diff --git a/apps/host-selfhost/src/execution.ts b/apps/host-selfhost/src/execution.ts
index 8dab577b9..e4523124f 100644
--- a/apps/host-selfhost/src/execution.ts
+++ b/apps/host-selfhost/src/execution.ts
@@ -68,9 +68,10 @@ export const SelfHostCodeExecutorProvider: Layer.Layer<CodeExecutorProvider> = L
   CodeExecutorProvider,
   () => {
     const { sandboxTimeoutMs } = loadConfig();
-    return makeQuickJsExecutor(
-      sandboxTimeoutMs === undefined ? {} : { timeoutMs: sandboxTimeoutMs },
-    );
+    return makeQuickJsExecutor({
+      ...(sandboxTimeoutMs === undefined ? {} : { timeoutMs: sandboxTimeoutMs }),
+      worker: true,
+    });
   },
 );
 
diff --git a/packages/kernel/runtime-quickjs/package.json b/packages/kernel/runtime-quickjs/package.json
index 5ea156acc..60d693ee1 100644
--- a/packages/kernel/runtime-quickjs/package.json
+++ b/packages/kernel/runtime-quickjs/package.json
@@ -31,6 +31,7 @@
   },
   "scripts": {
     "build": "tsup && (tsc --declaration --emitDeclarationOnly --outDir dist --rootDir src || true)",
+    "pretest": "bun run build",
     "typecheck": "tsgo --noEmit",
     "test": "vitest run",
     "test:watch": "vitest",
diff --git a/packages/kernel/runtime-quickjs/src/index.test.ts b/packages/kernel/runtime-quickjs/src/index.test.ts
index a55f022ac..c4137148c 100644
--- a/packages/kernel/runtime-quickjs/src/index.test.ts
+++ b/packages/kernel/runtime-quickjs/src/index.test.ts
@@ -433,3 +433,72 @@ describe("quickjs executor", () => {
     }),
   );
 });
+
+describe("quickjs worker executor (worker_threads)", () => {
+  const workerExecutor = makeQuickJsExecutor({ worker: true, timeoutMs: 5_000 });
+
+  it.effect("runs plain code and returns results in a worker thread", () =>
+    Effect.gen(function* () {
+      const result = yield* workerExecutor.execute(`return 10 + 20;`, makeTestInvoker({}));
+      expect(result.result).toBe(30);
+      expect(result.error).toBeUndefined();
+    }),
+  );
+
+  it.effect("handles bidirectional tool invocations across the worker thread boundary", () =>
+    Effect.gen(function* () {
+      const result = yield* workerExecutor.execute(
+        `
+        const user = await tools.users.get({ id: 42 });
+        return { name: user.name, doubled: user.id * 2 };
+        `,
+        makeTestInvoker({
+          "users.get": (args: any) => ({ id: args.id, name: "Alice" }),
+        }),
+      );
+
+      expect(result.error).toBeUndefined();
+      expect(result.result).toEqual({ name: "Alice", doubled: 84 });
+    }),
+  );
+
+  it.effect("handles tool errors across the worker thread boundary", () =>
+    Effect.gen(function* () {
+      const result = yield* workerExecutor.execute(
+        `
+        try {
+          await tools.db.query({ sql: "SELECT * FROM secrets" });
+          return "unexpected";
+        } catch (err) {
+          return err instanceof Error ? err.message : String(err);
+        }
+        `,
+        makeTestInvoker({
+          "db.query": () => {
+            throw new Error("connection refused");
+          },
+        }),
+      );
+
+      expect(result.error).toBeUndefined();
+      expect(result.result).toBe("Internal tool error");
+    }),
+  );
+
+  it.effect("preempts a CPU-bound loop in the worker thread without hanging the main thread", () =>
+    Effect.gen(function* () {
+      const timedWorkerExecutor = makeQuickJsExecutor({ worker: true, timeoutMs: 1_000 });
+      const result = yield* timedWorkerExecutor.execute(
+        `
+        const t0 = Date.now();
+        while (true) {}
+        return "unreachable";
+        `,
+        makeTestInvoker({}),
+      );
+
+      expect(result.result).toBeNull();
+      expect(result.error).toContain("QuickJS execution timed out after 1000ms");
+    }),
+  );
+});
diff --git a/packages/kernel/runtime-quickjs/src/index.ts b/packages/kernel/runtime-quickjs/src/index.ts
index 90de8461b..a5dc9543c 100644
--- a/packages/kernel/runtime-quickjs/src/index.ts
+++ b/packages/kernel/runtime-quickjs/src/index.ts
@@ -1,3 +1,6 @@
+import { existsSync } from "node:fs";
+import { fileURLToPath } from "node:url";
+
 import {
   recoverExecutionBody,
   stripTypeScript,
@@ -7,6 +10,7 @@ import {
   type SandboxToolInvoker,
 } from "@executor-js/codemode-core";
 import * as Data from "effect/Data";
+import * as Deferred from "effect/Deferred";
 import * as Effect from "effect/Effect";
 import {
   executeSealedBundle as runSealedBundleWith,
@@ -26,6 +30,7 @@ export type QuickJsExecutorOptions = {
   timeoutMs?: number;
   memoryLimitBytes?: number;
   maxStackSizeBytes?: number;
+  worker?: boolean;
 };
 
 // Allow pre-loading a QuickJS module (e.g. with custom WASM bytes for compiled binaries)
@@ -434,7 +439,7 @@ const drainAsync = async (
   drainJobs(context, runtime, deadline, timeoutMs);
 };
 
-const evaluateInQuickJs = async (
+export const evaluateInQuickJs = async (
   options: QuickJsExecutorOptions,
   code: string,
   toolInvoker: SandboxToolInvoker,
@@ -565,9 +570,136 @@ const runInQuickJs = (
     }),
   );
 
+export const resolveWorkerScriptPath = (): URL => {
+  const currentUrl = new URL(import.meta.url);
+  const devDistWorker = new URL("../dist/worker.js", currentUrl);
+  if (existsSync(fileURLToPath(devDistWorker))) {
+    return devDistWorker;
+  }
+  return new URL("./worker.js", currentUrl);
+};
+
+type NodeWorkerInstance = {
+  on(event: "message", listener: (value: unknown) => void): void;
+  on(event: "error", listener: (err: Error) => void): void;
+  on(event: "exit", listener: (exitCode: number) => void): void;
+  postMessage(value: unknown): void;
+  terminate(): Promise<number> | void;
+};
+
+const runInQuickJsWorker = (
+  options: QuickJsExecutorOptions,
+  code: string,
+  toolInvoker: SandboxToolInvoker,
+): Effect.Effect<ExecuteResult, QuickJsExecutionError> =>
+  Effect.gen(function* () {
+    const workerModule = yield* Effect.tryPromise({
+      try: () => import("node:worker_threads"),
+      catch: (err) =>
+        new QuickJsExecutionError({
+          message: `worker_threads module is not available in this runtime: ${String(err)}`,
+        }),
+    });
+
+    const context = yield* Effect.context<never>();
+    const runPromise = Effect.runPromiseWith(context);
+    const resultDeferred = yield* Deferred.make<ExecuteResult, QuickJsExecutionError>();
+
+    const scriptPath = resolveWorkerScriptPath();
+    const worker: NodeWorkerInstance = yield* Effect.try({
+      try: () =>
+        // oxlint-disable-next-line executor/no-double-cast -- boundary: dynamic worker_threads Worker instance bridged across heterogeneous runtime type environments
+        new workerModule.Worker(scriptPath, {
+          workerData: { code, options },
+        }) as unknown as NodeWorkerInstance,
+      catch: (cause) => new QuickJsExecutionError({ message: String(cause) }),
+    });
+
+    worker.on("message", (msg: unknown) => {
+      if (!msg || typeof msg !== "object") return;
+      const payload = msg as {
+        type?: string;
+        id?: number;
+        path?: string;
+        args?: Record<string, unknown>;
+        result?: ExecuteResult;
+        error?: string;
+      };
+      if (
+        payload.type === "tool_call" &&
+        typeof payload.id === "number" &&
+        typeof payload.path === "string"
+      ) {
+        void runPromise(toolInvoker.invoke({ path: payload.path, args: payload.args ?? {} })).then(
+          (value) => {
+            worker.postMessage({ type: "tool_result", id: payload.id, ok: true, value });
+          },
+          (cause) => {
+            worker.postMessage({
+              type: "tool_result",
+              id: payload.id,
+              ok: false,
+              error: cause instanceof Error ? cause.message : String(cause),
+            });
+          },
+        );
+      } else if (payload.type === "done" && payload.result) {
+        Effect.runSync(Deferred.complete(resultDeferred, Effect.succeed(payload.result)));
+        void worker.terminate();
+      } else if (payload.type === "error" && typeof payload.error === "string") {
+        Effect.runSync(
+          Deferred.complete(
+            resultDeferred,
+            Effect.fail(new QuickJsExecutionError({ message: payload.error })),
+          ),
+        );
+        void worker.terminate();
+      }
+    });
+
+    worker.on("error", (err: Error) => {
+      Effect.runSync(
+        Deferred.complete(
+          resultDeferred,
+          Effect.fail(new QuickJsExecutionError({ message: err.message })),
+        ),
+      );
+      void worker.terminate();
+    });
+
+    worker.on("exit", (exitCode: number) => {
+      if (exitCode !== 0) {
+        Effect.runSync(
+          Deferred.complete(
+            resultDeferred,
+            Effect.fail(
+              new QuickJsExecutionError({
+                message: `QuickJS worker thread exited unexpectedly with code ${exitCode}`,
+              }),
+            ),
+          ),
+        );
+      }
+    });
+
+    return yield* Deferred.await(resultDeferred).pipe(
+      Effect.ensuring(
+        Effect.sync(() => {
+          void worker.terminate();
+        }),
+      ),
+    );
+  }).pipe(
+    Effect.withSpan("executor.code.exec.quickjs_worker", {
+      attributes: { "executor.runtime": "quickjs-worker" },
+    }),
+  );
+
 export const makeQuickJsExecutor = (
   options: QuickJsExecutorOptions = {},
 ): CodeExecutor<QuickJsExecutionError> => ({
   execute: (code: string, toolInvoker: SandboxToolInvoker) =>
-    runInQuickJs(options, code, toolInvoker),
+    options.worker
+      ? runInQuickJsWorker(options, code, toolInvoker)
+      : runInQuickJs(options, code, toolInvoker),
 });
diff --git a/packages/kernel/runtime-quickjs/src/worker.ts b/packages/kernel/runtime-quickjs/src/worker.ts
new file mode 100644
index 000000000..dd8aaf29b
--- /dev/null
+++ b/packages/kernel/runtime-quickjs/src/worker.ts
@@ -0,0 +1,57 @@
+import { parentPort, workerData } from "node:worker_threads";
+import type { SandboxToolInvoker } from "@executor-js/codemode-core";
+import * as Effect from "effect/Effect";
+import { evaluateInQuickJs, type QuickJsExecutorOptions } from "./index";
+
+if (parentPort) {
+  const port = parentPort;
+  const { code, options } = workerData as {
+    code: string;
+    options: QuickJsExecutorOptions;
+  };
+
+  let nextCallId = 1;
+  const pendingCalls = new Map<
+    number,
+    {
+      resolve: (value: unknown) => void;
+      reject: (error: unknown) => void;
+    }
+  >();
+
+  port.on("message", (msg) => {
+    if (msg && typeof msg === "object" && msg.type === "tool_result") {
+      const pending = pendingCalls.get(msg.id);
+      if (pending) {
+        pendingCalls.delete(msg.id);
+        if (msg.ok) {
+          pending.resolve(msg.value);
+        } else {
+          pending.reject(msg.error);
+        }
+      }
+    }
+  });
+
+  const workerToolInvoker: SandboxToolInvoker = {
+    invoke: ({ path, args }) =>
+      Effect.promise(
+        () =>
+          new Promise((resolve, reject) => {
+            const id = nextCallId++;
+            pendingCalls.set(id, { resolve, reject });
+            port.postMessage({ type: "tool_call", id, path, args });
+          }),
+      ),
+  };
+
+  const runPromise = <A, E>(effect: Effect.Effect<A, E>): Promise<A> => Effect.runPromise(effect);
+
+  evaluateInQuickJs(options, code, workerToolInvoker, runPromise)
+    .then((result) => {
+      port.postMessage({ type: "done", result });
+    })
+    .catch((cause) => {
+      port.postMessage({ type: "error", error: String(cause) });
+    });
+}
diff --git a/packages/kernel/runtime-quickjs/tsup.config.ts b/packages/kernel/runtime-quickjs/tsup.config.ts
index 58569cf04..a9e977a7f 100644
--- a/packages/kernel/runtime-quickjs/tsup.config.ts
+++ b/packages/kernel/runtime-quickjs/tsup.config.ts
@@ -3,10 +3,11 @@ import { defineConfig } from "tsup";
 export default defineConfig({
   entry: {
     index: "src/index.ts",
+    worker: "src/worker.ts",
   },
   format: ["esm"],
   dts: false,
   sourcemap: true,
-  clean: true,
-  external: [/^@executor-js\//, /^effect/, /^@effect\//, "quickjs-emscripten"],
+  noExternal: [/^@executor-js\//],
+  external: [/^effect/, /^@effect\//, "quickjs-emscripten"],
 });