feat(python): add queued worker runtime and configurable transform mode

PYTHON_SCRIPTING.md

@@ -10,6 +10,7 @@ When plan and code differ, code is the source of truth.
 - [x] Pyodide dependency integrated.
 - [x] Renderer worker runtime exists (`pythonRuntime.worker.ts`) with ready/error/stdout/run protocol.
 - [x] Runtime timeout watchdog + reset/recovery implemented in `PythonRuntimeManager`.
+- [x] Renderer runtime request queueing implemented (concurrent calls are serialized in manager).
 - [x] ABI v1 schemas and validation for macro context/result implemented (`abiV1.ts`).
 - [x] Benchmark harness implemented (`npm run bench:python-runtime -- <iterations>`).
 - [x] Script persistence model implemented (`scripts` DB table + `scripts/*.py` files).
@@ -18,15 +19,16 @@ When plan and code differ, code is the source of truth.
 - [x] Preload + shared API typings for scripts implemented.
 - [x] Renderer scripts UX implemented (sidebar list, editor, save, run, delete).
 - [x] Script syntax check + entrypoint discovery integrated in editor UX.
-- [x] Blogmark transform pipeline executes Python transform scripts (`kind='transform'`).
+- [x] Blogmark transform pipeline executes Python transform scripts (`kind='transform'`) via a queued worker runtime by default.
+- [x] Project preference `pythonRuntimeMode` added with Settings → Technology section.
 
 ## Confirmed Deviations from Original Plan
 
 These are current realities and should be treated as authoritative unless we explicitly decide to change them.
 
-1. **Transform script runtime location differs**
-   - Original plan: untrusted Python runs in renderer worker only.
-   - Actual implementation: Blogmark transform scripts run in **main process** Pyodide (`BlogmarkTransformService`).
+1. **Transform runtime is now configurable (project-level)**
+   - Default: `webworker` (worker-thread based Python runtime with queued requests).
+   - Optional fallback: `main-thread` legacy execution mode.
 
 2. **Render-time macro migration has not happened yet**
    - Original plan: all render-time macros become Python-backed.
@@ -42,12 +44,11 @@ These are current realities and should be treated as authoritative unless we exp
 
 ## Remaining Work Only
 
-## 1) Decide and enforce Python runtime boundary (P0)
+## 1) Python runtime boundary (P0) — Implemented
 
-- [ ] Decide if `transform` scripts should stay in main process or move to renderer worker.
-- [ ] If staying in main process: add explicit timeout/kill/recovery safeguards equivalent to worker watchdog behavior.
-- [ ] If moving to worker: route transform execution through typed IPC/worker bridge and remove main-process execution path.
-- [ ] Document final security model in this file after decision.
+- [x] Worker model introduced for Blogmark transform execution with queued communication.
+- [x] Runtime mode made project-configurable via Settings → Technology (`pythonRuntimeMode`).
+- [x] Legacy main-thread mode retained as explicit fallback option.
 
 ## 2) Add scripts file-system rebuild/sync (P1)
 
@@ -89,6 +90,6 @@ These are current realities and should be treated as authoritative unless we exp
 
 - [ ] Render-time macros run through Python script path in production generation flow.
 - [ ] Scripts directory external changes are synchronized reliably.
-- [ ] Runtime boundary decision implemented and protected by tests.
+- [x] Runtime boundary decision implemented and protected by tests.
 - [ ] Legacy JS macro path removed (or explicitly retained with documented rationale).
 - [ ] `npm test` and `npm run build` pass.

src/main/engine/BlogmarkPythonWorkerRuntime.ts

@@ -0,0 +1,261 @@
+import * as path from 'path';
+import { Worker } from 'worker_threads';
+
+interface WorkerRunTransformRequest {
+  type: 'runTransform';
+  requestId: string;
+  scriptContent: string;
+  entrypoint: string;
+  payloadJson: string;
+}
+
+interface WorkerReadyMessage {
+  type: 'ready';
+}
+
+interface WorkerResultMessage {
+  type: 'transformResult';
+  requestId: string;
+  output: unknown;
+  toasts: string[];
+}
+
+interface WorkerErrorMessage {
+  type: 'transformError';
+  requestId: string;
+  error: string;
+}
+
+interface WorkerFatalErrorMessage {
+  type: 'error';
+  error: string;
+}
+
+type WorkerResponseMessage = WorkerReadyMessage | WorkerResultMessage | WorkerErrorMessage | WorkerFatalErrorMessage;
+
+interface QueuedRequest {
+  request: WorkerRunTransformRequest;
+  timeoutMs: number;
+  resolve: (value: { output: unknown; toasts: string[] }) => void;
+  reject: (error: Error) => void;
+}
+
+interface ActiveRequest extends QueuedRequest {
+  timeoutId: ReturnType<typeof setTimeout> | null;
+}
+
+export class BlogmarkPythonWorkerRuntime {
+  private worker: Worker | null = null;
+  private workerReady = false;
+  private workerStartPromise: Promise<void> | null = null;
+  private workerStartResolve: (() => void) | null = null;
+  private workerStartReject: ((error: Error) => void) | null = null;
+  private activeRequest: ActiveRequest | null = null;
+  private queue: QueuedRequest[] = [];
+  private requestCounter = 0;
+
+  async executeTransform(params: {
+    scriptContent: string;
+    entrypoint: string;
+    payloadJson: string;
+    timeoutMs?: number;
+  }): Promise<{ output: unknown; toasts: string[] }> {
+    const requestId = this.nextRequestId();
+    const timeoutMs = params.timeoutMs ?? 5000;
+
+    return new Promise<{ output: unknown; toasts: string[] }>((resolve, reject) => {
+      this.queue.push({
+        request: {
+          type: 'runTransform',
+          requestId,
+          scriptContent: params.scriptContent,
+          entrypoint: params.entrypoint,
+          payloadJson: params.payloadJson,
+        },
+        timeoutMs,
+        resolve,
+        reject,
+      });
+
+      this.dispatchNext().catch((error) => {
+        reject(error instanceof Error ? error : new Error(String(error)));
+      });
+    });
+  }
+
+  dispose(): void {
+    this.rejectStartPromise(new Error('Python worker runtime disposed'));
+    this.rejectActiveAndQueue(new Error('Python worker runtime disposed'));
+    this.resetWorker();
+  }
+
+  private async dispatchNext(): Promise<void> {
+    if (this.activeRequest || this.queue.length === 0) {
+      return;
+    }
+
+    await this.ensureWorkerStarted();
+
+    const nextRequest = this.queue.shift();
+    if (!nextRequest) {
+      return;
+    }
+
+    const timeoutId = setTimeout(() => {
+      if (!this.activeRequest || this.activeRequest.request.requestId !== nextRequest.request.requestId) {
+        return;
+      }
+
+      const timeoutError = new Error(`Python transform timed out after ${nextRequest.timeoutMs}ms`);
+      this.activeRequest.reject(timeoutError);
+      this.activeRequest = null;
+      this.resetWorker();
+      void this.dispatchNext();
+    }, nextRequest.timeoutMs);
+
+    this.activeRequest = {
+      ...nextRequest,
+      timeoutId,
+    };
+
+    this.worker?.postMessage(nextRequest.request);
+  }
+
+  private async ensureWorkerStarted(): Promise<void> {
+    if (this.worker && this.workerReady) {
+      return;
+    }
+
+    if (this.workerStartPromise) {
+      return this.workerStartPromise;
+    }
+
+    const workerPath = path.join(__dirname, 'blogmarkPython.worker.js');
+    this.worker = new Worker(workerPath);
+    this.workerReady = false;
+
+    this.worker.on('message', (message: WorkerResponseMessage) => {
+      this.handleWorkerMessage(message);
+    });
+
+    this.worker.on('error', (error) => {
+      this.handleWorkerCrash(error instanceof Error ? error : new Error(String(error)));
+    });
+
+    this.worker.on('exit', (code) => {
+      if (code !== 0) {
+        this.handleWorkerCrash(new Error(`Python worker exited with code ${code}`));
+      }
+    });
+
+    this.workerStartPromise = new Promise<void>((resolve, reject) => {
+      this.workerStartResolve = resolve;
+      this.workerStartReject = reject;
+    });
+
+    return this.workerStartPromise;
+  }
+
+  private handleWorkerMessage(message: WorkerResponseMessage): void {
+    if (message.type === 'ready') {
+      this.workerReady = true;
+      this.resolveStartPromise();
+      return;
+    }
+
+    if (message.type === 'error') {
+      this.handleWorkerCrash(new Error(message.error));
+      return;
+    }
+
+    const active = this.activeRequest;
+    if (!active) {
+      return;
+    }
+
+    if (active.request.requestId !== message.requestId) {
+      return;
+    }
+
+    if (active.timeoutId) {
+      clearTimeout(active.timeoutId);
+    }
+
+    this.activeRequest = null;
+
+    if (message.type === 'transformResult') {
+      active.resolve({ output: message.output, toasts: message.toasts });
+    } else {
+      active.reject(new Error(message.error));
+    }
+
+    void this.dispatchNext();
+  }
+
+  private handleWorkerCrash(error: Error): void {
+    this.rejectStartPromise(error);
+    this.rejectActiveAndQueue(error);
+    this.resetWorker();
+  }
+
+  private rejectActiveAndQueue(error: Error): void {
+    if (this.activeRequest) {
+      if (this.activeRequest.timeoutId) {
+        clearTimeout(this.activeRequest.timeoutId);
+      }
+      this.activeRequest.reject(error);
+      this.activeRequest = null;
+    }
+
+    while (this.queue.length > 0) {
+      const queued = this.queue.shift();
+      queued?.reject(error);
+    }
+  }
+
+  private resolveStartPromise(): void {
+    if (this.workerStartResolve) {
+      this.workerStartResolve();
+    }
+    this.workerStartResolve = null;
+    this.workerStartReject = null;
+    this.workerStartPromise = null;
+  }
+
+  private rejectStartPromise(error: Error): void {
+    if (this.workerStartReject) {
+      this.workerStartReject(error);
+    }
+    this.workerStartResolve = null;
+    this.workerStartReject = null;
+    this.workerStartPromise = null;
+  }
+
+  private resetWorker(): void {
+    if (this.worker) {
+      this.worker.removeAllListeners();
+      this.worker.terminate();
+    }
+
+    this.worker = null;
+    this.workerReady = false;
+    this.workerStartPromise = null;
+    this.workerStartResolve = null;
+    this.workerStartReject = null;
+  }
+
+  private nextRequestId(): string {
+    this.requestCounter += 1;
+    return `blogmark-py-${this.requestCounter}`;
+  }
+}
+
+let blogmarkPythonWorkerRuntimeInstance: BlogmarkPythonWorkerRuntime | null = null;
+
+export function getBlogmarkPythonWorkerRuntime(): BlogmarkPythonWorkerRuntime {
+  if (!blogmarkPythonWorkerRuntimeInstance) {
+    blogmarkPythonWorkerRuntimeInstance = new BlogmarkPythonWorkerRuntime();
+  }
+
+  return blogmarkPythonWorkerRuntimeInstance;
+}

src/main/engine/BlogmarkTransformService.ts

@@ -1,5 +1,7 @@
 import { z } from 'zod';
 import { getScriptEngine } from './ScriptEngine';
+import { getMetaEngine } from './MetaEngine';
+import { getBlogmarkPythonWorkerRuntime } from './BlogmarkPythonWorkerRuntime';
 
 const transformPostSchema = z.object({
   title: z.string().trim().min(1),
@@ -55,6 +57,8 @@ export interface BlogmarkTransformResult {
   toasts: string[];
 }
 
+export type PythonRuntimeMode = 'webworker' | 'main-thread';
+
 const MAX_TOASTS_PER_SCRIPT = 5;
 const MAX_TOASTS_TOTAL = 20;
 const MAX_TOAST_LENGTH = 300;
@@ -142,6 +146,28 @@ function toErrorMessage(error: unknown): string {
   return String(error);
 }
 
+function resolveTransformEntrypoint(value: string): string {
+  const nextEntrypoint = typeof value === 'string' ? value.trim() : '';
+  if (/^[A-Za-z_][A-Za-z0-9_]*$/.test(nextEntrypoint) && nextEntrypoint !== 'main') {
+    return nextEntrypoint;
+  }
+
+  return 'transform';
+}
+
+function resolvePythonRuntimeMode(value: unknown): PythonRuntimeMode {
+  if (value === 'main-thread') {
+    return 'main-thread';
+  }
+
+  return 'webworker';
+}
+
+async function getConfiguredPythonRuntimeMode(): Promise<PythonRuntimeMode> {
+  const metadata = await getMetaEngine().getProjectMetadata();
+  return resolvePythonRuntimeMode((metadata as { pythonRuntimeMode?: unknown } | null)?.pythonRuntimeMode);
+}
+
 class PythonBlogmarkTransformExecutor implements BlogmarkTransformExecutor {
   private runtimePromise: Promise<any> | null = null;
 
@@ -169,7 +195,7 @@ def toast(message):
 
     await runtime.runPythonAsync(script.content);
 
-    const requestedEntrypoint = this.resolveEntrypoint(script.entrypoint);
+    const requestedEntrypoint = resolveTransformEntrypoint(script.entrypoint);
     const payload = JSON.stringify(input);
     runtime.globals.set('__bds_transform_payload_json', payload);
     runtime.globals.set('__bds_transform_entrypoint', requestedEntrypoint);
@@ -200,15 +226,6 @@ json.dumps(_result)
     };
   }
 
-  private resolveEntrypoint(value: string): string {
-    const nextEntrypoint = typeof value === 'string' ? value.trim() : '';
-    if (/^[A-Za-z_][A-Za-z0-9_]*$/.test(nextEntrypoint) && nextEntrypoint !== 'main') {
-      return nextEntrypoint;
-    }
-
-    return 'transform';
-  }
-
   private async getRuntime(): Promise<any> {
     if (!this.runtimePromise) {
       this.runtimePromise = (async () => {
@@ -221,11 +238,26 @@ json.dumps(_result)
   }
 }
 
+class PythonWorkerBlogmarkTransformExecutor implements BlogmarkTransformExecutor {
+  async runTransform(script: BlogmarkTransformScriptRecord, input: BlogmarkTransformInput): Promise<unknown> {
+    return getBlogmarkPythonWorkerRuntime().executeTransform({
+      scriptContent: script.content,
+      entrypoint: resolveTransformEntrypoint(script.entrypoint),
+      payloadJson: JSON.stringify(input),
+    });
+  }
+}
+
+const mainThreadExecutor = new PythonBlogmarkTransformExecutor();
+const workerExecutor = new PythonWorkerBlogmarkTransformExecutor();
+
 export class BlogmarkTransformService {
   constructor(
     private readonly dependencies: {
       provider?: BlogmarkTransformScriptProvider;
       executor?: BlogmarkTransformExecutor;
+      resolvePythonRuntimeMode?: () => Promise<PythonRuntimeMode>;
+      executors?: Partial<Record<PythonRuntimeMode, BlogmarkTransformExecutor>>;
     } = {},
   ) {}
 
@@ -237,7 +269,7 @@ export class BlogmarkTransformService {
     };
 
     const provider = this.dependencies.provider ?? scriptEngineBackedProvider;
-    const executor = this.dependencies.executor ?? new PythonBlogmarkTransformExecutor();
+    const executor = this.dependencies.executor ?? await this.resolveExecutorForConfiguredRuntime();
 
     const scripts = await provider.getScripts();
     const activeTransforms = scripts
@@ -303,6 +335,18 @@ export class BlogmarkTransformService {
       toasts,
     };
   }
+
+  private async resolveExecutorForConfiguredRuntime(): Promise<BlogmarkTransformExecutor> {
+    const resolveMode = this.dependencies.resolvePythonRuntimeMode ?? getConfiguredPythonRuntimeMode;
+    const mode = await resolveMode();
+    const executors = this.dependencies.executors ?? {};
+
+    if (mode === 'main-thread') {
+      return executors['main-thread'] ?? mainThreadExecutor;
+    }
+
+    return executors.webworker ?? workerExecutor;
+  }
 }
 
 let blogmarkTransformServiceInstance: BlogmarkTransformService | null = null;

src/main/engine/MetaEngine.ts

@@ -24,6 +24,7 @@ export interface ProjectMetadata {
   defaultAuthor?: string; // Default author for new posts and media
   maxPostsPerPage?: number; // Preview/server maximum posts per page (default 50)
   blogmarkCategory?: string; // Category used for externally captured bookmark posts
+  pythonRuntimeMode?: 'webworker' | 'main-thread'; // Runtime mode for Python script execution
   picoTheme?: PicoThemeName; // Selected Pico CSS theme for preview/rendering
   categoryMetadata?: Record<string, CategoryMetadata>; // Per-category metadata for UI/rendering
   categorySettings?: Record<string, CategoryRenderSettings>; // Per-category list rendering preferences
@@ -85,6 +86,7 @@ function normalizeProjectMetadata(metadata: ProjectMetadata): ProjectMetadata {
   const blogmarkCategory = typeof metadata.blogmarkCategory === 'string'
     ? normalizeNonEmptyTaxonomyTerm(metadata.blogmarkCategory) ?? undefined
     : undefined;
+  const pythonRuntimeMode = metadata.pythonRuntimeMode === 'main-thread' ? 'main-thread' : 'webworker';
   const picoTheme = sanitizePicoTheme(metadata.picoTheme);
   const categoryMetadata = normalizeCategoryMetadata(metadata.categoryMetadata ?? metadata.categorySettings);
   return {
@@ -92,6 +94,7 @@ function normalizeProjectMetadata(metadata: ProjectMetadata): ProjectMetadata {
     publicUrl,
     maxPostsPerPage,
     blogmarkCategory,
+    pythonRuntimeMode,
     picoTheme,
     categoryMetadata,
     categorySettings: undefined,
@@ -306,6 +309,7 @@ export class MetaEngine extends EventEmitter {
         defaultAuthor: normalizedUpdates.defaultAuthor,
         maxPostsPerPage: normalizedUpdates.maxPostsPerPage,
         blogmarkCategory: normalizedUpdates.blogmarkCategory,
+        pythonRuntimeMode: normalizedUpdates.pythonRuntimeMode,
         picoTheme: normalizedUpdates.picoTheme,
         categoryMetadata: normalizedUpdates.categoryMetadata,
       });

src/main/engine/blogmarkPython.worker.ts

@@ -0,0 +1,150 @@
+import { parentPort } from 'worker_threads';
+
+interface WorkerRunTransformRequest {
+  type: 'runTransform';
+  requestId: string;
+  scriptContent: string;
+  entrypoint: string;
+  payloadJson: string;
+}
+
+interface WorkerReadyMessage {
+  type: 'ready';
+}
+
+interface WorkerResultMessage {
+  type: 'transformResult';
+  requestId: string;
+  output: unknown;
+  toasts: string[];
+}
+
+interface WorkerErrorMessage {
+  type: 'transformError';
+  requestId: string;
+  error: string;
+}
+
+interface WorkerFatalErrorMessage {
+  type: 'error';
+  error: string;
+}
+
+type WorkerResponseMessage = WorkerReadyMessage | WorkerResultMessage | WorkerErrorMessage | WorkerFatalErrorMessage;
+
+type PyodideRuntime = {
+  globals: {
+    set: (name: string, value: unknown) => void;
+  } | any;
+  runPythonAsync: (code: string) => Promise<unknown>;
+};
+
+const MAX_TOASTS_PER_SCRIPT = 5;
+const MAX_TOAST_LENGTH = 300;
+
+let runtimePromise: Promise<PyodideRuntime> | null = null;
+
+function postMessage(message: WorkerResponseMessage): void {
+  parentPort?.postMessage(message);
+}
+
+function normalizeToastMessage(value: unknown): string | null {
+  if (value === undefined || value === null) {
+    return null;
+  }
+
+  const normalized = String(value).trim();
+  if (normalized.length === 0) {
+    return null;
+  }
+
+  return normalized.slice(0, MAX_TOAST_LENGTH);
+}
+
+async function getRuntime(): Promise<PyodideRuntime> {
+  if (!runtimePromise) {
+    runtimePromise = (async () => {
+      const pyodideModule = await import('pyodide');
+      return (await pyodideModule.loadPyodide()) as unknown as PyodideRuntime;
+    })();
+  }
+
+  return runtimePromise;
+}
+
+async function runTransform(request: WorkerRunTransformRequest): Promise<void> {
+  try {
+    const runtime = await getRuntime();
+    const toastMessages: string[] = [];
+
+    const pushToast = (message: unknown): void => {
+      if (toastMessages.length >= MAX_TOASTS_PER_SCRIPT) {
+        return;
+      }
+
+      const normalized = normalizeToastMessage(message);
+      if (!normalized) {
+        return;
+      }
+
+      toastMessages.push(normalized);
+    };
+
+    runtime.globals.set('__bds_push_toast', pushToast);
+    await runtime.runPythonAsync(`
+def toast(message):
+    __bds_push_toast(str(message))
+`);
+
+    await runtime.runPythonAsync(request.scriptContent);
+    runtime.globals.set('__bds_transform_payload_json', request.payloadJson);
+    runtime.globals.set('__bds_transform_entrypoint', request.entrypoint);
+
+    const rawResult = await runtime.runPythonAsync(`
+import json
+_payload = json.loads(__bds_transform_payload_json)
+_entrypoint = __bds_transform_entrypoint
+_transform_fn = globals().get(_entrypoint)
+if _transform_fn is None or not callable(_transform_fn):
+    raise RuntimeError(f"Transform entrypoint '{_entrypoint}' is not callable")
+_post = _payload.get("post")
+if not isinstance(_post, dict):
+    raise RuntimeError("Transform payload is missing a valid 'post' object")
+_context = _payload.get("context")
+try:
+    _result = _transform_fn(_post, _context)
+except TypeError:
+    _result = _transform_fn(_post)
+if _result is None:
+    _result = _post
+json.dumps(_result)
+`);
+
+    postMessage({
+      type: 'transformResult',
+      requestId: request.requestId,
+      output: JSON.parse(String(rawResult)),
+      toasts: toastMessages,
+    });
+  } catch (error) {
+    const message = error instanceof Error ? error.message : String(error);
+    postMessage({ type: 'transformError', requestId: request.requestId, error: message });
+  }
+}
+
+parentPort?.on('message', (message: WorkerRunTransformRequest) => {
+  if (message.type !== 'runTransform') {
+    return;
+  }
+
+  void runTransform(message);
+});
+
+void getRuntime()
+  .then(() => {
+    postMessage({ type: 'ready' });
+  })
+  .catch((error) => {
+    const message = error instanceof Error ? error.message : String(error);
+    postMessage({ type: 'error', error: message });
+  });

src/main/ipc/handlers.ts

@@ -1006,7 +1006,7 @@ export function registerIpcHandlers(): void {
     return engine.getProjectMetadata();
   });
 
-  safeHandle('meta:updateProjectMetadata', async (_, updates: { name?: string; description?: string; dataPath?: string; publicUrl?: string; mainLanguage?: string; defaultAuthor?: string; maxPostsPerPage?: number; picoTheme?: import('../shared/picoThemes').PicoThemeName; categoryMetadata?: Record<string, { renderInLists: boolean; showTitle: boolean; title: string }>; categorySettings?: Record<string, { renderInLists: boolean; showTitle: boolean }> }) => {
+  safeHandle('meta:updateProjectMetadata', async (_, updates: { name?: string; description?: string; dataPath?: string; publicUrl?: string; mainLanguage?: string; defaultAuthor?: string; maxPostsPerPage?: number; blogmarkCategory?: string; pythonRuntimeMode?: 'webworker' | 'main-thread'; picoTheme?: import('../shared/picoThemes').PicoThemeName; categoryMetadata?: Record<string, { renderInLists: boolean; showTitle: boolean; title: string }>; categorySettings?: Record<string, { renderInLists: boolean; showTitle: boolean }> }) => {
     const engine = getMetaEngine();
     await ensureMetaContext(engine);
     await engine.updateProjectMetadata(updates);

src/main/preload.ts

@@ -172,7 +172,7 @@ export const electronAPI: ElectronAPI = {
     syncOnStartup: () => ipcRenderer.invoke('meta:syncOnStartup'),
     getProjectMetadata: () => ipcRenderer.invoke('meta:getProjectMetadata'),
     setProjectMetadata: (metadata: { name: string; description?: string }) => ipcRenderer.invoke('meta:setProjectMetadata', metadata),
-    updateProjectMetadata: (updates: { name?: string; description?: string; dataPath?: string; publicUrl?: string; mainLanguage?: string; defaultAuthor?: string; maxPostsPerPage?: number; blogmarkCategory?: string; picoTheme?: import('./shared/picoThemes').PicoThemeName; categoryMetadata?: Record<string, { renderInLists: boolean; showTitle: boolean; title: string }>; categorySettings?: Record<string, { renderInLists: boolean; showTitle: boolean }> }) => ipcRenderer.invoke('meta:updateProjectMetadata', updates),
+    updateProjectMetadata: (updates: { name?: string; description?: string; dataPath?: string; publicUrl?: string; mainLanguage?: string; defaultAuthor?: string; maxPostsPerPage?: number; blogmarkCategory?: string; pythonRuntimeMode?: 'webworker' | 'main-thread'; picoTheme?: import('./shared/picoThemes').PicoThemeName; categoryMetadata?: Record<string, { renderInLists: boolean; showTitle: boolean; title: string }>; categorySettings?: Record<string, { renderInLists: boolean; showTitle: boolean }> }) => ipcRenderer.invoke('meta:updateProjectMetadata', updates),
   },
 
   // Tag Management (advanced tag operations)

src/main/shared/electronApi.ts

@@ -43,6 +43,7 @@ export interface ProjectMetadata {
   defaultAuthor?: string;
   maxPostsPerPage?: number;
   blogmarkCategory?: string;
+  pythonRuntimeMode?: 'webworker' | 'main-thread';
   picoTheme?: import('./picoThemes').PicoThemeName;
   categoryMetadata?: Record<string, CategoryMetadata>;
   categorySettings?: Record<string, CategoryRenderSettings>;
@@ -619,7 +620,7 @@ export interface ElectronAPI {
     syncOnStartup: () => Promise<{ tags: string[]; categories: string[]; projectMetadata: ProjectMetadata | null }>;
     getProjectMetadata: () => Promise<ProjectMetadata | null>;
     setProjectMetadata: (metadata: { name: string; description?: string }) => Promise<ProjectMetadata | null>;
-    updateProjectMetadata: (updates: { name?: string; description?: string; dataPath?: string; publicUrl?: string; mainLanguage?: string; defaultAuthor?: string; maxPostsPerPage?: number; blogmarkCategory?: string; picoTheme?: import('./picoThemes').PicoThemeName; categoryMetadata?: Record<string, CategoryMetadata>; categorySettings?: Record<string, CategoryRenderSettings> }) => Promise<ProjectMetadata | null>;
+    updateProjectMetadata: (updates: { name?: string; description?: string; dataPath?: string; publicUrl?: string; mainLanguage?: string; defaultAuthor?: string; maxPostsPerPage?: number; blogmarkCategory?: string; pythonRuntimeMode?: 'webworker' | 'main-thread'; picoTheme?: import('./picoThemes').PicoThemeName; categoryMetadata?: Record<string, CategoryMetadata>; categorySettings?: Record<string, CategoryRenderSettings> }) => Promise<ProjectMetadata | null>;
   };
   tags: {
     getAll: () => Promise<TagData[]>;

src/renderer/components/SettingsView/SettingsView.tsx

@@ -10,7 +10,7 @@ import {
 import './SettingsView.css';
 
 // Export category IDs for sidebar navigation
-export type SettingsCategory = 'project' | 'editor' | 'content' | 'ai' | 'publishing' | 'data';
+export type SettingsCategory = 'project' | 'editor' | 'content' | 'ai' | 'technology' | 'publishing' | 'data';
 
 // Scroll to a settings section by category ID
 export const scrollToSettingsSection = (category: SettingsCategory) => {
@@ -150,6 +150,7 @@ export const SettingsView: React.FC = () => {
   const [projectDefaultAuthor, setProjectDefaultAuthor] = useState('');
   const [projectMaxPostsPerPage, setProjectMaxPostsPerPage] = useState(50);
   const [projectBlogmarkCategory, setProjectBlogmarkCategory] = useState('article');
+  const [projectPythonRuntimeMode, setProjectPythonRuntimeMode] = useState<'webworker' | 'main-thread'>('webworker');
 
   // Post categories management
   const [postCategories, setPostCategories] = useState<string[]>(DEFAULT_POST_CATEGORIES);
@@ -208,6 +209,9 @@ export const SettingsView: React.FC = () => {
         const incomingBlogmarkCategory = normalizeBlogmarkCategory((metadata as { blogmarkCategory?: unknown } | null)?.blogmarkCategory);
         setProjectBlogmarkCategory(incomingBlogmarkCategory || 'article');
 
+        const incomingPythonRuntimeMode = (metadata as { pythonRuntimeMode?: unknown } | null)?.pythonRuntimeMode;
+        setProjectPythonRuntimeMode(incomingPythonRuntimeMode === 'main-thread' ? 'main-thread' : 'webworker');
+
         const incomingCategoryMetadata = (metadata as any)?.categoryMetadata as Record<string, CategoryMetadata> | undefined;
         const incomingLegacyCategorySettings = (metadata as any)?.categorySettings as Record<string, { renderInLists: boolean; showTitle: boolean }> | undefined;
         setCategoryMetadata((current) => {
@@ -342,6 +346,7 @@ export const SettingsView: React.FC = () => {
           defaultAuthor: projectDefaultAuthor.trim() || undefined,
           maxPostsPerPage: Math.min(500, Math.max(1, Math.floor(projectMaxPostsPerPage || 50))),
           blogmarkCategory: normalizeBlogmarkCategory(projectBlogmarkCategory) || undefined,
+          pythonRuntimeMode: projectPythonRuntimeMode,
           categoryMetadata,
         });
       }
@@ -389,6 +394,7 @@ export const SettingsView: React.FC = () => {
   const editorKeywords = ['editor', 'mode', 'wysiwyg', 'markdown', 'preview', 'visual'];
   const contentKeywords = ['content', 'categories', 'post', 'article', 'picture', 'aside', 'page'];
   const aiKeywords = ['ai', 'assistant', 'chat', 'model', 'prompt', 'system', 'api', 'key', 'claude', 'gpt', 'opencode'];
+  const technologyKeywords = ['technology', 'python', 'runtime', 'worker', 'webworker', 'main thread', 'execution'];
   const publishingKeywords = ['publishing', 'ftp', 'ssh', 'deploy', 'server', 'host', 'upload'];
   const dataKeywords = ['data', 'database', 'rebuild', 'maintenance', 'posts', 'media', 'links', 'folder', 'filesystem'];
 
@@ -1023,6 +1029,30 @@ export const SettingsView: React.FC = () => {
     </SettingSection>
   );
 
+  const renderTechnologySettings = () => (
+    <SettingSection
+      id="settings-section-technology"
+      title={t('settings.technology.title')}
+      description={t('settings.technology.description')}
+      hidden={!sectionHasMatches(technologyKeywords)}
+    >
+      <SettingRow
+        id="project-python-runtime-mode"
+        label={t('settings.technology.pythonRuntimeModeLabel')}
+        description={t('settings.technology.pythonRuntimeModeDescription')}
+      >
+        <select
+          id="project-python-runtime-mode"
+          value={projectPythonRuntimeMode}
+          onChange={(event) => setProjectPythonRuntimeMode(event.target.value as 'webworker' | 'main-thread')}
+        >
+          <option value="webworker">{t('settings.technology.pythonRuntimeMode.webworker')}</option>
+          <option value="main-thread">{t('settings.technology.pythonRuntimeMode.mainThread')}</option>
+        </select>
+      </SettingRow>
+    </SettingSection>
+  );
+
   const renderPublishingSettings = () => (
     <>
       <SettingSection
@@ -1290,6 +1320,7 @@ export const SettingsView: React.FC = () => {
     sectionHasMatches(editorKeywords) ||
     sectionHasMatches(contentKeywords) ||
     sectionHasMatches(aiKeywords) ||
+    sectionHasMatches(technologyKeywords) ||
     sectionHasMatches(publishingKeywords) ||
     sectionHasMatches(dataKeywords);
 
@@ -1325,6 +1356,7 @@ export const SettingsView: React.FC = () => {
             {renderEditorSettings()}
             {renderContentSettings()}
             {renderAISettings()}
+            {renderTechnologySettings()}
             {renderPublishingSettings()}
             {renderDataSettings()}
           </>

src/renderer/components/Sidebar/Sidebar.tsx

@@ -1261,7 +1261,7 @@ const SettingsNav: React.FC = () => {
   const { tabs, activeTabId, openTab } = useAppStore();
   const [activeSection, setActiveSection] = useState<SettingsCategory | null>(() => {
     const persisted = getPersistedSidebarSection('settings');
-    if (persisted === 'project' || persisted === 'editor' || persisted === 'content' || persisted === 'ai' || persisted === 'publishing' || persisted === 'data') {
+    if (persisted === 'project' || persisted === 'editor' || persisted === 'content' || persisted === 'ai' || persisted === 'technology' || persisted === 'publishing' || persisted === 'data') {
       return persisted;
     }
     return null;
@@ -1322,6 +1322,13 @@ const SettingsNav: React.FC = () => {
           <span className="settings-nav-entry-icon">🤖</span>
           <span>{t('sidebar.nav.ai')}</span>
         </button>
+        <button
+          className={`settings-nav-entry ${activeSection === 'technology' ? 'active' : ''}`}
+          onClick={() => handleNavClick('technology')}
+        >
+          <span className="settings-nav-entry-icon">⚙️</span>
+          <span>{t('sidebar.nav.technology')}</span>
+        </button>
         <button 
           className={`settings-nav-entry ${activeSection === 'publishing' ? 'active' : ''}`}
           onClick={() => handleNavClick('publishing')}

src/renderer/i18n/locales/de.json

@@ -127,6 +127,12 @@
   "settings.content.showTitles": "Titel anzeigen",
   "settings.ai.title": "KI-Assistent",
   "settings.ai.noModels": "Keine Modelle verfügbar",
+  "settings.technology.title": "Technologie",
+  "settings.technology.description": "Konfiguriere das Laufzeitverhalten für die Python-Skriptausführung.",
+  "settings.technology.pythonRuntimeModeLabel": "Python-Laufzeitmodus",
+  "settings.technology.pythonRuntimeModeDescription": "Lege fest, wo Python-Skripte für Transformationspipelines ausgeführt werden.",
+  "settings.technology.pythonRuntimeMode.webworker": "Web Worker (empfohlen)",
+  "settings.technology.pythonRuntimeMode.mainThread": "Hauptthread (Legacy)",
   "settings.publishing.ftpTitle": "FTP-Veröffentlichung",
   "settings.publishing.sshTitle": "SSH-Veröffentlichung",
   "settings.data.title": "Datenbankwartung",
@@ -421,6 +427,7 @@
   "sidebar.nav.editor": "Texteditor",
   "sidebar.nav.content": "Inhalt",
   "sidebar.nav.ai": "KI-Assistent",
+  "sidebar.nav.technology": "Technologie",
   "sidebar.nav.publishing": "Veröffentlichung",
   "sidebar.nav.data": "Daten",
   "sidebar.nav.style": "Stil",

src/renderer/i18n/locales/en.json

@@ -127,6 +127,12 @@
   "settings.content.showTitles": "Show titles",
   "settings.ai.title": "AI Assistant",
   "settings.ai.noModels": "No models available",
+  "settings.technology.title": "Technology",
+  "settings.technology.description": "Configure runtime behavior for Python script execution.",
+  "settings.technology.pythonRuntimeModeLabel": "Python Runtime Mode",
+  "settings.technology.pythonRuntimeModeDescription": "Choose where Python scripts execute for transform pipelines.",
+  "settings.technology.pythonRuntimeMode.webworker": "Web Worker (Recommended)",
+  "settings.technology.pythonRuntimeMode.mainThread": "Main Thread (Legacy)",
   "settings.publishing.ftpTitle": "FTP Publishing",
   "settings.publishing.sshTitle": "SSH Publishing",
   "settings.data.title": "Database Maintenance",
@@ -421,6 +427,7 @@
   "sidebar.nav.editor": "Editor",
   "sidebar.nav.content": "Content",
   "sidebar.nav.ai": "AI Assistant",
+  "sidebar.nav.technology": "Technology",
   "sidebar.nav.publishing": "Publishing",
   "sidebar.nav.data": "Data",
   "sidebar.nav.style": "Style",

src/renderer/i18n/locales/es.json

@@ -127,6 +127,12 @@
   "settings.content.showTitles": "Mostrar títulos",
   "settings.ai.title": "Asistente IA",
   "settings.ai.noModels": "No hay modelos disponibles",
+  "settings.technology.title": "Tecnología",
+  "settings.technology.description": "Configura el comportamiento de ejecución para scripts de Python.",
+  "settings.technology.pythonRuntimeModeLabel": "Modo de ejecución de Python",
+  "settings.technology.pythonRuntimeModeDescription": "Elige dónde se ejecutan los scripts de Python para los flujos de transformación.",
+  "settings.technology.pythonRuntimeMode.webworker": "Web Worker (recomendado)",
+  "settings.technology.pythonRuntimeMode.mainThread": "Hilo principal (heredado)",
   "settings.publishing.ftpTitle": "Publicación FTP",
   "settings.publishing.sshTitle": "Publicación SSH",
   "settings.data.title": "Mantenimiento de base de datos",
@@ -421,6 +427,7 @@
   "sidebar.nav.editor": "Editor",
   "sidebar.nav.content": "Contenido",
   "sidebar.nav.ai": "Asistente IA",
+  "sidebar.nav.technology": "Tecnología",
   "sidebar.nav.publishing": "Publicación",
   "sidebar.nav.data": "Datos",
   "sidebar.nav.style": "Estilo",

src/renderer/i18n/locales/fr.json

@@ -127,6 +127,12 @@
   "settings.content.showTitles": "Afficher les titres",
   "settings.ai.title": "Assistant IA",
   "settings.ai.noModels": "Aucun modèle disponible",
+  "settings.technology.title": "Technologie",
+  "settings.technology.description": "Configurez le comportement d’exécution des scripts Python.",
+  "settings.technology.pythonRuntimeModeLabel": "Mode d’exécution Python",
+  "settings.technology.pythonRuntimeModeDescription": "Choisissez où les scripts Python s’exécutent pour les pipelines de transformation.",
+  "settings.technology.pythonRuntimeMode.webworker": "Web Worker (recommandé)",
+  "settings.technology.pythonRuntimeMode.mainThread": "Thread principal (hérité)",
   "settings.publishing.ftpTitle": "Publication FTP",
   "settings.publishing.sshTitle": "Publication SSH",
   "settings.data.title": "Maintenance de la base de données",
@@ -421,6 +427,7 @@
   "sidebar.nav.editor": "Éditeur",
   "sidebar.nav.content": "Contenu",
   "sidebar.nav.ai": "Assistant IA",
+  "sidebar.nav.technology": "Technologie",
   "sidebar.nav.publishing": "Publication",
   "sidebar.nav.data": "Données",
   "sidebar.nav.style": "Style",

src/renderer/i18n/locales/it.json

@@ -127,6 +127,12 @@
   "settings.content.showTitles": "Mostra titoli",
   "settings.ai.title": "Assistente IA",
   "settings.ai.noModels": "Nessun modello disponibile",
+  "settings.technology.title": "Tecnologia",
+  "settings.technology.description": "Configura il comportamento di runtime per l'esecuzione degli script Python.",
+  "settings.technology.pythonRuntimeModeLabel": "Modalità runtime Python",
+  "settings.technology.pythonRuntimeModeDescription": "Scegli dove eseguire gli script Python per le pipeline di trasformazione.",
+  "settings.technology.pythonRuntimeMode.webworker": "Web Worker (consigliato)",
+  "settings.technology.pythonRuntimeMode.mainThread": "Thread principale (legacy)",
   "settings.publishing.ftpTitle": "Pubblicazione FTP",
   "settings.publishing.sshTitle": "Pubblicazione SSH",
   "settings.data.title": "Manutenzione database",
@@ -421,6 +427,7 @@
   "sidebar.nav.editor": "Editor",
   "sidebar.nav.content": "Contenuto",
   "sidebar.nav.ai": "Assistente IA",
+  "sidebar.nav.technology": "Tecnologia",
   "sidebar.nav.publishing": "Pubblicazione",
   "sidebar.nav.data": "Dati",
   "sidebar.nav.style": "Stile",

src/renderer/python/PythonRuntimeManager.ts

@@ -54,6 +54,8 @@ export class PythonRuntimeManager {
   private initializeDeferred: InitializeDeferred | null = null;
   private ready = false;
   private pendingRuns = new Map<string, PendingRun>();
+  private requestQueue: PythonWorkerRequest[] = [];
+  private activeRequestId: string | null = null;
   private requestCounter = 0;
 
   constructor(private readonly workerFactory: WorkerFactory = createPythonRuntimeWorker) {}
@@ -123,7 +125,7 @@ export class PythonRuntimeManager {
         entrypoint: options?.entrypoint,
       };
 
-      this.worker!.postMessage(message);
+      this.enqueueRequest(message);
     });
   }
 
@@ -162,7 +164,7 @@ export class PythonRuntimeManager {
         cacheKey: options?.cacheKey,
       };
 
-      this.worker!.postMessage(message);
+      this.enqueueRequest(message);
     });
   }
 
@@ -198,7 +200,7 @@ export class PythonRuntimeManager {
         cacheKey: options?.cacheKey,
       };
 
-      this.worker!.postMessage(message);
+      this.enqueueRequest(message);
     });
   }
 
@@ -234,7 +236,7 @@ export class PythonRuntimeManager {
         cacheKey: options?.cacheKey,
       };
 
-      this.worker!.postMessage(message);
+      this.enqueueRequest(message);
     });
   }
 
@@ -262,6 +264,10 @@ export class PythonRuntimeManager {
 
     const pendingRun = this.pendingRuns.get(payload.requestId);
     if (!pendingRun) {
+      if (this.activeRequestId === payload.requestId && payload.type !== 'stdout') {
+        this.activeRequestId = null;
+        this.dispatchNextRequest();
+      }
       return;
     }
 
@@ -278,33 +284,40 @@ export class PythonRuntimeManager {
     if (payload.type === 'runResult') {
       if (pendingRun.kind !== 'run') {
         pendingRun.reject(new Error('Invalid response type for pending macro request'));
+        this.finishRequest(payload.requestId);
         return;
       }
       pendingRun.resolve({ result: payload.result, stdout: pendingRun.stdout });
+      this.finishRequest(payload.requestId);
       return;
     }
 
     if (payload.type === 'entrypoints') {
       if (pendingRun.kind !== 'inspect-entrypoints') {
         pendingRun.reject(new Error('Invalid response type for pending run request'));
+        this.finishRequest(payload.requestId);
         return;
       }
       pendingRun.resolve(payload.entrypoints);
+      this.finishRequest(payload.requestId);
       return;
     }
 
     if (payload.type === 'syntaxResult') {
       if (pendingRun.kind !== 'syntax-check') {
         pendingRun.reject(new Error('Invalid response type for pending syntax check request'));
+        this.finishRequest(payload.requestId);
         return;
       }
       pendingRun.resolve({ errors: payload.errors });
+      this.finishRequest(payload.requestId);
       return;
     }
 
     if (payload.type === 'macroResult') {
       if (pendingRun.kind !== 'macro-v1') {
         pendingRun.reject(new Error('Invalid response type for pending run request'));
+        this.finishRequest(payload.requestId);
         return;
       }
 
@@ -314,10 +327,12 @@ export class PythonRuntimeManager {
       } catch (error) {
         pendingRun.reject(error instanceof Error ? error : new Error(String(error)));
       }
+      this.finishRequest(payload.requestId);
       return;
     }
 
     pendingRun.reject(new Error(payload.error));
+    this.finishRequest(payload.requestId);
   }
 
   private handleWorkerError(error: Error): void {
@@ -334,6 +349,8 @@ export class PythonRuntimeManager {
     }
 
     this.pendingRuns.clear();
+    this.requestQueue = [];
+    this.activeRequestId = null;
     this.worker?.terminate();
     this.worker = null;
     this.initializingPromise = null;
@@ -356,12 +373,50 @@ export class PythonRuntimeManager {
     }
 
     this.pendingRuns.clear();
+    this.requestQueue = [];
+    this.activeRequestId = null;
     this.worker?.terminate();
     this.worker = null;
     this.initializingPromise = null;
     this.ready = false;
   }
 
+  private enqueueRequest(request: PythonWorkerRequest): void {
+    if (!this.worker || !this.ready) {
+      this.requestQueue.push(request);
+      return;
+    }
+
+    if (this.activeRequestId !== null) {
+      this.requestQueue.push(request);
+      return;
+    }
+
+    this.activeRequestId = request.requestId;
+    this.worker.postMessage(request);
+  }
+
+  private dispatchNextRequest(): void {
+    if (!this.worker || !this.ready || this.activeRequestId !== null || this.requestQueue.length === 0) {
+      return;
+    }
+
+    const nextRequest = this.requestQueue.shift();
+    if (!nextRequest) {
+      return;
+    }
+
+    this.activeRequestId = nextRequest.requestId;
+    this.worker.postMessage(nextRequest);
+  }
+
+  private finishRequest(requestId: string): void {
+    if (this.activeRequestId === requestId) {
+      this.activeRequestId = null;
+    }
+    this.dispatchNextRequest();
+  }
+
   private nextRequestId(): string {
     this.requestCounter += 1;
     return `req-${this.requestCounter}`;

tests/engine/BlogmarkTransformService.integration.test.ts

@@ -63,7 +63,10 @@ describe('BlogmarkTransformService (Pyodide integration)', () => {
       getScripts: async () => [createTransformScript()],
     };
 
-    const service = new BlogmarkTransformService({ provider });
+    const service = new BlogmarkTransformService({
+      provider,
+      resolvePythonRuntimeMode: async () => 'main-thread',
+    });
 
     const result = await service.applyTransforms(createInput());
 

tests/engine/BlogmarkTransformService.test.ts

@@ -247,67 +247,53 @@ describe('BlogmarkTransformService', () => {
     expect(result.toasts).toEqual(['Step finished', 'Step finished']);
   });
 
-  it('invokes python transform entrypoint with post payload shape', async () => {
-    const globalsStore = new Map<string, unknown>();
-    const runPythonAsync = vi.fn(async (code: string) => {
-      if (code.includes('json.dumps(_result)')) {
-        const payload = JSON.parse(String(globalsStore.get('__bds_transform_payload_json')));
-
-        if (code.includes('_transform_fn(_payload)')) {
-          return JSON.stringify(payload);
-        }
-
-        return JSON.stringify({
-          ...payload.post,
-          title: 'Normalized',
-          categories: ['spielelog', 'asides'],
-          tags: ['inbox', 'spielen'],
-        });
-      }
-
-      return null;
-    });
-
-    vi.doMock('pyodide', () => ({
-      loadPyodide: vi.fn(async () => ({
-        globals: {
-          set: (key: string, value: unknown) => {
-            globalsStore.set(key, value);
-          },
-        },
-        runPythonAsync,
-      })),
-    }));
-
-    const provider: BlogmarkTransformScriptProvider = {
-      getScripts: vi.fn(async () => [
-        createScript({
-          id: 'pyodide-transform',
-          slug: 'pyodide-transform',
-          title: 'Pyodide Transform',
-          kind: 'transform',
-          entrypoint: 'normalize_blogmark',
-          content: 'def normalize_blogmark(post):\n    return post',
-        }),
-      ]),
+  it('uses webworker executor when runtime mode resolves to webworker', async () => {
+    const webworkerExecutor: BlogmarkTransformExecutor = {
+      runTransform: vi.fn(async (_script, input) => ({ output: input.post, toasts: [] })),
+    };
+    const mainThreadExecutor: BlogmarkTransformExecutor = {
+      runTransform: vi.fn(async (_script, input) => ({ output: input.post, toasts: [] })),
     };
 
-    const service = new BlogmarkTransformService({ provider });
+    const service = new BlogmarkTransformService({
+      provider: {
+        getScripts: async () => [createScript({ id: 'worker-script', slug: 'worker-script' })],
+      },
+      resolvePythonRuntimeMode: async () => 'webworker',
+      executors: {
+        webworker: webworkerExecutor,
+        'main-thread': mainThreadExecutor,
+      },
+    });
 
-    const result = await service.applyTransforms(createInput());
+    await service.applyTransforms(createInput());
 
-    const transformInvocationCode = runPythonAsync.mock.calls
-      .map((call) => call[0])
-      .find((code) => typeof code === 'string' && String(code).includes('json.dumps(_result)'));
+    expect(webworkerExecutor.runTransform).toHaveBeenCalledTimes(1);
+    expect(mainThreadExecutor.runTransform).not.toHaveBeenCalled();
+  });
 
-    expect(result.post.title).toBe('Normalized');
-    expect(result.post.categories).toEqual(['spielelog', 'asides']);
-    expect(result.post.tags).toEqual(['inbox', 'spielen']);
-    expect(transformInvocationCode).toBeDefined();
-    expect(String(transformInvocationCode)).not.toContain('import inspect');
-    expect(String(transformInvocationCode)).toContain('\ntry:\n');
-    expect(String(transformInvocationCode)).toContain('\nexcept TypeError:\n');
-    expect(String(transformInvocationCode)).not.toContain('\n  try:\n');
-    expect(String(transformInvocationCode)).not.toContain('\n  except TypeError:\n');
+  it('uses main-thread executor when runtime mode resolves to main-thread', async () => {
+    const webworkerExecutor: BlogmarkTransformExecutor = {
+      runTransform: vi.fn(async (_script, input) => ({ output: input.post, toasts: [] })),
+    };
+    const mainThreadExecutor: BlogmarkTransformExecutor = {
+      runTransform: vi.fn(async (_script, input) => ({ output: input.post, toasts: [] })),
+    };
+
+    const service = new BlogmarkTransformService({
+      provider: {
+        getScripts: async () => [createScript({ id: 'main-script', slug: 'main-script' })],
+      },
+      resolvePythonRuntimeMode: async () => 'main-thread',
+      executors: {
+        webworker: webworkerExecutor,
+        'main-thread': mainThreadExecutor,
+      },
+    });
+
+    await service.applyTransforms(createInput());
+
+    expect(mainThreadExecutor.runTransform).toHaveBeenCalledTimes(1);
+    expect(webworkerExecutor.runTransform).not.toHaveBeenCalled();
   });
 });

tests/engine/MetaEngine.test.ts

@@ -739,6 +739,30 @@ describe('MetaEngine', () => {
       expect((metadata as any)?.blogmarkCategory).toBe('article');
     });
 
+    it('should set and get pythonRuntimeMode in project metadata', async () => {
+      await metaEngine.setProjectMetadata({
+        name: 'My Blog',
+        pythonRuntimeMode: 'main-thread',
+      } as any);
+
+      const metadata = await metaEngine.getProjectMetadata();
+      expect((metadata as any)?.pythonRuntimeMode).toBe('main-thread');
+    });
+
+    it('should persist pythonRuntimeMode to filesystem', async () => {
+      await metaEngine.setProjectMetadata({
+        name: 'Runtime Mode Project',
+        pythonRuntimeMode: 'webworker',
+      } as any);
+
+      const metaDir = metaEngine.getMetaDir();
+      const projectPath = normalizePath(`${metaDir}/project.json`);
+      const content = mockFiles.get(projectPath);
+      const parsed = JSON.parse(content!);
+
+      expect(parsed.pythonRuntimeMode).toBe('webworker');
+    });
+
     it('should persist blogmarkCategory to filesystem', async () => {
       await metaEngine.setProjectMetadata({
         name: 'Test Project',

tests/renderer/components/SettingsView.test.tsx

@@ -117,6 +117,33 @@ describe('SettingsView Diff Preferences', () => {
     );
   });
 
+  it('includes python runtime mode in metadata save payload', async () => {
+    (window as any).electronAPI.meta.getProjectMetadata = vi.fn().mockResolvedValue({
+      maxPostsPerPage: 75,
+      publicUrl: 'https://example.com',
+      pythonRuntimeMode: 'main-thread',
+      categorySettings: {
+        article: { renderInLists: true, showTitle: true },
+        picture: { renderInLists: true, showTitle: true },
+        aside: { renderInLists: true, showTitle: false },
+        page: { renderInLists: false, showTitle: true },
+      },
+    });
+
+    render(<SettingsView />);
+
+    await screen.findByDisplayValue('Main Thread (Legacy)');
+
+    const saveButton = screen.getByRole('button', { name: /save project settings/i });
+    fireEvent.click(saveButton);
+
+    await new Promise((resolve) => setTimeout(resolve, 0));
+
+    expect((window as any).electronAPI.meta.updateProjectMetadata).toHaveBeenCalledWith(
+      expect.objectContaining({ pythonRuntimeMode: 'main-thread' })
+    );
+  });
+
   it('renders category settings checkboxes with required defaults', async () => {
     render(<SettingsView />);
 

tests/renderer/python/PythonRuntimeManager.test.ts

@@ -191,6 +191,32 @@ describe('PythonRuntimeManager', () => {
     await expect(runPromise).rejects.toThrow('boom');
   });
 
+  it('queues concurrent execute calls and sends the next request only after completion', async () => {
+    const worker = new MockWorker();
+    const manager = new PythonRuntimeManager(() => worker as unknown as Worker);
+
+    const initPromise = manager.initialize();
+    worker.emitMessage({ type: 'ready' });
+    await initPromise;
+
+    const firstRun = manager.execute('1 + 1');
+    const secondRun = manager.execute('2 + 2');
+    await Promise.resolve();
+
+    expect(worker.postedMessages).toHaveLength(1);
+
+    const firstRequest = worker.postedMessages[0] as { requestId: string };
+    worker.emitMessage({ type: 'runResult', requestId: firstRequest.requestId, result: '2' });
+    await expect(firstRun).resolves.toEqual({ result: '2', stdout: '' });
+
+    await Promise.resolve();
+    expect(worker.postedMessages).toHaveLength(2);
+
+    const secondRequest = worker.postedMessages[1] as { requestId: string };
+    worker.emitMessage({ type: 'runResult', requestId: secondRequest.requestId, result: '4' });
+    await expect(secondRun).resolves.toEqual({ result: '4', stdout: '' });
+  });
+
   it('terminates timed out worker and recovers with a new worker', async () => {
     const workers: MockWorker[] = [];
     const manager = new PythonRuntimeManager(() => {