diff --git a/packages/cli-workflow/__tests__/thread-cli.test.ts b/packages/cli-workflow/__tests__/thread-cli.test.ts
index 5e36107..84fd465 100644
--- a/packages/cli-workflow/__tests__/thread-cli.test.ts
+++ b/packages/cli-workflow/__tests__/thread-cli.test.ts
@@ -187,6 +187,14 @@ describe("cli thread commands", () => {
     }
     expect(shown.value.includes('"threadId"')).toBe(true);
 
+    const parsed = JSON.parse(shown.value) as Record<string, unknown>;
+    expect(parsed.parentState).toBeNull();
+    const parsedSteps = parsed.steps as Array<Record<string, unknown>>;
+    for (const step of parsedSteps) {
+      expect(step).toHaveProperty("childThread");
+      expect(step.childThread).toBeNull();
+    }
+
     const removed = await cmdThreadRemove(storageRoot, threadId);
     expect(removed.ok).toBe(true);
 
diff --git a/packages/cli-workflow/src/commands/thread/show.ts b/packages/cli-workflow/src/commands/thread/show.ts
index fdc08a1..b03ae24 100644
--- a/packages/cli-workflow/src/commands/thread/show.ts
+++ b/packages/cli-workflow/src/commands/thread/show.ts
@@ -1,4 +1,4 @@
-import { createCasStore, getContentMerklePayload } from "@uncaged/workflow-cas";
+import { createCasStore, getContentMerklePayload, parseCasThreadNode } from "@uncaged/workflow-cas";
 import { FORK_BRANCH_ROLE, walkStateFramesNewestFirst } from "@uncaged/workflow-execute";
 import { err, ok, type Result } from "@uncaged/workflow-protocol";
 import { END } from "@uncaged/workflow-runtime";
@@ -6,6 +6,21 @@ import { getGlobalCasDir } from "@uncaged/workflow-util";
 
 import { resolveThreadRecord } from "../../thread-scan.js";
 
+async function readParentStateFromStartNode(
+  cas: { get(hash: string): Promise<string | null> },
+  startHash: string,
+): Promise<string | null> {
+  const yamlText = await cas.get(startHash);
+  if (yamlText === null) {
+    return null;
+  }
+  const parsed = parseCasThreadNode(yamlText);
+  if (parsed === null || parsed.kind !== "start") {
+    return null;
+  }
+  return parsed.node.payload.parentState;
+}
+
 export async function cmdThreadShow(
   storageRoot: string,
   threadId: string,
@@ -19,7 +34,15 @@ export async function cmdThreadShow(
   const frames = await walkStateFramesNewestFirst(cas, resolved.head);
   const chronological = [...frames].reverse();
 
-  const steps: Array<{ role: string; hash: string; timestamp: number; content: string }> = [];
+  const parentState = await readParentStateFromStartNode(cas, resolved.start);
+
+  const steps: Array<{
+    role: string;
+    hash: string;
+    timestamp: number;
+    content: string;
+    childThread: string | null;
+  }> = [];
   for (const fr of chronological) {
     if (fr.payload.role === END || fr.payload.role === FORK_BRANCH_ROLE) {
       continue;
@@ -33,6 +56,7 @@ export async function cmdThreadShow(
         payloadText !== null
           ? payloadText
           : `(content not in CAS; contentHash=${fr.payload.content})`,
+      childThread: fr.payload.childThread,
     });
   }
 
@@ -41,6 +65,7 @@ export async function cmdThreadShow(
     bundleHash: resolved.bundleHash,
     head: resolved.head,
     start: resolved.start,
+    parentState,
     source: resolved.source,
     steps,
   };
diff --git a/packages/workflow-agent-llm/__tests__/create-llm-adapter.test.ts b/packages/workflow-agent-llm/__tests__/create-llm-adapter.test.ts
index 8303efb..0a804b8 100644
--- a/packages/workflow-agent-llm/__tests__/create-llm-adapter.test.ts
+++ b/packages/workflow-agent-llm/__tests__/create-llm-adapter.test.ts
@@ -10,6 +10,7 @@ function makeCtx(userContent: string): AgentContext {
       content: userContent,
       meta: {},
       timestamp: 1,
+      parentState: null,
     },
     depth: 0,
     bundleHash: "TESTHASH00001",
diff --git a/packages/workflow-execute/src/engine/engine.ts b/packages/workflow-execute/src/engine/engine.ts
index b395f25..7fd9d52 100644
--- a/packages/workflow-execute/src/engine/engine.ts
+++ b/packages/workflow-execute/src/engine/engine.ts
@@ -499,6 +499,7 @@ export async function executeThread(
       content: input.prompt,
       meta: {},
       timestamp: nowMs,
+      parentState: options.parentStateHash,
     },
     steps: input.steps.map((out, i) => ({
       role: out.role,
diff --git a/packages/workflow-protocol/__tests__/moderator-table.test.ts b/packages/workflow-protocol/__tests__/moderator-table.test.ts
index 6d66a51..4b44542 100644
--- a/packages/workflow-protocol/__tests__/moderator-table.test.ts
+++ b/packages/workflow-protocol/__tests__/moderator-table.test.ts
@@ -27,6 +27,7 @@ function makeCtx(roles: (keyof TestMeta & string)[]): ModeratorContext<TestMeta>
       content: "test",
       meta: {},
       timestamp: Date.now(),
+      parentState: null,
     } as StartStep,
     steps,
   };
diff --git a/packages/workflow-protocol/src/types.ts b/packages/workflow-protocol/src/types.ts
index ed8988c..b7d002e 100644
--- a/packages/workflow-protocol/src/types.ts
+++ b/packages/workflow-protocol/src/types.ts
@@ -62,6 +62,7 @@ export type StartStep = {
   content: string;
   meta: Record<string, never>;
   timestamp: number;
+  parentState: string | null;
 };
 
 export type RoleStep<M extends RoleMeta> = {
diff --git a/packages/workflow-runtime/src/build-context.ts b/packages/workflow-runtime/src/build-context.ts
index 874c4a5..38c46fb 100644
--- a/packages/workflow-runtime/src/build-context.ts
+++ b/packages/workflow-runtime/src/build-context.ts
@@ -60,6 +60,7 @@ async function threadFromStartHead<M extends RoleMeta>(
       content: prompt,
       meta: {},
       timestamp: 0,
+      parentState: p.parentState,
     },
     steps: [],
   };
@@ -120,6 +121,7 @@ async function threadFromStateHead<M extends RoleMeta>(
       content: prompt,
       meta: {},
       timestamp: firstTs,
+      parentState: sp.parentState,
     },
     steps,
   };
diff --git a/packages/workflow-template-develop/__tests__/develop-template.test.ts b/packages/workflow-template-develop/__tests__/develop-template.test.ts
index a626d33..0013ea2 100644
--- a/packages/workflow-template-develop/__tests__/develop-template.test.ts
+++ b/packages/workflow-template-develop/__tests__/develop-template.test.ts
@@ -22,6 +22,7 @@ function makeStart(): ModeratorContext<DevelopMeta>["start"] {
     content: "Implement the feature",
     meta: {},
     timestamp: 0,
+    parentState: null,
   };
 }
 
diff --git a/packages/workflow-template-solve-issue/__tests__/solve-issue-template.test.ts b/packages/workflow-template-solve-issue/__tests__/solve-issue-template.test.ts
index c7137f2..6d2d1a1 100644
--- a/packages/workflow-template-solve-issue/__tests__/solve-issue-template.test.ts
+++ b/packages/workflow-template-solve-issue/__tests__/solve-issue-template.test.ts
@@ -107,6 +107,7 @@ function makeStart(): ModeratorContext<SolveIssueMeta>["start"] {
     content: "Fix the flaky login test",
     meta: {},
     timestamp: 0,
+    parentState: null,
   };
 }
 
@@ -188,6 +189,7 @@ function makeThread(prompt: string) {
       content: prompt,
       meta: {},
       timestamp: Date.now(),
+      parentState: null,
     },
     steps: [],
   };
diff --git a/packages/workflow-util-agent/__tests__/build-agent-prompt.test.ts b/packages/workflow-util-agent/__tests__/build-agent-prompt.test.ts
index 3601015..5b4e641 100644
--- a/packages/workflow-util-agent/__tests__/build-agent-prompt.test.ts
+++ b/packages/workflow-util-agent/__tests__/build-agent-prompt.test.ts
@@ -3,12 +3,13 @@ import { type AgentContext, START } from "@uncaged/workflow-runtime";
 
 import { buildAgentPrompt } from "../src/index.js";
 
-function startTask(content: string): AgentContext["start"] {
+function startTask(content: string, parentState: string | null = null): AgentContext["start"] {
   return {
     role: START,
     content,
     meta: {},
     timestamp: 1,
+    parentState,
   };
 }
 
@@ -95,6 +96,35 @@ describe("buildAgentPrompt", () => {
     expect(text).toContain("uncaged-workflow thread 01TEST000000000000000000TR");
   });
 
+  test("parentState null omits Parent Context section", async () => {
+    const ctx: AgentContext = {
+      start: startTask("top-level task"),
+      depth: 0,
+      bundleHash: "TESTHASH00001",
+      steps: [],
+      threadId: "01TEST000000000000000000TR",
+      currentRole: { name: START, systemPrompt: "You are an agent." },
+    };
+    const text = await buildAgentPrompt(ctx);
+    expect(text).not.toContain("## Parent Context");
+  });
+
+  test("parentState non-null includes Parent Context section with hash", async () => {
+    const parentHash = "01PARENTSTATE0000000000001";
+    const ctx: AgentContext = {
+      start: startTask("child task", parentHash),
+      depth: 1,
+      bundleHash: "TESTHASH00001",
+      steps: [],
+      threadId: "01TEST000000000000000000TR",
+      currentRole: { name: START, systemPrompt: "You are an agent." },
+    };
+    const text = await buildAgentPrompt(ctx);
+    expect(text).toContain("## Parent Context");
+    expect(text).toContain(parentHash);
+    expect(text).toContain(`uncaged-workflow cas get ${parentHash}`);
+  });
+
   test("middle steps show meta summary only and latest shows hash", async () => {
     const ha = "01HASHA00000000000000000001";
     const hb = "01HASHB00000000000000000001";
diff --git a/packages/workflow-util-agent/src/build-agent-prompt.ts b/packages/workflow-util-agent/src/build-agent-prompt.ts
index 2345ee1..2851f26 100644
--- a/packages/workflow-util-agent/src/build-agent-prompt.ts
+++ b/packages/workflow-util-agent/src/build-agent-prompt.ts
@@ -5,6 +5,19 @@ export async function buildAgentPrompt(ctx: AgentContext): Promise<string> {
   const lines: string[] = [];
   lines.push(ctx.currentRole.systemPrompt);
   lines.push("");
+
+  if (ctx.start.parentState !== null) {
+    lines.push("## Parent Context");
+    lines.push(
+      "This workflow was spawned by a parent workflow. The parent's state at spawn time is available at hash: " +
+        ctx.start.parentState,
+    );
+    lines.push(
+      `Use \`uncaged-workflow cas get ${ctx.start.parentState}\` to inspect the parent's context and trace back through its steps.`,
+    );
+    lines.push("");
+  }
+
   lines.push("## Task");
   lines.push(ctx.start.content);