diff --git a/engine_test.go b/engine_test.go
index 45bf9b3..1d92987 100644
--- a/engine_test.go
+++ b/engine_test.go
@@ -274,30 +274,183 @@ func TestGenerateRequestFormattingRedactsDirectAPIKey(t *testing.T) {
 	}
 }
 
-func TestPreparePreservesExplicitZeroExecutionOverrides(t *testing.T) {
-	engine := newContractEngine(t)
-	zeroFloat := 0.0
-	zeroInt := 0
-
-	prepared, err := engine.Prepare(context.Background(), scriptorium.RunRequest{
-		PromptID: frameworkMarkdownSummaryPromptID,
-		Inputs: map[string]scriptorium.ArtifactRef{
-			"transcript": scriptorium.File(frameworkTranscriptPath),
-			"glossary":   scriptorium.File(frameworkGlossaryPath),
-		},
-		Execution: &scriptorium.ExecutionTargetOverride{
-			Temperature:    &zeroFloat,
-			MaxTokens:      &zeroInt,
-			TopP:           &zeroFloat,
-			TimeoutSeconds: &zeroInt,
-		},
-	})
-	if err != nil {
-		t.Fatalf("expected prepare to succeed, got %v", err)
+func TestEngineExecutionSettingPrecedence(t *testing.T) {
+	floatPointer := func(value float64) *float64 {
+		return &value
 	}
-	target := prepared.EffectiveModelParams
-	if target.Temperature != 0 || target.MaxTokens != 0 || target.TopP != 0 || target.TimeoutSeconds != 0 {
-		t.Fatalf("expected explicit zero overrides in effective target, got %+v", target)
+	intPointer := func(value int) *int {
+		return &value
+	}
+
+	defaultsProfile := executionProfileFixture{
+		id:               "settings-defaults",
+		endpoint:         "http://profile-defaults.test/v1",
+		model:            "profile-defaults-model",
+		serviceTier:      "profile-defaults-tier",
+		reasoningEffort:  "profile-defaults-reasoning",
+		apiKeyEnv:        "SCRIPTORIUM_PRECEDENCE_DEFAULTS",
+		extraParamSource: "profile-defaults",
+	}
+	profileSettings := executionProfileFixture{
+		id:               "settings-profile",
+		endpoint:         "http://profile-settings.test/v1",
+		model:            "profile-settings-model",
+		temperature:      0.31,
+		maxTokens:        311,
+		topP:             0.61,
+		timeoutSeconds:   71,
+		serviceTier:      "profile-settings-tier",
+		reasoningEffort:  "profile-settings-reasoning",
+		apiKeyEnv:        "SCRIPTORIUM_PRECEDENCE_PROFILE",
+		extraParamSource: "profile-settings",
+	}
+	requestProfile := executionProfileFixture{
+		id:               "settings-request",
+		endpoint:         "http://profile-request.test/v1",
+		model:            "profile-request-model",
+		temperature:      0.29,
+		maxTokens:        299,
+		topP:             0.59,
+		timeoutSeconds:   79,
+		serviceTier:      "profile-request-tier",
+		reasoningEffort:  "profile-request-reasoning",
+		apiKeyEnv:        "SCRIPTORIUM_PRECEDENCE_REQUEST_PROFILE",
+		extraParamSource: "profile-request",
+	}
+	zeroOverrideProfile := executionProfileFixture{
+		id:               "settings-zero",
+		endpoint:         "http://profile-zero.test/v1",
+		model:            "profile-zero-model",
+		temperature:      0.43,
+		maxTokens:        433,
+		topP:             0.73,
+		timeoutSeconds:   83,
+		serviceTier:      "profile-zero-tier",
+		reasoningEffort:  "profile-zero-reasoning",
+		apiKeyEnv:        "SCRIPTORIUM_PRECEDENCE_ZERO",
+		extraParamSource: "profile-zero",
+	}
+
+	requestTarget := scriptorium.ExecutionTarget{
+		Endpoint:        "http://request-settings.test/v1",
+		Model:           "request-settings-model",
+		Temperature:     0.87,
+		MaxTokens:       877,
+		TopP:            0.97,
+		TimeoutSeconds:  177,
+		ServiceTier:     "request-settings-tier",
+		ReasoningEffort: "request-settings-reasoning",
+		APIKeyEnv:       "SCRIPTORIUM_PRECEDENCE_REQUEST",
+		ExtraParams:     map[string]any{"source": "request-settings"},
+	}
+	zeroOverrideTarget := executionTargetFromProfileFixture(zeroOverrideProfile)
+	zeroOverrideTarget.Temperature = 0
+	zeroOverrideTarget.MaxTokens = 0
+	zeroOverrideTarget.TopP = 0
+	zeroOverrideTarget.TimeoutSeconds = 0
+
+	tests := []struct {
+		name         string
+		profile      executionProfileFixture
+		override     *scriptorium.ExecutionTargetOverride
+		want         scriptorium.ExecutionTarget
+		wantPresence scriptorium.ExecutionTargetPresence
+	}{
+		{
+			name:    "framework defaults fill zero-valued profile settings",
+			profile: defaultsProfile,
+			want: scriptorium.ExecutionTarget{
+				Endpoint:        defaultsProfile.endpoint,
+				Model:           defaultsProfile.model,
+				Temperature:     0,
+				MaxTokens:       0,
+				TopP:            1,
+				TimeoutSeconds:  600,
+				ServiceTier:     defaultsProfile.serviceTier,
+				ReasoningEffort: defaultsProfile.reasoningEffort,
+				APIKeyEnv:       defaultsProfile.apiKeyEnv,
+				ExtraParams:     map[string]any{"source": defaultsProfile.extraParamSource},
+			},
+		},
+		{
+			name:    "profile settings replace framework defaults",
+			profile: profileSettings,
+			want:    executionTargetFromProfileFixture(profileSettings),
+		},
+		{
+			name:    "request settings replace profile settings",
+			profile: requestProfile,
+			override: &scriptorium.ExecutionTargetOverride{
+				Endpoint:        requestTarget.Endpoint,
+				Model:           requestTarget.Model,
+				Temperature:     floatPointer(requestTarget.Temperature),
+				MaxTokens:       intPointer(requestTarget.MaxTokens),
+				TopP:            floatPointer(requestTarget.TopP),
+				TimeoutSeconds:  intPointer(requestTarget.TimeoutSeconds),
+				ServiceTier:     requestTarget.ServiceTier,
+				ReasoningEffort: requestTarget.ReasoningEffort,
+				APIKeyEnv:       requestTarget.APIKeyEnv,
+				ExtraParams:     requestTarget.ExtraParams,
+			},
+			want:         requestTarget,
+			wantPresence: scriptorium.ExecutionTargetPresence{Temperature: true, MaxTokens: true, TopP: true, TimeoutSeconds: true},
+		},
+		{
+			name:    "explicit request zero replaces profile settings",
+			profile: zeroOverrideProfile,
+			override: &scriptorium.ExecutionTargetOverride{
+				Temperature:    floatPointer(0),
+				MaxTokens:      intPointer(0),
+				TopP:           floatPointer(0),
+				TimeoutSeconds: intPointer(0),
+			},
+			want:         zeroOverrideTarget,
+			wantPresence: scriptorium.ExecutionTargetPresence{Temperature: true, MaxTokens: true, TopP: true, TimeoutSeconds: true},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Setenv(tt.profile.apiKeyEnv, "set")
+			if tt.override != nil && tt.override.APIKeyEnv != "" {
+				t.Setenv(tt.override.APIKeyEnv, "set")
+			}
+
+			profileDir := t.TempDir()
+			writeExecutionProfileFixture(t, profileDir, tt.profile)
+			fake := &fakeLLMClient{response: &scriptorium.GenerateResponse{Content: "ok"}}
+			engine, err := scriptorium.NewEngine(scriptorium.Config{
+				PromptDir:  frameworkPromptDir,
+				ProfileDir: profileDir,
+				SchemaDir:  frameworkSchemaDir,
+			}, scriptorium.WithLLMClient(fake))
+			if err != nil {
+				t.Fatalf("construct engine: %v", err)
+			}
+
+			_, err = engine.Run(context.Background(), scriptorium.RunRequest{
+				PromptID:  frameworkMarkdownSummaryPromptID,
+				ProfileID: tt.profile.id,
+				Inputs: map[string]scriptorium.ArtifactRef{
+					"transcript": scriptorium.Inline("Nia labels the archive."),
+					"glossary":   scriptorium.Inline("archive: A catalogued collection."),
+				},
+				Execution: tt.override,
+			})
+			if err != nil {
+				t.Fatalf("run engine: %v", err)
+			}
+			if len(fake.requests) != 1 {
+				t.Fatalf("expected one generation request, got %d", len(fake.requests))
+			}
+			got := fake.requests[0]
+			if !reflect.DeepEqual(got.Target, tt.want) {
+				t.Fatalf("unexpected effective target:\ngot=%#v\nwant=%#v", got.Target, tt.want)
+			}
+			if got.TargetPresence != tt.wantPresence {
+				t.Fatalf("unexpected target presence: got=%+v want=%+v", got.TargetPresence, tt.wantPresence)
+			}
+		})
 	}
 }
 
@@ -419,25 +572,17 @@ func TestEngineRunWithDirectorySourcesAndFileInputs(t *testing.T) {
 func TestRunPassesPreparedRequestToInjectedLLMClient(t *testing.T) {
 	const directKey = "direct-injected-key"
 	fake := &fakeLLMClient{
-		response: &scriptorium.GenerateResponse{Content: "ok"},
+		response: &scriptorium.GenerateResponse{Content: `{"events":[{"title":"Archive labelled"}]}`},
 	}
 	engine := newContractEngineWithOptions(t, frameworkSchemaDir, scriptorium.WithLLMClient(fake))
-	zeroFloat := 0.0
-	zeroInt := 0
 
 	_, err := engine.Run(context.Background(), scriptorium.RunRequest{
-		PromptID: frameworkMarkdownSummaryPromptID,
+		PromptID: frameworkStructuredEventsPromptID,
 		APIKey:   directKey,
 		Inputs: map[string]scriptorium.ArtifactRef{
 			"transcript": scriptorium.Inline("Rin opens the gate."),
 			"glossary":   scriptorium.Inline("gate: A guarded passage."),
 		},
-		Execution: &scriptorium.ExecutionTargetOverride{
-			Temperature:    &zeroFloat,
-			MaxTokens:      &zeroInt,
-			TopP:           &zeroFloat,
-			TimeoutSeconds: &zeroInt,
-		},
 	})
 	if err != nil {
 		t.Fatalf("expected run to succeed, got %v", err)
@@ -449,14 +594,8 @@ func TestRunPassesPreparedRequestToInjectedLLMClient(t *testing.T) {
 	if len(req.Prompt.Messages) != 2 || !strings.Contains(req.Prompt.Messages[1].Content, "Rin opens the gate.") {
 		t.Fatalf("expected rendered prompt in generate request, got %+v", req.Prompt)
 	}
-	if req.Target.Model != "contract-fast-model" || req.Target.Temperature != 0 || req.Target.MaxTokens != 0 || req.Target.TopP != 0 || req.Target.TimeoutSeconds != 0 {
-		t.Fatalf("unexpected effective target: %+v", req.Target)
-	}
-	if !req.TargetPresence.Temperature || !req.TargetPresence.MaxTokens || !req.TargetPresence.TopP || !req.TargetPresence.TimeoutSeconds {
-		t.Fatalf("expected explicit zero target presence, got %+v", req.TargetPresence)
-	}
-	if req.StructuredOutput != nil {
-		t.Fatalf("did not expect structured output for markdown prompt: %+v", req.StructuredOutput)
+	if req.StructuredOutput == nil || req.StructuredOutput.Type != scriptorium.StructuredOutputJSONSchema || req.StructuredOutput.JSONSchema == nil {
+		t.Fatalf("expected structured output handoff, got %+v", req.StructuredOutput)
 	}
 	if req.APIKey != directKey {
 		t.Fatalf("expected direct key on injected generate request")
@@ -470,6 +609,78 @@ func TestRunPassesPreparedRequestToInjectedLLMClient(t *testing.T) {
 	}
 }
 
+func TestEngineRunPropagatesCallerCancellation(t *testing.T) {
+	started := make(chan struct{})
+	transport := roundTripFunc(func(req *http.Request) (*http.Response, error) {
+		close(started)
+		<-req.Context().Done()
+		return nil, req.Context().Err()
+	})
+	engine, err := scriptorium.NewEngine(scriptorium.Config{
+		PromptDir:  frameworkPromptDir,
+		ProfileDir: frameworkProfileDir,
+		SchemaDir:  frameworkSchemaDir,
+		HTTPClient: &http.Client{Transport: transport},
+	})
+	if err != nil {
+		t.Fatalf("construct engine: %v", err)
+	}
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	result := make(chan error, 1)
+	go func() {
+		_, err := engine.Run(ctx, scriptorium.RunRequest{
+			PromptID: frameworkMarkdownSummaryPromptID,
+			Inputs: map[string]scriptorium.ArtifactRef{
+				"transcript": scriptorium.Inline("Nia labels the archive."),
+				"glossary":   scriptorium.Inline("archive: A catalogued collection."),
+			},
+		})
+		result <- err
+	}()
+
+	<-started
+	cancel()
+	if err := <-result; !errors.Is(err, scriptorium.ErrLLMGenerate) {
+		t.Fatalf("expected ErrLLMGenerate after caller cancellation, got %v", err)
+	}
+}
+
+func TestRunRejectsReservedExtraParamsBeforeProviderCall(t *testing.T) {
+	called := false
+	transport := roundTripFunc(func(*http.Request) (*http.Response, error) {
+		called = true
+		return nil, errors.New("provider should not be called")
+	})
+	engine, err := scriptorium.NewEngine(scriptorium.Config{
+		PromptDir:  frameworkPromptDir,
+		ProfileDir: frameworkProfileDir,
+		SchemaDir:  frameworkSchemaDir,
+		HTTPClient: &http.Client{Transport: transport},
+	})
+	if err != nil {
+		t.Fatalf("construct engine: %v", err)
+	}
+
+	_, err = engine.Run(context.Background(), scriptorium.RunRequest{
+		PromptID: frameworkMarkdownSummaryPromptID,
+		Inputs: map[string]scriptorium.ArtifactRef{
+			"transcript": scriptorium.Inline("Nia labels the archive."),
+			"glossary":   scriptorium.Inline("archive: A catalogued collection."),
+		},
+		Execution: &scriptorium.ExecutionTargetOverride{
+			ExtraParams: map[string]any{"model": "collision"},
+		},
+	})
+	if !errors.Is(err, scriptorium.ErrInvalidRequest) {
+		t.Fatalf("expected ErrInvalidRequest, got %v", err)
+	}
+	if called {
+		t.Fatal("expected reserved provider parameter to fail before the provider call")
+	}
+}
+
 func TestRunUsesDirectAPIKeyWithDefaultLLMClient(t *testing.T) {
 	const directKey = "direct-public-key"
 	const missingEnv = "SCRIPTORIUM_PUBLIC_DIRECT_MISSING"
@@ -702,6 +913,18 @@ func TestPublicErrorsSupportErrorsIs(t *testing.T) {
 			client: &fakeLLMClient{err: llmErr},
 			want:   scriptorium.ErrLLMGenerate,
 		},
+		{
+			name: "nil llm response",
+			req: scriptorium.RunRequest{
+				PromptID: frameworkMarkdownSummaryPromptID,
+				Inputs: map[string]scriptorium.ArtifactRef{
+					"transcript": scriptorium.Inline("Rin opens the gate."),
+					"glossary":   scriptorium.Inline("gate: A guarded passage."),
+				},
+			},
+			client: &fakeLLMClient{},
+			want:   scriptorium.ErrLLMGenerate,
+		},
 		{
 			name: "validation runtime failure",
 			req: scriptorium.RunRequest{
@@ -2057,6 +2280,67 @@ func publicSchemaJSON() string {
 }`
 }
 
+type executionProfileFixture struct {
+	id               string
+	endpoint         string
+	model            string
+	temperature      float64
+	maxTokens        int
+	topP             float64
+	timeoutSeconds   int
+	serviceTier      string
+	reasoningEffort  string
+	apiKeyEnv        string
+	extraParamSource string
+}
+
+func executionTargetFromProfileFixture(profile executionProfileFixture) scriptorium.ExecutionTarget {
+	return scriptorium.ExecutionTarget{
+		Endpoint:        profile.endpoint,
+		Model:           profile.model,
+		Temperature:     profile.temperature,
+		MaxTokens:       profile.maxTokens,
+		TopP:            profile.topP,
+		TimeoutSeconds:  profile.timeoutSeconds,
+		ServiceTier:     profile.serviceTier,
+		ReasoningEffort: profile.reasoningEffort,
+		APIKeyEnv:       profile.apiKeyEnv,
+		ExtraParams:     map[string]any{"source": profile.extraParamSource},
+	}
+}
+
+func writeExecutionProfileFixture(t *testing.T, dir string, profile executionProfileFixture) {
+	t.Helper()
+	data := fmt.Sprintf(`id: %s
+endpoint: %s
+model: %s
+temperature: %g
+max_tokens: %d
+top_p: %g
+timeout_seconds: %d
+service_tier: %s
+reasoning_effort: %s
+api_key_env: %s
+extra_params:
+  source: %q
+`,
+		profile.id,
+		profile.endpoint,
+		profile.model,
+		profile.temperature,
+		profile.maxTokens,
+		profile.topP,
+		profile.timeoutSeconds,
+		profile.serviceTier,
+		profile.reasoningEffort,
+		profile.apiKeyEnv,
+		profile.extraParamSource,
+	)
+	if err := os.WriteFile(filepath.Join(dir, profile.id+".yaml"), []byte(data), 0o644); err != nil {
+		t.Fatalf("write execution profile fixture: %v", err)
+	}
+}
+
 type fakeLLMClient struct {
 	response *scriptorium.GenerateResponse
 	err      error