feat(cli): add background thread execution and running threads query

This commit implements issue #456, adding two related capabilities to the uwf CLI:

1. **Background execution mode** for `uwf thread step` (via `--background` flag)
   - Spawns agent execution in a detached background process
   - Returns immediately with thread ID and background status
   - Maintains marker files to track running processes
   - Supports `--count` option to run multiple steps in background
   - Prevents concurrent execution of the same thread

2. **Running threads query** command (`uwf thread running`)
   - Lists all threads currently executing in background
   - Returns thread ID, workflow, current role, PID, and start time
   - Automatically filters out stale markers (dead processes)
   - Empty list when no threads are running

**Key changes:**

- **workflow-protocol**: Added `RunningThreadItem`, `RunningThreadsOutput` types
  Updated `StepOutput` to include `background: boolean | null` field

- **cli-workflow/background**: New module for process management
  - Marker file creation/deletion (atomic operations)
  - PID liveness checking
  - Stale marker cleanup
  - Running threads query

- **cli-workflow/commands/thread**:
  - Updated `cmdThreadStep` to support `--background` and `--_background-worker` flags
  - Added `cmdThreadStepBackground` for spawning detached processes
  - Added `cmdThreadRunning` to list running threads
  - Updated `cmdThreadKill` to terminate background processes

- **cli-workflow/cli**: Added CLI routing for new commands and flags

**Integration:**
- `uwf thread kill` now terminates background processes before archiving
- Foreground execution checks for existing background process and fails if found
- Background worker creates/cleans up marker files automatically
- Marker files stored in `~/.uncaged/workflow/running/*.json`

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/cli-workflow/src/background/background.ts

@@ -0,0 +1,147 @@
+import { mkdir, readdir, readFile, rename, rm, writeFile } from "node:fs/promises";
+import { join } from "node:path";
+import type { RunningThreadItem, ThreadId } from "@uncaged/workflow-protocol";
+
+import type { RunningMarker } from "./types.js";
+
+/**
+ * Get the path to the running markers directory.
+ */
+export function getRunningDir(storageRoot: string): string {
+  return join(storageRoot, "running");
+}
+
+/**
+ * Get the path to a specific thread's marker file.
+ */
+export function getMarkerPath(storageRoot: string, threadId: ThreadId): string {
+  return join(getRunningDir(storageRoot), `${threadId}.json`);
+}
+
+/**
+ * Check if a PID is still running.
+ * Returns true if the process exists, false otherwise.
+ */
+export function isPidAlive(pid: number): boolean {
+  try {
+    // process.kill with signal 0 checks existence without killing
+    process.kill(pid, 0);
+    return true;
+  } catch {
+    // ESRCH means process doesn't exist
+    return false;
+  }
+}
+
+/**
+ * Create a marker file for a running thread.
+ * Writes to a temp file in the same directory, then atomically renames.
+ */
+export async function createMarker(storageRoot: string, marker: RunningMarker): Promise<void> {
+  const runningDir = getRunningDir(storageRoot);
+  await mkdir(runningDir, { recursive: true });
+
+  const markerPath = getMarkerPath(storageRoot, marker.thread);
+  const tempPath = join(runningDir, `.${marker.thread}-${process.pid}.tmp`);
+
+  const content = JSON.stringify(marker, null, 2);
+  await writeFile(tempPath, content, "utf8");
+  await rename(tempPath, markerPath);
+}
+
+/**
+ * Delete a marker file for a thread.
+ */
+export async function deleteMarker(storageRoot: string, threadId: ThreadId): Promise<void> {
+  const markerPath = getMarkerPath(storageRoot, threadId);
+  try {
+    await rm(markerPath);
+  } catch {
+    // Ignore errors if file doesn't exist
+  }
+}
+
+/**
+ * Read a marker file. Returns null if file doesn't exist or is invalid.
+ */
+export async function readMarker(
+  storageRoot: string,
+  threadId: ThreadId,
+): Promise<RunningMarker | null> {
+  const markerPath = getMarkerPath(storageRoot, threadId);
+  try {
+    const content = await readFile(markerPath, "utf8");
+    const marker = JSON.parse(content) as RunningMarker;
+    return marker;
+  } catch {
+    return null;
+  }
+}
+
+/**
+ * List all running threads, filtering out stale markers.
+ */
+export async function listRunningThreads(storageRoot: string): Promise<RunningThreadItem[]> {
+  const runningDir = getRunningDir(storageRoot);
+
+  let files: string[];
+  try {
+    files = await readdir(runningDir);
+  } catch {
+    // Directory doesn't exist or can't be read
+    return [];
+  }
+
+  const results: RunningThreadItem[] = [];
+
+  for (const filename of files) {
+    if (!filename.endsWith(".json")) {
+      continue;
+    }
+
+    const threadId = filename.slice(0, -5) as ThreadId;
+    const marker = await readMarker(storageRoot, threadId);
+
+    if (marker === null) {
+      // Invalid marker file
+      continue;
+    }
+
+    if (!isPidAlive(marker.pid)) {
+      // Stale marker - process no longer exists
+      await deleteMarker(storageRoot, threadId);
+      continue;
+    }
+
+    results.push({
+      thread: marker.thread,
+      workflow: marker.workflow,
+      pid: marker.pid,
+      startedAt: marker.startedAt,
+    });
+  }
+
+  return results;
+}
+
+/**
+ * Check if a thread is currently executing in the background.
+ * Returns the marker if running, null otherwise.
+ */
+export async function isThreadRunning(
+  storageRoot: string,
+  threadId: ThreadId,
+): Promise<RunningMarker | null> {
+  const marker = await readMarker(storageRoot, threadId);
+  if (marker === null) {
+    return null;
+  }
+
+  if (!isPidAlive(marker.pid)) {
+    // Stale marker
+    await deleteMarker(storageRoot, threadId);
+    return null;
+  }
+
+  return marker;
+}

packages/cli-workflow/src/background/index.ts

@@ -0,0 +1,11 @@
+export {
+  createMarker,
+  deleteMarker,
+  getMarkerPath,
+  getRunningDir,
+  isPidAlive,
+  isThreadRunning,
+  listRunningThreads,
+  readMarker,
+} from "./background.js";
+export type { RunningMarker } from "./types.js";

packages/cli-workflow/src/background/types.ts

@@ -0,0 +1,9 @@
+import type { CasRef, ThreadId } from "@uncaged/workflow-protocol";
+
+/** Marker file stored at ~/.uncaged/workflow/running/<thread-id>.json */
+export type RunningMarker = {
+  thread: ThreadId;
+  workflow: CasRef;
+  pid: number;
+  startedAt: number;
+};

packages/cli-workflow/src/cli.ts

@@ -22,6 +22,7 @@ import {
   cmdThreadKill,
   cmdThreadList,
   cmdThreadRead,
+  cmdThreadRunning,
   cmdThreadShow,
   cmdThreadStart,
   cmdThreadStep,
@@ -114,19 +115,41 @@ thread
   .argument("<thread-id>", "Thread ULID")
   .option("--agent <cmd>", "Override agent command")
   .option("-c, --count <number>", "Number of steps to run (default: 1)")
-  .action((threadId: string, opts: { agent: string | undefined; count: string | undefined }) => {
+  .option("--background", "Run in background and return immediately")
+  .option("--_background-worker", "Internal flag for background worker process", false)
+  .action(
+    (
+      threadId: string,
+      opts: {
+        agent: string | undefined;
+        count: string | undefined;
+        background: boolean;
+        _backgroundWorker: boolean;
+      },
+    ) => {
       const storageRoot = resolveStorageRoot();
       runAction(async () => {
         const agentOverride = opts.agent ?? null;
         const count = opts.count !== undefined ? Number(opts.count) : 1;
-      const results = await cmdThreadStep(storageRoot, threadId, agentOverride, count);
+        const background = opts.background ?? false;
+        const backgroundWorker = opts._backgroundWorker ?? false;
+
+        const results = await cmdThreadStep(
+          storageRoot,
+          threadId,
+          agentOverride,
+          count,
+          background,
+          backgroundWorker,
+        );
         if (results.length === 1) {
           writeOutput(results[0]);
         } else {
           writeOutput(results);
         }
       });
-  });
+    },
+  );
 
 thread
   .command("show")
@@ -152,6 +175,17 @@ thread
     });
   });
 
+thread
+  .command("running")
+  .description("List threads currently executing in the background")
+  .action(() => {
+    const storageRoot = resolveStorageRoot();
+    runAction(async () => {
+      const result = await cmdThreadRunning(storageRoot);
+      writeOutput(result);
+    });
+  });
+
 thread
   .command("kill")
   .description("Terminate and archive a thread")

packages/cli-workflow/src/commands/thread.ts

@@ -1,4 +1,4 @@
-import { execFileSync } from "node:child_process";
+import { execFileSync, spawn } from "node:child_process";
 import { access, readFile } from "node:fs/promises";
 import { dirname, isAbsolute, resolve as resolvePath } from "node:path";
 import type { Store as CasStore, JSONSchema } from "@uncaged/json-cas";
@@ -10,6 +10,7 @@ import type {
   AgentConfig,
   CasRef,
   ModeratorContext,
+  RunningThreadsOutput,
   StartEntry,
   StartNodePayload,
   StartOutput,
@@ -27,7 +28,12 @@ import type {
 import { createProcessLogger, generateUlid, type ProcessLogger } from "@uncaged/workflow-util";
 import { config as loadDotenv } from "dotenv";
 import { parse, stringify } from "yaml";
-
+import {
+  createMarker,
+  deleteMarker,
+  isThreadRunning,
+  listRunningThreads,
+} from "../background/index.js";
 import {
   appendThreadHistory,
   createUwfStore,
@@ -52,6 +58,7 @@ const PL_AGENT_SPAWN = "R5J2W8N4";
 const PL_AGENT_DONE = "C6P9E3H7";
 const PL_THREAD_ARCHIVED = "F4D8Q2K5";
 const PL_STEP_ERROR = "B8T5N1V6";
+const PL_BACKGROUND_START = "X7Q4W9M2";
 
 function failStep(plog: ProcessLogger, message: string): never {
   plog.log(PL_STEP_ERROR, message, null);
@@ -321,6 +328,7 @@ export async function cmdThreadShow(storageRoot: string, threadId: ThreadId): Pr
       thread: threadId,
       head: activeHead,
       done: false,
+      background: null,
     };
   }
 
@@ -331,6 +339,7 @@ export async function cmdThreadShow(storageRoot: string, threadId: ThreadId): Pr
       thread: threadId,
       head: hist.head,
       done: true,
+      background: null,
     };
   }
 
@@ -853,17 +862,45 @@ export async function cmdThreadStep(
   threadId: ThreadId,
   agentOverride: string | null,
   count: number,
+  background: boolean,
+  backgroundWorker: boolean,
 ): Promise<StepOutput[]> {
   if (count < 1 || !Number.isInteger(count)) {
     fail(`--count must be a positive integer, got: ${count}`);
   }
 
+  // Check if thread is already running in background (unless we ARE the background worker)
+  if (!backgroundWorker) {
+    const runningMarker = await isThreadRunning(storageRoot, threadId);
+    if (runningMarker !== null) {
+      fail(`thread already executing in background (PID: ${runningMarker.pid})`);
+    }
+  }
+
   const workflowHash = await resolveActiveThreadWorkflowHash(storageRoot, threadId);
   const plog = createProcessLogger({
     storageRoot,
     context: { thread: threadId, workflow: workflowHash },
   });
 
+  if (background && !backgroundWorker) {
+    // Spawn background process
+    return cmdThreadStepBackground(storageRoot, threadId, agentOverride, count, plog, workflowHash);
+  }
+
+  // If we're the background worker, create marker before execution
+  let markerCreated = false;
+  if (backgroundWorker) {
+    await createMarker(storageRoot, {
+      thread: threadId,
+      workflow: workflowHash,
+      pid: process.pid,
+      startedAt: Date.now(),
+    });
+    markerCreated = true;
+  }
+
+  try {
     const results: StepOutput[] = [];
     for (let i = 0; i < count; i++) {
       const result = await cmdThreadStepOnce(storageRoot, threadId, agentOverride, plog);
@@ -873,6 +910,12 @@ export async function cmdThreadStep(
       }
     }
     return results;
+  } finally {
+    // Cleanup marker if we created one
+    if (markerCreated) {
+      await deleteMarker(storageRoot, threadId);
+    }
+  }
 }
 
 async function resolveActiveThreadWorkflowHash(
@@ -889,6 +932,57 @@ async function resolveActiveThreadWorkflowHash(
   return chain.start.workflow;
 }
 
+async function cmdThreadStepBackground(
+  storageRoot: string,
+  threadId: ThreadId,
+  agentOverride: string | null,
+  count: number,
+  plog: ProcessLogger,
+  workflowHash: CasRef,
+): Promise<StepOutput[]> {
+  // Get current head to return to caller
+  const index = await loadThreadsIndex(storageRoot);
+  const headHash = index[threadId];
+  if (headHash === undefined) {
+    failStep(plog, `thread not active: ${threadId}`);
+  }
+
+  // Spawn detached background process
+  const scriptPath = process.argv[1];
+  if (scriptPath === undefined) {
+    failStep(plog, "unable to determine script path for background execution");
+  }
+
+  const args = ["thread", "step", threadId, "--count", String(count)];
+
+  if (agentOverride !== null) {
+    args.push("--agent", agentOverride);
+  }
+
+  // Internal flag to signal the background worker to create/cleanup markers
+  args.push("--_background-worker");
+
+  plog.log(PL_BACKGROUND_START, `spawning background process count=${count}`, null);
+
+  const child = spawn(scriptPath, args, {
+    detached: true,
+    stdio: "ignore",
+  });
+
+  child.unref();
+
+  // Return immediately with current state and background flag
+  return [
+    {
+      workflow: workflowHash,
+      thread: threadId,
+      head: headHash,
+      done: false,
+      background: true,
+    },
+  ];
+}
+
 async function cmdThreadStepOnce(
   storageRoot: string,
   threadId: ThreadId,
@@ -926,6 +1020,7 @@ async function cmdThreadStepOnce(
       thread: threadId,
       head: headHash,
       done: true,
+      background: null,
     };
   }
 
@@ -973,6 +1068,7 @@ async function cmdThreadStepOnce(
     thread: threadId,
     head: newHead,
     done,
+    background: null,
   };
 }
 
@@ -1109,6 +1205,17 @@ export async function cmdThreadKill(storageRoot: string, threadId: ThreadId): Pr
     fail(`thread not active: ${threadId}`);
   }
 
+  // Check if thread is running in background and terminate it
+  const runningMarker = await isThreadRunning(storageRoot, threadId);
+  if (runningMarker !== null) {
+    try {
+      process.kill(runningMarker.pid, "SIGTERM");
+    } catch {
+      // Process may have already exited, ignore error
+    }
+    await deleteMarker(storageRoot, threadId);
+  }
+
   const uwf = await createUwfStore(storageRoot);
   const workflow = resolveWorkflowFromHead(uwf, head);
   if (workflow === null) {
@@ -1128,3 +1235,8 @@ export async function cmdThreadKill(storageRoot: string, threadId: ThreadId): Pr
 
   return { thread: threadId, archived: true };
 }
+
+export async function cmdThreadRunning(storageRoot: string): Promise<RunningThreadsOutput> {
+  const threads = await listRunningThreads(storageRoot);
+  return { threads };
+}

packages/workflow-protocol/src/index.ts

@@ -15,6 +15,8 @@ export type {
   ProviderConfig,
   RoleDefinition,
   RoleName,
+  RunningThreadItem,
+  RunningThreadsOutput,
   Scenario,
   StartEntry,
   StartNodePayload,

packages/workflow-protocol/src/types.ts

@@ -84,6 +84,7 @@ export type StepOutput = {
   thread: ThreadId;
   head: CasRef;
   done: boolean;
+  background: boolean | null;
 };
 
 /** uwf thread steps — single step entry */
@@ -126,6 +127,19 @@ export type ThreadListItem = {
   head: CasRef;
 };
 
+/** uwf thread running — single running thread entry */
+export type RunningThreadItem = {
+  thread: ThreadId;
+  workflow: CasRef;
+  pid: number;
+  startedAt: number;
+};
+
+/** uwf thread running output */
+export type RunningThreadsOutput = {
+  threads: RunningThreadItem[];
+};
+
 // ── 4.6 配置 ────────────────────────────────────────────────────────
 
 /** Alias types for config references */