returns the single completed response as Undelivered;
+// cancellation after callback invocation does not replay that response. Context
+// cancellation takes precedence over a callback's returned action or error.
+// Start must be a captured exchange, Next non-nil, and AdditionalTurns
+// nonnegative; invalid values match ErrInvalidRequest. A nil engine returns
+// ErrInvalidConfig. The selected request is copied and its ContinueFrom is
+// replaced by the coordinator's current exchange. The request observer runs
+// before the coordinator observer for each continuation.
+func (e *Engine) Coordinate(ctx context.Context, req CoordinateRequest) (CoordinationResult, error) {
+	if e == nil || e.internal == nil {
+		return CoordinationResult{}, fmt.Errorf("%w: engine is nil", ErrInvalidConfig)
+	}
+	if req.Start == nil || req.Start.internal == nil || req.Next == nil || req.AdditionalTurns < 0 {
+		return CoordinationResult{}, fmt.Errorf("%w: invalid coordination request", ErrInvalidRequest)
+	}
+	outcome, err := usecase.Coordinate(ctx, req.Start.internal, req.AdditionalTurns,
+		func(ctx context.Context, turn usecase.ConversationTurn[RunResult]) (*RunRequest, error) {
+			return req.Next(ctx, ConversationTurn{Exchange: &Exchange{internal: turn.Exchange}, Result: turn.Result, RemainingTurns: turn.RemainingTurns})
+		},
+		func(ctx context.Context, current *domain.Exchange, selected *RunRequest) (*RunResult, *domain.Exchange, error) {
+			request := *selected
+			request.ContinueFrom = &Exchange{internal: current}
+			request.Observer = ComposeObservers(request.Observer, req.Observer)
+			result, runErr := e.Run(ctx, request)
+			if runErr != nil {
+				return nil, nil, runErr
+			}
+			return result, result.Exchange.internal, nil
+		})
+	return CoordinationResult{Outcome: CoordinationOutcome(outcome.Outcome), Undelivered: outcome.Undelivered}, err
+}

coordinate_contract_test.go

@@ -0,0 +1,61 @@
+package promptkit_test
+
+import (
+	"context"
+	"errors"
+	"testing"
+
+	"go.maximumdirect.net/promptkit"
+)
+
+func TestCoordinatePublicBoundary(t *testing.T) {
+	ctx := context.Background()
+	engine := newSimpleEngine(t, "question", promptkit.WithLLMClient(&fakeLLMClient{response: &promptkit.GenerateResponse{Content: "answer"}}))
+	initial, err := engine.Run(ctx, promptkit.RunRequest{PromptID: "prompt", CaptureExchange: true})
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, request := range []promptkit.CoordinateRequest{{}, {Start: &promptkit.Exchange{}, Next: func(context.Context, promptkit.ConversationTurn) (*promptkit.RunRequest, error) { return nil, nil }}, {Start: initial.Exchange, AdditionalTurns: -1, Next: func(context.Context, promptkit.ConversationTurn) (*promptkit.RunRequest, error) { return nil, nil }}} {
+		if _, err := engine.Coordinate(ctx, request); !errors.Is(err, promptkit.ErrInvalidRequest) {
+			t.Fatalf("invalid request error = %v", err)
+		}
+	}
+	if _, err := (*promptkit.Engine)(nil).Coordinate(ctx, promptkit.CoordinateRequest{}); !errors.Is(err, promptkit.ErrInvalidConfig) {
+		t.Fatalf("nil engine error = %v", err)
+	}
+	callbacks := 0
+	selected := &promptkit.RunRequest{PromptID: "prompt", ContinueFrom: &promptkit.Exchange{}}
+	result, err := engine.Coordinate(ctx, promptkit.CoordinateRequest{Start: initial.Exchange, AdditionalTurns: 2, Next: func(_ context.Context, turn promptkit.ConversationTurn) (*promptkit.RunRequest, error) {
+		callbacks++
+		if callbacks == 1 {
+			if turn.Result != nil || turn.RemainingTurns != 2 {
+				t.Fatalf("initial turn = %#v", turn)
+			}
+			return selected, nil
+		}
+		if turn.Result == nil || turn.Exchange.Details().ParentRunID == "" {
+			t.Fatalf("delivered turn = %#v", turn)
+		}
+		if callbacks == 2 {
+			turn.Result.Exchange = nil
+			*turn.Exchange = promptkit.Exchange{}
+			return selected, nil
+		}
+		if turn.Result.ParentRunID == initial.RunID {
+			t.Fatal("callback mutation redirected branch")
+		}
+		return nil, nil
+	}})
+	if err != nil || result.Outcome != promptkit.CoordinationStopped || callbacks != 3 || selected.ContinueFrom == nil {
+		t.Fatalf("result=%#v error=%v callbacks=%d", result, err, callbacks)
+	}
+	stop, err := engine.Coordinate(ctx, promptkit.CoordinateRequest{Start: initial.Exchange, Next: func(context.Context, promptkit.ConversationTurn) (*promptkit.RunRequest, error) { return nil, nil }})
+	if err != nil || stop.Outcome != promptkit.CoordinationStopped {
+		t.Fatalf("zero budget = %#v, %v", stop, err)
+	}
+	sentinel := errors.New("callback")
+	_, err = engine.Coordinate(ctx, promptkit.CoordinateRequest{Start: initial.Exchange, Next: func(context.Context, promptkit.ConversationTurn) (*promptkit.RunRequest, error) { return nil, sentinel }})
+	if !errors.Is(err, sentinel) {
+		t.Fatalf("callback error = %v", err)
+	}
+}

coordinate_controls_contract_test.go

@@ -0,0 +1,86 @@
+package promptkit_test
+
+import (
+	"context"
+	"errors"
+	"testing"
+	"testing/synctest"
+	"time"
+
+	"go.maximumdirect.net/promptkit"
+)
+
+func TestCoordinateUsesIndependentOperationBudgets(t *testing.T) {
+	synctest.Test(t, func(t *testing.T) {
+		engine := newBackendCapacityEngine(t, observationClientFunc(func(_ context.Context, req promptkit.GenerateRequest) (*promptkit.GenerateResponse, error) {
+			if req.Prompt.SessionID == "continuation" {
+				time.Sleep(2 * time.Second)
+			}
+			return &promptkit.GenerateResponse{Content: "answer", Usage: promptkit.TokenUsage{PromptTokens: 2}}, nil
+		}), 1, capacityInt(0), nil)
+		parent := context.Background()
+		initial, err := engine.Run(parent, promptkit.RunRequest{PromptID: "prompt", CaptureExchange: true})
+		if err != nil {
+			t.Fatal(err)
+		}
+		var group promptkit.ExecutionGroup
+		var child context.Context
+		var details []promptkit.PreparedRun
+		var requestEvents, coordinatorEvents []promptkit.ExecutionEvent
+		start := time.Now()
+		result, err := engine.Coordinate(parent, promptkit.CoordinateRequest{Start: initial.Exchange, AdditionalTurns: 2, Observer: promptkit.ComposeObservers(group.Observe, func(e promptkit.ExecutionEvent) { coordinatorEvents = append(coordinatorEvents, e) }), Next: func(ctx context.Context, turn promptkit.ConversationTurn) (*promptkit.RunRequest, error) {
+			if turn.Result != nil {
+				if ctx.Err() != nil || child == nil || !errors.Is(child.Err(), context.Canceled) {
+					t.Fatal("child was not cleaned up before callback")
+				}
+				if _, err := engine.Run(ctx, promptkit.RunRequest{PromptID: "prompt", OperationTimeout: time.Second}); err != nil {
+					t.Fatalf("assessment: %v", err)
+				}
+			}
+			if turn.RemainingTurns == 0 {
+				return nil, nil
+			}
+			return &promptkit.RunRequest{PromptID: "prompt", SessionID: "continuation", OperationTimeout: 3 * time.Second, Observer: func(e promptkit.ExecutionEvent) { requestEvents = append(requestEvents, e) }, OnPrepared: func(ctx context.Context, prepared promptkit.PreparedRun) {
+				child = ctx
+				details = append(details, prepared)
+			}}, nil
+		}})
+		if err != nil || result.Outcome != promptkit.CoordinationStopped || time.Since(start) != 4*time.Second || len(details) != 2 || len(details[0].Messages) != 3 || len(details[1].Messages) != 5 {
+			t.Fatalf("result=%+v error=%v details=%v elapsed=%v", result, err, len(details), time.Since(start))
+		}
+		summary := group.Snapshot()
+		if len(requestEvents) != 6 || len(coordinatorEvents) != 6 || len(summary.Members) != 2 || summary.Attempts != 2 || summary.Usage.PromptTokens.Value != 4 || !summary.TerminalCoverageComplete {
+			t.Fatalf("summary=%+v request=%d coordinator=%d", summary, len(requestEvents), len(coordinatorEvents))
+		}
+	})
+}
+
+func TestCoordinateTimeoutLeavesParentAndSiblingUsable(t *testing.T) {
+	synctest.Test(t, func(t *testing.T) {
+		engine := newBackendCapacityEngine(t, observationClientFunc(func(ctx context.Context, req promptkit.GenerateRequest) (*promptkit.GenerateResponse, error) {
+			if req.Prompt.SessionID == "slow" {
+				<-ctx.Done()
+				return nil, ctx.Err()
+			}
+			return &promptkit.GenerateResponse{Content: "answer"}, nil
+		}), 1, capacityInt(0), nil)
+		parent := context.Background()
+		initial, err := engine.Run(parent, promptkit.RunRequest{PromptID: "prompt", CaptureExchange: true})
+		if err != nil {
+			t.Fatal(err)
+		}
+		var details promptkit.PreparedRun
+		calls := 0
+		result, err := engine.Coordinate(parent, promptkit.CoordinateRequest{Start: initial.Exchange, AdditionalTurns: 1, Next: func(context.Context, promptkit.ConversationTurn) (*promptkit.RunRequest, error) {
+			calls++
+			return &promptkit.RunRequest{PromptID: "prompt", SessionID: "slow", OperationTimeout: time.Second, OnPrepared: func(_ context.Context, p promptkit.PreparedRun) { details = p }}, nil
+		}})
+		if !errors.Is(err, promptkit.ErrOperationTimeout) || !errors.Is(err, context.DeadlineExceeded) || calls != 1 || result.Outcome != "" || result.Undelivered != nil || details.ParentRunID != initial.RunID || parent.Err() != nil {
+			t.Fatalf("result=%+v error=%v callbacks=%d", result, err, calls)
+		}
+		sibling, err := engine.Run(parent, promptkit.RunRequest{PromptID: "prompt", ContinueFrom: initial.Exchange})
+		if err != nil || sibling.Exchange == nil {
+			t.Fatalf("sibling=%v error=%v", sibling, err)
+		}
+	})
+}

coordinate_lifecycle_contract_test.go

@@ -0,0 +1,170 @@
+package promptkit_test
+
+import (
+	"context"
+	"errors"
+	"net/http"
+	"testing"
+	"time"
+
+	"go.maximumdirect.net/promptkit"
+)
+
+func TestCoordinateReleasesCapacityBeforeCallback(t *testing.T) {
+	client := &preparedRecordingClient{responses: []*promptkit.GenerateResponse{{Content: "initial"}, {Content: ""}, {Content: " "}, {Content: "validator"}}}
+	engine := newBackendCapacityEngine(t, client, 1, capacityInt(0), nil)
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+	initial, err := engine.Run(ctx, promptkit.RunRequest{PromptID: "prompt", CaptureExchange: true})
+	if err != nil {
+		t.Fatal(err)
+	}
+	calls := 0
+	result, err := engine.Coordinate(ctx, promptkit.CoordinateRequest{Start: initial.Exchange, AdditionalTurns: 1, Next: func(ctx context.Context, turn promptkit.ConversationTurn) (*promptkit.RunRequest, error) {
+		calls++
+		if turn.Result == nil {
+			return &promptkit.RunRequest{PromptID: "prompt", Validation: &promptkit.OutputContract{Format: promptkit.FormatText, ValidationMode: promptkit.ValidationBasic, RepairAttempts: 1}}, nil
+		}
+		if turn.Result.Validation.Status != promptkit.ValidationFailed || turn.Result.Validation.RepairAttempts != 1 || turn.RemainingTurns != 0 {
+			t.Fatalf("delivered turn = %#v", turn)
+		}
+		_, err := engine.Run(ctx, promptkit.RunRequest{PromptID: "prompt"})
+		return nil, err
+	}})
+	if err != nil || result.Outcome != promptkit.CoordinationStopped || calls != 2 || len(client.snapshot()) != 4 {
+		t.Fatalf("outcome=%#v error=%v callbacks=%d calls=%d", result, err, calls, len(client.snapshot()))
+	}
+}
+
+func TestCoordinatePreservesCapacityAndProviderErrors(t *testing.T) {
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+	sourceEngine := newSimpleEngine(t, "source", promptkit.WithLLMClient(&fakeLLMClient{response: &promptkit.GenerateResponse{Content: "answer"}}))
+	initial, err := sourceEngine.Run(ctx, promptkit.RunRequest{PromptID: "prompt", CaptureExchange: true})
+	if err != nil {
+		t.Fatal(err)
+	}
+	release := make(chan struct{})
+	client := newCapacityGateClient(release, 1)
+	engine := newBackendCapacityEngine(t, client, 1, capacityInt(0), nil)
+	done := make(chan capacityRunResult, 1)
+	go runCapacityRequest(engine, ctx, promptkit.RunRequest{PromptID: "prompt"}, done)
+	select {
+	case <-client.started:
+	case <-ctx.Done():
+		t.Fatal(ctx.Err())
+	}
+	request := promptkit.CoordinateRequest{Start: initial.Exchange, AdditionalTurns: 1, Next: func(context.Context, promptkit.ConversationTurn) (*promptkit.RunRequest, error) {
+		return &promptkit.RunRequest{PromptID: "prompt"}, nil
+	}}
+	_, err = engine.Coordinate(ctx, request)
+	var capacity *promptkit.CapacityError
+	if !errors.As(err, &capacity) || capacity.BackendID != "limited" {
+		t.Fatalf("capacity error = %v", err)
+	}
+	close(release)
+	if finished := awaitCapacityRun(t, done); finished.err != nil {
+		t.Fatal(finished.err)
+	}
+	providerEngine := newBuiltInGenerationErrorEngine(t, http.StatusUnprocessableEntity, `{"error":{"message":"synthetic"}}`)
+	_, err = providerEngine.Coordinate(ctx, request)
+	var provider *promptkit.GenerationError
+	if !errors.As(err, &provider) || provider.StatusCode() != http.StatusUnprocessableEntity {
+		t.Fatalf("provider error = %v", err)
+	}
+}
+
+func TestCoordinateRetriesAndRepairsDeliverOnce(t *testing.T) {
+	initialEngine := newSimpleEngine(t, "source", promptkit.WithLLMClient(&fakeLLMClient{response: &promptkit.GenerateResponse{Content: "answer"}}))
+	initial, err := initialEngine.Run(context.Background(), promptkit.RunRequest{PromptID: "prompt", CaptureExchange: true})
+	if err != nil {
+		t.Fatal(err)
+	}
+	responses := []*http.Response{observedHTTPResponse(503, `{}`), observedHTTPResponse(200, `{"choices":[{"message":{"content":"invalid"}}]}`), observedHTTPResponse(200, `{"choices":[{"message":{"content":"{\"ok\":true}"}}]}`)}
+	calls := 0
+	one := 1
+	config := simpleConfig("follow-up")
+	config.TransientRetries = &one
+	config.HTTPClient = &http.Client{Transport: roundTripFunc(func(*http.Request) (*http.Response, error) {
+		response := responses[calls]
+		calls++
+		return response, nil
+	})}
+	engine, err := promptkit.NewEngine(config)
+	if err != nil {
+		t.Fatal(err)
+	}
+	callbacks := 0
+	result, err := engine.Coordinate(context.Background(), promptkit.CoordinateRequest{Start: initial.Exchange, AdditionalTurns: 1, Next: func(_ context.Context, turn promptkit.ConversationTurn) (*promptkit.RunRequest, error) {
+		callbacks++
+		if turn.Result == nil {
+			return &promptkit.RunRequest{PromptID: "prompt", Validation: &promptkit.OutputContract{Format: promptkit.FormatJSON, ValidationMode: promptkit.ValidationJSON, RepairAttempts: 1}}, nil
+		}
+		if !turn.Result.Validation.IsValid || turn.RemainingTurns != 0 {
+			t.Fatalf("turn = %#v", turn)
+		}
+		return nil, nil
+	}})
+	if err != nil || result.Outcome != promptkit.CoordinationStopped || callbacks != 2 || calls != 3 {
+		t.Fatalf("result=%#v error=%v callbacks=%d attempts=%d", result, err, callbacks, calls)
+	}
+}
+
+func TestCoordinateCancellationDelivery(t *testing.T) {
+	for _, cancelInside := range []bool{false, true} {
+		t.Run(map[bool]string{false: "before delivery", true: "inside callback"}[cancelInside], func(t *testing.T) {
+			client := &fakeLLMClient{response: &promptkit.GenerateResponse{Content: "answer"}}
+			engine := newSimpleEngine(t, "question", promptkit.WithLLMClient(client))
+			initial, err := engine.Run(context.Background(), promptkit.RunRequest{PromptID: "prompt", CaptureExchange: true})
+			if err != nil {
+				t.Fatal(err)
+			}
+			ctx, cancel := context.WithCancel(context.Background())
+			defer cancel()
+			calls := 0
+			result, err := engine.Coordinate(ctx, promptkit.CoordinateRequest{Start: initial.Exchange, AdditionalTurns: 1, Next: func(_ context.Context, turn promptkit.ConversationTurn) (*promptkit.RunRequest, error) {
+				calls++
+				if turn.Result != nil {
+					cancel()
+					return nil, nil
+				}
+				return &promptkit.RunRequest{PromptID: "prompt", Observer: func(event promptkit.ExecutionEvent) {
+					if !cancelInside && event.Kind == promptkit.ExecutionFinished {
+						cancel()
+					}
+				}}, nil
+			}})
+			if !errors.Is(err, context.Canceled) || result.Outcome != "" || (result.Undelivered != nil) == cancelInside {
+				t.Fatalf("result=%#v error=%v callbacks=%d", result, err, calls)
+			}
+			if !cancelInside && (calls != 1 || result.Undelivered.Exchange == nil) {
+				t.Fatalf("undelivered result=%#v calls=%d", result, calls)
+			}
+			if cancelInside && calls != 2 {
+				t.Fatalf("callbacks=%d", calls)
+			}
+		})
+	}
+}
+
+func TestCoordinateFailureLeavesSiblingUsable(t *testing.T) {
+	client := &fakeLLMClient{response: &promptkit.GenerateResponse{Content: "answer"}}
+	engine := newSimpleEngine(t, "question", promptkit.WithLLMClient(client))
+	ctx := context.Background()
+	initial, err := engine.Run(ctx, promptkit.RunRequest{PromptID: "prompt", CaptureExchange: true})
+	if err != nil {
+		t.Fatal(err)
+	}
+	sentinel := errors.New("provider")
+	client.err = sentinel
+	result, err := engine.Coordinate(ctx, promptkit.CoordinateRequest{Start: initial.Exchange, AdditionalTurns: 1, Next: func(context.Context, promptkit.ConversationTurn) (*promptkit.RunRequest, error) {
+		return &promptkit.RunRequest{PromptID: "prompt"}, nil
+	}})
+	if !errors.Is(err, sentinel) || !errors.Is(err, promptkit.ErrLLMGenerate) || result.Outcome != "" || result.Undelivered != nil {
+		t.Fatalf("failure=%#v, %v", result, err)
+	}
+	client.err = nil
+	if sibling, err := engine.Run(ctx, promptkit.RunRequest{PromptID: "prompt", ContinueFrom: initial.Exchange}); err != nil || sibling.Exchange == nil {
+		t.Fatalf("sibling=%#v, %v", sibling, err)
+	}
+}

doc.go

@@ -6,7 +6,10 @@
 // call [Engine.InspectConfiguration], [Engine.InspectPrompt],
 // [Engine.InspectRequirements], [Engine.InspectProfile], [Engine.InspectBackend],
 // [Engine.ListProfiles], [Engine.ListPrompts], [Engine.Prepare],
-// [Engine.PrepareExecution], [Engine.Run], or [Engine.RunPrepared]. Concrete registries, repositories,
+// [Engine.PrepareExecution], [Engine.Run], [Engine.RunPrepared], or
+// [Engine.Coordinate]. Captured [Exchange] values support independent
+// continuations, and [ExecutionGroup] collects explicitly observed runs.
+// Concrete registries, repositories,
 // validators, and the built-in OpenAI-compatible client remain internal
 // implementation details.
 //
@@ -14,7 +17,7 @@
 //
 // An Engine supports concurrent InspectConfiguration, InspectPrompt,
 // InspectRequirements, InspectProfile, InspectBackend, ListProfiles, ListPrompts,
-// Prepare, PrepareExecution, Run, and RunPrepared calls. Engine-local backend policies
+// Prepare, PrepareExecution, Run, RunPrepared, and Coordinate 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
@@ -43,7 +46,7 @@
 // that enforces their filesystem, authorization, and size policies.
 //
 // Rendered messages, input and output [Artifact] bodies, [RunResult.RawOutput],
-// and [ValidationResult.Errors] may contain sensitive data. Credential
+// [ValidationResult.Errors], and [ExchangeDetails.Messages] may contain sensitive data. Credential
 // exclusion and redaction do not sanitize those values. Applications and
 // injected collaborators are responsible for access control, retention,
 // logging, and secret handling appropriate to their data.

docs/consumers/pkg-promptkit.md

@@ -18,7 +18,7 @@ import "go.maximumdirect.net/promptkit"
 For migration from the former module path, see
 [Update Go Imports](configuration-migration.md#update-go-imports).
 
-Configuration APIs use `go.maximumdirect.net/configkit` v1.1.0. Its types are
+Configuration APIs use `go.maximumdirect.net/configkit` v1.2.0. Its types are
 not interchangeable with the old ConfigKit module path.
 
 The maintained [offline preparation example](../../examples/go-library/prepare/main.go)
@@ -102,6 +102,39 @@ new engine to observe configuration edits. See the [constructor GoDoc](../../eng
 for the exact public contract and the [framework format reference](../formats.md)
 for definition fields.
 
+### Load Companion Prompts Relative To The Main Configuration
+
+A non-file underlay can use the selected main configuration's directory for
+caller-declared imports. For example, this loads standalone prompt definitions
+from the adjacent `prompts/` directory into the `support` collection:
+
+```go
+main := configkit.Input{Source: configkit.File(configPath)}
+companion, err := promptkit.CollectionInput("support", configkit.Bytes("companion prompts", []byte(`
+config:
+  imports:
+    - dir: prompts
+      pattern: ["*.yaml", "*.yml"]
+      destination: [prompts]
+      placement: sequence-item
+      required: true
+`)))
+if err != nil { return err }
+companion.MainRelative = configkit.MainRelative()
+engine, err := promptkit.NewEngine(promptkit.Config{Sources: promptkit.ConfigurationSources{
+	Main: &main,
+	Underlays: []configkit.Underlay{{Inputs: []configkit.Input{companion}}},
+}})
+```
+
+`CollectionInput` enables imports; for a raw `configkit.Input`, enable them with
+`Imports: true`. The binding itself does not enable imports, and an explicit
+`Input.Filesystem` takes precedence. Imported prompt `content_file` references
+keep their source-local context. See ConfigKit's
+[layered-loading contract](https://forge.maximumdirect.net/base-layer/configkit/src/tag/v1.2.0/docs/config.md#layered-loading)
+for eligible inputs, fallback contexts, and behavior when the main has no usable
+filesystem context.
+
 ### Use Scoped Engine Views
 
 Create a view when an application wants short request IDs to resolve within a
@@ -181,7 +214,9 @@ input := configkit.Input{Source: document}
 null is not needed. Prompt and backend helpers, like profiles, support sibling
 inheritance via `Extends`; partial records can be completed by other layers.
 For relative prompt `content_file` references, supply the ordinary ConfigKit
-`Input.Filesystem` context for that source. The helpers do not read files or
+`Input.Filesystem` context for that source, or use a
+[main-relative underlay](#load-companion-prompts-relative-to-the-main-configuration).
+The helpers do not read files or
 validate final definitions. Profile and backend provider maps use ConfigKit Mapping input forms: built-in
 scalars, `[]any`, and `map[string]any` recursively. Convert `json.Number` or
 typed containers explicitly to an appropriate supported value, choosing numeric
@@ -300,7 +335,7 @@ for _, difference := range differences {
 ```
 
 See the [`ConfigurationInspection.Diff` GoDoc](../../configuration_inspection.go)
-and ConfigKit's [v1.1.0 effective-configuration diff guidance](https://forge.maximumdirect.net/base-layer/configkit/src/tag/v1.1.0/docs/config.md#effective-configuration-diffs)
+and ConfigKit's [effective-configuration diff guidance](https://forge.maximumdirect.net/base-layer/configkit/src/tag/v1.2.0/docs/config.md#effective-configuration-diffs)
 for the exact comparison contract. The operation compares only frozen effective
 configuration values: it does not reread external files or compare their
 contents, resolved credential contents, later execution preparation, or
@@ -546,6 +581,59 @@ retained callback without invoking it. The function's closure is retained by
 identity, so synchronize any shared state it uses. Prepared start events can
 include identities already resolved in the frozen snapshot.
 
+## Bound Execution And Retain Prepared Details
+
+Select the optional controls on [RunRequest](../../types.go). The operation
+budget is separate from the provider timeout and cancellation is cooperative.
+Notification is conditional on reaching the prepared point, synchronous, and
+runs while admission is held; its caller-owned details can be retained even if
+generation later fails. Do not rely on notification for preparation or admission
+failures. The application owns any subsequent diagnostic handling.
+
+```go
+var details *promptkit.PreparedRun
+result, err := engine.Run(ctx, promptkit.RunRequest{
+	PromptID: "answer", OperationTimeout: 30 * time.Second,
+	OnPrepared: func(_ context.Context, prepared promptkit.PreparedRun) { details = &prepared },
+})
+if errors.Is(err, promptkit.ErrOperationTimeout) && errors.Is(err, context.DeadlineExceeded) {
+	// details is available only if the notification was reached.
+}
+```
+
+For coordinated turns, put the controls on each returned request. Assessment
+in the next-turn callback runs after the previous admission and child scope
+are released, so it can select its own budget:
+
+```go
+next := func(ctx context.Context, turn promptkit.ConversationTurn) (*promptkit.RunRequest, error) {
+	if turn.Result != nil {
+		_, err := engine.Run(ctx, promptkit.RunRequest{PromptID: "assessment", OperationTimeout: 10 * time.Second})
+		return nil, err
+	}
+	return &promptkit.RunRequest{PromptID: "answer", OperationTimeout: 30 * time.Second,
+		OnPrepared: func(_ context.Context, prepared promptkit.PreparedRun) { details = &prepared }}, nil
+}
+```
+
+For split execution, [PrepareExecution](../../engine.go) retains the controls
+without starting their timer; idle time does not count. Use the same caller
+deadline for both calls if preparation and execution need one shared bound.
+
+```go
+handle, err := engine.PrepareExecution(ctx, promptkit.RunRequest{
+	PromptID: "answer", OperationTimeout: 30 * time.Second,
+	OnPrepared: func(_ context.Context, prepared promptkit.PreparedRun) { details = &prepared },
+})
+if err != nil { return err }
+defer handle.Discard()
+result, err := engine.RunPrepared(ctx, handle)
+```
+
+See the [observation example](../../examples/go-library/observe/main.go) for an
+offline coordinated workflow and [ErrOperationTimeout](../../engine.go) for
+timeout attribution.
+
 ## Execute And Validate
 
 [`Engine.Run`](../../engine.go) performs the same preparation, invokes the
@@ -660,6 +748,116 @@ application owns any semantic retry budget. When a
 pre-execution equality check is required, use `PrepareExecution` and compare
 its opaque rendered-prompt hash before invoking `RunPrepared`.
 
+### Capture And Continue A Conversation
+
+The [request GoDoc](../../types.go), [exchange GoDoc](../../exchange.go), and
+[prepared-execution GoDoc](../../prepared_execution.go) own the exact capture,
+continuation, inspection, and lifetime contracts.
+
+Capture records the messages actually executed and the final assistant content.
+An exchange is an immutable in-process capability; `Details()` returns detached,
+encodable history that may contain sensitive content. Ordinary result encoding
+does not include the exchange. Continuations imply capture and may branch or use
+a different engine, profile, output contract, credential, or session:
+
+```go
+initial, err := engine.Run(ctx, promptkit.RunRequest{PromptID: "proposal", CaptureExchange: true})
+if err != nil { return err }
+left, err := engine.Run(ctx, promptkit.RunRequest{PromptID: "refine", ContinueFrom: initial.Exchange})
+if err != nil { return err }
+right, err := anotherEngine.Run(ctx, promptkit.RunRequest{PromptID: "alternative", ContinueFrom: initial.Exchange})
+if err != nil { return err }
+_, _ = left, right // Independent branches; each result has its own exchange.
+```
+
+For a frozen execution, set `CaptureExchange` in the request passed to
+`PrepareExecution`, then obtain the exchange from `RunPrepared`'s result.
+Preparation and validation alone do not produce an exchange. Captured history
+precedes newly rendered definition messages and appended messages. A completed
+structurally invalid result still has an exchange; an operational error returns
+no result. The caller chooses what constitutes acceptance.
+
+```go
+prepared, err := engine.PrepareExecution(ctx, promptkit.RunRequest{PromptID: "proposal", CaptureExchange: true})
+if err != nil { return err }
+result, err := engine.RunPrepared(ctx, prepared)
+if err != nil { return err }
+details := result.Exchange.Details()
+_ = details // Explicitly encode or inspect when appropriate.
+```
+
+### Coordinate Application-Directed Turns
+
+The following sketches use illustrative application functions. The first records
+each newly delivered response before selecting another question:
+
+```go
+outcome, err := engine.Coordinate(ctx, promptkit.CoordinateRequest{
+	Start: initial.Exchange, AdditionalTurns: 3,
+	Next: func(ctx context.Context, turn promptkit.ConversationTurn) (*promptkit.RunRequest, error) {
+		if turn.Result != nil {
+			if !turn.Result.Validation.IsValid { return nil, fmt.Errorf("invalid response") }
+			analysis.Record(turn.Result)
+		}
+		question, ok := analysis.NextQuestion()
+		if !ok { return nil, nil }
+		return &promptkit.RunRequest{PromptID: "research/investigate", Vars: map[string]string{"question": question}}, nil
+	},
+})
+```
+
+An application can instead start with existing findings, assess each new
+candidate, and keep its decisions in a closure:
+
+```go
+outcome, err := engine.Coordinate(ctx, promptkit.CoordinateRequest{
+	Start: proposal.Exchange, AdditionalTurns: 3, Observer: branchGroup.Observe,
+	Next: func(ctx context.Context, turn promptkit.ConversationTurn) (*promptkit.RunRequest, error) {
+		if turn.Result != nil {
+			if !turn.Result.Validation.IsValid { return nil, fmt.Errorf("invalid revision") }
+			candidate, withdrawn, err := decodeRevision(turn.Result.RawOutput)
+			if err != nil { return nil, err }
+			if withdrawn { branch.MarkWithdrawn(); return nil, nil }
+			findings, err := assess(ctx, engine, candidate, validatorGroup.Observe)
+			if err != nil { return nil, err }
+			branch.Update(candidate, findings)
+			if branch.Done() { return nil, nil }
+		}
+		return &promptkit.RunRequest{PromptID: branch.FollowUpPrompt(), Vars: branch.Variables()}, nil
+	},
+})
+```
+
+`assess` is application code and explicitly selects its observer on any nested
+run. A completed turn releases its admission lease before the callback, so that
+assessment may use the same backend. `stopped` and `exhausted` describe control
+flow, not content acceptance. Cancellation before callback delivery can return one
+completed response in `Undelivered`; errors retain their ordinary identities.
+
+### Collect Selected Execution Facts
+
+Groups are independent of continuation and enrollment is explicit:
+
+```go
+var group, validatorGroup promptkit.ExecutionGroup
+request.Observer = promptkit.ComposeObservers(diagnosticObserver, group.Observe)
+prepared, err := engine.PrepareExecution(ctx, request) // no event yet
+if err != nil { return err }
+result, err := engine.RunPrepared(ctx, prepared)
+if err != nil { return err }
+_ = result
+// Set CoordinateRequest.Observer to group.Observe for its continuation runs.
+// A nested validator selects validatorGroup.Observe on its own request.
+summary, validatorSummary := group.Snapshot(), validatorGroup.Snapshot()
+_, _ = summary, validatorSummary
+```
+
+Only the first terminal event per run contributes counts and usage; coverage of
+currently observed members is separate from token-usage completeness. Empty
+groups report complete, known zero totals, while members without terminals do
+not invent totals. Overlapping groups are not generally additive. See the
+[offline example](../../examples/go-library/observe/main.go) for complete setup.
+
 ## Inputs, Profiles, And Overrides
 
 Use `File`, `Inline`, or `InlineWithURI` to construct request inputs. A request

docs/formats.md

@@ -6,7 +6,7 @@ This is the canonical reference for Promptkit's backend, profile, prompt, and
 schema definitions. The [Go consumer guide](consumers/pkg-promptkit.md) shows
 how to select sources and use an engine; the
 [OpenAI-compatible integration contract](integrations/openai-compatible-chat.md)
-owns outbound wire behavior. [ConfigKit](https://forge.maximumdirect.net/base-layer/configkit/src/tag/v1.1.0/docs/config.md)
+owns outbound wire behavior. [ConfigKit](https://forge.maximumdirect.net/base-layer/configkit/src/tag/v1.2.0/docs/config.md)
 owns the generic YAML grammar, imports, layering, inheritance, and diagnostics.
 This reference describes what Promptkit does with the final effective values.
 
@@ -429,7 +429,7 @@ references are exact IDs and use their qualified form when they select a
 collection resource. There is no Promptkit-specific depth limit or second
 base-chain resolver. Missing references and cycles fail at construction. For
 generic inheritance and layer mechanics, consult
-[ConfigKit](https://forge.maximumdirect.net/base-layer/configkit/src/tag/v1.1.0/docs/config.md).
+[ConfigKit](https://forge.maximumdirect.net/base-layer/configkit/src/tag/v1.2.0/docs/config.md).
 
 ## Schemas
 

docs/integrations/openai-compatible-chat.md

@@ -93,6 +93,8 @@ target's model must be available.
 Each ordinary message contains its `role` and string `content`. A
 cache-controlled message instead uses a text content block containing `type`,
 `text`, and `cache_control`; an empty cache-control TTL is omitted.
+For a continuation, captured history precedes the new definition and appended
+messages. Ancestry is local metadata and is not an upstream request field.
 
 Promptkit sends only `developer`, `system`, `user`, and `assistant` roles and
 does so without provider-specific translation. Tool and deprecated function

docs/internal/observation.md

@@ -14,6 +14,9 @@ The public projection retains private operation-owner metadata separately from
 inspectable child causes. Terminal classification uses that owner, preserving
 context precedence and generation-specific transport/provider details rather
 than mistaking a collaborator's nested lookup error for the parent's operation.
+Timeout attribution is finalized and the child scope released before terminal
+delivery, so observer latency cannot change the already selected outcome.
+Prepared-content notification is a separate opt-in callback, not an event.
 
 The facade masks inherited recorder and prepared-claim state when entering
 preparation or execution. Delegated calls from an injected collaborator retain
@@ -27,6 +30,8 @@ callback. The caller supplies normalized failure metadata; the recorder never
 parses an error string or receives provider bodies, credentials, headers, or
 other content. Events are values with no mutable containers. Callback panics
 propagate to the caller.
+The immediate parent run ID is set before start delivery and is retained as
+selection metadata changes. It describes ancestry, not implicit group membership.
 Resolved prompt and profile IDs are the exact runtime identities selected for
 the invocation, including namespace qualification when supplied by an engine
 view.
@@ -48,6 +53,13 @@ attempts above the first in a generation, and repair generations with at least
 one dispatch. A terminal event is an aggregate and must not be added to its
 attempt events.
 
+An execution group is a zero-ready synchronized collector of explicitly selected
+events. It enrolls by run ID and aggregates each run's first terminal counters and
+presence-aware usage exactly once. Snapshots copy members and totals under one
+lock; terminal coverage and token completeness are distinct. Observer fan-out
+copies the callback slice and invokes callbacks synchronously without holding a
+group lock. No group history, registry, or automatic nested membership exists.
+
 Smoke diagnostics mask inherited execution-recorder state while invoking
 generation or HTTP collaborators. Their private attempt evidence is assembled
 into a `SmokeTestReport`; it does not create ordinary run events or enter an

docs/internal/overview.md

@@ -14,11 +14,11 @@ contributor workflow and validation.
 | Root `promptkit` package | Provides the supported engine facade, ConfigKit source helpers, collection and resource mapping conveniences, injection options, public requests and results, resource and requirement inspection and identity inventories, concrete-request validation, explicit smoke diagnostics, observation values, opaque prepared-execution handles, value conversion, redacted formatting, and typed capacity and generation error mapping. | [Package GoDoc](../../doc.go), [observations](../../observations.go), [prepared execution](../../prepared_execution.go), [backend API](../../backends.go), [engine facade](../../engine.go), [smoke diagnostics](../../smoke_diagnostics.go), [identity listing](../../listing.go), [requirement facade](../../requirements.go) |
 | `examples/go-library/prepare` | Demonstrates offline preparation from two explicitly selected synthetic collections, source-relative imported content, a scoped view, filtered listings, and operator inventory. It is not a public library package. | [Example program](../../examples/go-library/prepare/main.go), [collection inputs](../../examples/go-library/prepare/collections/support/collection.yaml) |
 | `examples/go-library/run` | Demonstrates an offline downstream consumer using a prompt file, programmatic 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) |
-| `examples/go-library/observe` | Demonstrates offline built-in HTTP retry and synchronous metadata observation across structured proposal and assessment runs using embedded sources, two profiles on one finite backend, and an application decision. It is not a public library package. | [Example program](../../examples/go-library/observe/main.go) |
+| `examples/go-library/observe` | Demonstrates offline retry, observation, prepared notification, and separately budgeted coordinated proposal and assessment runs using two profiles on one finite backend. It is not a public library package. | [Example program](../../examples/go-library/observe/main.go) |
 | `internal/backend` | Holds a defensive, immutable lookup of final backend definitions, including diagnostic metadata, plus capacity policies validated by the configuration adapter. | [Backend registry](../../internal/backend/registry.go) |
 | `internal/configuration` | Loads root and namespaced resources through ConfigKit; validates final values, captures indexed resource views and source-relative prompt content, compiled templates, requirement metadata, identity inventories, backend and presence-aware execution profiles, and the read-only provenance inspection view. | [Typed loader](../../internal/configuration/load.go), [identity listing](../../internal/configuration/listing.go), [internal sources](sources.md#typed-configuration-loading) |
 | `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, requirement inspection, profiles, execution targets, rendering, generation, validation, and source-neutral smoke diagnostics, and owns source-neutral invariants for shared execution settings, OpenAI-compatible base endpoints, session identifiers, output contracts, and resource-identity syntax and qualification. Source parsing, required fields, other source-specific normalization, defaulting, and boundary-specific error classification remain with their callers. | [Domain declarations](../../internal/domain/domain.go), [requirement values](../../internal/domain/requirements.go), [endpoint invariant](../../internal/domain/endpoint.go) |
+| `internal/domain` | Defines internal framework values for requests, immutable conversation exchanges, artifacts, prompt definitions, requirement inspection, profiles, execution targets, rendering, generation, validation, and source-neutral smoke diagnostics, and owns source-neutral invariants for shared execution settings, OpenAI-compatible base endpoints, session identifiers, output contracts, and resource-identity syntax and qualification. Source parsing, required fields, other source-specific normalization, defaulting, and boundary-specific error classification remain with their callers. | [Domain declarations](../../internal/domain/domain.go), [exchange storage](../../internal/domain/exchange.go), [requirement values](../../internal/domain/requirements.go), [endpoint invariant](../../internal/domain/endpoint.go) |
 | `internal/engine` | Assembles caller-selected configuration into one snapshot, then constructs shared capacity, rendering, validation, generation, repair, the ordinary runner, and the smoke tester for each engine. Immutable namespace views share those collaborators and qualify caller-supplied execution, inspection, and diagnostic selections without rewriting captured references. Built-in engines provide the diagnostic HTTP collaborator; injected-client engines do not. It delegates resource inspection and identity listing directly to the captured configuration rather than the runner. | [Engine assembly](../../internal/engine/engine.go), [internal sources](sources.md#typed-configuration-loading), [internal runner](runner.md) |
 | `internal/generation` | Coordinates frozen built-in HTTP attempts and one-shot injected calls through active backend permits, including bounded status retries, cancellable backoff, ordinary attempt observations, and invocation-local evidence for diagnostics. | [Internal generation coordination](generation.md) |
 | `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) |
@@ -30,8 +30,8 @@ contributor workflow and validation.
 | `internal/artifact` | Resolves ordinary inline and unrestricted caller-selected file references into copied artifacts with metadata and hashes. | [Internal sources and validation](sources.md) |
 | `internal/validate` | Validates basic, JSON, and JSON Schema output using operating-system filesystem or `fs.FS` schema sources and creates operation-local validation plans with canonical contained schema resources. | [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 bounded response decoding, authentication, deadline handling, per-invocation HTTP evidence and built-in diagnostic probes, and ownership of the OpenAI-compatible reserved request-field policy. | [Internal model client](llm.md) |
-| `internal/observation` | Owns invocation-local run IDs, presence-aware usage and phase accounting, and synchronous metadata event delivery for ordinary and claimed prepared runs and generation attempts; smoke diagnostics keep their private evidence outside ordinary observer events. | [Internal execution observation](observation.md) |
-| `internal/usecase` | Selects captured prompts and profiles, enumerates requirements from captured prompt syntax, applies request overrides and completion checks, coordinates ordinary and single-claim prepared execution across rendering, artifact loading, operation-local validation plans, generation, capacity, and bounded repair, and runs explicit smoke diagnostics with private cases, shared generation/capacity, and optional built-in HTTP probes. | [Internal runner](runner.md), [requirement inspection](../../internal/usecase/requirements_inspection.go), [prepared-execution implementation](../../internal/usecase/prepared_execution.go) |
+| `internal/observation` | Owns invocation-local run IDs, presence-aware usage and phase accounting, synchronous metadata event delivery and composition, and explicit terminal-based group aggregation; smoke diagnostics keep their private evidence outside ordinary observer events. | [Internal execution observation](observation.md) |
+| `internal/usecase` | Selects captured prompts and profiles, enumerates requirements from captured prompt syntax, applies request overrides and completion checks, coordinates ordinary and single-claim prepared execution across rendering, artifact loading, operation-local validation plans, generation, capacity, and bounded repair, advances a single conversation branch, and runs explicit smoke diagnostics with private cases, shared generation/capacity, and optional built-in HTTP probes. | [Internal runner](runner.md), [requirement inspection](../../internal/usecase/requirements_inspection.go), [prepared-execution implementation](../../internal/usecase/prepared_execution.go) |
 
 The internal engine assembles these internal components without exposing their
 representations. Consumers depend only on the root facade.

docs/internal/runner.md

@@ -30,6 +30,24 @@ internal callers and tests.
 Each invocation carries its state in request, prepared-run, and result values.
 The runner has no durable run or session store.
 
+Captured exchanges are immutable parent-linked domain values. Each holds only
+its new rendered suffix, final assistant message, and run identities; flattening
+iteratively fills a single detached message buffer for preparation or projects
+directly into public inspection values. Preparation
+prepends this history to newly rendered definition messages and appended caller
+messages before computing the rendered hash. Follow-up selection and credentials
+remain those of the new request. Prepared execution retains capture and parent
+references in its single-claim payload, not its durable details; discard clears
+the payload. Completed runs capture the exact final output, including exhausted
+structural repair, while operational errors produce no exchange.
+
+The sequential coordinator uses ordinary facade runs and retains the current
+immutable exchange independently of callback-owned values. Each run releases its
+logical admission lease and finishes observation before application callback
+delivery. Context checks preserve a completed but undelivered result only when
+cancellation is observed before invoking the callback. Coordinator budgets count
+started runs, not transport attempts or structural repairs.
+
 ## Shared Prompt Selection
 
 The internal engine applies each namespace view to slashless caller selections
@@ -124,8 +142,8 @@ profile, or backend:
    schema metadata from it when required;
 2. load and hash input artifacts;
 3. render messages and the prompt-defined session;
-4. apply any direct session ID, then append already-normalized request messages
-   after the rendered definition messages;
+4. apply any direct session ID, then compose captured history, rendered
+   definition messages, and already-normalized request messages in that order;
 5. hash the effective rendered prompt; and
 6. construct the prepared value and preparation timing.
 
@@ -191,6 +209,16 @@ mapping and after the runner's deferred admission release. Preparation and
 prepared execution follows the same terminal mapping after a successful claim;
 invalid or discarded handles do not acquire an observer.
 
+The facade establishes an optional operation scope for ordinary runs before
+observation and preparation. A claimed prepared execution establishes its own
+scope from the retained duration; preparation and idle time do not consume it.
+The shared scope finalizer marks only a context failure caused by that scope's
+own deadline, preserving its underlying operation and cause. The scope is
+released before terminal delivery. At the shared prepared execution boundary,
+the runner conditionally invokes the adapter for the actual prepared snapshot
+while admission is held, with context checks before and after notification.
+The facade adapter projects a detached public value only when selected.
+
 Validation preparation and execution honor cancellation at every
 Promptkit-controlled boundary and do not publish a partial plan or result.
 Schema reads are bounded and context-checked between chunks; JSON decoding,

docs/internal/sources.md

@@ -36,12 +36,12 @@ owns the exact source-header and collection contracts.
 The `ConfigurationInspection.Diff` facade method compares two retained results
 through a private wrapper and a narrow `internal/configuration.Diff` bridge.
 The wrapper keeps `Result[document]` inside the adapter; the bridge delegates
-once to ConfigKit v1.1.0 and returns its public difference records. Comparison
+once to ConfigKit and returns its public difference records. Comparison
 uses only frozen effective values and does not reopen sources, resolve runtime
 credentials, or enter the execution path. ConfigKit applies sensitivity from
 both results; reports may still contain ordinary application data. See the
 [consumer comparison guide](../consumers/pkg-promptkit.md#inspect-the-captured-configuration)
-and ConfigKit's [v1.1.0 comparison contract](https://forge.maximumdirect.net/base-layer/configkit/src/tag/v1.1.0/docs/config.md#effective-configuration-diffs).
+and ConfigKit's [comparison contract](https://forge.maximumdirect.net/base-layer/configkit/src/tag/v1.2.0/docs/config.md#effective-configuration-diffs).
 
 After resource materialization and effective-profile snapshot construction, the
 adapter captures lexically ordered identity inventories for backends, profiles,

docs/oss/attribution.md

@@ -26,17 +26,17 @@ License and notices: [CC BY-SA 4.0](licenses/CC-BY-SA-4.0.txt) |
 
 ## ConfigKit
 
-Selected module: `go.maximumdirect.net/configkit` at `v1.1.0`.
+Selected module: `go.maximumdirect.net/configkit` at `v1.2.0`.
 
 Copyright (C) 2026 Eric Rakestraw
 
 SPDX-License-Identifier: LGPL-3.0-or-later
 
 License and notices: [LGPL 3.0](../../LICENSE) |
-[COPYRIGHT](https://forge.maximumdirect.net/base-layer/configkit/src/commit/191015e289d7f2b91abb9fdd9c4f2f5cf75b4de9/COPYRIGHT) |
-[LICENSE](https://forge.maximumdirect.net/base-layer/configkit/src/commit/191015e289d7f2b91abb9fdd9c4f2f5cf75b4de9/LICENSE).
+[COPYRIGHT](https://forge.maximumdirect.net/base-layer/configkit/src/commit/c1c7e87f2cdf77a0cbf3327831ac05578e756b9e/COPYRIGHT) |
+[LICENSE](https://forge.maximumdirect.net/base-layer/configkit/src/commit/c1c7e87f2cdf77a0cbf3327831ac05578e756b9e/LICENSE).
 
-[Source](https://forge.maximumdirect.net/base-layer/configkit/src/commit/191015e289d7f2b91abb9fdd9c4f2f5cf75b4de9).
+[Source](https://forge.maximumdirect.net/base-layer/configkit/src/commit/c1c7e87f2cdf77a0cbf3327831ac05578e756b9e).
 
 ## ReleaseKit
 

docs/releases/v0.21.0.md

@@ -0,0 +1,34 @@
+# Promptkit v0.21.0
+
+This minor release updates ConfigKit from v1.1.0 to v1.2.0, making main-relative
+companion imports available through Promptkit's existing configuration-source
+API. The Go consumer minimum remains 1.27.0.
+
+## Main-Relative Companion Imports
+
+Applications can bind a non-file underlay's declared imports to the selected
+main configuration's filesystem context. For example, an application can load
+prompt definitions from an adjacent `prompts/` directory without calculating
+that directory before loading the main configuration.
+
+Imports remain explicitly selected by the caller; the upgrade does not enable
+automatic discovery. Imported prompt message files retain their source-local
+context. The [consumer example](../consumers/pkg-promptkit.md#load-companion-prompts-relative-to-the-main-configuration)
+shows collection mounting and YAML filename filtering. ConfigKit's
+[layered-loading contract](https://forge.maximumdirect.net/base-layer/configkit/src/tag/v1.2.0/docs/config.md#layered-loading)
+defines context selection and fallback behavior.
+
+## Compatibility And Upgrade
+
+Promptkit's exported methods and request shapes are unchanged. No configuration
+format migration is required from v0.20.0. Applications constructing ConfigKit
+`Input` or `LayeredOptions` with positional struct literals must switch to
+field-named literals because those upstream types gained fields. See the
+[ConfigKit release notes](https://forge.maximumdirect.net/base-layer/configkit/src/tag/v1.2.0/docs/releases/v1.2.0.md)
+for upstream changes.
+
+```sh
+go get go.maximumdirect.net/promptkit@v0.21.0
+```
+
+[COPYRIGHT](https://forge.maximumdirect.net/base-layer/promptkit/src/tag/v0.21.0/COPYRIGHT) | [LICENSE](https://forge.maximumdirect.net/base-layer/promptkit/src/tag/v0.21.0/LICENSE) | [ATTRIBUTION](https://forge.maximumdirect.net/base-layer/promptkit/src/tag/v0.21.0/docs/oss/attribution.md)

docs/releases/v0.22.0.md

@@ -0,0 +1,67 @@
+# Promptkit v0.22.0
+
+This minor release adds captured conversation exchanges, optional sequential
+coordination, and explicit execution-group summaries. Applications can continue
+or branch a conversation without reconstructing rendered history, while keeping
+response interpretation and workflow decisions in application code.
+
+## Capture And Continue Conversations
+
+Set `RunRequest.CaptureExchange` to capture a completed run's effective messages
+and final assistant content. Supply its `RunResult.Exchange` as `ContinueFrom`
+on a new request to continue it. Exchanges are reusable, support independent
+branches and different engines, and expose detached history through `Details()`.
+Prepared execution also supports capture of the exact snapshot it executes.
+
+Internal structural-repair instructions and intermediate candidates are not
+conversation history. A completed result with failed structural validation can
+still be captured; an operational error returning no result creates no exchange.
+Follow-ups select their own profiles, output contracts, credentials, and sessions
+under ordinary request rules. See [capture and continuation guidance](../consumers/pkg-promptkit.md#capture-and-continue-a-conversation)
+and the [public exchange contract](../../exchange.go).
+
+## Coordinate Application-Directed Turns
+
+`Engine.Coordinate` advances one conversation branch using an application callback
+and an additional-turn budget. The application selects the next request or stops;
+Promptkit handles turn progression and cancellation. Each completed run releases
+backend admission before the callback, allowing assessment calls to use the same
+backend. Stop and budget exhaustion are control-flow outcomes, not judgments of
+response acceptance. Transport retries and structural repairs remain separate
+from the turn budget.
+
+See the [coordination walkthrough](../consumers/pkg-promptkit.md#coordinate-application-directed-turns)
+and [coordinator GoDoc](../../coordinate.go) for callback and cancellation behavior.
+
+## Collect Selected Execution Facts
+
+`ExecutionGroup` collects explicitly selected execution observations, while
+`ComposeObservers` combines groups with existing diagnostic observers. Detached
+group snapshots report member identities, terminal coverage, attempt/retry/repair
+counts, and available token totals without counting a run more than once.
+Terminal coverage remains distinct from token-usage completeness; membership is
+not inferred from ancestry or nested calls.
+
+See [execution-group guidance](../consumers/pkg-promptkit.md#collect-selected-execution-facts),
+the [group GoDoc](../../execution_group.go), and the updated
+[offline observation example](../../examples/go-library/observe/main.go).
+
+## Efficiency And Compatibility
+
+History assembly and inspection avoid intermediate full-history copies. Observer
+composition retains storage only for selected callbacks and avoids fan-out
+closures for zero or one selected callback.
+
+Capture and coordination are opt-in; existing execution workflows and
+configuration formats need no migration from v0.21.0. The Go consumer minimum
+remains 1.27.0, and dependency versions are unchanged. Exported request, prepared,
+result, and observation structs gained fields; consumers using positional struct
+literals must switch to keyed literals. Continuation metadata adds `parent_run_id`
+to prepared/result JSON when applicable; the opaque exchange itself is excluded.
+Exchanges are in-process capabilities, not serialized restartable sessions.
+
+```sh
+go get go.maximumdirect.net/promptkit@v0.22.0
+```
+
+[COPYRIGHT](https://forge.maximumdirect.net/base-layer/promptkit/src/tag/v0.22.0/COPYRIGHT) | [LICENSE](https://forge.maximumdirect.net/base-layer/promptkit/src/tag/v0.22.0/LICENSE) | [ATTRIBUTION](https://forge.maximumdirect.net/base-layer/promptkit/src/tag/v0.22.0/docs/oss/attribution.md)

docs/releases/v0.23.0.md

@@ -0,0 +1,62 @@
+# Promptkit v0.23.0
+
+This minor release adds optional whole-operation timeouts and prepared-request
+notifications to ordinary and coordinated execution. Applications can bound
+each continuation independently and retain the request selected for execution
+without replacing Promptkit's turn loop or preparing the same request twice.
+
+## Bound Individual Operations
+
+`RunRequest.OperationTimeout` selects a fresh cooperative budget covering
+preparation and execution, including capacity waiting, retries, validation,
+structural repair, and prepared-request notification. It is independent of the
+provider generation timeout. Zero or negative values select no additional
+timeout; existing parent cancellation and earlier deadlines remain effective.
+
+For split execution, `PrepareExecution` retains the duration without starting
+a timer. `RunPrepared` starts that budget after claiming the handle, so prior
+preparation and idle time do not consume it. Callers can still supply one shared
+deadline context to bound both calls together.
+
+Local budget failures match both `ErrOperationTimeout` and
+`context.DeadlineExceeded`. Each coordinated turn uses its selected budget;
+expiry stops that coordination through the existing operational-error path
+without canceling its parent or sibling work. Completed outcomes are fixed
+before terminal observation and are not changed by a slow terminal observer.
+Cancellation does not forcibly interrupt callbacks or collaborators that ignore
+their context.
+
+See the [execution-controls guide](../consumers/pkg-promptkit.md#bound-execution-and-retain-prepared-details)
+and the [public request contract](../../types.go) and
+[timeout error contract](../../engine.go) for exact behavior.
+
+## Retain Prepared Details Before Generation
+
+`RunRequest.OnPrepared` optionally receives a detached, credential-redacted
+`PreparedRun` and the operation context before first generation. It uses the
+actual prepared snapshot, including continuation history, without rereading
+inputs or rerendering. Applications may retain these details even if generation
+later fails without returning a result.
+
+Notification is synchronous, at most once per logical execution, and runs while
+logical admission is held. It is not repeated for retries or structural repair.
+Preparation or admission failure can prevent delivery. The application owns
+diagnostic storage and disclosure decisions; ordinary observers remain
+metadata-only. See the [OnPrepared GoDoc](../../types.go) and updated
+[offline observation example](../../examples/go-library/observe/main.go).
+
+## Compatibility And Upgrade
+
+Both controls are opt-in. Existing keyed requests and configuration files need
+no migration from v0.22.0. `Prepare` and `ValidateRequest` continue using only
+their supplied contexts and do not invoke `OnPrepared`. `RunRequest` gained
+fields; consumers using positional struct literals must switch to keyed
+literals. Prepared/result JSON representations are unchanged.
+
+The Go consumer minimum remains 1.27.0, and dependency versions are unchanged.
+
+```sh
+go get go.maximumdirect.net/promptkit@v0.23.0
+```
+
+[COPYRIGHT](https://forge.maximumdirect.net/base-layer/promptkit/src/tag/v0.23.0/COPYRIGHT) | [LICENSE](https://forge.maximumdirect.net/base-layer/promptkit/src/tag/v0.23.0/LICENSE) | [ATTRIBUTION](https://forge.maximumdirect.net/base-layer/promptkit/src/tag/v0.23.0/docs/oss/attribution.md)

engine.go

@@ -60,6 +60,13 @@ var (
 	// 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")
+	// ErrOperationTimeout identifies expiration of the timeout selected by
+	// RunRequest.OperationTimeout. Such errors also match context.DeadlineExceeded
+	// and retain their underlying operation and collaborator identities.
+	// Earlier parent cancellation or deadlines and provider-only deadlines do
+	// not acquire this identity. A completed result remains completed even if
+	// a terminal observer outlasts the selected budget.
+	ErrOperationTimeout = errors.New("operation timeout exceeded")
 	// ErrLLMGenerate identifies a model-client failure or a nil successful
 	// response. A built-in OpenAI-compatible non-2xx response is available as a
 	// [GenerationError]. Errors returned by an injected LLMClient remain
@@ -386,6 +393,7 @@ func (e *Engine) InspectBackend(ctx context.Context, backendID string) (*Backend
 // reported in the applicable operation category; no general errors.Is
 // relationship to ctx.Err is promised. Prepare returns no partial result on
 // error. It validates RunRequest.TransientRetries but makes no model attempts.
+// RunRequest.OperationTimeout and RunRequest.OnPrepared have no effect here.
 func (e *Engine) Prepare(ctx context.Context, req RunRequest) (*PreparedRun, error) {
 	prepared, err := e.prepareRequest(ctx, req)
 	if err != nil {
@@ -407,6 +415,7 @@ func (e *Engine) Prepare(ctx context.Context, req RunRequest) (*PreparedRun, err
 // matching ErrInvalidConfig when the Engine is nil. Success does not guarantee
 // provider acceptance or later execution success. Use [Engine.PrepareExecution]
 // when execution must use the snapshot produced by preparation.
+// Execution-only controls do not start a timer or invoke a notification here.
 func (e *Engine) ValidateRequest(ctx context.Context, req RunRequest) error {
 	_, err := e.prepareRequest(ctx, req)
 	return err
@@ -449,6 +458,8 @@ func (e *Engine) prepareRequest(ctx context.Context, req RunRequest) (*domain.Pr
 // ErrInvalidConfig. The effective transient retry budget is frozen privately
 // for RunPrepared and is absent from Details. A supplied Observer is retained
 // privately for RunPrepared; preparation itself emits no events.
+// RunRequest.OperationTimeout and RunRequest.OnPrepared are retained without
+// starting a timer or invoking the notification during preparation.
 func (e *Engine) PrepareExecution(ctx context.Context, req RunRequest) (*PreparedExecution, error) {
 	ctx = observation.WithoutExecutionState(ctx)
 	if e == nil || e.internal == nil {
@@ -479,6 +490,9 @@ func (e *Engine) PrepareExecution(ctx context.Context, req RunRequest) (*Prepare
 // returns the final failed validation result with cumulative usage and actual
 // repair attempts. An inability to generate or validate returns an error and
 // no partial result.
+// RunRequest.OperationTimeout selects a cooperative whole-operation deadline;
+// RunRequest.OnPrepared selects conditional notification before generation.
+// The outcome and timeout attribution are fixed before terminal observation.
 //
 // The built-in client retries eligible provider statuses using the effective
 // Config or request budget. With R additional transient attempts and P repair
@@ -510,18 +524,30 @@ func (e *Engine) PrepareExecution(ctx context.Context, req RunRequest) (*Prepare
 // result on error.
 func (e *Engine) Run(ctx context.Context, req RunRequest) (*RunResult, error) {
 	ctx = observation.WithoutExecutionState(ctx)
+	scope := usecase.NewOperationScope(ctx, req.OperationTimeout)
+	defer scope.Close()
+	ctx = scope.Context
 	if req.Observer == nil {
-		return e.run(ctx, req)
+		result, err := e.run(ctx, req)
+		err = mapPublicError(scope.Finalize(err))
+		scope.Close()
+		return result, err
 	}
 	recorder, idErr := observation.New(func(event domain.ExecutionEvent) { req.Observer(fromDomainExecutionEvent(event)) })
+	if req.ContinueFrom != nil && req.ContinueFrom.internal != nil {
+		recorder.SetParentRunID(req.ContinueFrom.internal.RunID())
+	}
 	recorder.Start()
 	if idErr != nil {
-		err := fmt.Errorf("failed to create run id: %w", idErr)
+		err := mapPublicError(scope.Finalize(fmt.Errorf("failed to create run id: %w", idErr)))
+		scope.Close()
 		terminal := observedFinish(nil, err)
 		recorder.Finish(observation.FinishData{Outcome: terminal.Outcome, Failure: terminal.Failure})
 		return nil, err
 	}
 	result, err := e.run(observation.WithRecorder(ctx, recorder), req)
+	err = mapPublicError(scope.Finalize(err))
+	scope.Close()
 	terminal := observedFinish(result, err)
 	recorder.Finish(observation.FinishData{Outcome: terminal.Outcome, Failure: terminal.Failure,
 		StatusCode: terminal.StatusCode, RetriesExhausted: terminal.RetriesExhausted})
@@ -540,7 +566,7 @@ func (e *Engine) run(ctx context.Context, req RunRequest) (*RunResult, error) {
 
 	result, err := e.internal.Run(ctx, domainReq)
 	if err != nil {
-		return nil, mapPublicError(err)
+		return nil, err
 	}
 	return fromDomainRunResult(result), nil
 }
@@ -584,6 +610,9 @@ func (e *Engine) run(ctx context.Context, req RunRequest) (*RunResult, error) {
 // mapping. Invalid, foreign, discarded, or reused handles emit no events.
 // Callbacks run outside the handle and capacity locks; panics propagate and
 // do not promise a terminal event.
+// The retained RunRequest.OperationTimeout starts after claim; the retained
+// RunRequest.OnPrepared is notified only if execution reaches its prepared
+// point. Final timeout attribution precedes terminal observation.
 func (e *Engine) RunPrepared(ctx context.Context, prepared *PreparedExecution) (*RunResult, error) {
 	ctx = observation.WithoutExecutionState(ctx)
 	if e == nil || e.internal == nil {

errors.go

@@ -96,6 +96,8 @@ func mapUnownedPublicError(err error) error {
 
 func publicErrorFor(err error) error {
 	switch {
+	case errors.Is(err, usecase.ErrOperationTimeout):
+		return ErrOperationTimeout
 	case errors.Is(err, promptdef.ErrPromptDefinitionNotFound):
 		return ErrPromptNotFound
 	case errors.Is(err, profile.ErrProfileNotFound):

examples/go-library/observe/main.go

@@ -9,6 +9,7 @@ import (
 	"net/http"
 	"os"
 	"strings"
+	"time"
 
 	"go.maximumdirect.net/configkit"
 	"go.maximumdirect.net/promptkit"
@@ -69,12 +70,18 @@ func (l *eventLog) terminal() (promptkit.ExecutionEvent, error) {
 }
 
 type summary struct {
-	ProposalAttempts           uint64 `json:"proposal_attempts"`
-	ProposalTransientRetries   uint64 `json:"proposal_transient_retries"`
-	ProposalUsageComplete      bool   `json:"proposal_usage_complete"`
-	AssessmentAttempts         uint64 `json:"assessment_attempts"`
-	AssessmentTransientRetries uint64 `json:"assessment_transient_retries"`
-	AssessmentOutcome          string `json:"assessment_outcome"`
+	ProposalAttempts           uint64                        `json:"proposal_attempts"`
+	ProposalTransientRetries   uint64                        `json:"proposal_transient_retries"`
+	ProposalUsageComplete      bool                          `json:"proposal_usage_complete"`
+	AssessmentAttempts         uint64                        `json:"assessment_attempts"`
+	AssessmentTransientRetries uint64                        `json:"assessment_transient_retries"`
+	AssessmentOutcome          string                        `json:"assessment_outcome"`
+	CoordinationOutcome        promptkit.CoordinationOutcome `json:"coordination_outcome"`
+	ExhaustedOutcome           promptkit.CoordinationOutcome `json:"exhausted_outcome"`
+	GroupMembers               int                           `json:"group_members"`
+	GroupAttempts              uint64                        `json:"group_attempts"`
+	PreparedMessages           int                           `json:"prepared_messages"`
+	PreparedParentMatches      bool                          `json:"prepared_parent_matches"`
 }
 
 func run() (summary, error) {
@@ -104,8 +111,9 @@ func run() (summary, error) {
 	}
 
 	var proposalLog eventLog
+	var executions promptkit.ExecutionGroup
 	proposal, err := engine.Run(context.Background(), promptkit.RunRequest{
-		PromptID: "example.proposal", Inputs: map[string]promptkit.ArtifactRef{"idea": promptkit.Inline("Reduce queue cleanup toil.")}, Observer: proposalLog.record,
+		PromptID: "example.proposal", Inputs: map[string]promptkit.ArtifactRef{"idea": promptkit.Inline("Reduce queue cleanup toil.")}, CaptureExchange: true, Observer: promptkit.ComposeObservers(proposalLog.record, executions.Observe),
 	})
 	if err != nil {
 		return summary{}, err
@@ -118,35 +126,56 @@ func run() (summary, error) {
 		return summary{}, err
 	}
 	var assessmentLog eventLog
-	assessment, err := engine.Run(context.Background(), promptkit.RunRequest{
-		PromptID: "example.assessment", Inputs: map[string]promptkit.ArtifactRef{"proposal": promptkit.Inline(proposal.RawOutput)}, Observer: assessmentLog.record,
+	var preparedDetails promptkit.PreparedRun
+	decision := "rejected"
+	coordination, err := engine.Coordinate(context.Background(), promptkit.CoordinateRequest{
+		Start: proposal.Exchange, AdditionalTurns: 1, Observer: executions.Observe,
+		Next: func(ctx context.Context, turn promptkit.ConversationTurn) (*promptkit.RunRequest, error) {
+			if turn.Result == nil {
+				return &promptkit.RunRequest{PromptID: "example.proposal", Inputs: map[string]promptkit.ArtifactRef{"idea": promptkit.Inline("Refine the proposal.")}, OperationTimeout: 10 * time.Second, OnPrepared: func(_ context.Context, details promptkit.PreparedRun) { preparedDetails = details }}, nil
+			}
+			if !turn.Result.Validation.IsValid {
+				return nil, fmt.Errorf("follow-up failed structural validation")
+			}
+			assessment, err := engine.Run(ctx, promptkit.RunRequest{PromptID: "example.assessment", Inputs: map[string]promptkit.ArtifactRef{"proposal": promptkit.Inline(turn.Result.RawOutput)}, OperationTimeout: 10 * time.Second, Observer: promptkit.ComposeObservers(assessmentLog.record, executions.Observe)})
+			if err != nil {
+				return nil, err
+			}
+			if !assessment.Validation.IsValid {
+				return nil, fmt.Errorf("assessment failed structural validation")
+			}
+			var proposalValue struct {
+				Risk string `json:"risk"`
+			}
+			var assessmentValue struct {
+				Recommendation string `json:"recommendation"`
+			}
+			if err := json.Unmarshal([]byte(turn.Result.RawOutput), &proposalValue); err != nil {
+				return nil, err
+			}
+			if err := json.Unmarshal([]byte(assessment.RawOutput), &assessmentValue); err != nil {
+				return nil, err
+			}
+			if proposalValue.Risk == "low" && assessmentValue.Recommendation == "accept" {
+				decision = "accepted"
+			}
+			return nil, nil
+		},
 	})
 	if err != nil {
 		return summary{}, err
 	}
-	if !assessment.Validation.IsValid {
-		return summary{}, fmt.Errorf("assessment failed structural validation")
+	exhausted, err := engine.Coordinate(context.Background(), promptkit.CoordinateRequest{Start: proposal.Exchange, Next: func(context.Context, promptkit.ConversationTurn) (*promptkit.RunRequest, error) {
+		return &promptkit.RunRequest{PromptID: "example.proposal"}, nil
+	}})
+	if err != nil {
+		return summary{}, err
 	}
 	assessmentTerminal, err := assessmentLog.terminal()
 	if err != nil {
 		return summary{}, err
 	}
-	var proposalValue struct {
-		Risk string `json:"risk"`
-	}
-	var assessmentValue struct {
-		Recommendation string `json:"recommendation"`
-	}
-	if err := json.Unmarshal([]byte(proposal.RawOutput), &proposalValue); err != nil {
-		return summary{}, err
-	}
-	if err := json.Unmarshal([]byte(assessment.RawOutput), &assessmentValue); err != nil {
-		return summary{}, err
-	}
-	decision := "rejected"
-	if proposalValue.Risk == "low" && assessmentValue.Recommendation == "accept" {
-		decision = "accepted"
-	}
+	group := executions.Snapshot()
 	return summary{
 		ProposalAttempts:           proposalTerminal.Attempts,
 		ProposalTransientRetries:   proposalTerminal.TransientRetries,
@@ -154,6 +183,12 @@ func run() (summary, error) {
 		AssessmentAttempts:         assessmentTerminal.Attempts,
 		AssessmentTransientRetries: assessmentTerminal.TransientRetries,
 		AssessmentOutcome:          decision,
+		CoordinationOutcome:        coordination.Outcome,
+		ExhaustedOutcome:           exhausted.Outcome,
+		GroupMembers:               len(group.Members),
+		GroupAttempts:              group.Attempts,
+		PreparedMessages:           len(preparedDetails.Messages),
+		PreparedParentMatches:      preparedDetails.ParentRunID == proposal.RunID,
 	}, nil
 }
 

examples/go-library/observe/main_test.go

@@ -12,19 +12,25 @@ func TestObserveExample(t *testing.T) {
 		t.Fatal(err)
 	}
 	var got struct {
-		ProposalAttempts   *int   `json:"proposal_attempts"`
-		ProposalRetries    *int   `json:"proposal_transient_retries"`
-		UsageComplete      *bool  `json:"proposal_usage_complete"`
-		AssessmentAttempts *int   `json:"assessment_attempts"`
-		AssessmentRetries  *int   `json:"assessment_transient_retries"`
-		Outcome            string `json:"assessment_outcome"`
+		ProposalAttempts      *int   `json:"proposal_attempts"`
+		ProposalRetries       *int   `json:"proposal_transient_retries"`
+		UsageComplete         *bool  `json:"proposal_usage_complete"`
+		AssessmentAttempts    *int   `json:"assessment_attempts"`
+		AssessmentRetries     *int   `json:"assessment_transient_retries"`
+		Outcome               string `json:"assessment_outcome"`
+		CoordinationOutcome   string `json:"coordination_outcome"`
+		ExhaustedOutcome      string `json:"exhausted_outcome"`
+		GroupMembers          int    `json:"group_members"`
+		GroupAttempts         int    `json:"group_attempts"`
+		PreparedMessages      int    `json:"prepared_messages"`
+		PreparedParentMatches bool   `json:"prepared_parent_matches"`
 	}
 	if err := json.Unmarshal(output.Bytes(), &got); err != nil {
 		t.Fatal(err)
 	}
 	if got.ProposalAttempts == nil || *got.ProposalAttempts != 2 || got.ProposalRetries == nil || *got.ProposalRetries != 1 ||
 		got.UsageComplete == nil || *got.UsageComplete || got.AssessmentAttempts == nil || *got.AssessmentAttempts != 1 ||
-		got.AssessmentRetries == nil || *got.AssessmentRetries != 0 || got.Outcome != "accepted" {
+		got.AssessmentRetries == nil || *got.AssessmentRetries != 0 || got.Outcome != "accepted" || got.CoordinationOutcome != "stopped" || got.ExhaustedOutcome != "exhausted" || got.GroupMembers != 3 || got.GroupAttempts != 4 || got.PreparedMessages < 3 || !got.PreparedParentMatches {
 		t.Fatalf("unexpected observation summary: %+v", got)
 	}
 }

exchange.go

@@ -0,0 +1,47 @@
+package promptkit
+
+import "go.maximumdirect.net/promptkit/internal/domain"
+
+const exchangeString = "promptkit.Exchange{opaque}"
+
+// Exchange is an immutable, reusable in-process snapshot of a completed
+// conversation. It is not a persistence or restart format. Different engines
+// can continue it, and independent branches do not alter one another.
+//
+// History preserves the effective request's message content, roles, and cache
+// metadata, followed by the exact final [RunResult.RawOutput] as an assistant
+// message. Intermediate structural-repair candidates and corrective instructions
+// are excluded. A continuation extends that logical history according to
+// [RunRequest.ContinueFrom], without changing its source exchange.
+type Exchange struct{ internal *domain.Exchange }
+
+// ExchangeDetails contains a detached copy of an exchange's logical history.
+type ExchangeDetails struct {
+	// RunID identifies the execution that produced this exchange.
+	RunID string `json:"run_id"`
+	// ParentRunID identifies the immediate source execution, or is empty.
+	ParentRunID string            `json:"parent_run_id,omitempty"`
+	Messages    []RenderedMessage `json:"messages"`
+}
+
+// Details returns detached history, or zero details for a nil or empty handle.
+// Messages include caller and model content and require appropriate handling.
+// Inspection uses only retained state, without rendering, filesystem reads, or
+// provider calls. Mutating returned messages or cache metadata cannot affect
+// this exchange, another branch, or subsequent Details calls.
+func (e *Exchange) Details() ExchangeDetails {
+	if e == nil || e.internal == nil {
+		return ExchangeDetails{}
+	}
+	messages := make([]RenderedMessage, e.internal.MessageCount())
+	e.internal.VisitMessages(func(index int, message domain.RenderedMessage) {
+		messages[index] = RenderedMessage{Role: message.Role, Content: message.Content, CacheControl: fromDomainCacheControl(message.CacheControl)}
+	})
+	return ExchangeDetails{RunID: e.internal.RunID(), ParentRunID: e.internal.ParentRunID(), Messages: messages}
+}
+
+// String returns an opaque representation without conversation content.
+func (e Exchange) String() string { return exchangeString }
+
+// GoString returns an opaque representation without conversation content.
+func (e Exchange) GoString() string { return exchangeString }

exchange_capture_contract_test.go

@@ -0,0 +1,113 @@
+package promptkit_test
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"reflect"
+	"strings"
+	"testing"
+
+	"go.maximumdirect.net/promptkit"
+)
+
+func TestExchangeCaptureAndPreparedContinuation(t *testing.T) {
+	client := &fakeLLMClient{response: &promptkit.GenerateResponse{Content: "answer"}}
+	engine := newSimpleEngine(t, "question", promptkit.WithLLMClient(client))
+	ctx := context.Background()
+	plain, err := engine.Run(ctx, promptkit.RunRequest{PromptID: "prompt"})
+	if err != nil || plain.Exchange != nil {
+		t.Fatalf("plain run = %#v, %v", plain, err)
+	}
+	initial, err := engine.Run(ctx, promptkit.RunRequest{PromptID: "prompt", CaptureExchange: true})
+	if err != nil || initial.Exchange == nil {
+		t.Fatalf("captured run = %#v, %v", initial, err)
+	}
+	want := []promptkit.RenderedMessage{{Role: promptkit.RoleUser, Content: "question"}, {Role: promptkit.RoleAssistant, Content: "answer"}}
+	if details := initial.Exchange.Details(); details.RunID != initial.RunID || !reflect.DeepEqual(details.Messages, want) {
+		t.Fatalf("details = %#v", details)
+	}
+	encoded, err := json.Marshal(initial)
+	if err != nil || strings.Contains(string(encoded), "exchange") || strings.Contains(string(encoded), "question") {
+		t.Fatalf("result JSON = %s, %v", encoded, err)
+	}
+	handle, err := engine.PrepareExecution(ctx, promptkit.RunRequest{PromptID: "prompt", ContinueFrom: initial.Exchange})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if handle.Details().ParentRunID != initial.RunID {
+		t.Fatal("prepared ancestry missing")
+	}
+	continued, err := engine.RunPrepared(ctx, handle)
+	if err != nil || continued.Exchange == nil || continued.ParentRunID != initial.RunID {
+		t.Fatalf("continued = %#v, %v", continued, err)
+	}
+	want = append(want, promptkit.RenderedMessage{Role: promptkit.RoleUser, Content: "question"}, promptkit.RenderedMessage{Role: promptkit.RoleAssistant, Content: "answer"})
+	if details := continued.Exchange.Details(); details.ParentRunID != initial.RunID || !reflect.DeepEqual(details.Messages, want) {
+		t.Fatalf("continued details = %#v", details)
+	}
+}
+
+func TestExchangeCapturesInvalidFinalOutput(t *testing.T) {
+	client := &fakeLLMClient{responses: []*promptkit.GenerateResponse{{Content: ""}, {Content: " "}}}
+	engine := newSimpleEngine(t, "question", promptkit.WithLLMClient(client))
+	result, err := engine.Run(context.Background(), promptkit.RunRequest{PromptID: "prompt", CaptureExchange: true,
+		Validation: &promptkit.OutputContract{Format: promptkit.FormatText, ValidationMode: promptkit.ValidationBasic, RepairAttempts: 1}})
+	if err != nil || result == nil || result.Validation.Status != promptkit.ValidationFailed || result.Exchange == nil {
+		t.Fatalf("invalid result = %#v, %v", result, err)
+	}
+	if messages := result.Exchange.Details().Messages; len(messages) != 2 || messages[1].Content != " " {
+		t.Fatalf("history = %#v", messages)
+	}
+}
+
+func TestExchangeFailuresLeaveSourceReusable(t *testing.T) {
+	client := &fakeLLMClient{response: &promptkit.GenerateResponse{Content: "answer"}}
+	engine := newSimpleEngine(t, "question", promptkit.WithLLMClient(client))
+	ctx := context.Background()
+	initial, err := engine.Run(ctx, promptkit.RunRequest{PromptID: "prompt", CaptureExchange: true})
+	if err != nil {
+		t.Fatal(err)
+	}
+	original := initial.Exchange.Details()
+	request := promptkit.RunRequest{PromptID: "prompt", ContinueFrom: initial.Exchange}
+	handle, err := engine.PrepareExecution(ctx, request)
+	if err != nil {
+		t.Fatal(err)
+	}
+	handle.Discard()
+	if result, err := engine.RunPrepared(ctx, handle); result != nil || !errors.Is(err, promptkit.ErrInvalidRequest) {
+		t.Fatalf("discarded = %#v, %v", result, err)
+	}
+	client.err = errors.New("provider failure")
+	if result, err := engine.Run(ctx, request); result != nil || !errors.Is(err, promptkit.ErrLLMGenerate) {
+		t.Fatalf("failure = %#v, %v", result, err)
+	}
+	client.err = nil
+	result, err := engine.Run(ctx, request)
+	if err != nil || result.Exchange == nil || !reflect.DeepEqual(initial.Exchange.Details(), original) {
+		t.Fatalf("reused source = %#v, %v", result, err)
+	}
+}
+
+func TestEmptyExchangeRejectedBeforePreparation(t *testing.T) {
+	client := &fakeLLMClient{response: &promptkit.GenerateResponse{Content: "answer"}}
+	engine := newSimpleEngine(t, "question", promptkit.WithLLMClient(client))
+	request := promptkit.RunRequest{PromptID: "missing", ContinueFrom: &promptkit.Exchange{}}
+	ctx := context.Background()
+	if _, err := engine.Prepare(ctx, request); !errors.Is(err, promptkit.ErrInvalidRequest) {
+		t.Fatalf("prepare error = %v", err)
+	}
+	if err := engine.ValidateRequest(ctx, request); !errors.Is(err, promptkit.ErrInvalidRequest) {
+		t.Fatalf("validate error = %v", err)
+	}
+	if _, err := engine.PrepareExecution(ctx, request); !errors.Is(err, promptkit.ErrInvalidRequest) {
+		t.Fatalf("prepare execution error = %v", err)
+	}
+	if _, err := engine.Run(ctx, request); !errors.Is(err, promptkit.ErrInvalidRequest) {
+		t.Fatalf("run error = %v", err)
+	}
+	if len(client.requests) != 0 {
+		t.Fatal("invalid exchange reached the model")
+	}
+}

exchange_test.go

@@ -0,0 +1,38 @@
+package promptkit
+
+import (
+	"encoding/json"
+	"fmt"
+	"strings"
+	"testing"
+
+	"go.maximumdirect.net/promptkit/internal/domain"
+)
+
+func TestExchangeDetailsAndRequestConversion(t *testing.T) {
+	var nilExchange *Exchange
+	if got := nilExchange.Details(); got.RunID != "" || got.Messages != nil {
+		t.Fatalf("nil details = %#v", got)
+	}
+	if _, err := toDomainRunRequest(RunRequest{ContinueFrom: &Exchange{}}); err == nil {
+		t.Fatal("empty exchange accepted")
+	}
+	history := domain.CaptureExchange(nil, "source", []domain.RenderedMessage{{Role: domain.RoleUser, Content: "private", CacheControl: &domain.CacheControl{Type: domain.CacheControlEphemeral, TTL: "1h"}}}, "answer")
+	handle := &Exchange{internal: history}
+	request, err := toDomainRunRequest(RunRequest{ContinueFrom: handle})
+	if err != nil || request.ContinueFrom != history || !request.CaptureExchange {
+		t.Fatalf("request = %#v, error = %v", request, err)
+	}
+	details := handle.Details()
+	details.Messages[0].CacheControl.TTL = "changed"
+	if handle.Details().Messages[0].CacheControl.TTL != "1h" {
+		t.Fatal("details changed exchange")
+	}
+	if strings.Contains(fmt.Sprintf("%v %#v", handle, handle), "private") {
+		t.Fatal("format exposed history")
+	}
+	encoded, err := json.Marshal(PreparedRun{ParentRunID: "source"})
+	if err != nil || !strings.Contains(string(encoded), `"parent_run_id":"source"`) {
+		t.Fatalf("prepared JSON = %s, %v", encoded, err)
+	}
+}

execution_budget_contract_test.go

@@ -0,0 +1,192 @@
+package promptkit_test
+
+import (
+	"context"
+	"errors"
+	"net/http"
+	"testing"
+	"testing/synctest"
+	"time"
+
+	"go.maximumdirect.net/promptkit"
+)
+
+func TestOperationBudgetCoversPreparationAndReleasesAdmission(t *testing.T) {
+	synctest.Test(t, func(t *testing.T) {
+		release := make(chan struct{})
+		reader := &capacityArtifactReader{entered: make(chan struct{}, 2), release: release}
+		client := newCapacityGateClient(closedCapacityChannel(), 2)
+		engine := newBackendCapacityEngine(t, client, 1, capacityInt(0), reader)
+		parent := context.Background()
+		req := capacityInputRequest("http://example.invalid/v1")
+		req.OperationTimeout = time.Second
+		_, err := engine.Run(parent, req)
+		if !errors.Is(err, promptkit.ErrOperationTimeout) || !errors.Is(err, context.DeadlineExceeded) || !errors.Is(err, promptkit.ErrArtifactLoad) || parent.Err() != nil {
+			t.Fatalf("preparation error=%v parent=%v", err, parent.Err())
+		}
+		close(release)
+		if result, err := engine.Run(parent, req); err != nil || result == nil {
+			t.Fatalf("capacity not released: %v", err)
+		}
+	})
+}
+
+func TestOperationBudgetCoversGenerationCapacityWait(t *testing.T) {
+	synctest.Test(t, func(t *testing.T) {
+		release := make(chan struct{})
+		client := newCapacityGateClient(release, 2)
+		engine := newBackendCapacityEngine(t, client, 1, capacityInt(1), nil)
+		results := make(chan capacityRunResult, 1)
+		go runCapacityRequest(engine, context.Background(), promptkit.RunRequest{PromptID: "prompt"}, results)
+		<-client.started
+		_, err := engine.Run(context.Background(), promptkit.RunRequest{PromptID: "prompt", OperationTimeout: time.Second})
+		if !errors.Is(err, promptkit.ErrOperationTimeout) || !errors.Is(err, promptkit.ErrLLMGenerate) || !errors.Is(err, context.DeadlineExceeded) {
+			t.Fatalf("capacity wait error=%v", err)
+		}
+		close(release)
+		if result := <-results; result.err != nil {
+			t.Fatal(result.err)
+		}
+		if _, err := engine.Run(context.Background(), promptkit.RunRequest{PromptID: "prompt"}); err != nil {
+			t.Fatal(err)
+		}
+	})
+}
+
+func TestOperationBudgetInterruptsRetryAndRepair(t *testing.T) {
+	for _, repair := range []bool{false, true} {
+		t.Run(map[bool]string{false: "retry", true: "repair"}[repair], func(t *testing.T) {
+			synctest.Test(t, func(t *testing.T) {
+				calls := 0
+				config := simpleConfig("message")
+				one := 1
+				config.TransientRetries = &one
+				config.HTTPClient = &http.Client{Transport: roundTripFunc(func(r *http.Request) (*http.Response, error) {
+					calls++
+					if repair && calls == 1 {
+						return retryHTTPResponse(200, "not json"), nil
+					}
+					if repair {
+						<-r.Context().Done()
+						return nil, r.Context().Err()
+					}
+					response := retryHTTPResponse(503, "")
+					response.Header = make(http.Header)
+					response.Header.Set("Retry-After", "5")
+					return response, nil
+				})}
+				engine, err := promptkit.NewEngine(config)
+				if err != nil {
+					t.Fatal(err)
+				}
+				req := simpleGenerationRequest()
+				req.OperationTimeout = time.Second
+				if repair {
+					req.Validation = &promptkit.OutputContract{Format: promptkit.FormatJSON, ValidationMode: promptkit.ValidationJSON, RepairAttempts: 1}
+				}
+				_, err = engine.Run(context.Background(), req)
+				if !errors.Is(err, promptkit.ErrOperationTimeout) || !errors.Is(err, promptkit.ErrLLMGenerate) || !errors.Is(err, context.DeadlineExceeded) {
+					t.Fatalf("error=%v calls=%d", err, calls)
+				}
+			})
+		})
+	}
+}
+
+func TestOperationOutcomeFrozenBeforeTerminalObserver(t *testing.T) {
+	for _, split := range []bool{false, true} {
+		t.Run(map[bool]string{false: "run", true: "prepared"}[split], func(t *testing.T) {
+			synctest.Test(t, func(t *testing.T) {
+				engine := observationEngine(t, observationClientFunc(func(context.Context, promptkit.GenerateRequest) (*promptkit.GenerateResponse, error) {
+					return &promptkit.GenerateResponse{Content: "ok"}, nil
+				}))
+				var terminal promptkit.ExecutionEvent
+				req := promptkit.RunRequest{PromptID: "prompt", OperationTimeout: time.Second, Observer: func(event promptkit.ExecutionEvent) {
+					if event.Kind == promptkit.ExecutionFinished {
+						time.Sleep(2 * time.Second)
+						terminal = event
+					}
+				}}
+				var result *promptkit.RunResult
+				var err error
+				if split {
+					var handle *promptkit.PreparedExecution
+					handle, err = engine.PrepareExecution(context.Background(), req)
+					if err == nil {
+						result, err = engine.RunPrepared(context.Background(), handle)
+					}
+				} else {
+					result, err = engine.Run(context.Background(), req)
+				}
+				if err != nil || result == nil || terminal.Outcome != promptkit.ExecutionOutcomeSuccess {
+					t.Fatalf("result=%v error=%v terminal=%+v", result, err, terminal)
+				}
+			})
+		})
+	}
+}
+
+func TestNotificationConsumesOperationBudget(t *testing.T) {
+	synctest.Test(t, func(t *testing.T) {
+		engine := observationEngine(t, observationClientFunc(func(context.Context, promptkit.GenerateRequest) (*promptkit.GenerateResponse, error) {
+			t.Fatal("generation ran")
+			return nil, nil
+		}))
+		_, err := engine.Run(context.Background(), promptkit.RunRequest{PromptID: "prompt", OperationTimeout: time.Second, OnPrepared: func(ctx context.Context, _ promptkit.PreparedRun) { <-ctx.Done() }})
+		if !errors.Is(err, promptkit.ErrOperationTimeout) || !errors.Is(err, promptkit.ErrLLMGenerate) || !errors.Is(err, context.DeadlineExceeded) {
+			t.Fatal(err)
+		}
+	})
+}
+
+func TestOperationTimeoutDoesNotRelabelOtherFailures(t *testing.T) {
+	synctest.Test(t, func(t *testing.T) {
+		failure := errors.New("independent failure")
+		engine := observationEngine(t, observationClientFunc(func(ctx context.Context, _ promptkit.GenerateRequest) (*promptkit.GenerateResponse, error) {
+			<-ctx.Done()
+			return nil, failure
+		}))
+		var terminal promptkit.ExecutionEvent
+		_, err := engine.Run(context.Background(), promptkit.RunRequest{PromptID: "prompt", OperationTimeout: time.Second, Observer: func(event promptkit.ExecutionEvent) {
+			if event.Kind == promptkit.ExecutionFinished {
+				terminal = event
+			}
+		}})
+		if !errors.Is(err, failure) || errors.Is(err, promptkit.ErrOperationTimeout) || terminal.Failure != promptkit.ExecutionFailureGeneration {
+			t.Fatalf("error=%v terminal=%+v", err, terminal)
+		}
+	})
+}
+
+func TestOperationTimeoutTerminalObserverSeesFinalFailure(t *testing.T) {
+	for _, split := range []bool{false, true} {
+		t.Run(map[bool]string{false: "run", true: "prepared"}[split], func(t *testing.T) {
+			synctest.Test(t, func(t *testing.T) {
+				engine := observationEngine(t, observationClientFunc(func(ctx context.Context, _ promptkit.GenerateRequest) (*promptkit.GenerateResponse, error) {
+					<-ctx.Done()
+					return nil, ctx.Err()
+				}))
+				var terminal promptkit.ExecutionEvent
+				req := promptkit.RunRequest{PromptID: "prompt", OperationTimeout: time.Second, Observer: func(event promptkit.ExecutionEvent) {
+					if event.Kind == promptkit.ExecutionFinished {
+						time.Sleep(time.Second)
+						terminal = event
+					}
+				}}
+				var err error
+				if split {
+					var handle *promptkit.PreparedExecution
+					handle, err = engine.PrepareExecution(context.Background(), req)
+					if err == nil {
+						_, err = engine.RunPrepared(context.Background(), handle)
+					}
+				} else {
+					_, err = engine.Run(context.Background(), req)
+				}
+				if !errors.Is(err, promptkit.ErrOperationTimeout) || !errors.Is(err, context.DeadlineExceeded) || terminal.Failure != promptkit.ExecutionFailureDeadline {
+					t.Fatalf("error=%v terminal=%+v", err, terminal)
+				}
+			})
+		})
+	}
+}

execution_controls_conversion_test.go

@@ -0,0 +1,38 @@
+package promptkit
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"go.maximumdirect.net/promptkit/internal/domain"
+)
+
+func TestExecutionControlsConversion(t *testing.T) {
+	ctx := context.WithValue(context.Background(), struct{}{}, "selected")
+	called := false
+	req := RunRequest{OperationTimeout: 3 * time.Second, OnPrepared: func(got context.Context, details PreparedRun) {
+		called = true
+		if got != ctx || details.PromptID != "prompt" {
+			t.Fatalf("unexpected notification: %v, %+v", got, details)
+		}
+		details.InputHashes["input"] = "changed"
+	}}
+	converted, err := toDomainRunRequest(req)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if converted.OperationTimeout != req.OperationTimeout || converted.OnPrepared == nil || called {
+		t.Fatal("execution controls were not retained without invocation")
+	}
+	req.OnPrepared = func(context.Context, PreparedRun) { t.Fatal("callback selection changed") }
+	prepared := &domain.PreparedRun{PromptID: "prompt", InputHashes: map[string]string{"input": "original"}}
+	converted.OnPrepared(ctx, prepared)
+	if !called || prepared.InputHashes["input"] != "original" {
+		t.Fatal("notification did not deliver detached details")
+	}
+	withoutCallback, err := toDomainRunRequest(RunRequest{})
+	if err != nil || withoutCallback.OnPrepared != nil {
+		t.Fatalf("nil callback was adapted: %v", err)
+	}
+}

execution_group.go

@@ -0,0 +1,56 @@
+package promptkit
+
+import (
+	"go.maximumdirect.net/promptkit/internal/domain"
+	"go.maximumdirect.net/promptkit/internal/observation"
+)
+
+// ExecutionGroup collects facts from explicitly selected execution events.
+// Its zero value is ready for concurrent use. Do not copy it after first use.
+type ExecutionGroup struct{ group observation.Group }
+
+// ExecutionGroupMember identifies an observed run and its terminal coverage.
+type ExecutionGroupMember struct {
+	RunID            string `json:"run_id"`
+	TerminalObserved bool   `json:"terminal_observed"`
+}
+
+// ExecutionGroupSummary is a detached snapshot of terminal-only totals.
+// Usage sums known terminal values, with Known as their logical OR and
+// Complete as their logical AND. Coverage describes whether every observed
+// member has a terminal, independently of usage completeness. An empty group
+// has complete coverage and known complete zero usage; a group with members
+// but no terminals has unknown, incomplete usage. Overlapping groups are not
+// generally additive.
+type ExecutionGroupSummary struct {
+	Members                  []ExecutionGroupMember `json:"members"`
+	TerminalCoverageComplete bool                   `json:"terminal_coverage_complete"`
+	Attempts                 uint64                 `json:"attempts"`
+	TransientRetries         uint64                 `json:"transient_retries"`
+	RepairCalls              uint64                 `json:"repair_calls"`
+	Usage                    ObservedTokenUsage     `json:"usage"`
+}
+
+// Observe enrolls an event's run, counting its first terminal event only.
+func (g *ExecutionGroup) Observe(event ExecutionEvent) {
+	count := func(c ObservedTokenCount) domain.ObservedTokenCount {
+		return domain.ObservedTokenCount{Value: c.Value, Known: c.Known, Complete: c.Complete}
+	}
+	g.group.Observe(domain.ExecutionEvent{Kind: domain.ExecutionEventKind(event.Kind), RunID: event.RunID, Attempts: event.Attempts, TransientRetries: event.TransientRetries, RepairCalls: event.RepairCalls,
+		Usage: domain.ObservedTokenUsage{PromptTokens: count(event.Usage.PromptTokens), CompletionTokens: count(event.Usage.CompletionTokens), TotalTokens: count(event.Usage.TotalTokens), CachedTokens: count(event.Usage.CachedTokens), CacheWriteTokens: count(event.Usage.CacheWriteTokens)}})
+}
+
+// Snapshot returns an internally consistent, caller-owned summary. Coverage
+// describes observed members only, not future members or application progress.
+func (g *ExecutionGroup) Snapshot() ExecutionGroupSummary {
+	value := g.group.Snapshot()
+	count := func(c domain.ObservedTokenCount) ObservedTokenCount {
+		return ObservedTokenCount{Value: c.Value, Known: c.Known, Complete: c.Complete}
+	}
+	members := make([]ExecutionGroupMember, len(value.Members))
+	for i, member := range value.Members {
+		members[i] = ExecutionGroupMember{RunID: member.RunID, TerminalObserved: member.TerminalObserved}
+	}
+	return ExecutionGroupSummary{Members: members, TerminalCoverageComplete: value.TerminalCoverageComplete, Attempts: value.Attempts, TransientRetries: value.TransientRetries, RepairCalls: value.RepairCalls,
+		Usage: ObservedTokenUsage{PromptTokens: count(value.Usage.PromptTokens), CompletionTokens: count(value.Usage.CompletionTokens), TotalTokens: count(value.Usage.TotalTokens), CachedTokens: count(value.Usage.CachedTokens), CacheWriteTokens: count(value.Usage.CacheWriteTokens)}}
+}

execution_group_contract_test.go

@@ -0,0 +1,151 @@
+package promptkit_test
+
+import (
+	"context"
+	"net/http"
+	"testing"
+
+	"go.maximumdirect.net/promptkit"
+)
+
+func TestExecutionGroupExplicitMembershipAndTerminalTotals(t *testing.T) {
+	responses := []*http.Response{
+		observedHTTPResponse(200, `{"choices":[{"message":{"content":"initial"}}],"usage":{"prompt_tokens":2}}`),
+		observedHTTPResponse(503, `{}`),
+		observedHTTPResponse(200, `{"choices":[{"message":{"content":"invalid"}}],"usage":{"prompt_tokens":3}}`),
+		observedHTTPResponse(200, `{"choices":[{"message":{"content":"{\"ok\":true}"}}],"usage":{"prompt_tokens":4}}`),
+		observedHTTPResponse(200, `{"choices":[{"message":{"content":"invalid"}}],"usage":{"prompt_tokens":5}}`),
+		observedHTTPResponse(422, `{"error":{}}`),
+		observedHTTPResponse(200, `{"choices":[{"message":{"content":"validator"}}],"usage":{"prompt_tokens":6}}`),
+	}
+	calls := 0
+	one := 1
+	config := simpleConfig("message")
+	config.TransientRetries = &one
+	config.HTTPClient = &http.Client{Transport: roundTripFunc(func(*http.Request) (*http.Response, error) {
+		response := responses[calls]
+		calls++
+		return response, nil
+	})}
+	engine, err := promptkit.NewEngine(config)
+	if err != nil {
+		t.Fatal(err)
+	}
+	ctx := context.Background()
+	var all, selected promptkit.ExecutionGroup
+	initial, err := engine.Run(ctx, promptkit.RunRequest{PromptID: "prompt", CaptureExchange: true, Observer: all.Observe})
+	if err != nil {
+		t.Fatal(err)
+	}
+	validation := &promptkit.OutputContract{Format: promptkit.FormatJSON, ValidationMode: promptkit.ValidationJSON, RepairAttempts: 1}
+	callbackCount := 0
+	var branchID, validatorID string
+	result, err := engine.Coordinate(ctx, promptkit.CoordinateRequest{Start: initial.Exchange, AdditionalTurns: 1, Observer: promptkit.ComposeObservers(all.Observe, selected.Observe), Next: func(ctx context.Context, turn promptkit.ConversationTurn) (*promptkit.RunRequest, error) {
+		callbackCount++
+		if turn.Result == nil {
+			return &promptkit.RunRequest{PromptID: "prompt", Validation: validation}, nil
+		}
+		branchID = turn.Result.RunID
+		return nil, nil
+	}})
+	if err != nil || result.Outcome != promptkit.CoordinationStopped || callbackCount != 2 {
+		t.Fatalf("coordinate=%#v, %v", result, err)
+	}
+	if failed, err := engine.Run(ctx, promptkit.RunRequest{PromptID: "prompt", ContinueFrom: initial.Exchange, Validation: validation, Observer: all.Observe}); err == nil || failed != nil {
+		t.Fatalf("failed branch=%#v, %v", failed, err)
+	}
+	validator, err := engine.Run(ctx, promptkit.RunRequest{PromptID: "prompt", Observer: selected.Observe})
+	if err != nil {
+		t.Fatal(err)
+	}
+	validatorID = validator.RunID
+	a, b := all.Snapshot(), selected.Snapshot()
+	if calls != 7 || len(a.Members) != 3 || a.Attempts != 6 || a.TransientRetries != 1 || a.RepairCalls != 2 || a.Usage.PromptTokens.Value != 14 || !a.Usage.PromptTokens.Known || !a.TerminalCoverageComplete {
+		t.Fatalf("all summary=%#v calls=%d", a, calls)
+	}
+	if len(b.Members) != 2 || b.Attempts != 4 || b.Usage.PromptTokens.Value != 13 || !b.TerminalCoverageComplete {
+		t.Fatalf("selected summary=%#v", b)
+	}
+	ids := map[string]bool{}
+	for _, member := range b.Members {
+		ids[member.RunID] = true
+	}
+	if !ids[branchID] || !ids[validatorID] || ids[initial.RunID] {
+		t.Fatalf("selected members=%#v", b.Members)
+	}
+}
+
+func TestExecutionGroupPreparedSilenceAndPanicCoverage(t *testing.T) {
+	engine := newSimpleEngine(t, "message", promptkit.WithLLMClient(&fakeLLMClient{response: &promptkit.GenerateResponse{Content: "ok"}}))
+	ctx := context.Background()
+	var group promptkit.ExecutionGroup
+	prepared, err := engine.PrepareExecution(ctx, promptkit.RunRequest{PromptID: "prompt", Observer: group.Observe})
+	if err != nil {
+		t.Fatal(err)
+	}
+	prepared.Discard()
+	_, _ = engine.RunPrepared(ctx, prepared)
+	if len(group.Snapshot().Members) != 0 {
+		t.Fatal("preparation or failed claim enrolled a run")
+	}
+	func() {
+		defer func() {
+			if recover() == nil {
+				t.Error("expected panic")
+			}
+		}()
+		_, _ = engine.Run(ctx, promptkit.RunRequest{PromptID: "prompt", Observer: promptkit.ComposeObservers(group.Observe, func(event promptkit.ExecutionEvent) {
+			_ = group.Snapshot()
+			if event.Kind == promptkit.ExecutionStarted {
+				panic("stop")
+			}
+		})})
+	}()
+	result := group.Snapshot()
+	if len(result.Members) != 1 || result.TerminalCoverageComplete || result.Members[0].TerminalObserved || result.Attempts != 0 {
+		t.Fatalf("panic summary=%#v", result)
+	}
+}
+
+func TestExecutionGroupPreparedAndCallbackMembership(t *testing.T) {
+	engine := newSimpleEngine(t, "message", promptkit.WithLLMClient(&fakeLLMClient{response: &promptkit.GenerateResponse{Content: "ok"}}))
+	ctx := context.Background()
+	var continuations, nested promptkit.ExecutionGroup
+	initial, err := engine.Run(ctx, promptkit.RunRequest{PromptID: "prompt", CaptureExchange: true})
+	if err != nil {
+		t.Fatal(err)
+	}
+	prepared, err := engine.PrepareExecution(ctx, promptkit.RunRequest{PromptID: "prompt", Observer: nested.Observe})
+	if err != nil {
+		t.Fatal(err)
+	}
+	preparedResult, err := engine.RunPrepared(ctx, prepared)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var callbackID string
+	_, err = engine.Coordinate(ctx, promptkit.CoordinateRequest{Start: initial.Exchange, AdditionalTurns: 1, Observer: continuations.Observe, Next: func(ctx context.Context, turn promptkit.ConversationTurn) (*promptkit.RunRequest, error) {
+		if turn.Result == nil {
+			return &promptkit.RunRequest{PromptID: "prompt"}, nil
+		}
+		callbackResult, err := engine.Run(ctx, promptkit.RunRequest{PromptID: "prompt", Observer: nested.Observe})
+		if err == nil {
+			callbackID = callbackResult.RunID
+		}
+		return nil, err
+	}})
+	if err != nil {
+		t.Fatal(err)
+	}
+	a, b := continuations.Snapshot(), nested.Snapshot()
+	if len(a.Members) != 1 || len(b.Members) != 2 || !a.TerminalCoverageComplete || !b.TerminalCoverageComplete {
+		t.Fatalf("summaries = %#v %#v", a, b)
+	}
+	ids := map[string]bool{}
+	for _, member := range b.Members {
+		ids[member.RunID] = true
+	}
+	if !ids[preparedResult.RunID] || !ids[callbackID] || ids[initial.RunID] {
+		t.Fatalf("members=%#v", b.Members)
+	}
+}

execution_timeout_contract_test.go

@@ -0,0 +1,100 @@
+package promptkit_test
+
+import (
+	"context"
+	"errors"
+	"testing"
+	"testing/synctest"
+	"time"
+
+	"go.maximumdirect.net/promptkit"
+)
+
+func TestRunOperationTimeoutAttribution(t *testing.T) {
+	synctest.Test(t, func(t *testing.T) {
+		engine := observationEngine(t, observationClientFunc(func(ctx context.Context, _ promptkit.GenerateRequest) (*promptkit.GenerateResponse, error) {
+			<-ctx.Done()
+			return nil, ctx.Err()
+		}))
+		var terminal promptkit.ExecutionEvent
+		_, err := engine.Run(context.Background(), promptkit.RunRequest{PromptID: "prompt", OperationTimeout: time.Second,
+			Observer: func(event promptkit.ExecutionEvent) {
+				if event.Kind == promptkit.ExecutionFinished {
+					terminal = event
+				}
+			}})
+		if !errors.Is(err, promptkit.ErrOperationTimeout) || !errors.Is(err, context.DeadlineExceeded) || !errors.Is(err, promptkit.ErrLLMGenerate) || terminal.Failure != promptkit.ExecutionFailureDeadline {
+			t.Fatalf("error=%v terminal=%+v", err, terminal)
+		}
+	})
+}
+
+func TestRunProviderDeadlineIsNotOperationTimeout(t *testing.T) {
+	engine := observationEngine(t, observationClientFunc(func(context.Context, promptkit.GenerateRequest) (*promptkit.GenerateResponse, error) {
+		return nil, context.DeadlineExceeded
+	}))
+	_, err := engine.Run(context.Background(), promptkit.RunRequest{PromptID: "prompt", OperationTimeout: time.Minute})
+	if !errors.Is(err, context.DeadlineExceeded) || errors.Is(err, promptkit.ErrOperationTimeout) {
+		t.Fatalf("provider deadline attribution: %v", err)
+	}
+}
+
+func TestPreparedExecutionRetainsTimeoutWithoutIdleTimer(t *testing.T) {
+	synctest.Test(t, func(t *testing.T) {
+		engine := observationEngine(t, observationClientFunc(func(ctx context.Context, _ promptkit.GenerateRequest) (*promptkit.GenerateResponse, error) {
+			<-ctx.Done()
+			return nil, ctx.Err()
+		}))
+		req := promptkit.RunRequest{PromptID: "prompt", OperationTimeout: time.Second}
+		handle, err := engine.PrepareExecution(context.Background(), req)
+		if err != nil {
+			t.Fatal(err)
+		}
+		req.OperationTimeout = 0
+		time.Sleep(2 * time.Second)
+		start := time.Now()
+		_, err = engine.RunPrepared(context.Background(), handle)
+		if time.Since(start) != time.Second || !errors.Is(err, promptkit.ErrOperationTimeout) || !errors.Is(err, context.DeadlineExceeded) || !errors.Is(err, promptkit.ErrLLMGenerate) {
+			t.Fatalf("elapsed=%v error=%v", time.Since(start), err)
+		}
+	})
+}
+
+func TestPreparedExecutionRespectsCallerDeadline(t *testing.T) {
+	synctest.Test(t, func(t *testing.T) {
+		engine := observationEngine(t, observationClientFunc(func(ctx context.Context, _ promptkit.GenerateRequest) (*promptkit.GenerateResponse, error) {
+			<-ctx.Done()
+			return nil, ctx.Err()
+		}))
+		ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
+		defer cancel()
+		handle, err := engine.PrepareExecution(ctx, promptkit.RunRequest{PromptID: "prompt", OperationTimeout: 5 * time.Second})
+		if err != nil {
+			t.Fatal(err)
+		}
+		time.Sleep(time.Second)
+		_, err = engine.RunPrepared(ctx, handle)
+		if !errors.Is(err, context.DeadlineExceeded) || errors.Is(err, promptkit.ErrOperationTimeout) {
+			t.Fatalf("caller deadline attribution: %v", err)
+		}
+	})
+}
+
+func TestPreparedExecutionDoesNotTimePreparation(t *testing.T) {
+	synctest.Test(t, func(t *testing.T) {
+		release := make(chan struct{})
+		reader := &capacityArtifactReader{entered: make(chan struct{}, 1), release: release}
+		engine := newBackendCapacityEngine(t, newCapacityGateClient(closedCapacityChannel(), 1), 1, capacityInt(0), reader)
+		req := capacityInputRequest("http://example.invalid/v1")
+		req.OperationTimeout = time.Second
+		go func() { time.Sleep(2 * time.Second); close(release) }()
+		start := time.Now()
+		handle, err := engine.PrepareExecution(context.Background(), req)
+		if err != nil || time.Since(start) != 2*time.Second {
+			t.Fatalf("prepare elapsed=%v error=%v", time.Since(start), err)
+		}
+		if _, err = engine.RunPrepared(context.Background(), handle); err != nil {
+			t.Fatalf("execution after preparation: %v", err)
+		}
+	})
+}

generation_error_contract_test.go

@@ -78,6 +78,7 @@ func TestBuiltInRepairGenerationError(t *testing.T) {
 		t.Fatalf("NewEngine: %v", err)
 	}
 	req := simpleGenerationRequest()
+	req.CaptureExchange = true
 	req.Validation = &promptkit.OutputContract{
 		Format:         promptkit.FormatJSON,
 		ValidationMode: promptkit.ValidationJSON,

go.mod

@@ -4,7 +4,7 @@ go 1.27.0
 
 require (
 	github.com/santhosh-tekuri/jsonschema/v6 v6.0.2
-	go.maximumdirect.net/configkit v1.1.0
+	go.maximumdirect.net/configkit v1.2.0
 	go.yaml.in/yaml/v4 v4.0.0-rc.6
 )
 

go.sum

@@ -4,8 +4,8 @@ github.com/santhosh-tekuri/jsonschema/v6 v6.0.2 h1:KRzFb2m7YtdldCEkzs6KqmJw4nqEV
 github.com/santhosh-tekuri/jsonschema/v6 v6.0.2/go.mod h1:JXeL+ps8p7/KNMjDQk3TCwPpBy0wYklyWTfbkIzdIFU=
 github.com/yuin/goldmark/v2 v2.1.6 h1:igy0q9JcqovoFXFx/XjtU8sHVhcUHavPj2sA3EoKJUE=
 github.com/yuin/goldmark/v2 v2.1.6/go.mod h1:G6M4/qOFtfn01/o14BU1UR2Lo5N3S9Qo7xCuz/sHjGQ=
-go.maximumdirect.net/configkit v1.1.0 h1:UzXN2ZMnEkcL0O4YSLqD9qd9M1gb/NWCHQR/H8AaQzA=
-go.maximumdirect.net/configkit v1.1.0/go.mod h1:s1zLJ9GGB/1Q4a+aOHMLISonJxWFgNWpA6SpPlOafeA=
+go.maximumdirect.net/configkit v1.2.0 h1:c4tDJ6Xeh/Jg90UZplLxsnf500lvrZBVj0E3i1ysir8=
+go.maximumdirect.net/configkit v1.2.0/go.mod h1:s1zLJ9GGB/1Q4a+aOHMLISonJxWFgNWpA6SpPlOafeA=
 go.maximumdirect.net/releasekit v0.9.0 h1:sK+Aoj6gS6ATU/1reocjwBekp4YETYENwkcX5ti6mrE=
 go.maximumdirect.net/releasekit v0.9.0/go.mod h1:QE4BLrAhVJTzi+/YTcVl21K/qZp9ojRLg5kuoy3Zb44=
 go.yaml.in/yaml/v4 v4.0.0-rc.6 h1:1h7H1ohdUh93/FyE4YaDa1Zh64K6VVbjF4K6WUxMtH4=

internal/domain/domain.go

@@ -1,6 +1,7 @@
 package domain
 
 import (
+	"context"
 	"time"
 )
 
@@ -60,6 +61,11 @@ type CacheControl struct {
 
 // RunRequest represents a request to generate a single artifact.
 type RunRequest struct {
+	OperationTimeout time.Duration
+	// OnPrepared receives borrowed, read-only internal state.
+	OnPrepared       func(context.Context, *PreparedRun)
+	CaptureExchange  bool
+	ContinueFrom     *Exchange
 	PromptID         string
 	ProfileID        string
 	SessionID        string
@@ -75,6 +81,8 @@ type RunRequest struct {
 
 // RunResult represents the complete result of a prompt execution run.
 type RunResult struct {
+	Exchange             *Exchange
+	ParentRunID          string
 	RunID                string
 	Artifact             Artifact
 	RawOutput            string
@@ -98,6 +106,7 @@ type RunResult struct {
 // PreparedRun contains pre-LLM execution state from the prepare/render phase.
 // It must never include resolved API key values, model output, or validation data.
 type PreparedRun struct {
+	ParentRunID          string
 	PromptID             string
 	PromptHash           string
 	SelectedProfileID    string

internal/domain/exchange.go

@@ -0,0 +1,92 @@
+package domain
+
+// Exchange is an immutable logical conversation ending in an assistant message.
+type Exchange struct {
+	parent      *Exchange
+	runID       string
+	parentRunID string
+	messages    []RenderedMessage
+	count       int
+}
+
+// CaptureExchange creates a new exchange without modifying its parent.
+func CaptureExchange(parent *Exchange, runID string, messages []RenderedMessage, output string) *Exchange {
+	turn := make([]RenderedMessage, len(messages)+1)
+	copyRenderedMessages(turn, messages)
+	turn[len(messages)] = RenderedMessage{Role: RoleAssistant, Content: output}
+	exchange := &Exchange{parent: parent, runID: runID, messages: turn, count: len(turn)}
+	if parent != nil {
+		exchange.parentRunID = parent.runID
+		exchange.count += parent.count
+	}
+	return exchange
+}
+
+// RunID returns the identity of the run that produced this exchange.
+func (e *Exchange) RunID() string {
+	if e == nil {
+		return ""
+	}
+	return e.runID
+}
+
+// ParentRunID returns the immediate source run identity, if present.
+func (e *Exchange) ParentRunID() string {
+	if e == nil {
+		return ""
+	}
+	return e.parentRunID
+}
+
+// MessageCount returns the number of messages in the logical history.
+func (e *Exchange) MessageCount() int {
+	if e == nil {
+		return 0
+	}
+	return e.count
+}
+
+// Messages returns a detached copy of the complete logical history.
+func (e *Exchange) Messages() []RenderedMessage {
+	if e == nil {
+		return nil
+	}
+	return e.MessagesWithSuffix()
+}
+
+// MessagesWithSuffix returns independently owned history followed by each part.
+// A nil exchange supplies no history.
+func (e *Exchange) MessagesWithSuffix(parts ...[]RenderedMessage) []RenderedMessage {
+	count := e.MessageCount()
+	for _, part := range parts {
+		count += len(part)
+	}
+	messages := make([]RenderedMessage, count)
+	e.visitChunks(func(start int, chunk []RenderedMessage) {
+		copyRenderedMessages(messages[start:], chunk)
+	})
+	offset := e.MessageCount()
+	for _, part := range parts {
+		copyRenderedMessages(messages[offset:], part)
+		offset += len(part)
+	}
+	return messages
+}
+
+// VisitMessages visits indexed history entries without materializing a slice.
+// Entries and their cache metadata are borrowed read-only values. Visits need
+// not be in index order; callers can fill a destination using the supplied index.
+func (e *Exchange) VisitMessages(visit func(int, RenderedMessage)) {
+	e.visitChunks(func(start int, chunk []RenderedMessage) {
+		for index, message := range chunk {
+			visit(start+index, message)
+		}
+	})
+}
+
+func (e *Exchange) visitChunks(visit func(int, []RenderedMessage)) {
+	for current := e; current != nil; current = current.parent {
+		start := current.count - len(current.messages)
+		visit(start, current.messages)
+	}
+}

internal/domain/exchange_test.go

@@ -0,0 +1,75 @@
+package domain
+
+import (
+	"fmt"
+	"reflect"
+	"testing"
+)
+
+func TestExchangeHistoryOwnershipAndBranches(t *testing.T) {
+	control := &CacheControl{Type: CacheControlEphemeral, TTL: "1h"}
+	source := []RenderedMessage{{Role: RoleSystem, Content: "", CacheControl: control}, {Role: RoleUser, Content: "question"}}
+	initial := CaptureExchange(nil, "first", source, " answer ")
+	first := CaptureExchange(initial, "second", []RenderedMessage{{Role: RoleUser, Content: "follow-up"}}, "reply")
+	second := CaptureExchange(initial, "third", []RenderedMessage{{Role: RoleDeveloper, Content: "other"}}, "")
+	source[0].Content = "changed"
+	control.TTL = "changed"
+	got := initial.Messages()
+	want := []RenderedMessage{{Role: RoleSystem, Content: "", CacheControl: &CacheControl{Type: CacheControlEphemeral, TTL: "1h"}}, {Role: RoleUser, Content: "question"}, {Role: RoleAssistant, Content: " answer "}}
+	if !reflect.DeepEqual(got, want) {
+		t.Fatalf("initial messages = %#v, want %#v", got, want)
+	}
+	got[0].CacheControl.TTL = "mutated"
+	got[1].Content = "mutated"
+	if !reflect.DeepEqual(initial.Messages(), want) {
+		t.Fatal("returned messages changed retained history")
+	}
+	if first.RunID() != "second" || first.ParentRunID() != "first" || first.MessageCount() != 5 {
+		t.Fatal("incorrect branch metadata")
+	}
+	if second.ParentRunID() != "first" || second.Messages()[3].Content != "other" || second.Messages()[4].Content != "" {
+		t.Fatal("incorrect sibling history")
+	}
+	if first.Messages()[3].Content != "follow-up" || !reflect.DeepEqual(initial.Messages(), want) {
+		t.Fatal("branches changed each other or their parent")
+	}
+}
+
+func TestExchangeManyTurns(t *testing.T) {
+	var exchange *Exchange
+	const turns = 2000
+	for i := 0; i < turns; i++ {
+		exchange = CaptureExchange(exchange, fmt.Sprint(i), []RenderedMessage{{Role: RoleUser, Content: fmt.Sprint(i)}}, fmt.Sprint(i))
+	}
+	messages := exchange.Messages()
+	if len(messages) != turns*2 || exchange.MessageCount() != len(messages) {
+		t.Fatalf("unexpected history length: %d", len(messages))
+	}
+	for i := 0; i < turns; i++ {
+		if messages[2*i].Content != fmt.Sprint(i) || messages[2*i+1].Role != RoleAssistant || messages[2*i+1].Content != fmt.Sprint(i) {
+			t.Fatalf("unexpected turn %d: %#v", i, messages[2*i:2*i+2])
+		}
+	}
+}
+
+func TestExchangeMessagesWithSuffix(t *testing.T) {
+	for _, history := range []*Exchange{nil, CaptureExchange(nil, "source", nil, "answer")} {
+		control := &CacheControl{Type: CacheControlEphemeral, TTL: "1h"}
+		first := []RenderedMessage{{Role: RoleUser, Content: "new", CacheControl: control}}
+		last := []RenderedMessage{{Role: RoleDeveloper, Content: "suffix"}}
+		got := history.MessagesWithSuffix(first, nil, last)
+		want := append(history.Messages(), first...)
+		want = append(want, last...)
+		if !reflect.DeepEqual(got, want) {
+			t.Fatalf("messages = %#v, want %#v", got, want)
+		}
+		got[history.MessageCount()].CacheControl.TTL = "changed"
+		got[history.MessageCount()].Content = "changed"
+		if first[0].Content != "new" || control.TTL != "1h" {
+			t.Fatal("assembled messages alias the caller suffix")
+		}
+		if history != nil && history.Messages()[0].Content != "answer" {
+			t.Fatal("assembly changed retained history")
+		}
+	}
+}

internal/domain/observation.go

@@ -61,11 +61,28 @@ type ObservedTokenUsage struct {
 	CacheWriteTokens ObservedTokenCount
 }
 
+// ExecutionGroupMember identifies an observed run and terminal coverage.
+type ExecutionGroupMember struct {
+	RunID            string
+	TerminalObserved bool
+}
+
+// ExecutionGroupSummary contains terminal-only aggregate execution facts.
+type ExecutionGroupSummary struct {
+	Members                  []ExecutionGroupMember
+	TerminalCoverageComplete bool
+	Attempts                 uint64
+	TransientRetries         uint64
+	RepairCalls              uint64
+	Usage                    ObservedTokenUsage
+}
+
 // ExecutionEvent contains metadata only; it carries no mutable containers or
 // error values. Nonterminal events leave aggregate counters at zero.
 type ExecutionEvent struct {
 	Kind               ExecutionEventKind
 	RunID              string
+	ParentRunID        string
 	Time               time.Time
 	PromptID           string
 	ProfileID          string

internal/observation/compose.go

@@ -0,0 +1,30 @@
+package observation
+
+// Compose copies and synchronously invokes non-nil observers in order.
+func Compose[T any, F ~func(T)](observers ...F) F {
+	count := 0
+	var single F
+	for _, observer := range observers {
+		if observer != nil {
+			count++
+			single = observer
+		}
+	}
+	if count == 0 {
+		return nil
+	}
+	if count == 1 {
+		return single
+	}
+	selected := make([]F, 0, count)
+	for _, observer := range observers {
+		if observer != nil {
+			selected = append(selected, observer)
+		}
+	}
+	return func(event T) {
+		for _, observer := range selected {
+			observer(event)
+		}
+	}
+}

internal/observation/group.go

@@ -0,0 +1,73 @@
+package observation
+
+import (
+	"sync"
+
+	"go.maximumdirect.net/promptkit/internal/domain"
+)
+
+// Group incrementally collects terminal facts for explicitly observed runs.
+// Its zero value is ready; it must not be copied after first use.
+type Group struct {
+	mu                         sync.Mutex
+	members                    map[string]bool
+	terminals                  int
+	attempts, retries, repairs uint64
+	usage                      domain.ObservedTokenUsage
+}
+
+// Observe enrolls a run and incorporates its first terminal event only.
+func (g *Group) Observe(event domain.ExecutionEvent) {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	if g.members == nil {
+		g.members = make(map[string]bool)
+	}
+	terminal := g.members[event.RunID]
+	if _, exists := g.members[event.RunID]; !exists {
+		g.members[event.RunID] = false
+	}
+	if event.Kind != domain.ExecutionFinished || terminal {
+		return
+	}
+	g.members[event.RunID] = true
+	g.attempts += event.Attempts
+	g.retries += event.TransientRetries
+	g.repairs += event.RepairCalls
+	if g.terminals == 0 {
+		complete := domain.ObservedTokenCount{Complete: true}
+		g.usage = domain.ObservedTokenUsage{PromptTokens: complete, CompletionTokens: complete, TotalTokens: complete, CachedTokens: complete, CacheWriteTokens: complete}
+	}
+	mergeGroupUsage(&g.usage, event.Usage)
+	g.terminals++
+}
+
+// Snapshot returns a detached consistent view of currently observed members.
+func (g *Group) Snapshot() domain.ExecutionGroupSummary {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	result := domain.ExecutionGroupSummary{Members: make([]domain.ExecutionGroupMember, 0, len(g.members)), TerminalCoverageComplete: len(g.members) == g.terminals, Attempts: g.attempts, TransientRetries: g.retries, RepairCalls: g.repairs, Usage: g.usage}
+	for id, terminal := range g.members {
+		result.Members = append(result.Members, domain.ExecutionGroupMember{RunID: id, TerminalObserved: terminal})
+	}
+	if len(g.members) == 0 {
+		zero := domain.ObservedTokenCount{Known: true, Complete: true}
+		result.Usage = domain.ObservedTokenUsage{PromptTokens: zero, CompletionTokens: zero, TotalTokens: zero, CachedTokens: zero, CacheWriteTokens: zero}
+	}
+	return result
+}
+
+func mergeGroupUsage(total *domain.ObservedTokenUsage, next domain.ObservedTokenUsage) {
+	merge := func(a *domain.ObservedTokenCount, b domain.ObservedTokenCount) {
+		if b.Known {
+			a.Value += b.Value
+		}
+		a.Known = a.Known || b.Known
+		a.Complete = a.Complete && b.Complete
+	}
+	merge(&total.PromptTokens, next.PromptTokens)
+	merge(&total.CompletionTokens, next.CompletionTokens)
+	merge(&total.TotalTokens, next.TotalTokens)
+	merge(&total.CachedTokens, next.CachedTokens)
+	merge(&total.CacheWriteTokens, next.CacheWriteTokens)
+}

internal/observation/group_test.go

@@ -0,0 +1,77 @@
+package observation
+
+import (
+	"fmt"
+	"sync"
+	"testing"
+
+	"go.maximumdirect.net/promptkit/internal/domain"
+)
+
+func TestGroupTerminalAccounting(t *testing.T) {
+	var group Group
+	empty := group.Snapshot()
+	if !empty.TerminalCoverageComplete || !empty.Usage.PromptTokens.Known || !empty.Usage.PromptTokens.Complete || empty.Usage.PromptTokens.Value != 0 {
+		t.Fatalf("empty = %#v", empty)
+	}
+	group.Observe(domain.ExecutionEvent{Kind: domain.ExecutionAttempt, RunID: "pending", Attempts: 99})
+	pending := group.Snapshot()
+	if pending.TerminalCoverageComplete || pending.Attempts != 0 || pending.Usage.PromptTokens.Known || pending.Usage.PromptTokens.Complete {
+		t.Fatalf("pending = %#v", pending)
+	}
+	zero := domain.ObservedTokenCount{Known: true, Complete: true}
+	known := domain.ObservedTokenCount{Value: 4, Known: true, Complete: true}
+	partial := domain.ObservedTokenCount{Value: 2, Known: true, Complete: false}
+	group.Observe(domain.ExecutionEvent{Kind: domain.ExecutionFinished, RunID: "first", Outcome: domain.ExecutionOutcomeError, Attempts: 2, TransientRetries: 1, RepairCalls: 1, Usage: domain.ObservedTokenUsage{PromptTokens: known, CompletionTokens: zero, TotalTokens: partial, CachedTokens: zero}})
+	group.Observe(domain.ExecutionEvent{Kind: domain.ExecutionFinished, RunID: "first", Attempts: 100})
+	group.Observe(domain.ExecutionEvent{Kind: domain.ExecutionStarted, RunID: "first"})
+	group.Observe(domain.ExecutionEvent{Kind: domain.ExecutionFinished, RunID: "pending", Attempts: 3, Usage: domain.ObservedTokenUsage{PromptTokens: partial, CompletionTokens: zero, TotalTokens: domain.ObservedTokenCount{}, CachedTokens: zero}})
+	got := group.Snapshot()
+	if !got.TerminalCoverageComplete || len(got.Members) != 2 || got.Attempts != 5 || got.TransientRetries != 1 || got.RepairCalls != 1 {
+		t.Fatalf("summary = %#v", got)
+	}
+	if got.Usage.PromptTokens != (domain.ObservedTokenCount{Value: 6, Known: true}) || got.Usage.CompletionTokens != zero || got.Usage.TotalTokens != partial || got.Usage.CachedTokens != zero || got.Usage.CacheWriteTokens != (domain.ObservedTokenCount{}) {
+		t.Fatalf("usage = %#v", got.Usage)
+	}
+	got.Members[0].RunID = "changed"
+	for _, member := range group.Snapshot().Members {
+		if member.RunID == "changed" || !member.TerminalObserved {
+			t.Fatalf("member = %#v", member)
+		}
+	}
+}
+
+func TestGroupConcurrentSnapshots(t *testing.T) {
+	var group Group
+	var wg sync.WaitGroup
+	const count = 100
+	for i := 0; i < count; i++ {
+		wg.Add(1)
+		go func(i int) {
+			defer wg.Done()
+			id := fmt.Sprint(i)
+			group.Observe(domain.ExecutionEvent{RunID: id, Kind: domain.ExecutionStarted})
+			group.Observe(domain.ExecutionEvent{RunID: id, Kind: domain.ExecutionFinished, Attempts: 1})
+		}(i)
+	}
+	for i := 0; i < count; i++ {
+		snapshot := group.Snapshot()
+		seen, terminals := map[string]bool{}, 0
+		for _, member := range snapshot.Members {
+			if seen[member.RunID] {
+				t.Fatal("duplicate member")
+			}
+			seen[member.RunID] = true
+			if member.TerminalObserved {
+				terminals++
+			}
+		}
+		if snapshot.Attempts != uint64(terminals) || snapshot.TerminalCoverageComplete != (terminals == len(snapshot.Members)) {
+			t.Fatalf("inconsistent snapshot = %#v", snapshot)
+		}
+	}
+	wg.Wait()
+	if final := group.Snapshot(); len(final.Members) != count || final.Attempts != count || !final.TerminalCoverageComplete {
+		t.Fatalf("final = %#v", final)
+	}
+}

internal/observation/recorder.go

@@ -16,6 +16,7 @@ import (
 // calls methods in execution order; it has no shared history or background work.
 type Recorder struct {
 	runID                string
+	parentRunID          string
 	startedAt            time.Time
 	now                  func() time.Time
 	observer             domain.ExecutionObserver
@@ -95,6 +96,9 @@ func NewRunID() (string, error) {
 // RunID returns this invocation's identity.
 func (r *Recorder) RunID() string { return r.runID }
 
+// SetParentRunID records the immediate source identity before start delivery.
+func (r *Recorder) SetParentRunID(id string) { r.parentRunID = id }
+
 // Start delivers the opening event with only identities already resolved.
 func (r *Recorder) Start() {
 	event := r.event(domain.ExecutionStarted)
@@ -177,7 +181,7 @@ func (r *Recorder) Finish(data FinishData) {
 
 func (r *Recorder) event(kind domain.ExecutionEventKind) domain.ExecutionEvent {
 	return domain.ExecutionEvent{
-		Kind: kind, RunID: r.runID, Time: r.now(), PromptID: r.selection.PromptID,
+		Kind: kind, RunID: r.runID, ParentRunID: r.parentRunID, Time: r.now(), PromptID: r.selection.PromptID,
 		ProfileID: r.selection.ProfileID,
 		BackendID: r.selection.BackendID, Model: r.selection.Model,
 	}

internal/usecase/coordinate.go

@@ -0,0 +1,66 @@
+package usecase
+
+import (
+	"context"
+
+	"go.maximumdirect.net/promptkit/internal/domain"
+)
+
+// ConversationTurn is one delivery to a coordinator callback.
+type ConversationTurn[R any] struct {
+	Exchange       *domain.Exchange
+	Result         *R
+	RemainingTurns int
+}
+
+// CoordinationOutcome describes a normal coordinator termination.
+type CoordinationOutcome string
+
+const (
+	CoordinationStopped   CoordinationOutcome = "stopped"
+	CoordinationExhausted CoordinationOutcome = "exhausted"
+)
+
+// CoordinationResult holds a normal outcome or a completed undelivered result.
+type CoordinationResult[R any] struct {
+	Outcome     CoordinationOutcome
+	Undelivered *R
+}
+
+// Coordinate advances one branch using the supplied callback and ordinary run.
+func Coordinate[Q, R any](ctx context.Context, start *domain.Exchange, turns int,
+	next func(context.Context, ConversationTurn[R]) (*Q, error),
+	run func(context.Context, *domain.Exchange, *Q) (*R, *domain.Exchange, error),
+) (CoordinationResult[R], error) {
+	current := start
+	var pending *R
+	for {
+		if err := ctx.Err(); err != nil {
+			return CoordinationResult[R]{Undelivered: pending}, err
+		}
+		request, err := next(ctx, ConversationTurn[R]{Exchange: current, Result: pending, RemainingTurns: turns})
+		pending = nil
+		if contextErr := ctx.Err(); contextErr != nil {
+			return CoordinationResult[R]{}, contextErr
+		}
+		if err != nil {
+			return CoordinationResult[R]{}, err
+		}
+		if request == nil {
+			return CoordinationResult[R]{Outcome: CoordinationStopped}, nil
+		}
+		if turns == 0 {
+			return CoordinationResult[R]{Outcome: CoordinationExhausted}, nil
+		}
+		if err := ctx.Err(); err != nil {
+			return CoordinationResult[R]{}, err
+		}
+		turns--
+		var nextExchange *domain.Exchange
+		pending, nextExchange, err = run(ctx, current, request)
+		if err != nil {
+			return CoordinationResult[R]{}, err
+		}
+		current = nextExchange
+	}
+}

internal/usecase/coordinate_test.go

@@ -0,0 +1,109 @@
+package usecase
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"testing"
+
+	"go.maximumdirect.net/promptkit/internal/domain"
+)
+
+func TestCoordinateBudgetAndDelivery(t *testing.T) {
+	for _, tc := range []struct {
+		name            string
+		budget          int
+		stop            bool
+		outcome         CoordinationOutcome
+		runs, callbacks int
+	}{
+		{"zero stop", 0, true, CoordinationStopped, 0, 1},
+		{"zero exhausted", 0, false, CoordinationExhausted, 0, 1},
+		{"stop after last", 2, true, CoordinationStopped, 2, 3},
+		{"request after last", 2, false, CoordinationExhausted, 2, 3},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			start := domain.CaptureExchange(nil, "initial", nil, "start")
+			runs, callbacks := 0, 0
+			result, err := Coordinate(context.Background(), start, tc.budget, func(_ context.Context, turn ConversationTurn[string]) (*string, error) {
+				callbacks++
+				if turn.RemainingTurns != tc.budget-runs || (runs == 0 && turn.Result != nil) || (runs > 0 && (turn.Result == nil || *turn.Result != fmt.Sprint(runs))) {
+					t.Fatalf("unexpected turn: %#v", turn)
+				}
+				if tc.stop && turn.RemainingTurns == 0 {
+					return nil, nil
+				}
+				value := "next"
+				return &value, nil
+			}, func(_ context.Context, current *domain.Exchange, _ *string) (*string, *domain.Exchange, error) {
+				runs++
+				value := fmt.Sprint(runs)
+				return &value, domain.CaptureExchange(current, value, nil, value), nil
+			})
+			if err != nil || result.Outcome != tc.outcome || result.Undelivered != nil || runs != tc.runs || callbacks != tc.callbacks {
+				t.Fatalf("result=%#v error=%v runs=%d callbacks=%d", result, err, runs, callbacks)
+			}
+		})
+	}
+}
+
+func TestCoordinateCancellationAndErrors(t *testing.T) {
+	start := domain.CaptureExchange(nil, "initial", nil, "start")
+	request := "continue"
+	for _, mode := range []string{"before", "after run", "inside callback", "callback error", "run error"} {
+		t.Run(mode, func(t *testing.T) {
+			ctx, cancel := context.WithCancel(context.Background())
+			defer cancel()
+			sentinel := errors.New("sentinel")
+			calls := 0
+			if mode == "before" {
+				cancel()
+			}
+			result, err := Coordinate(ctx, start, 1, func(_ context.Context, turn ConversationTurn[string]) (*string, error) {
+				calls++
+				if mode == "inside callback" && turn.Result != nil {
+					cancel()
+					return nil, sentinel
+				}
+				if mode == "callback error" {
+					return nil, fmt.Errorf("callback: %w", sentinel)
+				}
+				return &request, nil
+			}, func(_ context.Context, current *domain.Exchange, _ *string) (*string, *domain.Exchange, error) {
+				if mode == "run error" {
+					return nil, nil, fmt.Errorf("run: %w", sentinel)
+				}
+				value := "response"
+				if mode == "after run" {
+					cancel()
+				}
+				return &value, domain.CaptureExchange(current, "next", nil, value), nil
+			})
+			if mode == "before" || mode == "after run" || mode == "inside callback" {
+				if !errors.Is(err, context.Canceled) {
+					t.Fatalf("error = %v", err)
+				}
+			} else if !errors.Is(err, sentinel) {
+				t.Fatalf("error = %v", err)
+			}
+			if result.Outcome != "" || (mode == "after run") != (result.Undelivered != nil) {
+				t.Fatalf("result = %#v", result)
+			}
+			if mode == "before" && calls != 0 {
+				t.Fatal("callback invoked after cancellation")
+			}
+			if mode == "inside callback" && calls != 2 {
+				t.Fatalf("callbacks = %d", calls)
+			}
+		})
+	}
+}
+
+func TestCoordinatePropagatesPanic(t *testing.T) {
+	defer func() {
+		if recover() != "callback panic" {
+			t.Error("panic did not propagate")
+		}
+	}()
+	Coordinate[string, string](context.Background(), nil, 0, func(context.Context, ConversationTurn[string]) (*string, error) { panic("callback panic") }, nil)
+}

internal/usecase/operation_budget_test.go

@@ -0,0 +1,54 @@
+package usecase
+
+import (
+	"context"
+	"errors"
+	"testing"
+	"testing/synctest"
+	"time"
+
+	"go.maximumdirect.net/promptkit/internal/domain"
+	"go.maximumdirect.net/promptkit/internal/validate"
+)
+
+type deadlineValidation struct{ deadline time.Time }
+
+func (v *deadlineValidation) PrepareValidation(ctx context.Context, _ domain.OutputContract) (validate.PreparedValidation, error) {
+	v.deadline, _ = ctx.Deadline()
+	return v, nil
+}
+func (v *deadlineValidation) Validate(ctx context.Context, _ *domain.Artifact) (domain.ValidationResult, error) {
+	deadline, _ := ctx.Deadline()
+	if deadline != v.deadline {
+		return domain.ValidationResult{}, errors.New("validation deadline changed")
+	}
+	<-ctx.Done()
+	return domain.ValidationResult{}, ctx.Err()
+}
+func (*deadlineValidation) SchemaDocument() any { return nil }
+
+type deadlineModel struct{ validation *deadlineValidation }
+
+func (m deadlineModel) Generate(ctx context.Context, _ domain.GenerateRequest) (*domain.GenerateResponse, error) {
+	deadline, _ := ctx.Deadline()
+	if deadline != m.validation.deadline {
+		return nil, errors.New("generation deadline changed")
+	}
+	return &domain.GenerateResponse{Content: "ok"}, nil
+}
+
+func TestOperationDeadlineReachesPreparationGenerationAndValidation(t *testing.T) {
+	synctest.Test(t, func(t *testing.T) {
+		validation := &deadlineValidation{}
+		runner := NewRunner(&fakePromptRepo{def: promptDef(domain.FormatText, domain.ValidationBasic, 0)},
+			&fakeExecutionProfileRepo{profiles: map[string]*domain.ExecutionProfile{"exec": defaultExecutionProfile()}},
+			defaultArtifactReader(), defaultRenderer(), deadlineModel{validation}, validation, nil)
+		scope := NewOperationScope(context.Background(), time.Second)
+		defer scope.Close()
+		_, err := runner.Run(scope.Context, domain.RunRequest{PromptID: "p", ProfileID: "exec", Inputs: singleInputRef()})
+		err = scope.Finalize(err)
+		if !errors.Is(err, ErrOperationTimeout) || !errors.Is(err, ErrValidation) || !errors.Is(err, context.DeadlineExceeded) {
+			t.Fatalf("validation error=%v", err)
+		}
+	})
+}

internal/usecase/operation_scope.go

@@ -0,0 +1,36 @@
+package usecase
+
+import (
+	"context"
+	"errors"
+	"time"
+)
+
+// OperationScope owns an optional deadline and its invocation-specific cause.
+type OperationScope struct {
+	Context context.Context
+	cancel  context.CancelFunc
+	cause   error
+}
+
+// NewOperationScope creates a child only when the selected duration is positive.
+func NewOperationScope(parent context.Context, duration time.Duration) *OperationScope {
+	scope := &OperationScope{Context: parent, cancel: func() {}}
+	if duration > 0 {
+		scope.cause = errors.New("operation deadline")
+		scope.Context, scope.cancel = context.WithTimeoutCause(parent, duration, scope.cause)
+	}
+	return scope
+}
+
+// Finalize preserves the failure and marks it only when this scope expired.
+func (s *OperationScope) Finalize(err error) error {
+	if err != nil && s.cause != nil && s.Context.Err() == context.DeadlineExceeded &&
+		errors.Is(err, s.Context.Err()) && context.Cause(s.Context) == s.cause {
+		return &OperationError{Category: ErrOperationTimeout, Cause: err}
+	}
+	return err
+}
+
+// Close releases the timer and associated child context.
+func (s *OperationScope) Close() { s.cancel() }

internal/usecase/operation_scope_test.go

@@ -0,0 +1,52 @@
+package usecase
+
+import (
+	"context"
+	"errors"
+	"testing"
+	"testing/synctest"
+	"time"
+)
+
+func TestOperationScopeAttribution(t *testing.T) {
+	synctest.Test(t, func(t *testing.T) {
+		parent := context.Background()
+		disabled := NewOperationScope(parent, 0)
+		defer disabled.Close()
+		if disabled.Context != parent || disabled.Finalize(context.DeadlineExceeded) != context.DeadlineExceeded {
+			t.Fatal("disabled scope changed context or error")
+		}
+		outer := NewOperationScope(parent, time.Second)
+		defer outer.Close()
+		inner := NewOperationScope(outer.Context, 2*time.Second)
+		defer inner.Close()
+		time.Sleep(time.Second)
+		synctest.Wait()
+		if !errors.Is(outer.Finalize(context.DeadlineExceeded), ErrOperationTimeout) {
+			t.Fatal("own deadline not attributed")
+		}
+		if errors.Is(inner.Finalize(context.DeadlineExceeded), ErrOperationTimeout) {
+			t.Fatal("inherited scope misattributed")
+		}
+		other := errors.New("other")
+		if outer.Finalize(other) != other || outer.Finalize(nil) != nil {
+			t.Fatal("unrelated failure or completion changed")
+		}
+		canceled, cancel := context.WithCancel(parent)
+		child := NewOperationScope(canceled, time.Second)
+		defer child.Close()
+		cancel()
+		if errors.Is(child.Finalize(context.Canceled), ErrOperationTimeout) {
+			t.Fatal("parent cancellation misattributed")
+		}
+		deadline, stop := context.WithTimeout(parent, time.Second)
+		defer stop()
+		child = NewOperationScope(deadline, 2*time.Second)
+		defer child.Close()
+		time.Sleep(time.Second)
+		synctest.Wait()
+		if errors.Is(child.Finalize(context.DeadlineExceeded), ErrOperationTimeout) {
+			t.Fatal("parent deadline misattributed")
+		}
+	})
+}

internal/usecase/prepared_execution.go

@@ -29,7 +29,11 @@ type PreparedExecution struct {
 }
 
 type preparedExecutionPayload struct {
+	operationTimeout time.Duration
+	onPrepared       func(context.Context, *domain.PreparedRun)
 	prepared         *domain.PreparedRun
+	capture          bool
+	parent           *domain.Exchange
 	validation       validate.PreparedValidation
 	directKey        string
 	transientRetries uint64
@@ -54,7 +58,11 @@ func (r *Runner) PrepareExecution(ctx context.Context, req domain.RunRequest) (*
 		state:   preparedExecutionReady,
 		details: operation.run,
 		payload: &preparedExecutionPayload{
+			operationTimeout: req.OperationTimeout,
+			onPrepared:       req.OnPrepared,
 			prepared:         operation.run,
+			capture:          operation.capture,
+			parent:           operation.parent,
 			validation:       operation.validation,
 			directKey:        state.effectiveModel.APIKey,
 			transientRetries: state.transientRetries,
@@ -96,18 +104,23 @@ func (p *PreparedExecution) Discard() {
 }
 
 // RunPrepared claims and executes one prepared execution owned by this runner.
-func (r *Runner) RunPrepared(ctx context.Context, prepared *PreparedExecution) (*domain.RunResult, error) {
+func (r *Runner) RunPrepared(ctx context.Context, prepared *PreparedExecution) (result *domain.RunResult, err error) {
 	payload, err := prepared.claim(r)
 	if err != nil {
 		return nil, err
 	}
 	defer payload.clear()
+	scope := NewOperationScope(ctx, payload.operationTimeout)
+	defer scope.Close()
+	defer func() { err = scope.Finalize(err) }()
+	ctx = scope.Context
 
 	var recorder *observation.Recorder
 	var runID string
 	if payload.observer != nil {
 		var idErr error
 		recorder, idErr = observation.New(payload.observer)
+		recorder.SetParentRunID(payload.parent.RunID())
 		observation.RegisterClaim(ctx, recorder)
 		ctx = observation.WithRecorder(ctx, recorder)
 		selected := payload.prepared
@@ -139,7 +152,7 @@ func (r *Runner) RunPrepared(ctx context.Context, prepared *PreparedExecution) (
 	}
 	defer release()
 
-	return r.executePreparedRun(ctx, payload.prepared, payload.directKey, payload.transientRetries, runID, start, func(
+	return r.executePreparedRun(ctx, payload.prepared, payload.onPrepared, payload.capture, payload.parent, payload.directKey, payload.transientRetries, runID, start, func(
 		ctx context.Context,
 		artifact *domain.Artifact,
 		attemptsUsed int,
@@ -174,10 +187,14 @@ func (p *preparedExecutionPayload) clear() {
 		return
 	}
 	p.prepared = nil
+	p.capture = false
+	p.parent = nil
 	p.validation = nil
 	p.directKey = ""
 	p.transientRetries = 0
 	p.observer = nil
+	p.operationTimeout = 0
+	p.onPrepared = nil
 }
 
 type noOpPreparedValidation struct {

internal/usecase/prepared_notification_test.go

@@ -0,0 +1,42 @@
+package usecase
+
+import (
+	"context"
+	"errors"
+	"testing"
+	"time"
+
+	"go.maximumdirect.net/promptkit/internal/domain"
+)
+
+func TestPreparedNotificationChecksContextBeforeInvocation(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+	called := false
+	_, err := (&Runner{}).executePreparedRun(ctx, &domain.PreparedRun{}, func(context.Context, *domain.PreparedRun) { called = true }, false, nil, "", 0, "", time.Time{}, nil)
+	if called || !errors.Is(err, context.Canceled) || !errors.Is(err, ErrLLMGenerate) {
+		t.Fatalf("called=%v error=%v", called, err)
+	}
+}
+
+func TestPreparedNotificationPanicClearsClaimedPayload(t *testing.T) {
+	runner := NewRunner(&fakePromptRepo{def: promptDef(domain.FormatText, domain.ValidationNone, 0)},
+		&fakeExecutionProfileRepo{profiles: map[string]*domain.ExecutionProfile{"exec": defaultExecutionProfile()}},
+		defaultArtifactReader(), defaultRenderer(), &fakeLLM{resp: &domain.GenerateResponse{Content: "ok"}}, nil, nil)
+	handle, err := runner.PrepareExecution(context.Background(), domain.RunRequest{PromptID: "p", ProfileID: "exec", Inputs: singleInputRef(), OnPrepared: func(context.Context, *domain.PreparedRun) { panic("notification") }})
+	if err != nil {
+		t.Fatal(err)
+	}
+	payload := handle.payload
+	func() {
+		defer func() {
+			if recover() == nil {
+				t.Error("expected panic")
+			}
+		}()
+		_, _ = runner.RunPrepared(context.Background(), handle)
+	}()
+	if payload.prepared != nil || payload.onPrepared != nil || payload.directKey != "" || handle.payload != nil {
+		t.Fatal("claimed payload retained private state")
+	}
+}

internal/usecase/runner.go

@@ -39,6 +39,7 @@ var (
 	ErrPromptRender     = errors.New("failed to render prompt")
 	ErrLLMGenerate      = errors.New("failed to generate output")
 	ErrValidation       = errors.New("failed to validate output")
+	ErrOperationTimeout = errors.New("operation timeout exceeded")
 )
 
 // Runner executes the Promptkit core use case.
@@ -79,6 +80,8 @@ type preparationState struct {
 type preparedOperation struct {
 	run        *domain.PreparedRun
 	validation validate.PreparedValidation
+	capture    bool
+	parent     *domain.Exchange
 }
 
 func NewRunner(
@@ -163,7 +166,7 @@ func (r *Runner) Run(ctx context.Context, req domain.RunRequest) (*domain.RunRes
 	}
 	directAPIKey := state.effectiveModel.APIKey
 
-	return r.executePreparedRun(ctx, operation.run, directAPIKey, state.transientRetries, runID, start, func(
+	return r.executePreparedRun(ctx, operation.run, req.OnPrepared, operation.capture, operation.parent, directAPIKey, state.transientRetries, runID, start, func(
 		ctx context.Context,
 		artifact *domain.Artifact,
 		attemptsUsed int,
@@ -183,12 +186,24 @@ type preparedValidationFunc func(
 func (r *Runner) executePreparedRun(
 	ctx context.Context,
 	prepared *domain.PreparedRun,
+	onPrepared func(context.Context, *domain.PreparedRun),
+	capture bool,
+	parent *domain.Exchange,
 	directAPIKey string,
 	transientRetries uint64,
 	runID string,
 	start time.Time,
 	validateArtifact preparedValidationFunc,
 ) (*domain.RunResult, error) {
+	if onPrepared != nil {
+		if err := ctx.Err(); err != nil {
+			return nil, wrapGenerationError(err)
+		}
+		onPrepared(ctx, prepared)
+		if err := ctx.Err(); err != nil {
+			return nil, wrapGenerationError(err)
+		}
+	}
 	executionTarget := prepared.EffectiveModelParams
 	executionTarget.APIKey = directAPIKey
 	genResp, err := r.llm.Generate(ctx, newGenerationRequest(
@@ -252,8 +267,14 @@ func (r *Runner) executePreparedRun(
 
 	end := time.Now().UTC()
 	executionTarget.APIKey = ""
+	var exchange *domain.Exchange
+	if capture {
+		exchange = domain.CaptureExchange(parent, runID, prepared.Messages[parent.MessageCount():], genResp.Content)
+	}
 
 	return &domain.RunResult{
+		Exchange:             exchange,
+		ParentRunID:          prepared.ParentRunID,
 		RunID:                runID,
 		Artifact:             outputArtifact,
 		RawOutput:            genResp.Content,
@@ -399,7 +420,7 @@ func (r *Runner) completePreparation(
 	if err != nil {
 		return nil, err
 	}
-	return &preparedOperation{run: prepared, validation: validationPlan}, nil
+	return &preparedOperation{run: prepared, validation: validationPlan, capture: req.CaptureExchange, parent: req.ContinueFrom}, nil
 }
 
 func (r *Runner) prepareValidation(
@@ -472,12 +493,13 @@ func (r *Runner) completePreparationWithStructuredOutput(
 	if state.directSessionID != "" {
 		effectivePrompt.SessionID = state.directSessionID
 	}
-	effectivePrompt.Messages = domain.ConcatRenderedMessages(renderedPrompt.Messages, req.AppendedMessages)
+	effectivePrompt.Messages = req.ContinueFrom.MessagesWithSuffix(renderedPrompt.Messages, req.AppendedMessages)
 
 	end := time.Now().UTC()
 	effectiveModel := state.effectiveModel
 	effectiveModel.APIKey = ""
 	return &domain.PreparedRun{
+		ParentRunID:          req.ContinueFrom.RunID(),
 		PromptID:             state.definition.ID,
 		PromptHash:           state.promptDefinitionHash,
 		SelectedProfileID:    state.selectedProfileID,

internal/usecase/runner_test.go

@@ -1310,6 +1310,39 @@ func TestRunnerComposesAppendedMessagesIntoEveryRenderedPrompt(t *testing.T) {
 	}
 }
 
+func TestRunnerPreparesContinuationHistory(t *testing.T) {
+	renderer := &fakeRenderer{rendered: &domain.RenderedPrompt{SessionID: "session", Messages: []domain.RenderedMessage{{Role: domain.RoleUser, Content: "new"}}}}
+	client := &fakeLLM{resp: &domain.GenerateResponse{Content: "output"}}
+	runner := NewRunner(&fakePromptRepo{def: promptDef(domain.FormatText, domain.ValidationNone, 0)}, &fakeExecutionProfileRepo{profiles: map[string]*domain.ExecutionProfile{"exec": defaultExecutionProfile()}}, defaultArtifactReader(), renderer, client, nil, nil)
+	request := domain.RunRequest{PromptID: "p", ProfileID: "exec", Inputs: singleInputRef(), AppendedMessages: []domain.RenderedMessage{{Role: domain.RoleDeveloper, Content: "suffix"}}}
+	plain, err := runner.Prepare(context.Background(), request)
+	if err != nil {
+		t.Fatal(err)
+	}
+	request.ContinueFrom = domain.CaptureExchange(nil, "parent", []domain.RenderedMessage{{Role: domain.RoleSystem, Content: "old", CacheControl: &domain.CacheControl{Type: domain.CacheControlEphemeral, TTL: "1h"}}}, "prior")
+	prepared, err := runner.Prepare(context.Background(), request)
+	if err != nil {
+		t.Fatal(err)
+	}
+	want := []domain.RenderedMessage{{Role: domain.RoleSystem, Content: "old", CacheControl: &domain.CacheControl{Type: domain.CacheControlEphemeral, TTL: "1h"}}, {Role: domain.RoleAssistant, Content: "prior"}, {Role: domain.RoleUser, Content: "new"}, {Role: domain.RoleDeveloper, Content: "suffix"}}
+	if !reflect.DeepEqual(prepared.Messages, want) || prepared.ParentRunID != "parent" || prepared.PromptHash != plain.PromptHash || prepared.RenderedPromptHash == plain.RenderedPromptHash {
+		t.Fatalf("prepared = %#v, plain = %#v", prepared, plain)
+	}
+	if client.calls != 0 {
+		t.Fatal("preparation called the model")
+	}
+	execution, err := runner.PrepareExecution(context.Background(), request)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if execution.Details().ParentRunID != "parent" || !reflect.DeepEqual(execution.Details().Messages, want) {
+		t.Fatalf("prepared execution = %#v", execution.Details())
+	}
+	if client.calls != 0 {
+		t.Fatal("prepared execution called the model")
+	}
+}
+
 func TestRunnerRunSuccessful(t *testing.T) {
 	promptRepo := &fakePromptRepo{def: promptDef(domain.FormatMarkdown, domain.ValidationBasic, 0)}
 	execRepo := &fakeExecutionProfileRepo{profiles: map[string]*domain.ExecutionProfile{"exec": defaultExecutionProfile()}}
@@ -1693,12 +1726,13 @@ func TestRunnerReleasesAdmissionAcrossRunOutcomes(t *testing.T) {
 			)
 
 			result, err := runner.Run(context.Background(), domain.RunRequest{
-				PromptID:  "p",
-				ProfileID: "exec",
-				Inputs:    singleInputRef(),
+				PromptID:        "p",
+				ProfileID:       "exec",
+				Inputs:          singleInputRef(),
+				CaptureExchange: true,
 			})
 			if tc.wantError == nil {
-				if err != nil || result == nil {
+				if err != nil || result == nil || result.Exchange == nil {
 					t.Fatalf("successful run=(%+v, %v)", result, err)
 				}
 			} else {

observation_contract_test.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"encoding/json"
 	"errors"
+	"fmt"
 	"strings"
 	"testing"
 
@@ -15,6 +16,72 @@ func observationEngine(t *testing.T, client promptkit.LLMClient) *promptkit.Engi
 	return newSimpleEngine(t, "hello", promptkit.WithLLMClient(client))
 }
 
+func TestContinuationObservationAncestry(t *testing.T) {
+	client := &fakeLLMClient{response: &promptkit.GenerateResponse{Content: "private answer"}}
+	engine := observationEngine(t, client)
+	ctx := context.Background()
+	initial, err := engine.Run(ctx, promptkit.RunRequest{PromptID: "prompt", CaptureExchange: true})
+	if err != nil {
+		t.Fatal(err)
+	}
+	var events []promptkit.ExecutionEvent
+	observer := func(event promptkit.ExecutionEvent) { events = append(events, event) }
+	req := promptkit.RunRequest{PromptID: "prompt", ContinueFrom: initial.Exchange, Observer: observer}
+	prepared, err := engine.PrepareExecution(ctx, req)
+	if err != nil || len(events) != 0 {
+		t.Fatalf("preparation events=%#v error=%v", events, err)
+	}
+	check := func(result *promptkit.RunResult, runErr error) {
+		t.Helper()
+		if runErr != nil || result == nil || len(events) != 3 {
+			t.Fatalf("result=%#v error=%v events=%#v", result, runErr, events)
+		}
+		for _, event := range events {
+			if event.ParentRunID != initial.RunID || event.RunID != result.RunID || strings.Contains(fmt.Sprintf("%+v", event), "private answer") {
+				t.Fatalf("event = %#v", event)
+			}
+		}
+		events = nil
+	}
+	result, err := engine.Run(ctx, req)
+	check(result, err)
+	result, err = engine.RunPrepared(ctx, prepared)
+	check(result, err)
+	client.err = errors.New("failure")
+	if result, err := engine.Run(ctx, req); result != nil || err == nil || len(events) != 3 {
+		t.Fatalf("failure result=%#v error=%v events=%#v", result, err, events)
+	}
+	for _, event := range events {
+		if event.ParentRunID != initial.RunID {
+			t.Fatalf("failure event = %#v", event)
+		}
+	}
+}
+
+func TestContinuationNestedRunHasIndependentAncestry(t *testing.T) {
+	engine := observationEngine(t, &fakeLLMClient{response: &promptkit.GenerateResponse{Content: "ok"}})
+	ctx := context.Background()
+	initial, err := engine.Run(ctx, promptkit.RunRequest{PromptID: "prompt", CaptureExchange: true})
+	if err != nil {
+		t.Fatal(err)
+	}
+	var nested []promptkit.ExecutionEvent
+	var nestedErr error
+	_, err = engine.Run(ctx, promptkit.RunRequest{PromptID: "prompt", ContinueFrom: initial.Exchange, Observer: func(event promptkit.ExecutionEvent) {
+		if event.Kind == promptkit.ExecutionFinished {
+			_, nestedErr = engine.Run(ctx, promptkit.RunRequest{PromptID: "prompt", Observer: func(value promptkit.ExecutionEvent) { nested = append(nested, value) }})
+		}
+	}})
+	if err != nil || nestedErr != nil || len(nested) == 0 {
+		t.Fatalf("errors=%v, %v; events=%#v", err, nestedErr, nested)
+	}
+	for _, event := range nested {
+		if event.ParentRunID != "" {
+			t.Fatalf("nested event = %#v", event)
+		}
+	}
+}
+
 func TestRunObservationCoversFacadeErrors(t *testing.T) {
 	for _, tc := range []struct {
 		name    string

observations.go

@@ -6,6 +6,7 @@ import (
 	"time"
 
 	"go.maximumdirect.net/promptkit/internal/domain"
+	"go.maximumdirect.net/promptkit/internal/observation"
 )
 
 // ExecutionObserver receives immutable metadata events synchronously. Callers
@@ -16,6 +17,12 @@ import (
 // event, and Promptkit keeps no asynchronous queue or durable history.
 type ExecutionObserver func(ExecutionEvent)
 
+// ComposeObservers returns a synchronous ordered fan-out of non-nil observers.
+// It copies the input slice; a panic propagates and prevents later callbacks.
+func ComposeObservers(observers ...ExecutionObserver) ExecutionObserver {
+	return observation.Compose[ExecutionEvent](observers...)
+}
+
 // ExecutionEventKind identifies one point in an execution's lifecycle.
 type ExecutionEventKind string
 
@@ -102,6 +109,7 @@ type ObservedTokenUsage struct {
 type ExecutionEvent struct {
 	Kind               ExecutionEventKind
 	RunID              string
+	ParentRunID        string
 	Time               time.Time
 	PromptID           string
 	ProfileID          string
@@ -129,7 +137,7 @@ func fromDomainExecutionEvent(value domain.ExecutionEvent) ExecutionEvent {
 		return ObservedTokenCount{Value: c.Value, Known: c.Known, Complete: c.Complete}
 	}
 	return ExecutionEvent{
-		Kind: ExecutionEventKind(value.Kind), RunID: value.RunID, Time: value.Time,
+		Kind: ExecutionEventKind(value.Kind), RunID: value.RunID, ParentRunID: value.ParentRunID, Time: value.Time,
 		PromptID: value.PromptID, ProfileID: value.ProfileID,
 		BackendID: value.BackendID, Model: value.Model, Generation: value.Generation,
 		Attempt: value.Attempt, Dispatched: value.Dispatched, CapacityWait: value.CapacityWait,

observer_composition_contract_test.go

@@ -0,0 +1,103 @@
+package promptkit_test
+
+import (
+	"context"
+	"reflect"
+	"sync"
+	"testing"
+
+	"go.maximumdirect.net/promptkit"
+)
+
+func TestComposeObserversCopiesAndOrders(t *testing.T) {
+	if promptkit.ComposeObservers() != nil || promptkit.ComposeObservers(nil, nil) != nil {
+		t.Fatal("empty composition is not nil")
+	}
+	var calls []int
+	first := func(promptkit.ExecutionEvent) { calls = append(calls, 1) }
+	second := func(promptkit.ExecutionEvent) { calls = append(calls, 2) }
+	values := []promptkit.ExecutionObserver{first, nil, second, first}
+	observer := promptkit.ComposeObservers(values...)
+	values[0] = second
+	observer(promptkit.ExecutionEvent{})
+	if !reflect.DeepEqual(calls, []int{1, 2, 1}) {
+		t.Fatalf("calls=%v", calls)
+	}
+	defer func() {
+		if recover() != "panic" || len(calls) != 3 {
+			t.Errorf("panic or calls=%v", calls)
+		}
+	}()
+	promptkit.ComposeObservers(func(promptkit.ExecutionEvent) { panic("panic") }, second)(promptkit.ExecutionEvent{})
+}
+
+func TestComposeObserversSparseSelection(t *testing.T) {
+	for _, selected := range []int{0, 1, 2} {
+		calls := 0
+		observers := make([]promptkit.ExecutionObserver, 1024)
+		callback := func(promptkit.ExecutionEvent) { calls++ }
+		for index := 0; index < selected; index++ {
+			observers[len(observers)-1-index] = callback
+		}
+		composed := promptkit.ComposeObservers(observers...)
+		for index := range observers {
+			observers[index] = func(promptkit.ExecutionEvent) { calls += 100 }
+		}
+		if selected == 0 {
+			if composed != nil {
+				t.Fatal("all-nil composition must be nil")
+			}
+			continue
+		}
+		composed(promptkit.ExecutionEvent{})
+		if calls != selected {
+			t.Fatalf("selected %d callbacks, got %d calls", selected, calls)
+		}
+	}
+}
+
+func TestCoordinateObserverCompositionAndNestedSelection(t *testing.T) {
+	engine := newSimpleEngine(t, "question", promptkit.WithLLMClient(&fakeLLMClient{response: &promptkit.GenerateResponse{Content: "answer"}}))
+	ctx := context.Background()
+	initial, err := engine.Run(ctx, promptkit.RunRequest{PromptID: "prompt", CaptureExchange: true})
+	if err != nil {
+		t.Fatal(err)
+	}
+	var sequence []string
+	var nested int
+	request := &promptkit.RunRequest{PromptID: "prompt", Observer: func(promptkit.ExecutionEvent) { sequence = append(sequence, "request") }}
+	result, err := engine.Coordinate(ctx, promptkit.CoordinateRequest{Start: initial.Exchange, AdditionalTurns: 1, Observer: func(promptkit.ExecutionEvent) { sequence = append(sequence, "coordinator") }, Next: func(ctx context.Context, turn promptkit.ConversationTurn) (*promptkit.RunRequest, error) {
+		if turn.Result == nil {
+			return request, nil
+		}
+		_, err := engine.Run(ctx, promptkit.RunRequest{PromptID: "prompt", Observer: func(promptkit.ExecutionEvent) { nested++ }})
+		return nil, err
+	}})
+	if err != nil || result.Outcome != promptkit.CoordinationStopped || nested != 3 || !reflect.DeepEqual(sequence, []string{"request", "coordinator", "request", "coordinator", "request", "coordinator"}) {
+		t.Fatalf("result=%#v error=%v events=%v nested=%d", result, err, sequence, nested)
+	}
+	if request.ContinueFrom != nil {
+		t.Fatal("coordinator changed caller request")
+	}
+}
+
+func TestComposedObserversSupportConcurrentRuns(t *testing.T) {
+	var mu sync.Mutex
+	count := 0
+	observer := promptkit.ComposeObservers(func(promptkit.ExecutionEvent) { mu.Lock(); count++; mu.Unlock() })
+	engine := newSimpleEngine(t, "question", promptkit.WithLLMClient(&preparedRecordingClient{response: &promptkit.GenerateResponse{Content: "answer"}}))
+	var wg sync.WaitGroup
+	for i := 0; i < 8; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			if _, err := engine.Run(context.Background(), promptkit.RunRequest{PromptID: "prompt", Observer: observer}); err != nil {
+				t.Error(err)
+			}
+		}()
+	}
+	wg.Wait()
+	if count != 24 {
+		t.Fatalf("event count=%d", count)
+	}
+}

prepared_execution.go

@@ -15,6 +15,12 @@ const preparedExecutionString = "promptkit.PreparedExecution{opaque}"
 // A successful PrepareExecution may retain an Observer function in this
 // handle's private payload. The callback and other private execution settings
 // are cleared on claim completion or Discard. Details never exposes them.
+// It also retains RunRequest.CaptureExchange and RunRequest.ContinueFrom for
+// execution of the frozen snapshot. Only RunPrepared can produce its completed
+// [Exchange]; preparation alone does not.
+// RunRequest.OperationTimeout and RunRequest.OnPrepared are retained as
+// selections without an active idle timer; their execution lifecycle is
+// described on those fields.
 type PreparedExecution struct {
 	internal *usecase.PreparedExecution
 }

prepared_execution_contract_test.go

@@ -47,11 +47,36 @@ func TestPreparedExecutionFreezesSourcesAndReturnsIndependentDetails(t *testing.
 	}
 
 	temperature := 0.25
+	initial, err := newSimpleEngine(t, "history", promptkit.WithLLMClient(&fakeLLMClient{response: &promptkit.GenerateResponse{Content: "prior"}})).Run(context.Background(), promptkit.RunRequest{PromptID: "prompt", CaptureExchange: true})
+	if err != nil {
+		t.Fatal(err)
+	}
+	initialDetails := initial.Exchange.Details()
 	extraParams := map[string]any{
 		"nested": map[string]any{"source": "original"},
 	}
+	var notified *promptkit.PreparedRun
 	request := promptkit.RunRequest{
-		PromptID: "prepared",
+		PromptID:         "prepared",
+		APIKey:           "synthetic-direct-key",
+		ContinueFrom:     initial.Exchange,
+		SessionID:        "continuation-session",
+		AppendedMessages: []promptkit.RenderedMessage{{Role: promptkit.RoleUser, Content: "appended", CacheControl: &promptkit.CacheControl{Type: promptkit.CacheControlEphemeral}}},
+		OnPrepared: func(_ context.Context, details promptkit.PreparedRun) {
+			notified = &details
+			if details.ParentRunID != initial.RunID || details.SessionID != "continuation-session" || details.Messages[0].Content != "history" || details.Messages[2].Content != "Input=original artifact Label=original variable" || details.InputHashes["input"] != "original-input-hash" || details.StructuredOutput.JSONSchema.Schema.(map[string]any)["title"] != "original root" {
+				t.Error("notification did not describe the frozen continuation")
+			}
+			if strings.Contains(fmt.Sprintf("%#v", details), "synthetic-direct-key") {
+				t.Error("notification exposed credential")
+			}
+			details.Messages[0].Content = "mutated history"
+			details.Messages[2].Content = "mutated notification"
+			details.Messages[3].CacheControl.Type = "changed"
+			details.InputHashes["input"] = "mutated notification"
+			details.EffectiveModelParams.ExtraParams["nested"].(map[string]any)["source"] = "mutated notification"
+			details.StructuredOutput.JSONSchema.Schema.(map[string]any)["title"] = "mutated notification"
+		},
 		Inputs: map[string]promptkit.ArtifactRef{
 			"input": promptkit.Inline("original request input"),
 		},
@@ -87,14 +112,14 @@ func TestPreparedExecutionFreezesSourcesAndReturnsIndependentDetails(t *testing.
 	reader.set("changed artifact", "changed-input-hash")
 
 	first := prepared.Details()
-	first.Messages[0].Content = "changed details"
+	first.Messages[2].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.Messages[2].Content != "Input=original artifact Label=original variable" {
+		t.Fatalf("details message changed: %q", second.Messages[2].Content)
 	}
 	if second.InputHashes["input"] != "original-input-hash" {
 		t.Fatalf("details input hash changed: %q", second.InputHashes["input"])
@@ -119,13 +144,19 @@ func TestPreparedExecutionFreezesSourcesAndReturnsIndependentDetails(t *testing.
 	if reader.callCount() != 1 {
 		t.Fatalf("execution reopened artifact source: calls=%d", reader.callCount())
 	}
+	if notified == nil || prepared.Details().Messages[2].Content != second.Messages[2].Content || prepared.Details().Messages[3].CacheControl.Type != promptkit.CacheControlEphemeral || !reflect.DeepEqual(initial.Exchange.Details(), initialDetails) || result.Exchange.Details().Messages[0].Content != "history" || prepared.Details().StructuredOutput.JSONSchema.Schema.(map[string]any)["title"] != "original root" {
+		t.Fatal("notification changed prepared state")
+	}
 
 	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 ||
+	if generated.APIKey != "synthetic-direct-key" {
+		t.Fatal("direct credential did not reach client")
+	}
+	if generated.Prompt.Messages[0].Content != "history" || generated.Prompt.Messages[2].Content != second.Messages[2].Content || generated.Prompt.Messages[3].CacheControl.Type != promptkit.CacheControlEphemeral ||
 		generated.Target.Model != second.EffectiveModelParams.Model ||
 		!reflect.DeepEqual(generated.Target.ExtraParams, second.EffectiveModelParams.ExtraParams) ||
 		!reflect.DeepEqual(generated.StructuredOutput, second.StructuredOutput) {
@@ -276,7 +307,7 @@ func TestPreparedExecutionRepairsEmptyBasicOutput(t *testing.T) {
 	}}
 	engine := newPreparedContractEngine(t, client, "Summarize the source.")
 	prepared, err := engine.PrepareExecution(context.Background(), promptkit.RunRequest{
-		PromptID: "prepared",
+		PromptID: "prepared", CaptureExchange: true,
 		Validation: &promptkit.OutputContract{
 			Format:         promptkit.FormatMarkdown,
 			ValidationMode: promptkit.ValidationBasic,
@@ -296,6 +327,9 @@ func TestPreparedExecutionRepairsEmptyBasicOutput(t *testing.T) {
 		result.Validation.RepairAttempts != 1 || result.Usage != (promptkit.TokenUsage{PromptTokens: 10, CompletionTokens: 16, TotalTokens: 26}) {
 		t.Fatalf("repaired result = %+v", result)
 	}
+	if result.Exchange == nil || len(result.Exchange.Details().Messages) != len(details.Messages)+1 || result.Exchange.Details().Messages[len(details.Messages)].Content != "Corrected summary." {
+		t.Fatalf("repaired exchange = %#v", result.Exchange)
+	}
 	requests := client.snapshot()
 	if len(requests) != 2 || len(requests[1].Prompt.Messages) != len(details.Messages)+1 ||
 		!reflect.DeepEqual(requests[1].Prompt.Messages[:len(details.Messages)], details.Messages) ||
@@ -315,7 +349,8 @@ func TestPreparedExecutionIsolatesMutatingClientAcrossRepair(t *testing.T) {
 		t.Fatal(err)
 	}
 	prepared, err := engine.PrepareExecution(context.Background(), promptkit.RunRequest{
-		PromptID: "prepared",
+		PromptID:        "prepared",
+		CaptureExchange: true,
 		Execution: &promptkit.ExecutionTargetOverride{ExtraParams: map[string]any{
 			"nested": map[string]any{"value": "original"},
 		}},
@@ -333,6 +368,9 @@ func TestPreparedExecutionIsolatesMutatingClientAcrossRepair(t *testing.T) {
 	if !reflect.DeepEqual(before, prepared.Details()) {
 		t.Fatal("client mutation changed retained prepared metadata")
 	}
+	if result.Exchange == nil || result.Exchange.Details().Messages[0].Content != "original message" || result.Exchange.Details().Messages[1].CacheControl.TTL != "1h" || result.Exchange.Details().Messages[2].Content != `{"value":3}` {
+		t.Fatalf("mutating client changed exchange history: %#v", result.Exchange.Details())
+	}
 	result.EffectiveModelParams.ExtraParams["nested"].(map[string]any)["value"] = "result mutation"
 	if prepared.Details().EffectiveModelParams.ExtraParams["nested"].(map[string]any)["value"] != "original" {
 		t.Fatal("result mutation changed retained prepared metadata")

prepared_notification_contract_test.go

@@ -0,0 +1,246 @@
+package promptkit_test
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"net/http"
+	"reflect"
+	"strings"
+	"testing"
+
+	"go.maximumdirect.net/promptkit"
+)
+
+func TestPreparedNotificationSelectionAndOrdering(t *testing.T) {
+	for _, split := range []bool{false, true} {
+		t.Run(map[bool]string{false: "run", true: "prepared"}[split], func(t *testing.T) {
+			calls := 0
+			engine := observationEngine(t, observationClientFunc(func(_ context.Context, _ promptkit.GenerateRequest) (*promptkit.GenerateResponse, error) {
+				if calls != 1 {
+					t.Fatalf("generation before notification: %d", calls)
+				}
+				return &promptkit.GenerateResponse{Content: "ok"}, nil
+			}))
+			req := promptkit.RunRequest{PromptID: "prompt", OnPrepared: func(_ context.Context, details promptkit.PreparedRun) {
+				calls++
+				if details.PromptID != "prompt" || len(details.Messages) == 0 {
+					t.Fatalf("details=%+v", details)
+				}
+			}}
+			if split {
+				handle, err := engine.PrepareExecution(context.Background(), req)
+				if err != nil {
+					t.Fatal(err)
+				}
+				if calls != 0 {
+					t.Fatal("preparation notified")
+				}
+				req.OnPrepared = func(context.Context, promptkit.PreparedRun) { t.Fatal("selection changed") }
+				_, err = engine.RunPrepared(context.Background(), handle)
+				if err != nil {
+					t.Fatal(err)
+				}
+			} else if _, err := engine.Run(context.Background(), req); err != nil {
+				t.Fatal(err)
+			}
+			if calls != 1 {
+				t.Fatalf("notifications=%d", calls)
+			}
+		})
+	}
+}
+
+func TestNotificationRedactsEnvironmentCredential(t *testing.T) {
+	t.Setenv("PROMPTKIT_NOTIFICATION_KEY", "synthetic-environment-key")
+	config := simpleConfig("message")
+	config.HTTPClient = &http.Client{Transport: roundTripFunc(func(req *http.Request) (*http.Response, error) {
+		if req.Header.Get("Authorization") != "Bearer synthetic-environment-key" {
+			t.Error("environment credential did not reach provider")
+		}
+		return retryHTTPResponse(200, "ok"), nil
+	})}
+	engine, err := promptkit.NewEngine(config)
+	if err != nil {
+		t.Fatal(err)
+	}
+	req := simpleGenerationRequest()
+	req.Execution = &promptkit.ExecutionTargetOverride{APIKeyEnv: "PROMPTKIT_NOTIFICATION_KEY"}
+	req.OnPrepared = func(_ context.Context, details promptkit.PreparedRun) {
+		if strings.Contains(fmt.Sprintf("%#v", details), "synthetic-environment-key") {
+			t.Error("notification exposed credential")
+		}
+	}
+	if _, err := engine.Run(context.Background(), req); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestContinuationNotificationIsDetachedAndSurvivesFailure(t *testing.T) {
+	failure := errors.New("provider failure")
+	calls := 0
+	engine := newSimpleEngine(t, "question", promptkit.WithLLMClient(observationClientFunc(func(_ context.Context, req promptkit.GenerateRequest) (*promptkit.GenerateResponse, error) {
+		calls++
+		if calls == 1 {
+			return &promptkit.GenerateResponse{Content: "answer"}, nil
+		}
+		if req.Prompt.Messages[0].Content != "question" || req.Prompt.Messages[3].Content != "follow-up" || req.APIKey != "synthetic-key" {
+			t.Fatalf("generation request=%+v", req)
+		}
+		return nil, failure
+	})))
+	initial, err := engine.Run(context.Background(), promptkit.RunRequest{PromptID: "prompt", CaptureExchange: true})
+	if err != nil {
+		t.Fatal(err)
+	}
+	before := initial.Exchange.Details()
+	var retained promptkit.PreparedRun
+	req := promptkit.RunRequest{PromptID: "prompt", ContinueFrom: initial.Exchange, APIKey: "synthetic-key", SessionID: "session", AppendedMessages: []promptkit.RenderedMessage{{Role: promptkit.RoleUser, Content: "follow-up", CacheControl: &promptkit.CacheControl{Type: promptkit.CacheControlEphemeral}}}, OnPrepared: func(_ context.Context, details promptkit.PreparedRun) {
+		retained = details
+		details.Messages[0].Content = "changed"
+		details.Messages[3].Content = "changed"
+		details.Messages[3].CacheControl.Type = "changed"
+	}}
+	result, err := engine.Run(context.Background(), req)
+	if result != nil || !errors.Is(err, failure) || retained.ParentRunID != initial.RunID || retained.SessionID != "session" || !reflect.DeepEqual(before, initial.Exchange.Details()) {
+		t.Fatalf("result=%v error=%v retained=%+v", result, err, retained)
+	}
+}
+
+func TestPreparedNotificationInvalidHandlesAndPreparationOnlyCalls(t *testing.T) {
+	calls := 0
+	engine := newPreparedContractEngine(t, &preparedRecordingClient{response: &promptkit.GenerateResponse{Content: "ok"}}, "content")
+	foreign := newPreparedContractEngine(t, &preparedRecordingClient{response: &promptkit.GenerateResponse{Content: "ok"}}, "other")
+	req := promptkit.RunRequest{PromptID: "prepared", OnPrepared: func(context.Context, promptkit.PreparedRun) { calls++ }}
+	if _, err := engine.Prepare(context.Background(), req); err != nil {
+		t.Fatal(err)
+	}
+	if err := engine.ValidateRequest(context.Background(), req); err != nil {
+		t.Fatal(err)
+	}
+	handle, err := engine.PrepareExecution(context.Background(), req)
+	if err != nil {
+		t.Fatal(err)
+	}
+	_ = handle.Details()
+	if _, err := foreign.RunPrepared(context.Background(), handle); !errors.Is(err, promptkit.ErrInvalidRequest) {
+		t.Fatal(err)
+	}
+	if _, err := engine.RunPrepared(context.Background(), nil); !errors.Is(err, promptkit.ErrInvalidRequest) {
+		t.Fatal(err)
+	}
+	handle.Discard()
+	if _, err := engine.RunPrepared(context.Background(), handle); !errors.Is(err, promptkit.ErrInvalidRequest) {
+		t.Fatal(err)
+	}
+	if _, err := engine.Run(context.Background(), promptkit.RunRequest{PromptID: "missing", OnPrepared: req.OnPrepared}); err == nil {
+		t.Fatal("expected preparation failure")
+	}
+	if calls != 0 {
+		t.Fatalf("unexpected notifications=%d", calls)
+	}
+	handle, err = engine.PrepareExecution(context.Background(), req)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := engine.RunPrepared(context.Background(), handle); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := engine.RunPrepared(context.Background(), handle); !errors.Is(err, promptkit.ErrInvalidRequest) || calls != 1 {
+		t.Fatalf("reused handle error=%v notifications=%d", err, calls)
+	}
+}
+
+func TestNotificationHoldsAdmission(t *testing.T) {
+	engine := newBackendCapacityEngine(t, observationClientFunc(func(context.Context, promptkit.GenerateRequest) (*promptkit.GenerateResponse, error) {
+		return &promptkit.GenerateResponse{Content: "ok"}, nil
+	}), 1, capacityInt(0), nil)
+	_, err := engine.Run(context.Background(), promptkit.RunRequest{PromptID: "prompt", OnPrepared: func(ctx context.Context, _ promptkit.PreparedRun) {
+		if _, err := engine.Run(ctx, promptkit.RunRequest{PromptID: "prompt"}); !errors.Is(err, promptkit.ErrCapacityExceeded) {
+			t.Errorf("nested admission error=%v", err)
+		}
+	}})
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestNotificationPanicReleasesCapacityAndConsumesHandle(t *testing.T) {
+	engine := newBackendCapacityEngine(t, observationClientFunc(func(context.Context, promptkit.GenerateRequest) (*promptkit.GenerateResponse, error) {
+		return &promptkit.GenerateResponse{Content: "ok"}, nil
+	}), 1, capacityInt(0), nil)
+	for _, split := range []bool{false, true} {
+		req := promptkit.RunRequest{PromptID: "prompt", OnPrepared: func(context.Context, promptkit.PreparedRun) { panic("notification") }}
+		var handle *promptkit.PreparedExecution
+		if split {
+			var err error
+			handle, err = engine.PrepareExecution(context.Background(), req)
+			if err != nil {
+				t.Fatal(err)
+			}
+		}
+		func() {
+			defer func() {
+				if recover() == nil {
+					t.Error("expected panic")
+				}
+			}()
+			if split {
+				_, _ = engine.RunPrepared(context.Background(), handle)
+			} else {
+				_, _ = engine.Run(context.Background(), req)
+			}
+		}()
+		if split {
+			if _, err := engine.RunPrepared(context.Background(), handle); !errors.Is(err, promptkit.ErrInvalidRequest) {
+				t.Fatal("handle reusable after panic")
+			}
+		}
+		if _, err := engine.Run(context.Background(), promptkit.RunRequest{PromptID: "prompt"}); err != nil {
+			t.Fatalf("capacity not released: %v", err)
+		}
+	}
+}
+
+func TestNotificationSkippedOnAdmissionAndPreExecutionCancellation(t *testing.T) {
+	release := make(chan struct{})
+	client := newCapacityGateClient(release, 1)
+	engine := newBackendCapacityEngine(t, client, 1, capacityInt(0), nil)
+	results := make(chan capacityRunResult, 1)
+	go runCapacityRequest(engine, context.Background(), promptkit.RunRequest{PromptID: "prompt"}, results)
+	<-client.started
+	calls := 0
+	req := promptkit.RunRequest{PromptID: "prompt", OnPrepared: func(context.Context, promptkit.PreparedRun) { calls++ }}
+	if _, err := engine.Run(context.Background(), req); !errors.Is(err, promptkit.ErrCapacityExceeded) {
+		t.Fatalf("admission error=%v", err)
+	}
+	close(release)
+	if result := <-results; result.err != nil {
+		t.Fatal(result.err)
+	}
+	handle, err := engine.PrepareExecution(context.Background(), req)
+	if err != nil {
+		t.Fatal(err)
+	}
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+	if _, err = engine.RunPrepared(ctx, handle); !errors.Is(err, context.Canceled) {
+		t.Fatalf("cancellation error=%v", err)
+	}
+	if calls != 0 {
+		t.Fatalf("notifications=%d", calls)
+	}
+}
+
+func TestCancellationInPreparedNotificationPreventsGeneration(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	engine := observationEngine(t, observationClientFunc(func(context.Context, promptkit.GenerateRequest) (*promptkit.GenerateResponse, error) {
+		t.Fatal("generation after cancellation")
+		return nil, nil
+	}))
+	_, err := engine.Run(ctx, promptkit.RunRequest{PromptID: "prompt", OnPrepared: func(context.Context, promptkit.PreparedRun) { cancel() }})
+	if !errors.Is(err, context.Canceled) || !errors.Is(err, promptkit.ErrLLMGenerate) {
+		t.Fatalf("error=%v", err)
+	}
+}

reliable_execution_contract_test.go

@@ -46,10 +46,12 @@ func TestPublicRetryRepairSequenceKeepsUsageAndEventsConsistent(t *testing.T) {
 	}
 	var events []promptkit.ExecutionEvent
 	req := simpleGenerationRequest()
+	notifications := 0
+	req.OnPrepared = func(context.Context, promptkit.PreparedRun) { notifications++ }
 	req.Validation = &promptkit.OutputContract{Format: promptkit.FormatJSON, ValidationMode: promptkit.ValidationJSON, RepairAttempts: 1}
 	req.Observer = func(event promptkit.ExecutionEvent) { events = append(events, event) }
 	result, err := engine.Run(context.Background(), req)
-	if err != nil || result == nil || !result.Validation.IsValid || calls != 4 {
+	if err != nil || result == nil || !result.Validation.IsValid || calls != 4 || notifications != 1 {
 		t.Fatalf("result=%+v error=%v calls=%d", result, err, calls)
 	}
 	if len(events) != 8 {

types.go

@@ -85,6 +85,43 @@ const (
 // own private execution snapshot for RunPrepared. Excessively deep or large
 // JSON-shaped values are rejected for safety.
 type RunRequest struct {
+	// OperationTimeout selects a whole-operation budget when positive. Run
+	// applies it across preparation and execution; PrepareExecution retains it
+	// for RunPrepared, without counting preparation or idle time. Zero or a
+	// negative value selects no additional timeout. This is independent of the
+	// provider generation timeout and respects earlier parent cancellation or
+	// deadlines. Local timeout failures match ErrOperationTimeout and
+	// context.DeadlineExceeded. Cancellation remains cooperative; callbacks or
+	// collaborators that ignore context cannot be forcibly interrupted.
+	OperationTimeout time.Duration `json:"-" yaml:"-"`
+	// OnPrepared synchronously receives detached, credential-redacted details of
+	// the logical request after preparation and admission, before the first
+	// generation, at most once per logical execution if it reaches that point.
+	// Transient retries and structural repair do not repeat the notification.
+	// Its context is the operation-scoped context, subject to OperationTimeout
+	// and parent cancellation/deadlines. In [Engine.Coordinate], this differs
+	// from the parent context passed to the next-turn callback.
+	// Admission remains held, and callback time consumes OperationTimeout. The
+	// callback owns the delivered mutable values; its closure is shared and must
+	// synchronize any shared state. It has no error return, and a panic propagates
+	// after resource cleanup. Prepare and ValidateRequest do not invoke it;
+	// PrepareExecution retains the selected function for RunPrepared and Discard
+	// releases it.
+	OnPrepared func(context.Context, PreparedRun) `json:"-" yaml:"-"`
+	// CaptureExchange requests an immutable snapshot of a completed run, even
+	// when its structural validation fails. Operational errors return no result.
+	// The snapshot is returned in RunResult.Exchange; see [Exchange] for its
+	// logical history contract. Prepare and ValidateRequest create no exchange.
+	// PrepareExecution retains this choice for its eventual RunPrepared call.
+	CaptureExchange bool
+	// ContinueFrom supplies captured history and implies capture of the new run.
+	// A non-nil zero-value exchange is invalid. Follow-up settings are selected
+	// normally and are not inherited from the exchange.
+	// Messages are assembled as captured history, rendered definition messages,
+	// then AppendedMessages. Prior templates and inputs are not rendered or read
+	// again. Continuation adds no history-size limit or truncation; upstream
+	// rejection follows the ordinary generation-error contract.
+	ContinueFrom *Exchange `json:"-" yaml:"-"`
 	// PromptID is the required prompt identifier. A namespace-scoped engine view
 	// qualifies a slashless caller value; otherwise it selects the exact
 	// configured ID.
@@ -153,13 +190,16 @@ type RunRequest struct {
 
 // 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
+// API key values. RunRequest.OnPrepared also supplies this detached value.
+// It does not include 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
 // opaque.
 type PreparedRun struct {
+	// ParentRunID identifies the immediate source exchange, if any.
+	ParentRunID string `json:"parent_run_id,omitempty"`
 	// PromptID is the selected prompt identifier.
 	PromptID string `json:"prompt_id"`
 	// PromptHash is an opaque equality value for the selected definition.
@@ -187,8 +227,8 @@ type PreparedRun struct {
 	// RenderedPromptHash is an opaque equality value for SessionID and Messages.
 	RenderedPromptHash string `json:"rendered_prompt_hash"`
 	// Messages are the complete effective messages that Run or RunPrepared passes
-	// to the LLM client: definition messages in rendered order followed by any
-	// RunRequest.AppendedMessages in caller order.
+	// to the LLM client: captured history, if any, followed by definition
+	// messages and RunRequest.AppendedMessages in caller order.
 	Messages []RenderedMessage `json:"messages"`
 	// StartTime is the UTC time at which preparation began.
 	StartTime time.Time `json:"start_time,omitempty"`
@@ -207,6 +247,10 @@ type PreparedRun struct {
 // use RFC 3339 and zero timing values are omitted. Run IDs and hash formats are
 // opaque.
 type RunResult struct {
+	// Exchange holds captured conversation history when requested.
+	Exchange *Exchange `json:"-" yaml:"-"`
+	// ParentRunID identifies the immediate source exchange, if any.
+	ParentRunID string `json:"parent_run_id,omitempty"`
 	// RunID is an opaque identifier for this invocation.
 	RunID string `json:"run_id"`
 	// Artifact contains the generated output and derived metadata.