feat(worktree): extend --resume context restore to headless + ACP modes

Phase C task 7 originally placed the worktree-restore logic in
AppContainer.tsx (TUI only). E2E Group C exposed that headless and ACP
modes never run AppContainer, so stale sidecars accumulate and the model
loses worktree context after --resume.

Refactor to a shared `restoreWorktreeContext` helper in core, then wire
the three entry points:

- TUI (AppContainer): keep historyManager.addItem(INFO) UX, route via
  the helper.
- Headless (nonInteractiveCli): prepend the notice as a system-reminder
  block on the user prompt; emit a `worktree_restored` system message to
  the JSON adapter so SDK consumers can react.
- ACP (Session.pendingWorktreeNotice): set by acpAgent.loadSession on
  resume, consumed and cleared exactly once on the next #executePrompt.

All three modes call the same helper, so stale-sidecar cleanup is
consistent. Helper covers: missing sidecar, live worktree dir,
deleted worktree dir, regular file at worktreePath, malformed JSON.

5 new unit tests for restoreWorktreeContext (13/13 pass total).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

packages/cli/src/acp-integration/acpAgent.ts

@@ -22,6 +22,7 @@ import {
   SessionStartSource,
   SessionEndReason,
   type PermissionMode,
+  restoreWorktreeContext,
 } from '@qwen-code/qwen-code-core';
 import {
   AgentSideConnection,
@@ -325,7 +326,26 @@ class QwenAgent implements Agent {
     this.setupFileSystem(config);
 
     const sessionData = config.getResumedSessionData();
-    await this.createAndStoreSession(config, sessionData?.conversation);
+    const session = await this.createAndStoreSession(
+      config,
+      sessionData?.conversation,
+    );
+
+    // Phase C: restore worktree context on --resume. Cleans up stale
+    // sidecars (worktree dir deleted out-of-band) and queues a notice for
+    // the next prompt when the worktree is alive. Best-effort: failures
+    // don't block session load.
+    try {
+      const sessionPath = config
+        .getSessionService()
+        .getWorktreeSessionPath(config.getSessionId());
+      const restored = await restoreWorktreeContext(sessionPath);
+      if (restored.contextMessage) {
+        session.pendingWorktreeNotice = restored.contextMessage;
+      }
+    } catch (error) {
+      debugLogger.warn(`ACP worktree restore failed: ${error}`);
+    }
 
     const modesData = this.buildModesData(config);
     const availableModels = this.buildAvailableModels(config);

packages/cli/src/acp-integration/session/Session.ts

@@ -158,6 +158,16 @@ export class Session implements SessionContext {
   // Message rewrite middleware (optional, installed after history replay)
   messageRewriter?: MessageRewriteMiddleware;
 
+  /**
+   * Phase C worktree restore notice. Set by acpAgent.loadSession when a
+   * resumed session has a live worktree sidecar; prepended to the next
+   * #executePrompt call as a <system-reminder>, then cleared. TUI uses
+   * historyManager.addItem(INFO) for the equivalent UX hint and headless
+   * prepends to the single shot prompt — all three modes share the
+   * `restoreWorktreeContext` helper that produces this string.
+   */
+  pendingWorktreeNotice: string | null = null;
+
   // Implement SessionContext interface
   readonly sessionId: string;
 
@@ -530,6 +540,20 @@ export class Session implements SessionContext {
           parts = [...systemReminders, ...parts];
         }
 
+        // Phase C: one-shot worktree restore notice, set by acpAgent on
+        // --resume / loadSession when the session's worktree is still alive.
+        // Prepended exactly once, then cleared so it doesn't repeat on
+        // subsequent turns.
+        if (this.pendingWorktreeNotice) {
+          parts = [
+            {
+              text: `<system-reminder>\n${this.pendingWorktreeNotice}\n</system-reminder>\n\n`,
+            },
+            ...parts,
+          ];
+          this.pendingWorktreeNotice = null;
+        }
+
         let nextMessage: Content | null = { role: 'user', parts };
 
         while (nextMessage !== null) {

packages/cli/src/nonInteractiveCli.ts

@@ -26,6 +26,7 @@ import {
   parseAndFormatApiError,
   createDebugLogger,
   SendMessageType,
+  restoreWorktreeContext,
 } from '@qwen-code/qwen-code-core';
 import type { Content, Part, PartListUnion } from '@google/genai';
 import type { CLIUserMessage, PermissionMode } from './nonInteractive/types.js';
@@ -375,6 +376,39 @@ export async function runNonInteractive(
         initialPartList = [{ text: input }];
       }
 
+      // Phase C: when --resume restored a session with an active worktree,
+      // prepend a system-reminder block to the user prompt so the model
+      // knows to keep using the worktree path. Stale sidecars (worktree
+      // dir deleted between sessions) are cleaned up inside the helper.
+      // TUI does this via historyManager.addItem(INFO); headless does it
+      // here because there is no UI history to write into.
+      if (config.getResumedSessionData()) {
+        try {
+          const sessionPath = config
+            .getSessionService()
+            .getWorktreeSessionPath(sessionId);
+          const restored = await restoreWorktreeContext(sessionPath);
+          if (restored.contextMessage) {
+            const reminderPart: Part = {
+              text: `<system-reminder>\n${restored.contextMessage}\n</system-reminder>\n\n`,
+            };
+            const partsArr = Array.isArray(initialPartList)
+              ? initialPartList
+              : [initialPartList];
+            initialPartList = [reminderPart, ...partsArr];
+            // Also surface the notice in the JSON stream so SDK consumers
+            // can react to it (logging, UI hints, etc.).
+            adapter.emitSystemMessage('worktree_restored', {
+              slug: restored.session?.slug,
+              path: restored.session?.worktreePath,
+              branch: restored.session?.worktreeBranch,
+            });
+          }
+        } catch (error) {
+          debugLogger.warn(`worktree restore failed (non-fatal):`, error);
+        }
+      }
+
       const initialParts = normalizePartList(initialPartList);
       let currentMessages: Content[] = [{ role: 'user', parts: initialParts }];
 

packages/cli/src/ui/AppContainer.tsx

@@ -60,11 +60,10 @@ import {
   ToolConfirmationOutcome,
   type WaitingToolCall,
   ToolNames,
-  readWorktreeSession,
   clearWorktreeSession,
+  restoreWorktreeContext,
   GitWorktreeService,
 } from '@qwen-code/qwen-code-core';
-import * as fsPromises from 'node:fs/promises';
 import { buildResumedHistoryItems } from './utils/resumeHistoryUtils.js';
 import {
   getStickyTodos,
@@ -509,37 +508,22 @@ export const AppContainer = (props: AppContainerProps) => {
           setSessionName(title);
         }
 
-        // If the resumed session had an active worktree, inject a context
-        // message so the model immediately knows to keep using the
-        // worktree path for file operations (qwen-code can't `chdir` the
-        // way claude-code does — Config.targetDir is immutable).
-        //
-        // Stale sidecars (worktree dir deleted between sessions) get
-        // cleaned up so Footer / useWorktreeSession don't show a phantom
-        // worktree indicator.
+        // Restore worktree context (shared logic — headless and ACP use
+        // the same helper). Stale sidecars get cleaned up; live ones
+        // produce an INFO message the model sees on the next turn.
         try {
           const sessionPath = config
             .getSessionService()
             .getWorktreeSessionPath(config.getSessionId());
-          const ws = await readWorktreeSession(sessionPath);
-          if (ws) {
-            const worktreeAlive = await fsPromises
-              .stat(ws.worktreePath)
-              .then((s) => s.isDirectory())
-              .catch(() => false);
-            if (worktreeAlive) {
-              historyManager.addItem(
-                {
-                  type: MessageType.INFO,
-                  text:
-                    `[Resumed] Active worktree: "${ws.slug}" at ${ws.worktreePath} ` +
-                    `(branch: ${ws.worktreeBranch}). Continue using this path for all file operations.`,
-                },
-                Date.now(),
-              );
-            } else {
-              await clearWorktreeSession(sessionPath);
-            }
+          const restored = await restoreWorktreeContext(sessionPath, (err) => {
+            // eslint-disable-next-line no-console
+            console.debug('worktree session restore warning:', err);
+          });
+          if (restored.contextMessage) {
+            historyManager.addItem(
+              { type: MessageType.INFO, text: restored.contextMessage },
+              Date.now(),
+            );
           }
         } catch (error) {
           // Best-effort: failures here only affect UI hint visibility,

packages/core/src/services/worktreeSessionService.test.ts

@@ -12,6 +12,7 @@ import {
   readWorktreeSession,
   writeWorktreeSession,
   clearWorktreeSession,
+  restoreWorktreeContext,
   type WorktreeSession,
 } from './worktreeSessionService.js';
 
@@ -84,3 +85,58 @@ describe('clearWorktreeSession', () => {
     await expect(clearWorktreeSession(filePath)).resolves.not.toThrow();
   });
 });
+
+describe('restoreWorktreeContext', () => {
+  it('returns nulls when no sidecar exists', async () => {
+    const result = await restoreWorktreeContext(filePath);
+    expect(result.session).toBeNull();
+    expect(result.contextMessage).toBeNull();
+  });
+
+  it('returns context message + session when worktree dir is alive', async () => {
+    // Point sample at a real existing directory (tmpDir itself).
+    const live: WorktreeSession = { ...sample, worktreePath: tmpDir };
+    await writeWorktreeSession(filePath, live);
+    const result = await restoreWorktreeContext(filePath);
+
+    expect(result.session).toEqual(live);
+    expect(result.contextMessage).toContain(`"${live.slug}"`);
+    expect(result.contextMessage).toContain(live.worktreePath);
+    expect(result.contextMessage).toContain(live.worktreeBranch);
+    // Sidecar should remain on disk so subsequent reads still see it.
+    expect(await readWorktreeSession(filePath)).toEqual(live);
+  });
+
+  it('cleans up stale sidecar when worktree dir is gone', async () => {
+    // sample.worktreePath points at /repo/.qwen/... which does not exist.
+    await writeWorktreeSession(filePath, sample);
+    expect(await readWorktreeSession(filePath)).toEqual(sample);
+
+    const result = await restoreWorktreeContext(filePath);
+    expect(result.session).toBeNull();
+    expect(result.contextMessage).toBeNull();
+    // Sidecar should be deleted.
+    expect(await readWorktreeSession(filePath)).toBeNull();
+  });
+
+  it('treats a regular file at worktreePath as not-a-worktree', async () => {
+    const filePathTarget = path.join(tmpDir, 'pretend-worktree');
+    await fs.writeFile(filePathTarget, 'not a dir', 'utf-8');
+    const bogus: WorktreeSession = { ...sample, worktreePath: filePathTarget };
+    await writeWorktreeSession(filePath, bogus);
+
+    const result = await restoreWorktreeContext(filePath);
+    expect(result.session).toBeNull();
+    expect(await readWorktreeSession(filePath)).toBeNull();
+  });
+
+  it('forwards warning when sidecar JSON is malformed', async () => {
+    await fs.writeFile(filePath, 'not valid json {', 'utf-8');
+    const warnings: unknown[] = [];
+    const result = await restoreWorktreeContext(filePath, (e) =>
+      warnings.push(e),
+    );
+    expect(result.session).toBeNull();
+    expect(warnings.length).toBe(1);
+  });
+});

packages/core/src/services/worktreeSessionService.ts

@@ -59,3 +59,69 @@ export async function clearWorktreeSession(filePath: string): Promise<void> {
     throw error;
   }
 }
+
+export interface WorktreeRestoreResult {
+  /**
+   * When non-null, the worktree directory is still alive — callers should
+   * surface this one-line context message so the model continues using
+   * the worktree path for file operations after a `--resume`.
+   *
+   * Each entry point chooses its own injection mechanism:
+   * - TUI: `historyManager.addItem({ type: INFO, text })`
+   * - Headless: prepend as a `<system-reminder>` block to the user prompt
+   * - ACP: emit as a `system` message and prepend to the next prompt
+   */
+  contextMessage: string | null;
+  /** Active worktree session, or null when no sidecar / sidecar was stale. */
+  session: WorktreeSession | null;
+}
+
+/**
+ * Reads the WorktreeSession sidecar for the current session, validates
+ * that the worktree directory still exists on disk, and either:
+ *
+ * - returns a context message + the live session, or
+ * - deletes the stale sidecar and returns nulls.
+ *
+ * Shared by TUI / headless / ACP entry points so all three behave
+ * consistently on `--resume`. Failures are logged via the supplied
+ * `onWarn` callback but never thrown — worktree restore is best-effort,
+ * the session itself must still load.
+ */
+export async function restoreWorktreeContext(
+  sidecarPath: string,
+  onWarn?: (error: unknown) => void,
+): Promise<WorktreeRestoreResult> {
+  let session: WorktreeSession | null = null;
+  try {
+    session = await readWorktreeSession(sidecarPath);
+  } catch (error) {
+    onWarn?.(error);
+    return { contextMessage: null, session: null };
+  }
+  if (!session) return { contextMessage: null, session: null };
+
+  let worktreeAlive = false;
+  try {
+    const stat = await fs.stat(session.worktreePath);
+    worktreeAlive = stat.isDirectory();
+  } catch {
+    worktreeAlive = false;
+  }
+
+  if (!worktreeAlive) {
+    try {
+      await clearWorktreeSession(sidecarPath);
+    } catch (error) {
+      onWarn?.(error);
+    }
+    return { contextMessage: null, session: null };
+  }
+
+  return {
+    session,
+    contextMessage:
+      `[Resumed] Active worktree: "${session.slug}" at ${session.worktreePath} ` +
+      `(branch: ${session.worktreeBranch}). Continue using this path for all file operations.`,
+  };
+}