feat(cli): add thread poke command

Re-runs the head step's agent with a supplementary prompt and replaces
the head step (rewires new step's prev to old head's prev) instead of
appending. Skips moderator re-route — the role of the head step is
reused.

Fixes #144

.changeset/thread-poke.md

@@ -0,0 +1,11 @@
+---
+"@united-workforce/cli": minor
+---
+
+feat(cli): add `uwf thread poke` command
+
+New subcommand `uwf thread poke <thread-id> -p <prompt>` re-runs the head step's
+agent with a supplementary prompt, replacing the head step's output. Unlike
+`thread resume`, poke skips the moderator and rewrites the new step's `prev`
+pointer so the new head replaces (not appends to) the old head. Works on idle
+and suspended threads. Resolves issue #144 (Phase 1).

packages/cli/src/__tests__/thread-poke.test.ts

@@ -0,0 +1,549 @@
+import { execFileSync } from "node:child_process";
+import { mkdir, mkdtemp, readFile, rm, writeFile } from "node:fs/promises";
+import { tmpdir } from "node:os";
+import { dirname, join } from "node:path";
+import { fileURLToPath } from "node:url";
+import { putSchema } from "@ocas/core";
+import { openStore } from "@ocas/fs";
+import type {
+  CasRef,
+  StepNodePayload,
+  ThreadId,
+  ThreadIndexEntry,
+} from "@united-workforce/protocol";
+import { afterEach, beforeEach, describe, expect, test } from "vitest";
+import { registerUwfSchemas } from "../schemas.js";
+import { seedThreads } from "./thread-test-helpers.js";
+
+const OUTPUT_SCHEMA = {
+  type: "object" as const,
+  properties: {
+    $status: { type: "string" as const },
+    note: { type: "string" as const },
+  },
+  required: ["$status"],
+  additionalProperties: false,
+};
+
+const THREAD_ID = "01POKESTEPTEST00000000" as ThreadId;
+
+let tmpDir: string;
+
+beforeEach(async () => {
+  tmpDir = await mkdtemp(join(tmpdir(), "cli-uwf-poke-test-"));
+});
+
+afterEach(async () => {
+  await rm(tmpDir, { recursive: true, force: true });
+});
+
+type SetupResult = {
+  casDir: string;
+  oldStepHash: CasRef;
+  oldStepPrev: CasRef | null;
+  oldStepCompletedAtMs: number;
+  startHash: CasRef;
+  workflowHash: CasRef;
+  mockAgentPath: string;
+  failingAgentPath: string;
+  promptCapturePath: string;
+  envCapturePath: string;
+};
+
+type SetupOpts = {
+  threadStatus: ThreadIndexEntry["status"];
+  multipleSteps: boolean;
+  newCompletedAtMs: number;
+  newStatus: string;
+  // The agent name to record in the head StepNode.agent field. Defaults to mockAgentPath.
+  stepAgentNameOverride: string | null;
+  // Whether to seed an actual head StepNode (false → only StartNode is the head).
+  withHeadStep: boolean;
+};
+
+async function setupThread(opts: Partial<SetupOpts> = {}): Promise<SetupResult> {
+  const cfg: SetupOpts = {
+    threadStatus: opts.threadStatus ?? "idle",
+    multipleSteps: opts.multipleSteps ?? false,
+    newCompletedAtMs: opts.newCompletedAtMs ?? 1716600005000,
+    newStatus: opts.newStatus ?? "ok",
+    stepAgentNameOverride: opts.stepAgentNameOverride ?? null,
+    withHeadStep: opts.withHeadStep ?? true,
+  };
+
+  const casDir = join(tmpDir, "cas");
+  await mkdir(casDir, { recursive: true });
+
+  const store = await openStore(casDir);
+  const schemas = await registerUwfSchemas(store);
+  const outputSchemaHash = await putSchema(store, OUTPUT_SCHEMA);
+
+  const workflowHash = await store.cas.put(schemas.workflow, {
+    name: "test-poke",
+    description: "poke command integration test",
+    roles: {
+      worker: {
+        description: "Worker role",
+        goal: "Work",
+        capabilities: [],
+        procedure: "work",
+        output: "result",
+        frontmatter: outputSchemaHash,
+      },
+      reviewer: {
+        description: "Reviewer role",
+        goal: "Review",
+        capabilities: [],
+        procedure: "review",
+        output: "result",
+        frontmatter: outputSchemaHash,
+      },
+    },
+    graph: {
+      $START: {
+        new: { role: "worker", prompt: "Start work", location: null },
+        resume: { role: "worker", prompt: "Resume the work", location: null },
+      },
+      worker: {
+        ok: { role: "reviewer", prompt: "Review the work", location: null },
+        needs_input: {
+          role: "$SUSPEND",
+          prompt: "Please clarify",
+          location: null,
+        },
+      },
+      reviewer: { done: { role: "$END", prompt: "Done", location: null } },
+    },
+  });
+
+  const startHash = await store.cas.put(schemas.startNode, {
+    workflow: workflowHash,
+    prompt: "Test poke task",
+    cwd: tmpDir,
+  });
+
+  process.env.OCAS_HOME = casDir;
+
+  // Paths for mock agent and capture files (set early so we can use mockAgentPath as the recorded agent name)
+  const promptCapturePath = join(tmpDir, "captured-prompt.txt");
+  const envCapturePath = join(tmpDir, "captured-env.txt");
+  const mockAgentPath = join(tmpDir, "mock-agent.sh");
+  const failingAgentPath = join(tmpDir, "failing-agent.sh");
+
+  // Build head StepNode chain
+  let oldStepPrev: CasRef | null = null;
+  if (cfg.multipleSteps) {
+    // First step: prev=null
+    const firstOutputHash = await store.cas.put(outputSchemaHash, { $status: "ok" });
+    const firstDetailHash = await store.cas.put(schemas.text, "first detail");
+    const firstStepHash = await store.cas.put(schemas.stepNode, {
+      start: startHash,
+      prev: null,
+      role: "worker",
+      output: firstOutputHash,
+      detail: firstDetailHash,
+      agent: cfg.stepAgentNameOverride ?? mockAgentPath,
+      edgePrompt: "Start work",
+      startedAtMs: 1716600000000,
+      completedAtMs: 1716600001000,
+      cwd: tmpDir,
+      assembledPrompt: null,
+      usage: null,
+    });
+    oldStepPrev = firstStepHash;
+  }
+
+  let oldStepHash: CasRef = startHash;
+  const oldStepCompletedAtMs = 1716600002000;
+  if (cfg.withHeadStep) {
+    const outputHash = await store.cas.put(outputSchemaHash, { $status: "ok" });
+    const detailHash = await store.cas.put(schemas.text, "head step detail");
+    oldStepHash = await store.cas.put(schemas.stepNode, {
+      start: startHash,
+      prev: oldStepPrev,
+      role: "worker",
+      output: outputHash,
+      detail: detailHash,
+      agent: cfg.stepAgentNameOverride ?? mockAgentPath,
+      edgePrompt: "Start work",
+      startedAtMs: 1716600001500,
+      completedAtMs: oldStepCompletedAtMs,
+      cwd: tmpDir,
+      assembledPrompt: null,
+      usage: null,
+    });
+  }
+
+  // Seed thread index entry. For "running" we let the test create the marker separately.
+  await seedThreads(tmpDir, {
+    [THREAD_ID]: {
+      head: oldStepHash,
+      status: cfg.threadStatus,
+      suspendedRole: cfg.threadStatus === "suspended" ? "worker" : null,
+      suspendMessage: cfg.threadStatus === "suspended" ? "Please clarify" : null,
+      completedAt:
+        cfg.threadStatus === "completed" || cfg.threadStatus === "cancelled"
+          ? oldStepCompletedAtMs
+          : null,
+    },
+  });
+
+  // Mock agent always emits a stepNode keyed off the current thread head (which we
+  // observe through OCAS_HOME). The script writes prompt/env captures and then prints
+  // an adapter JSON that references a pre-built stepHash.
+  // We pre-build the agent's stepHash with prev=oldStepHash (normal append behaviour).
+  const newOutputHash = await store.cas.put(outputSchemaHash, {
+    $status: cfg.newStatus,
+    note: "poked output",
+  });
+  const newDetailHash = await store.cas.put(schemas.text, "poked detail");
+  const agentStepHash = await store.cas.put(schemas.stepNode, {
+    start: startHash,
+    prev: cfg.withHeadStep ? oldStepHash : null,
+    role: "worker",
+    output: newOutputHash,
+    detail: newDetailHash,
+    agent: "mock-agent-output",
+    edgePrompt: "poke prompt placeholder",
+    startedAtMs: cfg.newCompletedAtMs - 100,
+    completedAtMs: cfg.newCompletedAtMs,
+    cwd: tmpDir,
+    assembledPrompt: null,
+    usage: null,
+  });
+
+  const adapterJson = JSON.stringify({
+    stepHash: agentStepHash,
+    detailHash: newDetailHash,
+    role: "worker",
+    frontmatter: { $status: cfg.newStatus, note: "poked output" },
+    body: "",
+    startedAtMs: cfg.newCompletedAtMs - 100,
+    completedAtMs: cfg.newCompletedAtMs,
+    usage: null,
+  });
+
+  await writeFile(
+    mockAgentPath,
+    `#!/bin/sh
+prompt=""
+while [ $# -gt 0 ]; do
+  if [ "$1" = "--prompt" ]; then
+    prompt="$2"
+    shift 2
+  else
+    shift
+  fi
+done
+printf '%s' "$prompt" > '${promptCapturePath}'
+printf 'OCAS_HOME=%s\\n' "$OCAS_HOME" > '${envCapturePath}'
+echo '${adapterJson}'
+`,
+    { mode: 0o755 },
+  );
+
+  await writeFile(
+    failingAgentPath,
+    `#!/bin/sh
+echo "boom" >&2
+exit 7
+`,
+    { mode: 0o755 },
+  );
+
+  const configPath = join(tmpDir, "config.yaml");
+  await writeFile(
+    configPath,
+    `defaultAgent: uwf-hermes\ndefaultModel: test-model\nagentOverrides: null\nagents: {}\nproviders: {}\nmodels: {}\n`,
+  );
+
+  return {
+    casDir,
+    oldStepHash,
+    oldStepPrev,
+    oldStepCompletedAtMs,
+    startHash,
+    workflowHash,
+    mockAgentPath,
+    failingAgentPath,
+    promptCapturePath,
+    envCapturePath,
+  };
+}
+
+function runUwf(
+  args: string[],
+  casDir: string,
+): { stdout: string; stderr: string; status: number } {
+  const cliPath = join(dirname(fileURLToPath(import.meta.url)), "..", "..", "dist", "cli.js");
+  try {
+    const stdout = execFileSync(process.execPath, [cliPath, ...args], {
+      encoding: "utf8",
+      stdio: ["ignore", "pipe", "pipe"],
+      env: {
+        ...process.env,
+        UWF_HOME: tmpDir,
+        OCAS_HOME: casDir,
+      },
+      cwd: tmpDir,
+      timeout: 30000,
+    });
+    return { stdout, stderr: "", status: 0 };
+  } catch (error) {
+    const err = error as NodeJS.ErrnoException & {
+      stdout?: string | Buffer;
+      stderr?: string | Buffer;
+      status?: number;
+    };
+    return {
+      stdout: typeof err.stdout === "string" ? err.stdout : (err.stdout?.toString("utf8") ?? ""),
+      stderr: typeof err.stderr === "string" ? err.stderr : (err.stderr?.toString("utf8") ?? ""),
+      status: err.status ?? 1,
+    };
+  }
+}
+
+// ── Group 1: CLI argument validation ───────────────────────────────────────
+
+describe("uwf thread poke - CLI argument validation", () => {
+  test("1.1 missing -p flag exits non-zero", async () => {
+    const { casDir } = await setupThread();
+    const result = runUwf(["thread", "poke", THREAD_ID], casDir);
+    expect(result.status).not.toBe(0);
+    expect(result.stderr.toLowerCase()).toMatch(/required|missing|prompt/);
+  });
+
+  test("1.2 -p without --agent succeeds", async () => {
+    const { casDir } = await setupThread();
+    const result = runUwf(["thread", "poke", THREAD_ID, "-p", "do it again"], casDir);
+    expect(result.status).toBe(0);
+  });
+
+  test("1.3 -p with --agent succeeds", async () => {
+    const { casDir, mockAgentPath } = await setupThread();
+    const result = runUwf(
+      ["thread", "poke", THREAD_ID, "-p", "do it again", "--agent", mockAgentPath],
+      casDir,
+    );
+    expect(result.status).toBe(0);
+  });
+});
+
+// ── Group 2: Guard errors ──────────────────────────────────────────────────
+
+describe("uwf thread poke - guard errors", () => {
+  test("2.1 thread not found", async () => {
+    const { casDir } = await setupThread();
+    const result = runUwf(["thread", "poke", "01NOSUCHTHREAD0000000A", "-p", "prompt"], casDir);
+    expect(result.status).not.toBe(0);
+    expect(result.stderr.toLowerCase()).toMatch(/not found|not active/);
+  });
+
+  test("2.2 thread running rejects poke", async () => {
+    const { casDir, workflowHash } = await setupThread();
+    // Create background marker to simulate running
+    const { createMarker } = await import("../background/index.js");
+    await createMarker(tmpDir, {
+      thread: THREAD_ID,
+      workflow: workflowHash,
+      pid: process.pid,
+      startedAt: Date.now(),
+    });
+
+    const result = runUwf(["thread", "poke", THREAD_ID, "-p", "prompt"], casDir);
+    expect(result.status).not.toBe(0);
+    expect(result.stderr.toLowerCase()).toContain("already executing");
+  });
+
+  test("2.3 completed thread rejects poke", async () => {
+    const { casDir } = await setupThread({ threadStatus: "completed" });
+    const result = runUwf(["thread", "poke", THREAD_ID, "-p", "prompt"], casDir);
+    expect(result.status).not.toBe(0);
+    expect(result.stderr.toLowerCase()).toMatch(/cannot be poked|completed/);
+  });
+
+  test("2.4 cancelled thread rejects poke", async () => {
+    const { casDir } = await setupThread({ threadStatus: "cancelled" });
+    const result = runUwf(["thread", "poke", THREAD_ID, "-p", "prompt"], casDir);
+    expect(result.status).not.toBe(0);
+    expect(result.stderr.toLowerCase()).toMatch(/cannot be poked|cancelled/);
+  });
+
+  test("2.5 thread head is StartNode (no StepNode) rejects poke", async () => {
+    const { casDir } = await setupThread({ withHeadStep: false });
+    const result = runUwf(["thread", "poke", THREAD_ID, "-p", "prompt"], casDir);
+    expect(result.status).not.toBe(0);
+    expect(result.stderr.toLowerCase()).toMatch(/no step|cannot be poked/);
+  });
+});
+
+// ── Group 3: Success happy path ────────────────────────────────────────────
+
+describe("uwf thread poke - success", () => {
+  test("3.1, 3.4 idle thread → new head differs from old, thread index updated", async () => {
+    const { casDir, oldStepHash, mockAgentPath } = await setupThread();
+    const result = runUwf(
+      ["thread", "poke", THREAD_ID, "-p", "redo", "--agent", mockAgentPath],
+      casDir,
+    );
+    expect(result.status).toBe(0);
+    const cliOutput = JSON.parse(result.stdout.trim());
+    expect(cliOutput.head).not.toBe(oldStepHash);
+
+    const { createUwfStore, getThread } = await import("../store.js");
+    const uwf = await createUwfStore(tmpDir);
+    const entry = getThread(uwf.varStore, THREAD_ID);
+    expect(entry?.head).toBe(cliOutput.head);
+  });
+
+  test("3.2 new step's prev equals old head's prev (replace, not append)", async () => {
+    const { casDir, oldStepPrev, mockAgentPath } = await setupThread({ multipleSteps: true });
+    const result = runUwf(
+      ["thread", "poke", THREAD_ID, "-p", "redo", "--agent", mockAgentPath],
+      casDir,
+    );
+    expect(result.status).toBe(0);
+    const cliOutput = JSON.parse(result.stdout.trim());
+
+    const { createUwfStore } = await import("../store.js");
+    const uwf = await createUwfStore(tmpDir);
+    const node = uwf.store.cas.get(cliOutput.head as CasRef);
+    expect(node).not.toBeNull();
+    expect(node?.type).toBe(uwf.schemas.stepNode);
+    const payload = node?.payload as StepNodePayload;
+    expect(payload.prev).toBe(oldStepPrev);
+  });
+
+  test("3.2b new step's prev is null when old head was the first step", async () => {
+    // multipleSteps:false means oldHead.prev = null
+    const { casDir, mockAgentPath } = await setupThread({ multipleSteps: false });
+    const result = runUwf(
+      ["thread", "poke", THREAD_ID, "-p", "redo", "--agent", mockAgentPath],
+      casDir,
+    );
+    expect(result.status).toBe(0);
+    const cliOutput = JSON.parse(result.stdout.trim());
+
+    const { createUwfStore } = await import("../store.js");
+    const uwf = await createUwfStore(tmpDir);
+    const node = uwf.store.cas.get(cliOutput.head as CasRef);
+    const payload = node?.payload as StepNodePayload;
+    expect(payload.prev).toBeNull();
+  });
+
+  test("3.3 new step's completedAtMs is later than old", async () => {
+    const { casDir, oldStepCompletedAtMs, mockAgentPath } = await setupThread();
+    const result = runUwf(
+      ["thread", "poke", THREAD_ID, "-p", "redo", "--agent", mockAgentPath],
+      casDir,
+    );
+    expect(result.status).toBe(0);
+    const cliOutput = JSON.parse(result.stdout.trim());
+
+    const { createUwfStore } = await import("../store.js");
+    const uwf = await createUwfStore(tmpDir);
+    const node = uwf.store.cas.get(cliOutput.head as CasRef);
+    const payload = node?.payload as StepNodePayload;
+    expect(payload.completedAtMs).toBeGreaterThan(oldStepCompletedAtMs);
+  });
+
+  test("3.5 status remains idle after poke (no completion/suspend)", async () => {
+    const { casDir, mockAgentPath } = await setupThread();
+    const result = runUwf(
+      ["thread", "poke", THREAD_ID, "-p", "redo", "--agent", mockAgentPath],
+      casDir,
+    );
+    expect(result.status).toBe(0);
+    const cliOutput = JSON.parse(result.stdout.trim());
+    expect(cliOutput.status).toBe("idle");
+    expect(cliOutput.done).toBe(false);
+    expect(cliOutput.suspendedRole).toBeNull();
+    expect(cliOutput.suspendMessage).toBeNull();
+  });
+
+  test("3.6 currentRole unchanged after poke (no moderator re-route)", async () => {
+    // Before poke: idle thread with worker step having $status=ok → moderator would route to reviewer.
+    // After poke (mock returns same $status=ok), moderator routing remains the same.
+    const { casDir, mockAgentPath } = await setupThread();
+    const result = runUwf(
+      ["thread", "poke", THREAD_ID, "-p", "redo", "--agent", mockAgentPath],
+      casDir,
+    );
+    expect(result.status).toBe(0);
+    const cliOutput = JSON.parse(result.stdout.trim());
+    expect(cliOutput.currentRole).toBe("reviewer");
+  });
+});
+
+// ── Group 4: Agent resolution ──────────────────────────────────────────────
+
+describe("uwf thread poke - agent resolution", () => {
+  test("4.1 without --agent, agent command read from head step's agent field", async () => {
+    // Head step's agent field points at mockAgentPath (default in setupThread)
+    const { casDir, promptCapturePath } = await setupThread();
+    const result = runUwf(["thread", "poke", THREAD_ID, "-p", "redo"], casDir);
+    expect(result.status).toBe(0);
+    const captured = await readFile(promptCapturePath, "utf8");
+    expect(captured).toBe("redo");
+  });
+
+  test("4.2 with --agent, explicit override is used", async () => {
+    // Head step records "uwf-mock" (which is not a real binary). Override with mockAgentPath.
+    const { casDir, mockAgentPath } = await setupThread({ stepAgentNameOverride: "uwf-mock" });
+    const result = runUwf(
+      ["thread", "poke", THREAD_ID, "-p", "redo", "--agent", mockAgentPath],
+      casDir,
+    );
+    expect(result.status).toBe(0);
+  });
+});
+
+// ── Group 5: Prompt passthrough ────────────────────────────────────────────
+
+describe("uwf thread poke - prompt passthrough", () => {
+  test("5.1 -p value is passed to agent as --prompt", async () => {
+    const { casDir, mockAgentPath, promptCapturePath } = await setupThread();
+    const supplement = "Use the REST API instead.";
+    const result = runUwf(
+      ["thread", "poke", THREAD_ID, "-p", supplement, "--agent", mockAgentPath],
+      casDir,
+    );
+    expect(result.status).toBe(0);
+    const captured = await readFile(promptCapturePath, "utf8");
+    expect(captured).toBe(supplement);
+  });
+});
+
+// ── Group 6: Edge cases ────────────────────────────────────────────────────
+
+describe("uwf thread poke - edge cases", () => {
+  test("6.1 poke succeeds on suspended thread", async () => {
+    const { casDir, oldStepHash, mockAgentPath } = await setupThread({
+      threadStatus: "suspended",
+    });
+    const result = runUwf(
+      ["thread", "poke", THREAD_ID, "-p", "redo", "--agent", mockAgentPath],
+      casDir,
+    );
+    expect(result.status).toBe(0);
+    const cliOutput = JSON.parse(result.stdout.trim());
+    expect(cliOutput.head).not.toBe(oldStepHash);
+    expect(cliOutput.status).toBe("idle");
+    expect(cliOutput.suspendedRole).toBeNull();
+    expect(cliOutput.suspendMessage).toBeNull();
+  });
+
+  test("6.2 agent failure leaves thread head unchanged", async () => {
+    const { casDir, oldStepHash, failingAgentPath } = await setupThread();
+    const result = runUwf(
+      ["thread", "poke", THREAD_ID, "-p", "redo", "--agent", failingAgentPath],
+      casDir,
+    );
+    expect(result.status).not.toBe(0);
+
+    const { createUwfStore, getThread } = await import("../store.js");
+    const uwf = await createUwfStore(tmpDir);
+    const entry = getThread(uwf.varStore, THREAD_ID);
+    expect(entry?.head).toBe(oldStepHash);
+  });
+});

packages/cli/src/cli.ts

@@ -17,6 +17,7 @@ import {
   cmdThreadCancel,
   cmdThreadExec,
   cmdThreadList,
+  cmdThreadPoke,
   cmdThreadRead,
   cmdThreadResume,
   cmdThreadShow,
@@ -290,6 +291,26 @@ thread
     });
   });
 
+thread
+  .command("poke")
+  .description("Re-run the head step's agent with a supplementary prompt (replaces head step)")
+  .argument("<thread-id>", "Thread ULID")
+  .requiredOption("-p, --prompt <text>", "Supplementary prompt for the agent")
+  .option("--agent <cmd>", "Override agent command (defaults to head step's agent)")
+  .action((threadId: string, opts: { prompt: string; agent: string | undefined }) => {
+    const storageRoot = resolveStorageRoot();
+    runAction(async () => {
+      const agentOverride = opts.agent ?? null;
+      const result = await cmdThreadPoke(
+        storageRoot,
+        threadId as ThreadId,
+        opts.prompt,
+        agentOverride,
+      );
+      writeOutput(result);
+    });
+  });
+
 thread
   .command("stop")
   .description("Stop background execution of a thread (keep thread active)")

packages/cli/src/commands/thread.ts

@@ -199,6 +199,7 @@ const PL_THREAD_ARCHIVED = "F4D8Q2K5";
 const PL_STEP_ERROR = "B8T5N1V6";
 const PL_BACKGROUND_START = "X7Q4W9M2";
 const PL_THREAD_RESUME = "K2R7M4N8";
+const PL_THREAD_POKE = "P4Q9R3X7";
 
 type ResumeStepConfig = {
   role: string;
@@ -1135,6 +1136,147 @@ export async function cmdThreadResume(
   });
 }
 
+/**
+ * Validate that a thread can be poked. Returns the existing entry and the head StepNode payload.
+ * Fails (process exit) when the thread is missing, running, completed, cancelled, or has no
+ * StepNode at its head.
+ */
+async function validatePokePreconditions(
+  storageRoot: string,
+  uwf: UwfStore,
+  threadId: ThreadId,
+): Promise<{ entry: ThreadIndexEntry; oldHead: CasRef; oldHeadPayload: StepNodePayload }> {
+  const runningMarker = await isThreadRunning(storageRoot, threadId);
+  if (runningMarker !== null) {
+    fail(`thread already executing in background (PID: ${runningMarker.pid})`);
+  }
+
+  const entry = getThread(uwf.varStore, threadId);
+  if (entry === null) {
+    fail(`thread not active: ${threadId}`);
+  }
+
+  if (entry.status === "completed" || entry.status === "cancelled") {
+    fail(`thread cannot be poked: ${threadId} (status: ${entry.status})`);
+  }
+
+  const oldHead = entry.head;
+  const oldHeadNode = uwf.store.cas.get(oldHead);
+  if (oldHeadNode === null) {
+    fail(`CAS node not found: ${oldHead}`);
+  }
+  if (oldHeadNode.type !== uwf.schemas.stepNode) {
+    fail("thread cannot be poked: no step to replace (head is StartNode)");
+  }
+
+  return { entry, oldHead, oldHeadPayload: oldHeadNode.payload as StepNodePayload };
+}
+
+/**
+ * Resolve the next role from the post-poke chain state, used for the StepOutput.currentRole field.
+ * Returns null when the next role is $END, evaluation fails, or the result is a suspend.
+ */
+function resolveCurrentRoleFromChain(
+  uwfAfter: UwfStore,
+  workflow: WorkflowPayload,
+  replacedHash: CasRef,
+): string | null {
+  const chainAfter = walkChain(uwfAfter, replacedHash);
+  const { lastRole, lastOutput } = resolveEvaluateArgs(uwfAfter, chainAfter);
+  const afterResult = evaluate(workflow.graph, lastRole, lastOutput);
+  if (!afterResult.ok || isSuspendResult(afterResult.value)) {
+    return null;
+  }
+  if (afterResult.value.role === END_ROLE) {
+    return null;
+  }
+  return afterResult.value.role;
+}
+
+/**
+ * Poke a thread: re-run the agent on the head step with a supplementary prompt,
+ * replacing the head step's output. The new step's `prev` points to the OLD head's
+ * `prev` — semantically replacing (not appending to) the head. The moderator is NOT
+ * re-evaluated for routing; the role of the head step is re-used.
+ */
+export async function cmdThreadPoke(
+  storageRoot: string,
+  threadId: ThreadId,
+  prompt: string,
+  agentOverride: string | null,
+): Promise<StepOutput> {
+  const uwf = await createUwfStore(storageRoot);
+  const { entry, oldHeadPayload } = await validatePokePreconditions(storageRoot, uwf, threadId);
+
+  const chain = walkChain(uwf, entry.head);
+  const workflowHash = chain.start.workflow;
+  const threadCwd = chain.start.cwd;
+
+  const plog = createProcessLogger({
+    storageRoot,
+    context: { thread: threadId, workflow: workflowHash },
+  });
+
+  // Resolve the agent: --agent override wins; otherwise read from old head step's `agent` field.
+  const config = await loadWorkflowConfig(storageRoot);
+  const workflow = loadWorkflowPayload(uwf, workflowHash);
+  const role = oldHeadPayload.role;
+  const agent =
+    agentOverride !== null
+      ? resolveAgentConfig(config, workflow, role, agentOverride)
+      : parseAgentOverride(oldHeadPayload.agent);
+
+  const effectiveCwd = oldHeadPayload.cwd !== "" ? oldHeadPayload.cwd : threadCwd;
+
+  plog.log(PL_THREAD_POKE, `poke role=${role} agent=${agent.command}`, null);
+  plog.log(PL_AGENT_SPAWN, `spawning agent command=${agent.command}`, {
+    args: [...agent.args, threadId, role].join(" "),
+  });
+
+  loadDotenv({ path: getEnvPath(storageRoot) });
+
+  // Spawn the agent. The agent will create a new StepNode with prev=oldHead (it reads
+  // the active thread head). After the agent returns, we rewrite that node's prev so
+  // that the new head replaces the old head instead of appending after it.
+  const agentResult = spawnAgent(plog, agent, threadId, role, prompt, effectiveCwd);
+  const agentStepHash = agentResult.stepHash as CasRef;
+
+  plog.log(PL_AGENT_DONE, `agent returned head=${agentStepHash}`, null);
+
+  const uwfAfter = await createUwfStore(storageRoot);
+  const agentNode = uwfAfter.store.cas.get(agentStepHash);
+  if (agentNode === null || agentNode.type !== uwfAfter.schemas.stepNode) {
+    failStep(plog, `agent returned hash that is not a StepNode: ${agentStepHash}`);
+  }
+  const agentPayload = agentNode.payload as StepNodePayload;
+
+  // Rewrite the new step so that its `prev` points to the OLD head's prev (replace semantics).
+  const replacedPayload: StepNodePayload = {
+    ...agentPayload,
+    prev: oldHeadPayload.prev,
+  };
+  const replacedHash = await uwfAfter.store.cas.put(uwfAfter.schemas.stepNode, replacedPayload);
+  const replacedNode = uwfAfter.store.cas.get(replacedHash);
+  if (replacedNode === null || !validate(uwfAfter.store, replacedNode)) {
+    failStep(plog, "rewritten StepNode failed schema validation");
+  }
+
+  // Update thread head to the replaced step. Status becomes idle (no moderator re-route).
+  setThread(uwfAfter.varStore, threadId, updateThreadHead(entry, replacedHash));
+
+  return {
+    workflow: workflowHash,
+    thread: threadId,
+    head: replacedHash,
+    status: "idle",
+    currentRole: resolveCurrentRoleFromChain(uwfAfter, workflow, replacedHash),
+    suspendedRole: null,
+    suspendMessage: null,
+    done: false,
+    background: null,
+  };
+}
+
 export function validateCount(count: number): void {
   if (count < 1 || !Number.isInteger(count)) {
     throw new Error(`--count must be a positive integer, got: ${count}`);