feat(#194): Phase 2 — Engine layer Merkle call stack wiring

- Protocol: AgentFnResult = string | { output, childThread }, RoleOutput.childThread,
  ThreadContext.bundleHash for parent state lookup
- Runtime: create-workflow normalizes AgentFnResult, propagates childThread in RoleOutput
- Engine: ExecuteThreadOptions.parentStateHash, appendStateForStep writes childThread,
  putStartNode uses parentStateHash
- workflowAsAgent: reads parent head state from threads.json, passes parentStateHash
  to child, returns { output, childThread: rootHash }
- Integration test: 4 cases verifying bidirectional Merkle links (306 lines)

Phase 2 of #194 (Merkle Call Stack). Closes #196.

小橘 <xiaoju@shazhou.work>

packages/cli-workflow/__tests__/gc-cli.test.ts

@@ -45,8 +45,8 @@ describe("gc cli and garbageCollectCas", () => {
       {
         name: "demo",
         hash: bundleHash,
-
         depth: 0,
+        parentState: null,
       },
       promptHash,
     );
@@ -100,8 +100,8 @@ describe("gc cli and garbageCollectCas", () => {
       {
         name: "demo",
         hash: bundleHash,
-
         depth: 0,
+        parentState: null,
       },
       promptHash,
     );
@@ -135,8 +135,8 @@ describe("gc cli and garbageCollectCas", () => {
       {
         name: "demo",
         hash: bundleHash,
-
         depth: 0,
+        parentState: null,
       },
       promptHash,
     );

packages/workflow-agent-llm/__tests__/create-llm-adapter.test.ts

@@ -12,6 +12,7 @@ function makeCtx(userContent: string): AgentContext {
       timestamp: 1,
     },
     depth: 0,
+    bundleHash: "TESTHASH00001",
     steps: [],
     threadId: "01TEST000000000000000000TR",
     currentRole: { name: "planner", systemPrompt: "system instructions" },

packages/workflow-cas/__tests__/collect-refs.test.ts

@@ -14,6 +14,7 @@ function payload(
     ancestors: partial.ancestors ?? [],
     compact: partial.compact ?? null,
     timestamp: partial.timestamp ?? 0,
+    childThread: partial.childThread ?? null,
   };
 }
 
@@ -62,4 +63,32 @@ describe("collectRefs", () => {
     );
     expect(refs).toEqual(["S2", "C2"]);
   });
+
+  test("includes childThread hash when childThread is non-null", () => {
+    const refs = collectRefs(
+      payload({
+        role: "developer",
+        start: "S3",
+        content: "C3",
+        ancestors: ["A3"],
+        compact: null,
+        childThread: "CHILDEND000000000000001",
+      }),
+    );
+    expect(refs).toEqual(["S3", "C3", "A3", "CHILDEND000000000000001"]);
+  });
+
+  test("does not include childThread when childThread is null", () => {
+    const refs = collectRefs(
+      payload({
+        role: "developer",
+        start: "S4",
+        content: "C4",
+        ancestors: [],
+        compact: null,
+        childThread: null,
+      }),
+    );
+    expect(refs).toEqual(["S4", "C4"]);
+  });
 });

packages/workflow-cas/__tests__/nodes.test.ts

@@ -0,0 +1,161 @@
+import { afterEach, beforeEach, describe, expect, test } from "bun:test";
+import { mkdtemp, rm } from "node:fs/promises";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { stringify } from "yaml";
+
+import { createCasStore } from "../src/cas.js";
+import { parseCasThreadNode, putStartNode, putStateNode } from "../src/nodes.js";
+
+describe("putStartNode — parentState in refs", () => {
+  let dir: string;
+
+  beforeEach(async () => {
+    dir = await mkdtemp(join(tmpdir(), "wf-cas-nodes-"));
+  });
+
+  afterEach(async () => {
+    await rm(dir, { recursive: true, force: true });
+  });
+
+  test("refs contains only promptHash when parentState is null", async () => {
+    const cas = createCasStore(join(dir, "cas"));
+    const promptHash = await cas.put("hello");
+    const startHash = await putStartNode(
+      cas,
+      { name: "demo", hash: "BUNDLEAAAAAAAAA", depth: 0, parentState: null },
+      promptHash,
+    );
+
+    const blob = await cas.get(startHash);
+    expect(blob).not.toBeNull();
+    const parsed = parseCasThreadNode(blob ?? "");
+    expect(parsed).not.toBeNull();
+    expect(parsed?.kind).toBe("start");
+    if (parsed?.kind !== "start") return;
+
+    expect(parsed.node.refs).toEqual([promptHash]);
+    expect(parsed.node.payload.parentState).toBeNull();
+  });
+
+  test("refs contains [promptHash, parentStateHash] when parentState is set", async () => {
+    const cas = createCasStore(join(dir, "cas"));
+    const parentStateHash = await cas.put("fake-parent-state");
+    const promptHash = await cas.put("child-prompt");
+    const startHash = await putStartNode(
+      cas,
+      { name: "develop", hash: "BUNDLEBBBBBBBBB", depth: 1, parentState: parentStateHash },
+      promptHash,
+    );
+
+    const blob = await cas.get(startHash);
+    expect(blob).not.toBeNull();
+    const parsed = parseCasThreadNode(blob ?? "");
+    expect(parsed).not.toBeNull();
+    expect(parsed?.kind).toBe("start");
+    if (parsed?.kind !== "start") return;
+
+    expect(parsed.node.refs).toEqual([promptHash, parentStateHash]);
+    expect(parsed.node.payload.parentState).toBe(parentStateHash);
+  });
+});
+
+describe("putStateNode — childThread in refs", () => {
+  let dir: string;
+
+  beforeEach(async () => {
+    dir = await mkdtemp(join(tmpdir(), "wf-cas-nodes-state-"));
+  });
+
+  afterEach(async () => {
+    await rm(dir, { recursive: true, force: true });
+  });
+
+  test("refs does not include childThread when childThread is null", async () => {
+    const cas = createCasStore(join(dir, "cas"));
+    const startHash = await cas.put("start");
+    const contentHash = await cas.put("content");
+    const stateHash = await putStateNode(cas, {
+      role: "planner",
+      meta: {},
+      start: startHash,
+      content: contentHash,
+      ancestors: [],
+      compact: null,
+      timestamp: 1000,
+      childThread: null,
+    });
+
+    const blob = await cas.get(stateHash);
+    expect(blob).not.toBeNull();
+    const parsed = parseCasThreadNode(blob ?? "");
+    expect(parsed?.kind).toBe("state");
+    if (parsed?.kind !== "state") return;
+
+    expect(parsed.node.refs).not.toContain("anything-else");
+    expect(parsed.node.refs).toEqual([startHash, contentHash]);
+    expect(parsed.node.payload.childThread).toBeNull();
+  });
+
+  test("refs includes childThread hash when childThread is set", async () => {
+    const cas = createCasStore(join(dir, "cas"));
+    const startHash = await cas.put("start");
+    const contentHash = await cas.put("content");
+    const childEndHash = await cas.put("child-end-state");
+    const stateHash = await putStateNode(cas, {
+      role: "developer",
+      meta: { pr: 42 },
+      start: startHash,
+      content: contentHash,
+      ancestors: [],
+      compact: null,
+      timestamp: 2000,
+      childThread: childEndHash,
+    });
+
+    const blob = await cas.get(stateHash);
+    expect(blob).not.toBeNull();
+    const parsed = parseCasThreadNode(blob ?? "");
+    expect(parsed?.kind).toBe("state");
+    if (parsed?.kind !== "state") return;
+
+    expect(parsed.node.refs).toContain(childEndHash);
+    expect(parsed.node.payload.childThread).toBe(childEndHash);
+  });
+});
+
+describe("parseCasThreadNode — legacy node compatibility", () => {
+  test("start node without parentState field defaults to null", () => {
+    const yaml = stringify({
+      type: "start",
+      payload: { name: "demo", hash: "BUNDLEAAAAAAAAA", depth: 0 },
+      refs: ["PROMPTHASH00001"],
+    });
+    const parsed = parseCasThreadNode(yaml);
+    expect(parsed).not.toBeNull();
+    expect(parsed?.kind).toBe("start");
+    if (parsed?.kind !== "start") return;
+    expect(parsed.node.payload.parentState).toBeNull();
+  });
+
+  test("state node without childThread field defaults to null", () => {
+    const yaml = stringify({
+      type: "state",
+      payload: {
+        role: "planner",
+        meta: {},
+        start: "STARTHASH00001",
+        content: "CONTENTHASH0001",
+        ancestors: [],
+        compact: null,
+        timestamp: 1000,
+      },
+      refs: ["STARTHASH00001", "CONTENTHASH0001"],
+    });
+    const parsed = parseCasThreadNode(yaml);
+    expect(parsed).not.toBeNull();
+    expect(parsed?.kind).toBe("state");
+    if (parsed?.kind !== "state") return;
+    expect(parsed.node.payload.childThread).toBeNull();
+  });
+});

packages/workflow-cas/src/collect-refs.ts

@@ -9,5 +9,8 @@ export function collectRefs(payload: StateNode["payload"]): string[] {
   if (payload.compact !== null) {
     out.push(payload.compact);
   }
+  if (payload.childThread !== null) {
+    out.push(payload.childThread);
+  }
   return out;
 }

packages/workflow-cas/src/nodes.ts

@@ -18,6 +18,10 @@ function isStartPayload(value: unknown): value is StartNodePayload {
   if (!isRecord(value)) {
     return false;
   }
+  const parentState = value.parentState;
+  if (parentState !== undefined && parentState !== null && typeof parentState !== "string") {
+    return false;
+  }
   return (
     typeof value.name === "string" &&
     typeof value.hash === "string" &&
@@ -25,6 +29,16 @@ function isStartPayload(value: unknown): value is StartNodePayload {
   );
 }
 
+/** Normalizes a raw start payload, defaulting `parentState` to `null` for legacy nodes. */
+function normalizeStartPayload(raw: StartNodePayload): StartNodePayload {
+  return {
+    name: raw.name,
+    hash: raw.hash,
+    depth: raw.depth,
+    parentState: raw.parentState ?? null,
+  };
+}
+
 function isStatePayload(value: unknown): value is StateNodePayload {
   if (!isRecord(value)) {
     return false;
@@ -41,6 +55,10 @@ function isStatePayload(value: unknown): value is StateNodePayload {
   if (!isRecord(meta)) {
     return false;
   }
+  const childThread = value.childThread;
+  if (childThread !== undefined && childThread !== null && typeof childThread !== "string") {
+    return false;
+  }
   return (
     typeof value.role === "string" &&
     typeof value.start === "string" &&
@@ -49,6 +67,20 @@ function isStatePayload(value: unknown): value is StateNodePayload {
   );
 }
 
+/** Normalizes a raw state payload, defaulting `childThread` to `null` for legacy nodes. */
+function normalizeStatePayload(raw: StateNodePayload): StateNodePayload {
+  return {
+    role: raw.role,
+    meta: raw.meta,
+    start: raw.start,
+    content: raw.content,
+    ancestors: raw.ancestors,
+    compact: raw.compact,
+    timestamp: raw.timestamp,
+    childThread: raw.childThread ?? null,
+  };
+}
+
 /** Parses a YAML CAS blob into a typed RFC v3 thread node (or legacy content layout with `children`). */
 export function parseCasThreadNode(yamlText: string): ParsedCasThreadNode | null {
   let raw: unknown;
@@ -86,14 +118,22 @@ export function parseCasThreadNode(yamlText: string): ParsedCasThreadNode | null
     if (!isStartPayload(raw.payload)) {
       return null;
     }
-    const node: StartNode = { type: "start", payload: raw.payload, refs: [...refs] };
+    const node: StartNode = {
+      type: "start",
+      payload: normalizeStartPayload(raw.payload),
+      refs: [...refs],
+    };
     return { kind: "start", node };
   }
 
   if (!isStatePayload(raw.payload)) {
     return null;
   }
-  const node: StateNode = { type: "state", payload: raw.payload, refs: [...refs] };
+  const node: StateNode = {
+    type: "state",
+    payload: normalizeStatePayload(raw.payload),
+    refs: [...refs],
+  };
   return { kind: "state", node };
 }
 
@@ -143,10 +183,14 @@ export async function putStartNode(
   payload: StartNode["payload"],
   promptHash: string,
 ): Promise<string> {
+  const refs = [promptHash];
+  if (payload.parentState !== null) {
+    refs.push(payload.parentState);
+  }
   const node: StartNode = {
     type: "start",
     payload,
-    refs: [promptHash],
+    refs,
   };
   return store.put(serializeCasNode(node));
 }

packages/workflow-execute/__tests__/engine.test.ts

@@ -35,6 +35,7 @@ function noLogger(): (tag: string, content: string) => void {
 function makeOptions(overrides: Partial<ExecuteThreadOptions>): ExecuteThreadOptions {
   return {
     depth: 0,
+    parentStateHash: null,
     signal: new AbortController().signal,
     awaitAfterEachYield: async () => {},
     forkSourceThreadId: null,
@@ -144,9 +145,9 @@ describe("executeThread (Phase 2 — CAS thread storage)", () => {
       runtime: WorkflowRuntime,
     ): AsyncGenerator<RoleOutput, WorkflowCompletion> {
       const h1 = await runtime.cas.put("plan-text");
-      yield { role: "planner", contentHash: h1, meta: { plan: 1 }, refs: [h1] };
+      yield { role: "planner", contentHash: h1, meta: { plan: 1 }, refs: [h1], childThread: null };
       const h2 = await runtime.cas.put("code-text");
-      yield { role: "coder", contentHash: h2, meta: { diff: "y" }, refs: [h2] };
+      yield { role: "coder", contentHash: h2, meta: { diff: "y" }, refs: [h2], childThread: null };
       return { returnCode: 0, summary: "done" };
     };
 
@@ -210,7 +211,7 @@ describe("executeThread (Phase 2 — CAS thread storage)", () => {
       runtime: WorkflowRuntime,
     ): AsyncGenerator<RoleOutput, WorkflowCompletion> {
       const h = await runtime.cas.put("only-step");
-      yield { role: "only", contentHash: h, meta: {}, refs: [h] };
+      yield { role: "only", contentHash: h, meta: {}, refs: [h], childThread: null };
       return { returnCode: 0, summary: "completed" };
     };
 
@@ -261,7 +262,7 @@ describe("executeThread (Phase 2 — CAS thread storage)", () => {
       runtime: WorkflowRuntime,
     ): AsyncGenerator<RoleOutput, WorkflowCompletion> {
       const h = await runtime.cas.put("step");
-      yield { role: "only", contentHash: h, meta: {}, refs: [h] };
+      yield { role: "only", contentHash: h, meta: {}, refs: [h], childThread: null };
       return { returnCode: 0, summary: "done" };
     };
 

packages/workflow-execute/__tests__/gc-mark-sweep.test.ts

@@ -46,6 +46,7 @@ describe("garbageCollectCas (mark-and-sweep)", () => {
         name: "demo",
         hash: bundleHash,
         depth: 0,
+        parentState: null,
       },
       promptHash,
     );
@@ -59,6 +60,7 @@ describe("garbageCollectCas (mark-and-sweep)", () => {
       ancestors: [],
       compact: null,
       timestamp: 1,
+      childThread: null,
     } satisfies StateNodePayload);
 
     const c2 = await putContentNodeWithRefs(cas, "c1", []);
@@ -70,6 +72,7 @@ describe("garbageCollectCas (mark-and-sweep)", () => {
       ancestors: [h1],
       compact: null,
       timestamp: 2,
+      childThread: null,
     } satisfies StateNodePayload);
 
     const ec = await putContentNodeWithRefs(cas, "", []);
@@ -81,6 +84,7 @@ describe("garbageCollectCas (mark-and-sweep)", () => {
       ancestors: [h1],
       compact: null,
       timestamp: 3,
+      childThread: null,
     } satisfies StateNodePayload);
 
     await upsertThreadEntry(bundleDir, "THREAD_AAAAAAA", {

packages/workflow-execute/__tests__/merkle-call-stack.test.ts

@@ -0,0 +1,306 @@
+import { afterEach, beforeEach, describe, expect, test } from "bun:test";
+import { mkdir, mkdtemp, readFile, rm, writeFile } from "node:fs/promises";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import type { CasStore } from "@uncaged/workflow-cas";
+import { createCasStore, parseCasThreadNode } from "@uncaged/workflow-cas";
+import type { StartNode, StateNode } from "@uncaged/workflow-protocol";
+import type {
+  RoleOutput,
+  ThreadContext,
+  WorkflowCompletion,
+  WorkflowFn,
+  WorkflowRuntime,
+} from "@uncaged/workflow-runtime";
+
+import { executeThread } from "../src/engine/engine.js";
+import type { ExecuteThreadIo, ExecuteThreadOptions } from "../src/engine/types.js";
+
+const TEST_REGISTRY_YAML = `config:
+  maxDepth: 3
+  supervisorInterval: 0
+  providers:
+    stub:
+      baseUrl: http://127.0.0.1:9
+      apiKey: test
+  models:
+    default: stub/m
+workflows: {}
+`;
+
+function noLogger(): (tag: string, content: string) => void {
+  return () => {};
+}
+
+function makeOptions(overrides: Partial<ExecuteThreadOptions>): ExecuteThreadOptions {
+  return {
+    depth: 0,
+    parentStateHash: null,
+    signal: new AbortController().signal,
+    awaitAfterEachYield: async () => {},
+    forkSourceThreadId: null,
+    prefilledDiskSteps: null,
+    forkContinuation: null,
+    replayTimestamps: null,
+    storageRoot: "/tmp/never",
+    ...overrides,
+  };
+}
+
+async function setupStorage(): Promise<{
+  storageRoot: string;
+  casDir: string;
+}> {
+  const storageRoot = await mkdtemp(join(tmpdir(), "uncaged-wf-merkle-"));
+  await writeFile(join(storageRoot, "workflow.yaml"), TEST_REGISTRY_YAML, "utf8");
+  const casDir = join(storageRoot, "cas");
+  await mkdir(casDir, { recursive: true });
+  return { storageRoot, casDir };
+}
+
+async function loadStartNode(cas: CasStore, endHash: string): Promise<StartNode> {
+  const endBlob = await cas.get(endHash);
+  const endParsed = parseCasThreadNode(endBlob ?? "");
+  if (endParsed?.kind !== "state") throw new Error("expected state node");
+  const startBlob = await cas.get(endParsed.node.payload.start);
+  const startParsed = parseCasThreadNode(startBlob ?? "");
+  if (startParsed?.kind !== "start") throw new Error("expected start node");
+  return startParsed.node;
+}
+
+async function loadStateNode(cas: CasStore, hash: string): Promise<StateNode> {
+  const blob = await cas.get(hash);
+  const parsed = parseCasThreadNode(blob ?? "");
+  if (parsed?.kind !== "state") throw new Error("expected state node");
+  return parsed.node;
+}
+
+describe("Merkle call stack — cross-thread DAG linking (Phase 2)", () => {
+  let storageRoot: string;
+  let casDir: string;
+
+  beforeEach(async () => {
+    const setup = await setupStorage();
+    storageRoot = setup.storageRoot;
+    casDir = setup.casDir;
+  });
+
+  afterEach(async () => {
+    await rm(storageRoot, { recursive: true, force: true });
+  });
+
+  test("parentStateHash is written into child start node's parentState and refs", async () => {
+    const cas = createCasStore(casDir);
+
+    // biome-ignore lint/correctness/useYield: testing start-only path
+    const parentWf: WorkflowFn = async function* (
+      _thread: ThreadContext,
+      _runtime: WorkflowRuntime,
+    ): AsyncGenerator<RoleOutput, WorkflowCompletion> {
+      return { returnCode: 0, summary: "parent done" };
+    };
+
+    const parentResult = await executeThread(
+      parentWf,
+      "parent-wf",
+      { prompt: "parent task", steps: [] },
+      makeOptions({ storageRoot }),
+      {
+        threadId: "P_THREAD_01",
+        hash: "PARENTHASH0001",
+        infoJsonlPath: join(storageRoot, "logs", "PARENTHASH0001", "P1.info.jsonl"),
+        cas,
+      },
+      noLogger(),
+    );
+
+    // biome-ignore lint/correctness/useYield: testing start-only path
+    const childWf: WorkflowFn = async function* (
+      _thread: ThreadContext,
+      _runtime: WorkflowRuntime,
+    ): AsyncGenerator<RoleOutput, WorkflowCompletion> {
+      return { returnCode: 0, summary: "child done" };
+    };
+
+    const childResult = await executeThread(
+      childWf,
+      "child-wf",
+      { prompt: "child task", steps: [] },
+      makeOptions({ storageRoot, depth: 1, parentStateHash: parentResult.rootHash }),
+      {
+        threadId: "C_THREAD_01",
+        hash: "CHILDHASH00001",
+        infoJsonlPath: join(storageRoot, "logs", "CHILDHASH00001", "C1.info.jsonl"),
+        cas,
+      },
+      noLogger(),
+    );
+
+    const childStart = await loadStartNode(cas, childResult.rootHash);
+    expect(childStart.payload.parentState).toBe(parentResult.rootHash);
+    expect(childStart.refs).toContain(parentResult.rootHash);
+  });
+
+  test("childThread on parent state node points to child's final state and is in refs", async () => {
+    const cas = createCasStore(casDir);
+    const childFinalHash = "CHILD_FINAL_001";
+
+    const parentWf: WorkflowFn = async function* (
+      _thread: ThreadContext,
+      runtime: WorkflowRuntime,
+    ): AsyncGenerator<RoleOutput, WorkflowCompletion> {
+      const h = await runtime.cas.put("developer output");
+      yield {
+        role: "developer",
+        contentHash: h,
+        meta: { action: "delegate" },
+        refs: [h],
+        childThread: childFinalHash,
+      };
+      return { returnCode: 0, summary: "parent complete" };
+    };
+
+    const result = await executeThread(
+      parentWf,
+      "parent-wf",
+      { prompt: "parent task", steps: [] },
+      makeOptions({ storageRoot }),
+      {
+        threadId: "P_THREAD_02",
+        hash: "CTHREAD_TEST01",
+        infoJsonlPath: join(storageRoot, "logs", "CTHREAD_TEST01", "P2.info.jsonl"),
+        cas,
+      },
+      noLogger(),
+    );
+
+    const endNode = await loadStateNode(cas, result.rootHash);
+    const devStateHash = endNode.payload.ancestors[0] ?? "";
+    const devNode = await loadStateNode(cas, devStateHash);
+
+    expect(devNode.payload.role).toBe("developer");
+    expect(devNode.payload.childThread).toBe(childFinalHash);
+    expect(devNode.refs).toContain(childFinalHash);
+  });
+
+  test("parent state with no child has childThread: null", async () => {
+    const cas = createCasStore(casDir);
+
+    const wf: WorkflowFn = async function* (
+      _thread: ThreadContext,
+      runtime: WorkflowRuntime,
+    ): AsyncGenerator<RoleOutput, WorkflowCompletion> {
+      const h = await runtime.cas.put("prep output");
+      yield { role: "preparer", contentHash: h, meta: {}, refs: [h], childThread: null };
+      return { returnCode: 0, summary: "done" };
+    };
+
+    const result = await executeThread(
+      wf,
+      "test-wf",
+      { prompt: "task", steps: [] },
+      makeOptions({ storageRoot }),
+      {
+        threadId: "NULL_CT_01",
+        hash: "NULLCT_TEST001",
+        infoJsonlPath: join(storageRoot, "logs", "NULLCT_TEST001", "N1.info.jsonl"),
+        cas,
+      },
+      noLogger(),
+    );
+
+    const endNode = await loadStateNode(cas, result.rootHash);
+    const prepHash = endNode.payload.ancestors[0] ?? "";
+    const prepNode = await loadStateNode(cas, prepHash);
+
+    expect(prepNode.payload.childThread).toBeNull();
+    expect(prepNode.refs).not.toContain(null);
+  });
+
+  test("full bidirectional: child parentState is traversable to parent's context", async () => {
+    const cas = createCasStore(casDir);
+    const parentHash = "BIDIR_PARENT01";
+
+    const parentWf: WorkflowFn = async function* (
+      _thread: ThreadContext,
+      runtime: WorkflowRuntime,
+    ): AsyncGenerator<RoleOutput, WorkflowCompletion> {
+      const h1 = await runtime.cas.put("preparation output");
+      yield {
+        role: "preparer",
+        contentHash: h1,
+        meta: { repoPath: "/test" },
+        refs: [h1],
+        childThread: null,
+      };
+      const h2 = await runtime.cas.put("developer output");
+      yield {
+        role: "developer",
+        contentHash: h2,
+        meta: { action: "code" },
+        refs: [h2],
+        childThread: "CHILD_END_HASH1",
+      };
+      return { returnCode: 0, summary: "all done" };
+    };
+
+    const observedHeads: string[] = [];
+    const opts = makeOptions({
+      storageRoot,
+      awaitAfterEachYield: async () => {
+        const bundleDir = join(storageRoot, "bundles", parentHash);
+        const text = await readFile(join(bundleDir, "threads.json"), "utf8");
+        const parsed = JSON.parse(text) as Record<string, { head: string }>;
+        const head = parsed.BIDIR_T_001?.head ?? null;
+        if (head !== null) observedHeads.push(head);
+      },
+    });
+
+    await executeThread(
+      parentWf,
+      "bidir-wf",
+      { prompt: "bidir test", steps: [] },
+      opts,
+      {
+        threadId: "BIDIR_T_001",
+        hash: parentHash,
+        infoJsonlPath: join(storageRoot, "logs", parentHash, "BD1.info.jsonl"),
+        cas,
+      },
+      noLogger(),
+    );
+
+    expect(observedHeads.length).toBe(2);
+    const preparerStateHash = observedHeads[0] ?? "";
+
+    // Execute child with parentState pointing to parent's preparer state
+    // biome-ignore lint/correctness/useYield: testing start-only path
+    const childWf: WorkflowFn = async function* (
+      _t: ThreadContext,
+      _r: WorkflowRuntime,
+    ): AsyncGenerator<RoleOutput, WorkflowCompletion> {
+      return { returnCode: 0, summary: "child ok" };
+    };
+
+    const childResult = await executeThread(
+      childWf,
+      "bidir-child",
+      { prompt: "child bidir", steps: [] },
+      makeOptions({ storageRoot, depth: 1, parentStateHash: preparerStateHash }),
+      {
+        threadId: "BIDIR_C_001",
+        hash: "BIDIR_CHILD001",
+        infoJsonlPath: join(storageRoot, "logs", "BIDIR_CHILD001", "BC1.info.jsonl"),
+        cas,
+      },
+      noLogger(),
+    );
+
+    // Upward traversal: child start → parentState → preparer state → meta.repoPath
+    const childStart = await loadStartNode(cas, childResult.rootHash);
+    expect(childStart.payload.parentState).toBe(preparerStateHash);
+
+    const parentPrep = await loadStateNode(cas, preparerStateHash);
+    expect(parentPrep.payload.meta.repoPath).toBe("/test");
+  });
+});

packages/workflow-execute/src/engine/engine.ts

@@ -94,6 +94,7 @@ async function appendStateForStep(params: {
   meta: Record<string, unknown>;
   refs: readonly string[];
   timestamp: number;
+  childThread: string | null;
 }): Promise<{ stateHash: string; chain: ChainState }> {
   const text = await getContentMerklePayload(params.cas, params.contentHash);
   if (text === null) {
@@ -112,6 +113,7 @@ async function appendStateForStep(params: {
     ancestors,
     compact: null,
     timestamp: params.timestamp,
+    childThread: params.childThread,
   };
   const stateHash = await putStateNode(params.cas, payload);
   return {
@@ -137,6 +139,7 @@ async function appendEndState(params: {
     ancestors,
     compact: null,
     timestamp: params.timestamp,
+    childThread: null,
   };
   return putStateNode(params.cas, payload);
 }
@@ -329,6 +332,7 @@ async function driveWorkflowGenerator(params: {
       meta: step.meta,
       refs: step.refs,
       timestamp: ts,
+      childThread: step.childThread ?? null,
     });
     chain = written_.chain;
     await publishHead({ bundleDir, threadId, startHash, headHash: written_.stateHash });
@@ -439,6 +443,7 @@ export async function executeThread(
         name: workflowName,
         hash: io.hash,
         depth: options.depth,
+        parentState: options.parentStateHash,
       },
       promptHash,
     );
@@ -466,6 +471,7 @@ export async function executeThread(
         meta: row.meta,
         refs: row.refs,
         timestamp: row.timestamp,
+        childThread: null,
       });
       chain = written.chain;
       await publishHead({
@@ -487,6 +493,7 @@ export async function executeThread(
   const thread: ThreadContext = {
     threadId: io.threadId,
     depth: options.depth,
+    bundleHash: io.hash,
     start: {
       role: START,
       content: input.prompt,

packages/workflow-execute/src/engine/fork-thread.ts

@@ -144,6 +144,7 @@ async function payloadToRoleOutput(cas: CasStore, payload: StateNodePayload): Pr
     contentHash: payload.content,
     meta: payload.meta,
     refs,
+    childThread: payload.childThread,
   };
 }
 
@@ -240,6 +241,7 @@ async function buildForkContinuation(params: {
     ancestors: ancestorsMarker,
     compact: null,
     timestamp: Date.now(),
+    childThread: null,
   };
   const markerHash = await putStateNode(cas, markerPayload);
 

packages/workflow-execute/src/engine/types.ts

@@ -41,6 +41,8 @@ export type PrefilledDiskStep = {
 export type ExecuteThreadOptions = {
   /** Passed to the bundle thread context as `ThreadContext.depth`. */
   depth: number;
+  /** Parent thread's head state hash at spawn time; `null` for top-level threads. */
+  parentStateHash: string | null;
   signal: AbortSignal;
   /** Invoked after each successful yield (and outer-loop checks); used for pause/resume. */
   awaitAfterEachYield: () => Promise<void>;

packages/workflow-execute/src/engine/worker.ts

@@ -72,11 +72,13 @@ function parseRoleOutputRecord(obj: Record<string, unknown>): RoleOutput | null
   if (meta === null || typeof meta !== "object") {
     return null;
   }
+  const childThread = obj.childThread;
   return {
     role,
     contentHash,
     meta: meta as Record<string, unknown>,
     refs: normalizeRefsField(obj.refs),
+    childThread: typeof childThread === "string" ? childThread : null,
   };
 }
 
@@ -497,6 +499,7 @@ async function main(): Promise<void> {
         { prompt: cmd.prompt, steps: cmd.steps },
         {
           ...cmd.options,
+          parentStateHash: null,
           signal: ac.signal,
           awaitAfterEachYield: () => pauseGate.awaitAfterYield(),
           forkSourceThreadId: cmd.forkSourceThreadId,

packages/workflow-execute/src/workflow-as-agent.ts

@@ -6,7 +6,7 @@ import {
   getRegisteredWorkflow,
   readWorkflowRegistry,
 } from "@uncaged/workflow-register";
-import type { AgentContext, AgentFn } from "@uncaged/workflow-runtime";
+import type { AgentContext, AgentFn, AgentFnResult } from "@uncaged/workflow-runtime";
 import {
   createLogger,
   generateUlid,
@@ -14,7 +14,7 @@ import {
   getGlobalCasDir,
 } from "@uncaged/workflow-util";
 import type { ExecuteThreadIo } from "./engine/index.js";
-import { executeThread } from "./engine/index.js";
+import { executeThread, getBundleDir, readThreadsIndex } from "./engine/index.js";
 
 const DEFAULT_WORKFLOW_AS_AGENT_MAX_DEPTH = 3;
 
@@ -37,6 +37,13 @@ function resolveWorkflowAsAgentStorageRoot(options: WorkflowAsAgentOptions | nul
   return getDefaultWorkflowStorageRoot();
 }
 
+async function readParentHeadState(storageRoot: string, ctx: AgentContext): Promise<string | null> {
+  const bundleDir = getBundleDir(storageRoot, ctx.bundleHash);
+  const index = await readThreadsIndex(bundleDir);
+  const entry = index[ctx.threadId] ?? null;
+  return entry !== null ? entry.head : null;
+}
+
 /**
  * Returns an {@link AgentFn} that runs another registered workflow in a new thread,
  * using the parent thread's initial prompt (`ctx.start.content`) as the child prompt.
@@ -45,7 +52,7 @@ export function workflowAsAgent(
   workflowName: string,
   options: WorkflowAsAgentOptions | null = null,
 ): AgentFn {
-  return async (ctx: AgentContext): Promise<string> => {
+  return async (ctx: AgentContext): Promise<AgentFnResult> => {
     const nextDepth = ctx.depth + 1;
 
     const storageRoot = resolveWorkflowAsAgentStorageRoot(options);
@@ -89,6 +96,8 @@ export function workflowAsAgent(
     const logger = createLogger({ sink: { kind: "file", path: infoJsonlPath } });
     const signalNever = new AbortController();
 
+    const parentHeadState = await readParentHeadState(storageRoot, ctx);
+
     try {
       const result = await executeThread(
         bundleExportsResult.value.run,
@@ -96,6 +105,7 @@ export function workflowAsAgent(
         input,
         {
           depth: nextDepth,
+          parentStateHash: parentHeadState,
           signal: signalNever.signal,
           awaitAfterEachYield: async () => {},
           forkSourceThreadId: ctx.threadId,
@@ -107,7 +117,8 @@ export function workflowAsAgent(
         io,
         logger,
       );
-      return `Child workflow "${workflowName}" completed (returnCode=${result.returnCode}).\n\nSummary: ${result.summary}\n\nChild thread root hash: ${result.rootHash}`;
+      const summary = `Child workflow "${workflowName}" completed (returnCode=${result.returnCode}).\n\nSummary: ${result.summary}\n\nChild thread root hash: ${result.rootHash}`;
+      return { output: summary, childThread: result.rootHash };
     } catch (e) {
       const message = e instanceof Error ? e.message : String(e);
       return `ERROR: ${message}`;

packages/workflow-protocol/__tests__/moderator-table.test.ts

@@ -21,6 +21,7 @@ function makeCtx(roles: (keyof TestMeta & string)[]): ModeratorContext<TestMeta>
   return {
     threadId: "test-thread",
     depth: 0,
+    bundleHash: "TESTHASH00001",
     start: {
       role: START,
       content: "test",

packages/workflow-protocol/src/cas-types.ts

@@ -4,6 +4,8 @@ export type StartNodePayload = {
   name: string;
   hash: string;
   depth: number;
+  /** Parent thread's head state hash at spawn time. `null` for top-level workflows. */
+  parentState: string | null;
 };
 
 export type StartNode = {
@@ -20,6 +22,8 @@ export type StateNodePayload = {
   ancestors: string[];
   compact: string | null;
   timestamp: number;
+  /** Child thread's final state hash (workflow-as-agent). `null` when no child spawned. */
+  childThread: string | null;
 };
 
 export type StateNode = {

packages/workflow-protocol/src/index.ts

@@ -13,6 +13,7 @@ export type {
   AgentBinding,
   AgentContext,
   AgentFn,
+  AgentFnResult,
   CasStore,
   ExtractFn,
   ExtractResult,

packages/workflow-protocol/src/types.ts

@@ -41,6 +41,7 @@ export type RoleOutput = {
   contentHash: string;
   meta: Record<string, unknown>;
   refs: string[];
+  childThread: string | null;
 };
 
 export type StartStep = {
@@ -63,6 +64,7 @@ export type RoleStep<M extends RoleMeta> = {
 export type ThreadContext<M extends RoleMeta = RoleMeta> = {
   threadId: string;
   depth: number;
+  bundleHash: string;
   start: StartStep;
   steps: RoleStep<M>[];
 };
@@ -127,7 +129,9 @@ export type ExtractFn = <T extends Record<string, unknown>>(
   contentHash: string,
 ) => Promise<ExtractResult<T>>;
 
-export type AgentFn = (ctx: AgentContext) => Promise<string>;
+export type AgentFnResult = string | { output: string; childThread: string | null };
+
+export type AgentFn = (ctx: AgentContext) => Promise<AgentFnResult>;
 
 export type AgentBinding = {
   agent: AgentFn;

packages/workflow-runtime/__tests__/build-context.test.ts

@@ -27,7 +27,7 @@ describe("buildThreadContext", () => {
     const bundleHash = "BHAAAAAAAAAAA";
     const startHash = await putStartNode(
       cas,
-      { name: "demo", hash: bundleHash, depth: 2 },
+      { name: "demo", hash: bundleHash, depth: 2, parentState: null },
       promptHash,
     );
 
@@ -41,6 +41,7 @@ describe("buildThreadContext", () => {
       ancestors: [],
       compact: null,
       timestamp: 1000,
+      childThread: null,
     });
 
     const chCode = await putContentNodeWithRefs(cas, "code body", []);
@@ -52,6 +53,7 @@ describe("buildThreadContext", () => {
       ancestors: [statePlan],
       compact: null,
       timestamp: 2000,
+      childThread: null,
     });
 
     const ctx = await buildThreadContext(stateCode, cas);
@@ -71,7 +73,7 @@ describe("buildThreadContext", () => {
     const promptHash = await cas.put("only-prompt");
     const startHash = await putStartNode(
       cas,
-      { name: "solo", hash: "BHBBBBBBBBBBB", depth: 1 },
+      { name: "solo", hash: "BHBBBBBBBBBBB", depth: 1, parentState: null },
       promptHash,
     );
 
@@ -87,7 +89,7 @@ describe("buildThreadContext", () => {
     const bundleHash = "BHCCCCCCCCCCC";
     const startHash = await putStartNode(
       cas,
-      { name: "demo", hash: bundleHash, depth: 0 },
+      { name: "demo", hash: bundleHash, depth: 0, parentState: null },
       promptHash,
     );
 
@@ -100,6 +102,7 @@ describe("buildThreadContext", () => {
       ancestors: [],
       compact: null,
       timestamp: 500,
+      childThread: null,
     });
 
     const endContent = await putContentNodeWithRefs(cas, "finished", []);
@@ -111,6 +114,7 @@ describe("buildThreadContext", () => {
       ancestors: [state1],
       compact: null,
       timestamp: 600,
+      childThread: null,
     });
 
     const ctx = await buildThreadContext(endState, cas);

packages/workflow-runtime/src/build-context.ts

@@ -54,6 +54,7 @@ async function threadFromStartHead<M extends RoleMeta>(
   return {
     threadId: "",
     depth: p.depth,
+    bundleHash: p.hash,
     start: {
       role: START,
       content: prompt,
@@ -113,6 +114,7 @@ async function threadFromStateHead<M extends RoleMeta>(
   return {
     threadId: "",
     depth: sp.depth,
+    bundleHash: sp.hash,
     start: {
       role: START,
       content: prompt,

packages/workflow-runtime/src/create-workflow.ts

@@ -6,6 +6,7 @@ import {
   type AgentBinding,
   type AgentContext,
   type AgentFn,
+  type AgentFnResult,
   END,
   type ModeratorContext,
   type RoleDefinition,
@@ -49,6 +50,16 @@ function mergeUniqueHashes(a: readonly string[], b: readonly string[]): string[]
   return out;
 }
 
+function normalizeAgentResult(result: AgentFnResult): {
+  output: string;
+  childThread: string | null;
+} {
+  if (typeof result === "string") {
+    return { output: result, childThread: null };
+  }
+  return result;
+}
+
 function agentForRole(binding: AgentBinding, roleName: string): AgentFn {
   const overrides = binding.overrides;
   const overrideFn: AgentFn | undefined =
@@ -86,9 +97,9 @@ async function advanceOneRound<M extends RoleMeta>(
   };
 
   const agent = agentForRole(binding, next);
-  const raw = await agent(agentCtx as unknown as AgentContext);
+  const agentResult = normalizeAgentResult(await agent(agentCtx as unknown as AgentContext));
 
-  const agentContentHash = await putContentNodeWithRefs(runtime.cas, raw, []);
+  const agentContentHash = await putContentNodeWithRefs(runtime.cas, agentResult.output, []);
 
   const extracted = await runtime.extract(
     roleDef.schema as z.ZodType<Record<string, unknown>>,
@@ -122,6 +133,7 @@ async function advanceOneRound<M extends RoleMeta>(
       contentHash: step.contentHash,
       meta: step.meta,
       refs: step.refs,
+      childThread: agentResult.childThread,
     },
     step,
   };

packages/workflow-runtime/src/index.ts

@@ -5,6 +5,7 @@ export type {
   AgentBinding,
   AgentContext,
   AgentFn,
+  AgentFnResult,
   CasStore,
   ExtractFn,
   ExtractResult,

packages/workflow-runtime/src/types.ts

@@ -7,6 +7,7 @@ export type {
   AgentBinding,
   AgentContext,
   AgentFn,
+  AgentFnResult,
   CasStore,
   ExtractFn,
   ExtractResult,

packages/workflow-template-develop/__tests__/develop-template.test.ts

@@ -26,6 +26,7 @@ function makeCtx(steps: ModeratorContext<DevelopMeta>["steps"]): ModeratorContex
   return {
     threadId: "01TEST000000000000000000TR",
     depth: 0,
+    bundleHash: "TESTHASH00001",
     start: makeStart(),
     steps,
   };

packages/workflow-template-solve-issue/__tests__/solve-issue-template.test.ts

@@ -113,6 +113,7 @@ function makeCtx(
   return {
     threadId: "01TEST000000000000000000TR",
     depth: 0,
+    bundleHash: "TESTHASH00001",
     start: makeStart(),
     steps,
   };
@@ -178,6 +179,7 @@ function makeThread(prompt: string) {
   return {
     threadId: "01TEST000000000000000000TR",
     depth: 0,
+    bundleHash: "TESTHASH00001",
     start: {
       role: START,
       content: prompt,

packages/workflow-util-agent/__tests__/build-agent-prompt.test.ts

@@ -17,6 +17,7 @@ describe("buildAgentPrompt", () => {
     const ctx: AgentContext = {
       start: startTask("fix the bug"),
       depth: 0,
+      bundleHash: "TESTHASH00001",
       steps: [],
       threadId: "01TEST000000000000000000TR",
       currentRole: { name: START, systemPrompt: "You are an agent." },
@@ -33,6 +34,7 @@ describe("buildAgentPrompt", () => {
     const ctx: AgentContext = {
       start: startTask("user task"),
       depth: 0,
+      bundleHash: "TESTHASH00001",
       threadId: "01TEST000000000000000000TR",
       currentRole: { name: "coder", systemPrompt: "Be helpful." },
       steps: [
@@ -61,6 +63,7 @@ describe("buildAgentPrompt", () => {
     const ctx: AgentContext = {
       start: startTask("first message full: task content here"),
       depth: 0,
+      bundleHash: "TESTHASH00001",
       threadId: "01TEST000000000000000000TR",
       currentRole: { name: "coder", systemPrompt: "System." },
       steps: [
@@ -99,6 +102,7 @@ describe("buildAgentPrompt", () => {
     const ctx: AgentContext = {
       start: startTask("start"),
       depth: 0,
+      bundleHash: "TESTHASH00001",
       threadId: "01TEST000000000000000000TR",
       currentRole: { name: "c", systemPrompt: "S" },
       steps: [