diff --git a/docs/rfc-001-workflow-engine.md b/docs/rfc-001-workflow-engine.md
index 3f95bcc..6b7f892 100644
--- a/docs/rfc-001-workflow-engine.md
+++ b/docs/rfc-001-workflow-engine.md
@@ -35,9 +35,15 @@ type WorkflowResult = {
   summary: string;
 };
 
+/** Input to a workflow — prompt + optional historical steps for fork/resume. */
+type ThreadInput = {
+  prompt: string;
+  steps: RoleOutput[];   // [] for new thread, pre-filled for fork/resume
+};
+
 /** The bundle contract — an AsyncGenerator, not a Promise. */
 type WorkflowFn = (
-  prompt: string,
+  input: ThreadInput,
   options: { isDryRun: boolean; maxRounds: number }
 ) => AsyncGenerator<RoleOutput, WorkflowResult>;
 ```
@@ -49,8 +55,8 @@ exporting a framework-specific shape:
 
 ```typescript
 // Example bundle — zero framework dependency
-export default async function* (prompt, options) {
-  const plan = await callLLM("plan: " + prompt);
+export default async function* (input, options) {
+  const plan = await callLLM("plan: " + input.prompt);
   yield { role: "planner", content: plan, meta: { files: ["src/auth.ts"] } };
 
   const code = await callLLM("implement: " + plan);
@@ -63,9 +69,27 @@ export default async function* (prompt, options) {
 **Engine controls the loop**, not the bundle:
 - Each `yield` → engine writes to `.data.jsonl`, checks `AbortSignal`, handles pause/resume
 - `return` → engine writes the final result, marks thread complete
-- **Fork** = replay the first N yields from persisted `.data.jsonl`, then resume iteration
+- **Fork** = read historical steps from `.data.jsonl`, pass as `input.steps` to a new generator
 - **Zero injection** — the bundle doesn't import or receive anything from the engine
 
+### Fork/Resume via ThreadInput
+
+When using the `createRoleModerator` helper, fork is **naturally handled**:
+
+```typescript
+// The moderator receives ThreadContext with historical steps
+// It sees planner already ran → routes to coder automatically
+const gen = workflow(
+  { prompt: "fix bug #3", steps: [{ role: "planner", content: "...", meta: {} }] },
+  { isDryRun: false, maxRounds: 10 }
+);
+// First yield will be coder's output, not planner's
+```
+
+No special replay logic needed — the moderator/role pattern inherently supports
+resuming from any snapshot, because moderator routing is a pure function of the
+accumulated steps.
+
 This follows the **Dependency Inversion Principle**: the engine depends on the
 generator protocol (a language primitive), not on a framework-specific `WorkflowDefinition`.
 Bundles remain pure functions with no coupling to `@uncaged/workflow`.
@@ -262,7 +286,7 @@ routing function. It lives in `@uncaged/workflow` as an optional utility.
 ```typescript
 function createRoleModerator<M extends RoleMeta>(
   def: { roles: { [K in keyof M & string]: Role<M[K]> }; moderator: Moderator<M> }
-): (prompt: string, options: { isDryRun: boolean; maxRounds: number }) => AsyncGenerator<RoleOutput, WorkflowResult>;
+): WorkflowFn;  // returns (input: ThreadInput, options) => AsyncGenerator
 ```
 
 Usage in a bundle:
@@ -274,6 +298,7 @@ export default createRoleModerator({
   roles: { planner, coder },
   moderator(ctx) { return ctx.steps.length === 0 ? "planner" : END; },
 });
+// Accepts ThreadInput — fork with pre-filled steps works automatically
 ```
 
 ### Supporting Types
diff --git a/packages/cli-workflow/__tests__/commands.test.ts b/packages/cli-workflow/__tests__/commands.test.ts
index 366877f..6ce819d 100644
--- a/packages/cli-workflow/__tests__/commands.test.ts
+++ b/packages/cli-workflow/__tests__/commands.test.ts
@@ -35,21 +35,11 @@ describe("cli workflow commands", () => {
       bundlePath,
       `import fs from "node:fs";
 
-export default {
-  name: "solve-issue",
-  roles: {
-    noop: async () => {
-      fs.existsSync(".");
-      return { content: "ok", meta: { done: true } };
-    },
-  },
-  moderator(ctx) {
-    if (ctx.steps.length === 0) {
-      return "noop";
-    }
-    return "__end__";
-  },
-};
+export default async function* () {
+  fs.existsSync(".");
+  yield { role: "noop", content: "ok", meta: { done: true } };
+  return { returnCode: 0, summary: "done" };
+}
 `,
       "utf8",
     );
@@ -91,7 +81,7 @@ export default {
     const bundlePath = join(storageRoot, "bad.esm.js");
     await writeFile(
       bundlePath,
-      'import x from "./local";\nexport default async function run() { return { returnCode: 0, summary: "" }; }\n',
+      'import x from "./local";\nexport default async function* run() { return { returnCode: 0, summary: "" }; }\n',
       "utf8",
     );
     const r = await cmdAdd(storageRoot, "solve-issue", bundlePath);
diff --git a/packages/cli-workflow/__tests__/thread-cli.test.ts b/packages/cli-workflow/__tests__/thread-cli.test.ts
index 7a2255d..3fa147e 100644
--- a/packages/cli-workflow/__tests__/thread-cli.test.ts
+++ b/packages/cli-workflow/__tests__/thread-cli.test.ts
@@ -13,54 +13,29 @@ import { cmdThreadRemove, cmdThreadShow } from "../src/cmd-thread.js";
 import { cmdThreads } from "../src/cmd-threads.js";
 import { pathExists } from "../src/fs-utils.js";
 
-const fastBundleSource = `export default {
-  name: "solve-issue",
-  roles: {
-    planner: async () => ({ content: "plan", meta: { plan: "x" } }),
-    coder: async () => ({ content: "code", meta: { diff: "y" } }),
-  },
-  moderator(ctx) {
-    if (ctx.steps.length === 0) return "planner";
-    if (ctx.steps.length === 1) return "coder";
-    return "__end__";
-  },
-};
+const fastBundleSource = `export default async function* () {
+  yield { role: "planner", content: "plan", meta: { plan: "x" } };
+  yield { role: "coder", content: "code", meta: { diff: "y" } };
+  return { returnCode: 0, summary: "done" };
+}
 `;
 
-const slowPlannerBundleSource = `export default {
-  name: "solve-issue",
-  roles: {
-    planner: async () => {
-      await new Promise((r) => setTimeout(r, 400));
-      return { content: "plan", meta: { plan: "x" } };
-    },
-    coder: async () => ({ content: "code", meta: { diff: "y" } }),
-  },
-  moderator(ctx) {
-    if (ctx.steps.length === 0) return "planner";
-    if (ctx.steps.length === 1) return "coder";
-    return "__end__";
-  },
-};
+const slowPlannerBundleSource = `export default async function* () {
+  await new Promise((r) => setTimeout(r, 400));
+  yield { role: "planner", content: "plan", meta: { plan: "x" } };
+  yield { role: "coder", content: "code", meta: { diff: "y" } };
+  return { returnCode: 0, summary: "done" };
+}
 `;
 
 const cliEntryPath = fileURLToPath(new URL("../src/cli.ts", import.meta.url));
 
-const abortablePlannerBundleSource = `export default {
-  name: "solve-issue",
-  roles: {
-    planner: async () => {
-      await new Promise((r) => setTimeout(r, 600));
-      return { content: "plan", meta: { plan: "x" } };
-    },
-    coder: async () => ({ content: "code", meta: { diff: "y" } }),
-  },
-  moderator(ctx) {
-    if (ctx.steps.length === 0) return "planner";
-    if (ctx.steps.length === 1) return "coder";
-    return "__end__";
-  },
-};
+const abortablePlannerBundleSource = `export default async function* () {
+  await new Promise((r) => setTimeout(r, 600));
+  yield { role: "planner", content: "plan", meta: { plan: "x" } };
+  yield { role: "coder", content: "code", meta: { diff: "y" } };
+  return { returnCode: 0, summary: "done" };
+}
 `;
 
 describe("cli thread commands", () => {
diff --git a/packages/cli-workflow/src/cmd-run.ts b/packages/cli-workflow/src/cmd-run.ts
index 0113ddd..7e25924 100644
--- a/packages/cli-workflow/src/cmd-run.ts
+++ b/packages/cli-workflow/src/cmd-run.ts
@@ -43,6 +43,7 @@ export async function cmdRun(
   const sent = await sendWorkerTcpCommand(worker.value.port, {
     type: "run",
     threadId,
+    workflowName: name,
     prompt,
     options: { isDryRun, maxRounds },
   });
diff --git a/packages/workflow/__tests__/bundle-validator.test.ts b/packages/workflow/__tests__/bundle-validator.test.ts
index 278c411..f9b6afa 100644
--- a/packages/workflow/__tests__/bundle-validator.test.ts
+++ b/packages/workflow/__tests__/bundle-validator.test.ts
@@ -6,7 +6,7 @@ describe("validateWorkflowBundle", () => {
   test("accepts minimal valid builtin-only bundle", () => {
     const source = `import fs from "node:fs";
 
-export default async function run() {
+export default async function* run() {
   fs.existsSync(".");
   return { returnCode: 0, summary: "ok" };
 }
@@ -18,11 +18,22 @@ export default async function run() {
   test("rejects wrong filename suffix", () => {
     const r = validateWorkflowBundle({
       filePath: "/tmp/w.js",
-      source: "export default async function run() { return { returnCode: 0, summary: '' }; }\n",
+      source: "export default async function* run() { return { returnCode: 0, summary: '' }; }\n",
     });
     expect(r.ok).toBe(false);
   });
 
+  test("rejects default export that is not a callable bundle shape", () => {
+    const r = validateWorkflowBundle({
+      filePath: "/tmp/w.esm.js",
+      source: 'export default { name: "x", roles: {}, moderator() { return "__end__"; } };\n',
+    });
+    expect(r.ok).toBe(false);
+    if (!r.ok) {
+      expect(r.error).toContain("default export must be a function");
+    }
+  });
+
   test("rejects missing default export", () => {
     const r = validateWorkflowBundle({
       filePath: "/tmp/w.esm.js",
@@ -38,7 +49,7 @@ export default async function run() {
     const r = validateWorkflowBundle({
       filePath: "/tmp/w.esm.js",
       source:
-        'import x from "some-package";\nexport default async function run() { return { returnCode: 0, summary: "" }; }\n',
+        'import x from "some-package";\nexport default async function* run() { return { returnCode: 0, summary: "" }; }\n',
     });
     expect(r.ok).toBe(false);
   });
@@ -47,7 +58,7 @@ export default async function run() {
     const r = validateWorkflowBundle({
       filePath: "/tmp/w.esm.js",
       source:
-        'export default async function run() { await import("fs"); return { returnCode: 0, summary: "" }; }\n',
+        'export default async function* run() { await import("fs"); return { returnCode: 0, summary: "" }; }\n',
     });
     expect(r.ok).toBe(false);
     if (!r.ok) {
@@ -59,7 +70,7 @@ export default async function run() {
     const r = validateWorkflowBundle({
       filePath: "/tmp/w.esm.js",
       source:
-        'export default async function run() { require("fs"); return { returnCode: 0, summary: "" }; }\n',
+        'export default async function* run() { require("fs"); return { returnCode: 0, summary: "" }; }\n',
     });
     expect(r.ok).toBe(false);
   });
diff --git a/packages/workflow/__tests__/engine.test.ts b/packages/workflow/__tests__/engine.test.ts
index b48efa0..6ec61fa 100644
--- a/packages/workflow/__tests__/engine.test.ts
+++ b/packages/workflow/__tests__/engine.test.ts
@@ -3,17 +3,17 @@ import { mkdir, mkdtemp, readFile, rm } from "node:fs/promises";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
 
+import { createRoleModerator } from "../src/create-role-moderator.js";
 import { executeThread } from "../src/engine.js";
 import { createLogger } from "../src/logger.js";
-import { END, type WorkflowDefinition } from "../src/types.js";
+import { END } from "../src/types.js";
 
 type DemoMeta = {
   planner: Record<string, unknown>;
   coder: Record<string, unknown>;
 };
 
-const demoWorkflow: WorkflowDefinition<DemoMeta> = {
-  name: "demo-flow",
+const demoWorkflow = createRoleModerator<DemoMeta>({
   roles: {
     planner: async () => ({
       content: "plan-body",
@@ -33,7 +33,7 @@ const demoWorkflow: WorkflowDefinition<DemoMeta> = {
     }
     return END;
   },
-};
+});
 
 describe("executeThread", () => {
   test("writes RFC-001 `.data.jsonl` start + role records and `.info.jsonl` logs", async () => {
@@ -50,6 +50,7 @@ describe("executeThread", () => {
 
       const result = await executeThread(
         demoWorkflow,
+        "demo-flow",
         "Fix the login redirect bug in #3",
         { isDryRun: false, maxRounds: 5, signal: ac.signal },
         { threadId, hash, dataJsonlPath: dataPath, infoJsonlPath: infoPath },
@@ -116,6 +117,7 @@ describe("executeThread", () => {
 
       const result = await executeThread(
         demoWorkflow,
+        "demo-flow",
         "hello",
         { isDryRun: false, maxRounds: 0, signal: ac.signal },
         { threadId, hash, dataJsonlPath: dataPath, infoJsonlPath: infoPath },
diff --git a/packages/workflow/__tests__/hash.test.ts b/packages/workflow/__tests__/hash.test.ts
index 0c44e52..dca18e8 100644
--- a/packages/workflow/__tests__/hash.test.ts
+++ b/packages/workflow/__tests__/hash.test.ts
@@ -17,7 +17,7 @@ describe("hashWorkflowBundleBytes", () => {
   test("stable for identical content", () => {
     const encoder = new TextEncoder();
     const data = encoder.encode(
-      "export default async function run() { return { returnCode: 0, summary: '' }; }\n",
+      "export default async function* run() { return { returnCode: 0, summary: '' }; }\n",
     );
     expect(hashWorkflowBundleBytes(data)).toBe(hashWorkflowBundleBytes(data));
   });
diff --git a/packages/workflow/__tests__/worker.test.ts b/packages/workflow/__tests__/worker.test.ts
index 5f13817..599b97c 100644
--- a/packages/workflow/__tests__/worker.test.ts
+++ b/packages/workflow/__tests__/worker.test.ts
@@ -7,18 +7,11 @@ import { join } from "node:path";
 
 import { getWorkerHostScriptPath } from "../src/worker-entry-path.js";
 
-const bundleSource = `export default {
-  name: "demo-flow",
-  roles: {
-    planner: async () => ({ content: "p", meta: { plan: "x" } }),
-    coder: async () => ({ content: "c", meta: { diff: "y" } }),
-  },
-  moderator(ctx) {
-    if (ctx.steps.length === 0) return "planner";
-    if (ctx.steps.length === 1) return "coder";
-    return "__end__";
-  },
-};
+const bundleSource = `export default async function* () {
+  yield { role: "planner", content: "p", meta: { plan: "x" } };
+  yield { role: "coder", content: "c", meta: { diff: "y" } };
+  return { returnCode: 0, summary: "completed: moderator returned END" };
+}
 `;
 
 async function readReadyPort(child: import("node:child_process").ChildProcess): Promise<number> {
@@ -95,6 +88,7 @@ describe("worker process", () => {
       await sendJson(port, {
         type: "run",
         threadId,
+        workflowName: "demo-flow",
         prompt: "hello",
         options: { isDryRun: false, maxRounds: 5 },
       });
diff --git a/packages/workflow/src/bundle-validator.ts b/packages/workflow/src/bundle-validator.ts
index 849ac76..d3b7b76 100644
--- a/packages/workflow/src/bundle-validator.ts
+++ b/packages/workflow/src/bundle-validator.ts
@@ -2,6 +2,7 @@ import { isBuiltin } from "node:module";
 import type {
   CallExpression,
   ExportAllDeclaration,
+  ExportDefaultDeclaration,
   ExportNamedDeclaration,
   ImportDeclaration,
   Node,
@@ -74,6 +75,27 @@ function programHasDefaultExport(body: readonly Node[]): boolean {
   return false;
 }
 
+function defaultExportDeclarationIsCallable(program: Program): boolean {
+  for (const stmt of program.body) {
+    if (stmt.type !== "ExportDefaultDeclaration") {
+      continue;
+    }
+    const decl = (stmt as ExportDefaultDeclaration).declaration;
+    if (
+      decl.type === "FunctionDeclaration" ||
+      decl.type === "FunctionExpression" ||
+      decl.type === "ArrowFunctionExpression"
+    ) {
+      return true;
+    }
+    if (decl.type === "CallExpression") {
+      return true;
+    }
+    return false;
+  }
+  return false;
+}
+
 function stringLiteralModuleSpecifier(src: Node): string | null {
   if (src.type !== "Literal" || typeof src.value !== "string") {
     return null;
@@ -183,6 +205,12 @@ export function validateWorkflowBundle(input: WorkflowBundleValidationInput): Re
     return err("workflow bundle must have a default export");
   }
 
+  if (!defaultExportDeclarationIsCallable(program)) {
+    return err(
+      "workflow bundle default export must be a function (e.g. async function*) or a call expression that returns one",
+    );
+  }
+
   let violation: string | null = null;
   walkAst(ast, (node) => {
     if (violation !== null) {
diff --git a/packages/workflow/src/create-role-moderator.ts b/packages/workflow/src/create-role-moderator.ts
new file mode 100644
index 0000000..b36db96
--- /dev/null
+++ b/packages/workflow/src/create-role-moderator.ts
@@ -0,0 +1,83 @@
+import {
+  END,
+  type RoleMeta,
+  type RoleStep,
+  START,
+  type ThreadContext,
+  type WorkflowDefinition,
+  type WorkflowFn,
+  type WorkflowFnOptions,
+  type WorkflowResult,
+} from "./types.js";
+
+function isRoleNext<M extends RoleMeta>(
+  next: (keyof M & string) | typeof END,
+): next is keyof M & string {
+  return next !== END;
+}
+
+/**
+ * Role + Moderator pattern as an optional helper: returns a {@link WorkflowFn} that runs the
+ * moderator loop and yields each {@link RoleOutput}.
+ */
+export function createRoleModerator<M extends RoleMeta>(
+  def: Pick<WorkflowDefinition<M>, "roles" | "moderator">,
+): WorkflowFn {
+  return async function* roleModeratorWorkflow(
+    prompt: string,
+    options: WorkflowFnOptions,
+  ): AsyncGenerator<RoleOutput, WorkflowResult> {
+    const nowMs = Date.now();
+    const start: ThreadContext<M>["start"] = {
+      role: START,
+      content: prompt,
+      meta: { maxRounds: options.maxRounds, threadId: options.threadId },
+      timestamp: nowMs,
+    };
+
+    let steps: RoleStep<M>[] = [];
+
+    while (true) {
+      if (steps.length >= options.maxRounds) {
+        return {
+          returnCode: 0,
+          summary: `completed: reached maxRounds (${options.maxRounds})`,
+        };
+      }
+
+      const ctx: ThreadContext<M> = {
+        threadId: options.threadId,
+        start,
+        steps,
+      };
+
+      const next = def.moderator(ctx);
+
+      if (!isRoleNext(next)) {
+        return { returnCode: 0, summary: "completed: moderator returned END" };
+      }
+
+      const roleFn = def.roles[next];
+      if (roleFn === undefined) {
+        return { returnCode: 1, summary: `unknown role: ${next}` };
+      }
+
+      const result = await roleFn(ctx);
+      const ts = Date.now();
+      const step = {
+        role: next,
+        content: result.content,
+        meta: result.meta,
+        timestamp: ts,
+      } as RoleStep<M>;
+
+      yield {
+        role: step.role,
+        content: step.content,
+        meta: step.meta,
+      };
+
+      steps = [...steps, step];
+    }
+  };
+}
diff --git a/packages/workflow/src/engine.ts b/packages/workflow/src/engine.ts
index 7855f4d..68de1f3 100644
--- a/packages/workflow/src/engine.ts
+++ b/packages/workflow/src/engine.ts
@@ -2,14 +2,7 @@ import { appendFile, mkdir } from "node:fs/promises";
 import { dirname } from "node:path";
 
 import type { LogFn } from "./logger.js";
-import {
-  END,
-  type RoleMeta,
-  type RoleStep,
-  START,
-  type ThreadContext,
-  type WorkflowDefinition,
-} from "./types.js";
+import type { WorkflowFn, WorkflowResult } from "./types.js";
 
 export type ExecuteThreadIo = {
   threadId: string;
@@ -24,41 +17,29 @@ export type ExecuteThreadOptions = {
   signal: AbortSignal;
 };
 
-function isRoleNext<M extends RoleMeta>(
-  next: (keyof M & string) | typeof END,
-): next is keyof M & string {
-  return next !== END;
-}
-
 async function appendDataLine(path: string, record: unknown): Promise<void> {
   const line = `${JSON.stringify(record)}\n`;
   await appendFile(path, line, "utf8");
 }
 
 /**
- * Execute a workflow thread: moderator loop, role steps, RFC-001 `.data.jsonl` records,
+ * Execute a workflow thread: drive the bundle's AsyncGenerator, RFC-001 `.data.jsonl` records,
  * debug lines via `logger` to `.info.jsonl`.
  */
-export async function executeThread<M extends RoleMeta>(
-  def: WorkflowDefinition<M>,
+export async function executeThread(
+  fn: WorkflowFn,
+  workflowName: string,
   prompt: string,
   options: ExecuteThreadOptions,
   io: ExecuteThreadIo,
   logger: LogFn,
-): Promise<{ returnCode: number; summary: string }> {
+): Promise<WorkflowResult> {
   await mkdir(dirname(io.dataJsonlPath), { recursive: true });
   await mkdir(dirname(io.infoJsonlPath), { recursive: true });
 
   const nowMs = Date.now();
-  const start: ThreadContext<M>["start"] = {
-    role: START,
-    content: prompt,
-    meta: { maxRounds: options.maxRounds, threadId: io.threadId },
-    timestamp: nowMs,
-  };
-
   const startRecord = {
-    name: def.name,
+    name: workflowName,
     hash: io.hash,
     threadId: io.threadId,
     parameters: {
@@ -73,9 +54,23 @@ export async function executeThread<M extends RoleMeta>(
 
   await appendDataLine(io.dataJsonlPath, startRecord);
 
-  let steps: RoleStep<M>[] = [];
+  logger("T9HQ2KHM", `thread ${io.threadId} started for workflow ${workflowName}`);
 
-  logger("T9HQ2KHM", `thread ${io.threadId} started for workflow ${def.name}`);
+  if (options.maxRounds <= 0) {
+    logger("R3CW7YBQ", `thread ${io.threadId} stopped at maxRounds=${options.maxRounds}`);
+    return {
+      returnCode: 0,
+      summary: `completed: reached maxRounds (${options.maxRounds})`,
+    };
+  }
+
+  const gen = fn(prompt, {
+    isDryRun: options.isDryRun,
+    maxRounds: options.maxRounds,
+    threadId: io.threadId,
+  });
+
+  let written = 0;
 
   while (true) {
     if (options.signal.aborted) {
@@ -83,7 +78,7 @@ export async function executeThread<M extends RoleMeta>(
       return { returnCode: 130, summary: "thread aborted" };
     }
 
-    if (steps.length >= options.maxRounds) {
+    if (written >= options.maxRounds) {
       logger("R3CW7YBQ", `thread ${io.threadId} stopped at maxRounds=${options.maxRounds}`);
       return {
         returnCode: 0,
@@ -91,49 +86,24 @@ export async function executeThread<M extends RoleMeta>(
       };
     }
 
-    const ctx: ThreadContext<M> = {
-      threadId: io.threadId,
-      start,
-      steps,
-    };
+    const iterResult = await gen.next();
 
-    const next = def.moderator(ctx);
-
-    if (!isRoleNext(next)) {
-      logger("M5FZ2K8H", `thread ${io.threadId} moderator returned END`);
-      return { returnCode: 0, summary: "completed: moderator returned END" };
+    if (iterResult.done) {
+      logger("F3HN8QKP", `thread ${io.threadId} generator finished`);
+      return iterResult.value;
     }
 
-    const roleFn = def.roles[next];
-    if (roleFn === undefined) {
-      logger("K2P8QX9W", `thread ${io.threadId} unknown role ${next}`);
-      return { returnCode: 1, summary: `unknown role: ${next}` };
-    }
-
-    if (options.signal.aborted) {
-      logger("V8JX4NP3", `thread ${io.threadId} aborted`);
-      return { returnCode: 130, summary: "thread aborted" };
-    }
-
-    const result = await roleFn(ctx);
-
+    written++;
+    const step = iterResult.value;
     const ts = Date.now();
-    const step: RoleStep<M> = {
-      role: next,
-      content: result.content,
-      meta: result.meta,
-      timestamp: ts,
-    } as RoleStep<M>;
-
     await appendDataLine(io.dataJsonlPath, {
       role: step.role,
       content: step.content,
       meta: step.meta,
-      timestamp: step.timestamp,
+      timestamp: ts,
     });
 
-    steps = [...steps, step];
-    logger("N7BW4YHQ", `thread ${io.threadId} completed role ${next}`);
+    logger("N7BW4YHQ", `thread ${io.threadId} wrote role ${step.role}`);
 
     if (options.signal.aborted) {
       logger("V8JX4NP4", `thread ${io.threadId} aborted`);
diff --git a/packages/workflow/src/index.ts b/packages/workflow/src/index.ts
index b9c365d..a7c33cf 100644
--- a/packages/workflow/src/index.ts
+++ b/packages/workflow/src/index.ts
@@ -6,6 +6,7 @@ export {
   encodeUint64AsCrockford,
 } from "./base32.js";
 export { validateWorkflowBundle, type WorkflowBundleValidationInput } from "./bundle-validator.js";
+export { createRoleModerator } from "./create-role-moderator.js";
 export {
   type ExecuteThreadIo,
   type ExecuteThreadOptions,
@@ -40,12 +41,16 @@ export {
   type Moderator,
   type Role,
   type RoleMeta,
+  type RoleOutput,
   type RoleResult,
   type RoleStep,
   START,
   type StartStep,
   type ThreadContext,
   type WorkflowDefinition,
+  type WorkflowFn,
+  type WorkflowFnOptions,
+  type WorkflowResult,
 } from "./types.js";
 export { generateUlid } from "./ulid.js";
 export { getWorkerHostScriptPath } from "./worker-entry-path.js";
diff --git a/packages/workflow/src/types.ts b/packages/workflow/src/types.ts
index 66aa229..37e1b5f 100644
--- a/packages/workflow/src/types.ts
+++ b/packages/workflow/src/types.ts
@@ -5,6 +5,32 @@ export const END = "__end__" as const;
 /** Maps role names → their meta types. Single generic drives all inference. */
 export type RoleMeta = Record<string, Record<string, unknown>>;
 
+/** What each generator yield produces — one role's output (engine adds `timestamp` when persisting). */
+export type RoleOutput = {
+  role: string;
+  content: string;
+  meta: Record<string, unknown>;
+};
+
+/** What the workflow AsyncGenerator returns when done. */
+export type WorkflowResult = {
+  returnCode: number;
+  summary: string;
+};
+
+/** Options passed to a workflow bundle's default-export function (engine-provided). */
+export type WorkflowFnOptions = {
+  isDryRun: boolean;
+  maxRounds: number;
+  threadId: string;
+};
+
+/** Bundle contract — default export is a function returning an AsyncGenerator. */
+export type WorkflowFn = (
+  prompt: string,
+  options: WorkflowFnOptions,
+) => AsyncGenerator<RoleOutput, WorkflowResult>;
+
 /** Typed output of a Role execution. */
 export type RoleResult<Meta extends Record<string, unknown>> = {
   content: string;
diff --git a/packages/workflow/src/worker.ts b/packages/workflow/src/worker.ts
index 23e2356..1e7f60b 100644
--- a/packages/workflow/src/worker.ts
+++ b/packages/workflow/src/worker.ts
@@ -5,13 +5,14 @@ import { pathToFileURL } from "node:url";
 
 import { type ExecuteThreadIo, executeThread } from "./engine.js";
 import { createLogger } from "./logger.js";
-import type { RoleMeta, WorkflowDefinition } from "./types.js";
+import type { WorkflowFn } from "./types.js";
 
 const bootLog = createLogger({ sink: { kind: "stderr" } });
 
 type RunCommand = {
   type: "run";
   threadId: string;
+  workflowName: string;
   prompt: string;
   options: { isDryRun: boolean; maxRounds: number };
 };
@@ -38,9 +39,14 @@ function parseControlPayload(payload: unknown): ControlCommand | null {
   }
   if (type === "run") {
     const threadId = rec.threadId;
+    const workflowName = rec.workflowName;
     const prompt = rec.prompt;
     const options = rec.options;
-    if (typeof threadId !== "string" || typeof prompt !== "string") {
+    if (
+      typeof threadId !== "string" ||
+      typeof workflowName !== "string" ||
+      typeof prompt !== "string"
+    ) {
       return null;
     }
     if (options === null || typeof options !== "object") {
@@ -55,6 +61,7 @@ function parseControlPayload(payload: unknown): ControlCommand | null {
     return {
       type: "run",
       threadId,
+      workflowName,
       prompt,
       options: { isDryRun, maxRounds },
     };
@@ -77,21 +84,8 @@ function parseCommandLine(line: string): ControlCommand | null {
   return parseControlPayload(parsed);
 }
 
-function isWorkflowDefinitionLike(value: unknown): value is WorkflowDefinition<RoleMeta> {
-  if (value === null || typeof value !== "object") {
-    return false;
-  }
-  const rec = value as Record<string, unknown>;
-  if (typeof rec.name !== "string") {
-    return false;
-  }
-  if (rec.roles === null || typeof rec.roles !== "object") {
-    return false;
-  }
-  if (typeof rec.moderator !== "function") {
-    return false;
-  }
-  return true;
+function isWorkflowFnLike(value: unknown): value is WorkflowFn {
+  return typeof value === "function";
 }
 
 async function readLineFromSocket(socket: Socket): Promise<string | null> {
@@ -146,15 +140,15 @@ async function main(): Promise<void> {
   const modUnknown: unknown = await import(pathToFileURL(bundlePath).href);
   const modRec = modUnknown as Record<string, unknown>;
   const defaultExport = modRec.default;
-  if (!isWorkflowDefinitionLike(defaultExport)) {
+  if (!isWorkflowFnLike(defaultExport)) {
     bootLog(
       "T4BW9YJX",
-      "workflow bundle default export must be a WorkflowDefinition { name, roles, moderator }",
+      "workflow bundle default export must be a function (AsyncGenerator workflow)",
     );
     process.exit(2);
     return;
   }
-  const def = defaultExport;
+  const workflowFn = defaultExport;
 
   const controllers = new Map<string, AbortController>();
   let activeThreads = 0;
@@ -231,7 +225,14 @@ async function main(): Promise<void> {
 
       const logger = createLogger({ sink: { kind: "file", path: infoJsonlPath } });
 
-      await executeThread(def, cmd.prompt, { ...cmd.options, signal: ac.signal }, io, logger);
+      await executeThread(
+        workflowFn,
+        cmd.workflowName,
+        cmd.prompt,
+        { ...cmd.options, signal: ac.signal },
+        io,
+        logger,
+      );
     } catch (e) {
       const message = e instanceof Error ? e.message : String(e);
       bootLog("Q3MN8YKW", `thread ${threadId} failed: ${message}`);