diff --git a/internal/jsonvalue/jsonvalue.go b/internal/jsonvalue/jsonvalue.go
index c9878c1..922a9d4 100644
--- a/internal/jsonvalue/jsonvalue.go
+++ b/internal/jsonvalue/jsonvalue.go
@@ -18,13 +18,19 @@ type visit struct {
 	ptr uintptr
 }
 
+// Copy validates and deeply copies a JSON-compatible value while preserving
+// compatible concrete map, slice, array, scalar, and number types.
+func Copy(src any) (any, error) {
+	return copyValue(reflect.ValueOf(src), "value", make(map[visit]struct{}), true)
+}
+
 // CopyMap validates and deeply copies an extra-parameter map while preserving
 // compatible concrete map, slice, array, scalar, and number types.
 func CopyMap(src map[string]any) (map[string]any, error) {
 	if src == nil {
 		return nil, nil
 	}
-	copied, err := copyValue(reflect.ValueOf(src), "extra_params", make(map[visit]struct{}))
+	copied, err := copyValue(reflect.ValueOf(src), "extra_params", make(map[visit]struct{}), false)
 	if err != nil {
 		return nil, err
 	}
@@ -35,7 +41,12 @@ func CopyMap(src map[string]any) (map[string]any, error) {
 	return out, nil
 }
 
-func copyValue(value reflect.Value, path string, seen map[visit]struct{}) (any, error) {
+func copyValue(
+	value reflect.Value,
+	path string,
+	seen map[visit]struct{},
+	allowEmptyMapKeys bool,
+) (any, error) {
 	if !value.IsValid() {
 		return nil, nil
 	}
@@ -43,7 +54,7 @@ func copyValue(value reflect.Value, path string, seen map[visit]struct{}) (any,
 		if value.IsNil() {
 			return nil, nil
 		}
-		return copyValue(value.Elem(), path, seen)
+		return copyValue(value.Elem(), path, seen, allowEmptyMapKeys)
 	}
 	if !value.CanInterface() {
 		return nil, fmt.Errorf("%s: value cannot be copied", path)
@@ -88,22 +99,27 @@ func copyValue(value reflect.Value, path string, seen map[visit]struct{}) (any,
 		}
 		seen[current] = struct{}{}
 		defer delete(seen, current)
-		return copyValue(value.Elem(), path, seen)
+		return copyValue(value.Elem(), path, seen, allowEmptyMapKeys)
 	case reflect.Map:
-		return copyMapValue(value, path, seen)
+		return copyMapValue(value, path, seen, allowEmptyMapKeys)
 	case reflect.Slice:
 		if value.IsNil() {
 			return nil, nil
 		}
-		return copySequenceValue(value, path, seen)
+		return copySequenceValue(value, path, seen, allowEmptyMapKeys)
 	case reflect.Array:
-		return copySequenceValue(value, path, seen)
+		return copySequenceValue(value, path, seen, allowEmptyMapKeys)
 	default:
 		return nil, fmt.Errorf("%s: unsupported JSON value type %s", path, value.Type())
 	}
 }
 
-func copyMapValue(value reflect.Value, path string, seen map[visit]struct{}) (any, error) {
+func copyMapValue(
+	value reflect.Value,
+	path string,
+	seen map[visit]struct{},
+	allowEmptyMapKeys bool,
+) (any, error) {
 	if value.IsNil() {
 		return nil, nil
 	}
@@ -133,10 +149,10 @@ func copyMapValue(value reflect.Value, path string, seen map[visit]struct{}) (an
 	elementType := value.Type().Elem()
 	for _, key := range keys {
 		name := key.String()
-		if name == "" {
+		if name == "" && !allowEmptyMapKeys {
 			return nil, fmt.Errorf("%s: map key must not be empty", path)
 		}
-		copied, err := copyValue(value.MapIndex(key), path+"."+name, seen)
+		copied, err := copyValue(value.MapIndex(key), path+"."+name, seen, allowEmptyMapKeys)
 		if err != nil {
 			return nil, err
 		}
@@ -171,7 +187,12 @@ func copyMapValue(value reflect.Value, path string, seen map[visit]struct{}) (an
 	return out, nil
 }
 
-func copySequenceValue(value reflect.Value, path string, seen map[visit]struct{}) (any, error) {
+func copySequenceValue(
+	value reflect.Value,
+	path string,
+	seen map[visit]struct{},
+	allowEmptyMapKeys bool,
+) (any, error) {
 	var current visit
 	if value.Kind() == reflect.Slice {
 		current = visit{typ: value.Type(), ptr: value.Pointer()}
@@ -186,7 +207,12 @@ func copySequenceValue(value reflect.Value, path string, seen map[visit]struct{}
 	preserveType := true
 	elementType := value.Type().Elem()
 	for i := 0; i < value.Len(); i++ {
-		copied, err := copyValue(value.Index(i), fmt.Sprintf("%s[%d]", path, i), seen)
+		copied, err := copyValue(
+			value.Index(i),
+			fmt.Sprintf("%s[%d]", path, i),
+			seen,
+			allowEmptyMapKeys,
+		)
 		if err != nil {
 			return nil, err
 		}
diff --git a/internal/jsonvalue/jsonvalue_test.go b/internal/jsonvalue/jsonvalue_test.go
index 8ca084f..bafd373 100644
--- a/internal/jsonvalue/jsonvalue_test.go
+++ b/internal/jsonvalue/jsonvalue_test.go
@@ -44,6 +44,21 @@ func TestCopyMapPreservesTypesAndIsolatesMutations(t *testing.T) {
 	}
 }
 
+func TestCopyAllowsEmptyObjectKeysAndIsolatesMutations(t *testing.T) {
+	nested := map[string]any{"": []any{"original"}}
+
+	copiedValue, err := jsonvalue.Copy(nested)
+	if err != nil {
+		t.Fatalf("copy value: %v", err)
+	}
+	nested[""].([]any)[0] = "changed"
+
+	copied := copiedValue.(map[string]any)
+	if got := copied[""].([]any)[0]; got != "original" {
+		t.Fatalf("copied value was not isolated: %v", got)
+	}
+}
+
 func TestCopyMapRejectsInvalidValues(t *testing.T) {
 	cyclicMap := map[string]any{}
 	cyclicMap["self"] = cyclicMap
diff --git a/internal/usecase/prepared_execution.go b/internal/usecase/prepared_execution.go
new file mode 100644
index 0000000..f6b7af3
--- /dev/null
+++ b/internal/usecase/prepared_execution.go
@@ -0,0 +1,300 @@
+package usecase
+
+import (
+	"context"
+	"fmt"
+	"sync"
+	"time"
+
+	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
+	"gitea.maximumdirect.net/eric/promptkit/internal/jsonvalue"
+	"gitea.maximumdirect.net/eric/promptkit/internal/validate"
+)
+
+type preparedExecutionState uint8
+
+const (
+	preparedExecutionReady preparedExecutionState = iota
+	preparedExecutionClaimed
+	preparedExecutionDiscarded
+)
+
+// PreparedExecution owns one frozen, single-use runner execution.
+type PreparedExecution struct {
+	owner   *Runner
+	mu      sync.Mutex
+	state   preparedExecutionState
+	details *domain.PreparedRun
+	payload *preparedExecutionPayload
+}
+
+type preparedExecutionPayload struct {
+	prepared   *domain.PreparedRun
+	validation validate.PreparedValidation
+}
+
+// PrepareExecution completes preparation without generation or admission and
+// returns a runner-bound, single-use execution.
+func (r *Runner) PrepareExecution(ctx context.Context, req domain.RunRequest) (*PreparedExecution, error) {
+	state, err := r.resolvePreparation(ctx, req, time.Now().UTC())
+	if err != nil {
+		return nil, err
+	}
+
+	validationPlan, err := r.prepareValidation(ctx, state.effectiveContract)
+	if err != nil {
+		return nil, err
+	}
+
+	structuredOutput, err := r.structuredOutputFromValidationPlan(
+		state.definition,
+		state.effectiveContract,
+		validationPlan,
+	)
+	if err != nil {
+		return nil, err
+	}
+	prepared, err := r.completePreparationWithStructuredOutput(ctx, req, state, structuredOutput)
+	if err != nil {
+		return nil, err
+	}
+
+	executionSnapshot, err := clonePreparedRun(prepared)
+	if err != nil {
+		prepared.EffectiveModelParams.APIKey = ""
+		return nil, fmt.Errorf("%w: failed to copy prepared execution: %v", ErrInvalidRequest, err)
+	}
+	executionSnapshot.StructuredOutput = prepared.StructuredOutput
+	prepared.EffectiveModelParams.APIKey = ""
+
+	details, err := clonePreparedRun(executionSnapshot)
+	if err != nil {
+		executionSnapshot.EffectiveModelParams.APIKey = ""
+		return nil, fmt.Errorf("%w: failed to copy prepared execution details: %v", ErrInvalidRequest, err)
+	}
+	details.EffectiveModelParams.APIKey = ""
+
+	return &PreparedExecution{
+		owner:   r,
+		state:   preparedExecutionReady,
+		details: details,
+		payload: &preparedExecutionPayload{
+			prepared:   executionSnapshot,
+			validation: validationPlan,
+		},
+	}, nil
+}
+
+func (r *Runner) prepareValidation(
+	ctx context.Context,
+	contract domain.OutputContract,
+) (validate.PreparedValidation, error) {
+	if r.validator == nil {
+		return noOpPreparedValidation{contract: contract}, nil
+	}
+	preparer, ok := r.validator.(validate.ValidationPreparer)
+	if !ok {
+		return nil, fmt.Errorf("%w: validator does not support prepared validation", ErrValidation)
+	}
+	plan, err := preparer.PrepareValidation(ctx, contract)
+	if err != nil {
+		return nil, fmt.Errorf("%w: %w", ErrValidation, err)
+	}
+	if plan == nil {
+		return nil, fmt.Errorf("%w: validator returned nil prepared validation", ErrValidation)
+	}
+	return plan, nil
+}
+
+func (r *Runner) structuredOutputFromValidationPlan(
+	def *domain.PromptDefinition,
+	contract domain.OutputContract,
+	plan validate.PreparedValidation,
+) (*domain.StructuredOutputSpec, error) {
+	if contract.ValidationMode != domain.ValidationJSONSchema {
+		return nil, nil
+	}
+
+	schemaDocument := plan.SchemaDocument()
+	if schemaDocument == nil {
+		if r.validator == nil {
+			return nil, nil
+		}
+		return nil, fmt.Errorf("%w: prepared json_schema validation has no schema document", ErrValidation)
+	}
+	return structuredOutputSpec(def, schemaDocument), nil
+}
+
+// Details returns a fresh credential-redacted copy of the prepared run.
+func (p *PreparedExecution) Details() *domain.PreparedRun {
+	if p == nil {
+		return nil
+	}
+	p.mu.Lock()
+	detailsSnapshot := p.details
+	p.mu.Unlock()
+
+	details, err := clonePreparedRun(detailsSnapshot)
+	if err != nil {
+		return nil
+	}
+	return details
+}
+
+// Discard invalidates an unclaimed execution and drops its private payload.
+func (p *PreparedExecution) Discard() {
+	if p == nil {
+		return
+	}
+
+	p.mu.Lock()
+	if p.state != preparedExecutionReady {
+		p.mu.Unlock()
+		return
+	}
+	p.state = preparedExecutionDiscarded
+	payload := p.payload
+	p.payload = nil
+	p.mu.Unlock()
+
+	payload.clear()
+}
+
+// RunPrepared claims and executes one prepared execution owned by this runner.
+func (r *Runner) RunPrepared(ctx context.Context, prepared *PreparedExecution) (*domain.RunResult, error) {
+	payload, err := prepared.claim(r)
+	if err != nil {
+		return nil, err
+	}
+	defer payload.clear()
+
+	runID, err := newRunID()
+	if err != nil {
+		return nil, fmt.Errorf("failed to create run id: %w", err)
+	}
+	start := time.Now().UTC()
+
+	target := payload.prepared.EffectiveModelParams
+	if err := validateAPIKey(target.APIKeyEnv, target.APIKey, target.APIKeyRequired); err != nil {
+		return nil, fmt.Errorf("%w: %w", ErrInvalidRequest, err)
+	}
+
+	release, err := r.admitRun(ctx, target.BackendID)
+	if err != nil {
+		return nil, err
+	}
+	defer release()
+
+	return r.executePreparedRun(ctx, payload.prepared, runID, start, func(
+		ctx context.Context,
+		artifact *domain.Artifact,
+		attemptsUsed int,
+	) (domain.ValidationResult, error) {
+		result, validationErr := payload.validation.Validate(ctx, artifact)
+		if validationErr != nil {
+			return domain.ValidationResult{}, validationErr
+		}
+		result.RepairAttempts = attemptsUsed
+		return result, nil
+	})
+}
+
+func (p *PreparedExecution) claim(owner *Runner) (*preparedExecutionPayload, error) {
+	if p == nil || owner == nil || p.owner != owner {
+		return nil, fmt.Errorf("%w: prepared execution does not belong to this runner", ErrInvalidRequest)
+	}
+
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	if p.state != preparedExecutionReady || p.payload == nil {
+		return nil, fmt.Errorf("%w: prepared execution is not ready", ErrInvalidRequest)
+	}
+	p.state = preparedExecutionClaimed
+	payload := p.payload
+	p.payload = nil
+	return payload, nil
+}
+
+func (p *preparedExecutionPayload) clear() {
+	if p == nil {
+		return
+	}
+	if p.prepared != nil {
+		p.prepared.EffectiveModelParams.APIKey = ""
+	}
+	p.prepared = nil
+	p.validation = nil
+}
+
+type noOpPreparedValidation struct {
+	contract domain.OutputContract
+}
+
+func (p noOpPreparedValidation) Validate(
+	ctx context.Context,
+	_ *domain.Artifact,
+) (domain.ValidationResult, error) {
+	if err := ctx.Err(); err != nil {
+		return domain.ValidationResult{}, err
+	}
+	return domain.ValidationResult{
+		Status:         domain.ValidationSkipped,
+		Mode:           p.contract.ValidationMode,
+		SchemaPath:     p.contract.SchemaPath,
+		RepairAttempts: p.contract.RepairAttempts,
+		IsValid:        true,
+	}, nil
+}
+
+func (noOpPreparedValidation) SchemaDocument() any {
+	return nil
+}
+
+func clonePreparedRun(source *domain.PreparedRun) (*domain.PreparedRun, error) {
+	if source == nil {
+		return nil, nil
+	}
+
+	copied := *source
+	extraParams, err := jsonvalue.CopyMap(source.EffectiveModelParams.ExtraParams)
+	if err != nil {
+		return nil, err
+	}
+	copied.EffectiveModelParams.ExtraParams = extraParams
+
+	if source.InputHashes != nil {
+		copied.InputHashes = make(map[string]string, len(source.InputHashes))
+		for name, hash := range source.InputHashes {
+			copied.InputHashes[name] = hash
+		}
+	}
+	if source.Messages != nil {
+		copied.Messages = make([]domain.RenderedMessage, len(source.Messages))
+		for i, message := range source.Messages {
+			copied.Messages[i] = message
+			if message.CacheControl != nil {
+				cacheControl := *message.CacheControl
+				copied.Messages[i].CacheControl = &cacheControl
+			}
+		}
+	}
+
+	if source.StructuredOutput != nil {
+		structuredOutput := *source.StructuredOutput
+		copied.StructuredOutput = &structuredOutput
+		if source.StructuredOutput.JSONSchema != nil {
+			jsonSchema := *source.StructuredOutput.JSONSchema
+			copied.StructuredOutput.JSONSchema = &jsonSchema
+			schema, copyErr := cloneJSONValue(source.StructuredOutput.JSONSchema.Schema)
+			if copyErr != nil {
+				return nil, copyErr
+			}
+			copied.StructuredOutput.JSONSchema.Schema = schema
+		}
+	}
+	return &copied, nil
+}
+
+func cloneJSONValue(source any) (any, error) {
+	return jsonvalue.Copy(source)
+}
diff --git a/internal/usecase/prepared_execution_test.go b/internal/usecase/prepared_execution_test.go
new file mode 100644
index 0000000..d0649d7
--- /dev/null
+++ b/internal/usecase/prepared_execution_test.go
@@ -0,0 +1,468 @@
+package usecase
+
+import (
+	"context"
+	"errors"
+	"os"
+	"reflect"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
+	"gitea.maximumdirect.net/eric/promptkit/internal/validate"
+)
+
+type recordingPreparedValidation struct {
+	contract       domain.OutputContract
+	schemaDocument any
+	results        []domain.ValidationResult
+	errs           []error
+	artifacts      []string
+}
+
+func (p *recordingPreparedValidation) Validate(
+	_ context.Context,
+	artifact *domain.Artifact,
+) (domain.ValidationResult, error) {
+	p.artifacts = append(p.artifacts, string(artifact.Body))
+	index := len(p.artifacts) - 1
+	if index < len(p.errs) && p.errs[index] != nil {
+		return domain.ValidationResult{}, p.errs[index]
+	}
+	if len(p.results) == 0 {
+		return domain.ValidationResult{
+			Status:  domain.ValidationPassed,
+			Mode:    p.contract.ValidationMode,
+			IsValid: true,
+		}, nil
+	}
+	if index >= len(p.results) {
+		index = len(p.results) - 1
+	}
+	return p.results[index], nil
+}
+
+func (p *recordingPreparedValidation) SchemaDocument() any {
+	return p.schemaDocument
+}
+
+type recordingValidationPreparer struct {
+	plan                *recordingPreparedValidation
+	prepareErr          error
+	prepareCalls        int
+	directValidateCalls int
+}
+
+func (v *recordingValidationPreparer) Validate(
+	context.Context,
+	*domain.Artifact,
+	domain.OutputContract,
+) (domain.ValidationResult, error) {
+	v.directValidateCalls++
+	return domain.ValidationResult{}, errors.New("live validation must not be used")
+}
+
+func (v *recordingValidationPreparer) PrepareValidation(
+	_ context.Context,
+	contract domain.OutputContract,
+) (validate.PreparedValidation, error) {
+	v.prepareCalls++
+	if v.prepareErr != nil {
+		return nil, v.prepareErr
+	}
+	v.plan.contract = contract
+	return v.plan, nil
+}
+
+func TestRunnerPrepareExecutionCompletesWithoutAdmissionOrGeneration(t *testing.T) {
+	schemaDocument := map[string]any{
+		"type": "object",
+		"properties": map[string]any{
+			"value": map[string]any{"type": "string"},
+			"":      map[string]any{"type": "boolean"},
+		},
+	}
+	def := promptDef(domain.FormatJSON, domain.ValidationJSONSchema, 0)
+	def.Validation.SchemaPath = "schema.json"
+	reader := defaultArtifactReader()
+	renderer := &fakeRenderer{rendered: &domain.RenderedPrompt{
+		SessionID: "prepared-session",
+		Messages: []domain.RenderedMessage{{
+			Role:    "user",
+			Content: "original message",
+		}},
+	}}
+	llmClient := &fakeLLM{forbid: true}
+	validator := &recordingValidationPreparer{
+		plan: &recordingPreparedValidation{schemaDocument: schemaDocument},
+	}
+	admitter := &fakeRunAdmitter{}
+	profile := defaultExecutionProfile()
+	profile.ExtraParams = map[string]any{
+		"metadata": map[string]any{"source": "original"},
+	}
+	runner := NewRunner(
+		&fakePromptRepo{def: def},
+		&fakeExecutionProfileRepo{profiles: map[string]*domain.ExecutionProfile{"exec": profile}},
+		nil,
+		reader,
+		renderer,
+		llmClient,
+		validator,
+		admitter,
+	)
+
+	prepared, err := runner.PrepareExecution(context.Background(), domain.RunRequest{
+		PromptID:  "p",
+		ProfileID: "exec",
+		APIKey:    "direct-test-key",
+		Inputs:    singleInputRef(),
+	})
+	if err != nil {
+		t.Fatalf("prepare execution: %v", err)
+	}
+	defer prepared.Discard()
+
+	if validator.prepareCalls != 1 || validator.directValidateCalls != 0 {
+		t.Fatalf(
+			"validation calls=(prepare=%d direct=%d), want (1, 0)",
+			validator.prepareCalls,
+			validator.directValidateCalls,
+		)
+	}
+	if reader.calls != 1 || renderer.calls != 1 {
+		t.Fatalf("completion calls=(artifact=%d render=%d), want (1, 1)", reader.calls, renderer.calls)
+	}
+	if len(admitter.backendIDs) != 0 || llmClient.calls != 0 {
+		t.Fatalf("prepare invoked execution collaborators: admission=%v generation=%d", admitter.backendIDs, llmClient.calls)
+	}
+
+	first := prepared.Details()
+	if first == nil {
+		t.Fatal("prepared details are nil")
+	}
+	if first.EffectiveModelParams.APIKey != "" {
+		t.Fatal("prepared details retained the direct API key")
+	}
+	if first.StructuredOutput == nil ||
+		first.StructuredOutput.JSONSchema == nil ||
+		!reflect.DeepEqual(first.StructuredOutput.JSONSchema.Schema, schemaDocument) {
+		t.Fatalf("prepared details have unexpected structured output: %#v", first.StructuredOutput)
+	}
+
+	first.Messages[0].Content = "caller mutation"
+	first.InputHashes["input"] = "caller mutation"
+	first.EffectiveModelParams.ExtraParams["metadata"].(map[string]any)["source"] = "caller mutation"
+	first.StructuredOutput.JSONSchema.Schema.(map[string]any)["type"] = "string"
+	renderer.rendered.Messages[0].Content = "source mutation"
+
+	second := prepared.Details()
+	if second.Messages[0].Content != "original message" ||
+		second.InputHashes["input"] == "caller mutation" ||
+		second.EffectiveModelParams.ExtraParams["metadata"].(map[string]any)["source"] != "original" ||
+		second.StructuredOutput.JSONSchema.Schema.(map[string]any)["type"] != "object" {
+		t.Fatalf("details did not preserve an independent snapshot: %#v", second)
+	}
+}
+
+func TestRunnerRunPreparedRechecksEnvironmentCredentialBeforeAdmission(t *testing.T) {
+	const environmentName = "PROMPTKIT_PREPARED_EXECUTION_TEST_KEY"
+	t.Setenv(environmentName, "available-during-preparation")
+
+	profile := defaultExecutionProfile()
+	profile.APIKeyEnv = environmentName
+	validator := &recordingValidationPreparer{plan: &recordingPreparedValidation{}}
+	admitter := &fakeRunAdmitter{}
+	llmClient := &fakeLLM{resp: &domain.GenerateResponse{Content: "unexpected"}}
+	runner := NewRunner(
+		&fakePromptRepo{def: promptDef(domain.FormatText, domain.ValidationNone, 0)},
+		&fakeExecutionProfileRepo{profiles: map[string]*domain.ExecutionProfile{"exec": profile}},
+		nil,
+		defaultArtifactReader(),
+		defaultRenderer(),
+		llmClient,
+		validator,
+		admitter,
+	)
+	prepared, err := runner.PrepareExecution(context.Background(), domain.RunRequest{
+		PromptID:  "p",
+		ProfileID: "exec",
+		Inputs:    singleInputRef(),
+	})
+	if err != nil {
+		t.Fatalf("prepare execution: %v", err)
+	}
+	if err := os.Unsetenv(environmentName); err != nil {
+		t.Fatalf("unset credential environment: %v", err)
+	}
+
+	result, err := runner.RunPrepared(context.Background(), prepared)
+	if result != nil {
+		t.Fatalf("credential failure returned partial result: %+v", result)
+	}
+	if !errors.Is(err, ErrInvalidRequest) || !errors.Is(err, ErrAPIKeyEnvMissing) {
+		t.Fatalf("credential error identities are missing: %v", err)
+	}
+	if len(admitter.backendIDs) != 0 || llmClient.calls != 0 {
+		t.Fatalf("credential failure reached admission or generation: admission=%v generation=%d", admitter.backendIDs, llmClient.calls)
+	}
+	if _, err := runner.RunPrepared(context.Background(), prepared); !errors.Is(err, ErrInvalidRequest) {
+		t.Fatalf("credential failure did not consume execution: %v", err)
+	}
+}
+
+func TestRunnerRunPreparedUsesFrozenValidationForInitialAndRepairOutputs(t *testing.T) {
+	validator := &recordingValidationPreparer{
+		plan: &recordingPreparedValidation{
+			results: []domain.ValidationResult{
+				{
+					Status:  domain.ValidationFailed,
+					Mode:    domain.ValidationJSON,
+					Errors:  []string{"invalid"},
+					IsValid: false,
+				},
+				{
+					Status:  domain.ValidationPassed,
+					Mode:    domain.ValidationJSON,
+					IsValid: true,
+				},
+			},
+		},
+	}
+	repairer := &fakeRepairer{
+		responses: []*domain.GenerateResponse{{Content: `{"repaired":true}`}},
+	}
+	admitter := &fakeRunAdmitter{}
+	reader := defaultArtifactReader()
+	renderer := defaultRenderer()
+	runner := NewRunnerWithRepairer(
+		&fakePromptRepo{def: promptDef(domain.FormatJSON, domain.ValidationJSON, 1)},
+		&fakeExecutionProfileRepo{profiles: map[string]*domain.ExecutionProfile{"exec": defaultExecutionProfile()}},
+		nil,
+		reader,
+		renderer,
+		&fakeLLM{resp: &domain.GenerateResponse{Content: `{"broken":true}`}},
+		validator,
+		repairer,
+		admitter,
+	)
+	prepared, err := runner.PrepareExecution(context.Background(), domain.RunRequest{
+		PromptID:  "p",
+		ProfileID: "exec",
+		Inputs:    singleInputRef(),
+	})
+	if err != nil {
+		t.Fatalf("prepare execution: %v", err)
+	}
+
+	result, err := runner.RunPrepared(context.Background(), prepared)
+	if err != nil {
+		t.Fatalf("run prepared: %v", err)
+	}
+	if !reflect.DeepEqual(validator.plan.artifacts, []string{`{"broken":true}`, `{"repaired":true}`}) {
+		t.Fatalf("prepared validation artifacts=%#v", validator.plan.artifacts)
+	}
+	if validator.directValidateCalls != 0 || repairer.calls != 1 {
+		t.Fatalf("validation/repair calls=(direct=%d repair=%d), want (0, 1)", validator.directValidateCalls, repairer.calls)
+	}
+	if result.Validation.Status != domain.ValidationPassed || result.Validation.RepairAttempts != 1 {
+		t.Fatalf("unexpected repaired validation result: %+v", result.Validation)
+	}
+	if admitter.releaseCalls != 1 {
+		t.Fatalf("admission releases=%d, want 1", admitter.releaseCalls)
+	}
+	if reader.calls != 1 || renderer.calls != 1 {
+		t.Fatalf("execution reopened preparation sources: artifact=%d render=%d", reader.calls, renderer.calls)
+	}
+}
+
+func TestRunnerRunPreparedReleasesAdmissionAcrossExecutionErrors(t *testing.T) {
+	generationFailure := errors.New("generation failed")
+	validationFailure := errors.New("validation failed")
+	repairFailure := errors.New("repair failed")
+
+	tests := []struct {
+		name          string
+		generationErr error
+		validation    *recordingPreparedValidation
+		repairer      *fakeRepairer
+		wantError     error
+	}{
+		{
+			name:          "generation failure",
+			generationErr: generationFailure,
+			validation:    &recordingPreparedValidation{},
+			wantError:     ErrLLMGenerate,
+		},
+		{
+			name: "validation failure",
+			validation: &recordingPreparedValidation{
+				errs: []error{validationFailure},
+			},
+			wantError: ErrValidation,
+		},
+		{
+			name: "repair failure",
+			validation: &recordingPreparedValidation{
+				results: []domain.ValidationResult{{
+					Status:  domain.ValidationFailed,
+					Mode:    domain.ValidationJSON,
+					Errors:  []string{"invalid"},
+					IsValid: false,
+				}},
+			},
+			repairer:  &fakeRepairer{err: repairFailure},
+			wantError: ErrValidation,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			def := promptDef(domain.FormatJSON, domain.ValidationJSON, 1)
+			if test.repairer == nil {
+				def.Validation.RepairAttempts = 0
+			}
+			validator := &recordingValidationPreparer{plan: test.validation}
+			admitter := &fakeRunAdmitter{}
+			runner := NewRunnerWithRepairer(
+				&fakePromptRepo{def: def},
+				&fakeExecutionProfileRepo{profiles: map[string]*domain.ExecutionProfile{"exec": defaultExecutionProfile()}},
+				nil,
+				defaultArtifactReader(),
+				defaultRenderer(),
+				&fakeLLM{
+					resp: &domain.GenerateResponse{Content: `{"value":true}`},
+					err:  test.generationErr,
+				},
+				validator,
+				test.repairer,
+				admitter,
+			)
+			prepared, err := runner.PrepareExecution(context.Background(), domain.RunRequest{
+				PromptID:  "p",
+				ProfileID: "exec",
+				Inputs:    singleInputRef(),
+			})
+			if err != nil {
+				t.Fatalf("prepare execution: %v", err)
+			}
+
+			result, err := runner.RunPrepared(context.Background(), prepared)
+			if result != nil || !errors.Is(err, test.wantError) {
+				t.Fatalf("run prepared=(%+v, %v), want %v", result, err, test.wantError)
+			}
+			if len(admitter.backendIDs) != 1 || admitter.releaseCalls != 1 {
+				t.Fatalf(
+					"admission calls=%#v releases=%d, want one each",
+					admitter.backendIDs,
+					admitter.releaseCalls,
+				)
+			}
+		})
+	}
+}
+
+func TestRunnerPreparedExecutionOwnershipUseAndDiscard(t *testing.T) {
+	newRunner := func() *Runner {
+		return NewRunner(
+			&fakePromptRepo{def: promptDef(domain.FormatText, domain.ValidationNone, 0)},
+			&fakeExecutionProfileRepo{profiles: map[string]*domain.ExecutionProfile{"exec": defaultExecutionProfile()}},
+			nil,
+			defaultArtifactReader(),
+			defaultRenderer(),
+			&fakeLLM{resp: &domain.GenerateResponse{Content: "ok"}},
+			&recordingValidationPreparer{plan: &recordingPreparedValidation{}},
+			nil,
+		)
+	}
+	request := domain.RunRequest{
+		PromptID:  "p",
+		ProfileID: "exec",
+		Inputs:    singleInputRef(),
+	}
+
+	owner := newRunner()
+	prepared, err := owner.PrepareExecution(context.Background(), request)
+	if err != nil {
+		t.Fatalf("prepare execution: %v", err)
+	}
+	if _, err := newRunner().RunPrepared(context.Background(), prepared); !errors.Is(err, ErrInvalidRequest) {
+		t.Fatalf("foreign runner error=%v, want ErrInvalidRequest", err)
+	}
+	if _, err := owner.RunPrepared(context.Background(), prepared); err != nil {
+		t.Fatalf("owner run prepared: %v", err)
+	}
+	if _, err := owner.RunPrepared(context.Background(), prepared); !errors.Is(err, ErrInvalidRequest) {
+		t.Fatalf("second owner run error=%v, want ErrInvalidRequest", err)
+	}
+	if prepared.Details() == nil {
+		t.Fatal("details unavailable after execution")
+	}
+
+	discarded, err := owner.PrepareExecution(context.Background(), request)
+	if err != nil {
+		t.Fatalf("prepare discarded execution: %v", err)
+	}
+	discarded.Discard()
+	discarded.Discard()
+	if _, err := owner.RunPrepared(context.Background(), discarded); !errors.Is(err, ErrInvalidRequest) {
+		t.Fatalf("discarded execution error=%v, want ErrInvalidRequest", err)
+	}
+	if discarded.Details() == nil {
+		t.Fatal("details unavailable after discard")
+	}
+}
+
+func TestRunnerPreparedExecutionWithoutValidatorSkipsValidation(t *testing.T) {
+	runner := NewRunner(
+		&fakePromptRepo{def: promptDef(domain.FormatJSON, domain.ValidationJSON, 0)},
+		&fakeExecutionProfileRepo{profiles: map[string]*domain.ExecutionProfile{"exec": defaultExecutionProfile()}},
+		nil,
+		defaultArtifactReader(),
+		defaultRenderer(),
+		&fakeLLM{resp: &domain.GenerateResponse{Content: `{}`}},
+		nil,
+		nil,
+	)
+	prepared, err := runner.PrepareExecution(context.Background(), domain.RunRequest{
+		PromptID:  "p",
+		ProfileID: "exec",
+		Inputs:    singleInputRef(),
+	})
+	if err != nil {
+		t.Fatalf("prepare execution: %v", err)
+	}
+
+	result, err := runner.RunPrepared(context.Background(), prepared)
+	if err != nil {
+		t.Fatalf("run prepared: %v", err)
+	}
+	if result.Validation.Status != domain.ValidationSkipped || !result.Validation.IsValid {
+		t.Fatalf("unexpected no-validator result: %+v", result.Validation)
+	}
+}
+
+func TestRunnerPrepareExecutionRequiresValidationPreparer(t *testing.T) {
+	reader := defaultArtifactReader()
+	runner := NewRunner(
+		&fakePromptRepo{def: promptDef(domain.FormatText, domain.ValidationBasic, 0)},
+		&fakeExecutionProfileRepo{profiles: map[string]*domain.ExecutionProfile{"exec": defaultExecutionProfile()}},
+		nil,
+		reader,
+		defaultRenderer(),
+		&fakeLLM{forbid: true},
+		&fakeValidator{},
+		nil,
+	)
+
+	prepared, err := runner.PrepareExecution(context.Background(), domain.RunRequest{
+		PromptID:  "p",
+		ProfileID: "exec",
+		Inputs:    singleInputRef(),
+	})
+	if prepared != nil || !errors.Is(err, ErrValidation) {
+		t.Fatalf("prepare execution=(%+v, %v), want ErrValidation", prepared, err)
+	}
+	if reader.calls != 0 {
+		t.Fatalf("unsupported validator allowed completion, artifact calls=%d", reader.calls)
+	}
+}
diff --git a/internal/usecase/runner.go b/internal/usecase/runner.go
index 23738a5..f60fd6a 100644
--- a/internal/usecase/runner.go
+++ b/internal/usecase/runner.go
@@ -131,26 +131,39 @@ func (r *Runner) Run(ctx context.Context, req domain.RunRequest) (*domain.RunRes
 		return nil, err
 	}
 
-	if r.admitter != nil {
-		release, admitErr := r.admitter.Admit(ctx, state.effectiveModel.BackendID)
-		if admitErr != nil {
-			if errors.Is(admitErr, capacity.ErrCapacityExceeded) {
-				return nil, fmt.Errorf(
-					"backend %q admission: %w",
-					state.effectiveModel.BackendID,
-					admitErr,
-				)
-			}
-			return nil, admitErr
-		}
-		defer release()
+	release, err := r.admitRun(ctx, state.effectiveModel.BackendID)
+	if err != nil {
+		return nil, err
 	}
+	defer release()
 
 	prepared, err := r.completePreparation(ctx, req, state)
 	if err != nil {
 		return nil, err
 	}
 
+	return r.executePreparedRun(ctx, prepared, runID, start, func(
+		ctx context.Context,
+		artifact *domain.Artifact,
+		attemptsUsed int,
+	) (domain.ValidationResult, error) {
+		return r.validateOutput(ctx, artifact, prepared.OutputContract, attemptsUsed)
+	})
+}
+
+type preparedValidationFunc func(
+	context.Context,
+	*domain.Artifact,
+	int,
+) (domain.ValidationResult, error)
+
+func (r *Runner) executePreparedRun(
+	ctx context.Context,
+	prepared *domain.PreparedRun,
+	runID string,
+	start time.Time,
+	validateArtifact preparedValidationFunc,
+) (*domain.RunResult, error) {
 	genResp, err := r.llm.Generate(ctx, domain.GenerateRequest{
 		Prompt:           domain.RenderedPrompt{SessionID: prepared.SessionID, Messages: prepared.Messages},
 		Target:           prepared.EffectiveModelParams,
@@ -165,7 +178,7 @@ func (r *Runner) Run(ctx context.Context, req domain.RunRequest) (*domain.RunRes
 	}
 
 	outputArtifact := buildOutputArtifact(genResp.Content, prepared.OutputContract.Format)
-	validationResult, err := r.validateOutput(ctx, &outputArtifact, prepared.OutputContract, 0)
+	validationResult, err := validateArtifact(ctx, &outputArtifact, 0)
 	if err != nil {
 		return nil, fmt.Errorf("%w: %w", ErrValidation, err)
 	}
@@ -195,7 +208,7 @@ func (r *Runner) Run(ctx context.Context, req domain.RunRequest) (*domain.RunRes
 			genResp = repairResp
 			outputArtifact = buildOutputArtifact(genResp.Content, prepared.OutputContract.Format)
 
-			validationResult, err = r.validateOutput(ctx, &outputArtifact, prepared.OutputContract, attemptsUsed)
+			validationResult, err = validateArtifact(ctx, &outputArtifact, attemptsUsed)
 			if err != nil {
 				return nil, fmt.Errorf("%w: %w", ErrValidation, err)
 			}
@@ -324,7 +337,15 @@ func (r *Runner) completePreparation(
 	if err != nil {
 		return nil, err
 	}
+	return r.completePreparationWithStructuredOutput(ctx, req, state, structuredOutput)
+}
 
+func (r *Runner) completePreparationWithStructuredOutput(
+	ctx context.Context,
+	req domain.RunRequest,
+	state *preparationState,
+	structuredOutput *domain.StructuredOutputSpec,
+) (*domain.PreparedRun, error) {
 	resolvedInputs := make(map[string]*domain.Artifact, len(req.Inputs))
 	inputHashes := make(map[string]string, len(req.Inputs))
 	for name, ref := range req.Inputs {
@@ -374,6 +395,20 @@ func (r *Runner) completePreparation(
 	}, nil
 }
 
+func (r *Runner) admitRun(ctx context.Context, backendID string) (func(), error) {
+	if r.admitter == nil {
+		return func() {}, nil
+	}
+	release, err := r.admitter.Admit(ctx, backendID)
+	if err != nil {
+		if errors.Is(err, capacity.ErrCapacityExceeded) {
+			return nil, fmt.Errorf("backend %q admission: %w", backendID, err)
+		}
+		return nil, err
+	}
+	return release, nil
+}
+
 func (r *Runner) resolveStructuredOutput(ctx context.Context, def *domain.PromptDefinition, contract domain.OutputContract) (*domain.StructuredOutputSpec, error) {
 	if contract.ValidationMode != domain.ValidationJSONSchema {
 		return nil, nil
@@ -389,14 +424,18 @@ func (r *Runner) resolveStructuredOutput(ctx context.Context, def *domain.Prompt
 		return nil, fmt.Errorf("%w: failed to load json schema for structured output: %v", ErrValidation, err)
 	}
 
+	return structuredOutputSpec(def, schemaDoc), nil
+}
+
+func structuredOutputSpec(def *domain.PromptDefinition, schemaDocument any) *domain.StructuredOutputSpec {
 	return &domain.StructuredOutputSpec{
 		Type: domain.StructuredOutputJSONSchema,
 		JSONSchema: &domain.StructuredOutputJSONSpec{
 			Name:   deriveStructuredSchemaName(def.ID, def.Version),
 			Strict: true,
-			Schema: schemaDoc,
+			Schema: schemaDocument,
 		},
-	}, nil
+	}
 }
 
 func deriveStructuredSchemaName(promptID string, promptVersion string) string {