fix(worktree): fetch trunk before creating worktree to avoid stale base

apps/code/src/main/services/workspace/service.ts

@@ -590,6 +590,7 @@ export class WorkspaceService extends TypedEventEmitter<WorkspaceServiceEvents>
         worktree = await worktreeManager.createWorktree({
           baseBranch: defaultBranch,
           onOutput,
+          fetchBeforeCreate: true,
         });
         log.info(
           `Created detached worktree from trunk: ${worktree.worktreeName} at ${worktree.worktreePath}`,

packages/git/src/queries.ts

@@ -1023,6 +1023,28 @@ export async function fetch(
   );
 }
 
+export async function hasRef(git: GitLike, ref: string): Promise<boolean> {
+  try {
+    await git.revparse(["--verify", "--quiet", `${ref}^{commit}`]);
+    return true;
+  } catch {
+    return false;
+  }
+}
+
+export async function fetchRef(
+  git: GitLike,
+  remote: string,
+  ref: string,
+): Promise<boolean> {
+  try {
+    await git.raw(["fetch", "--quiet", "--no-tags", remote, ref]);
+    return true;
+  } catch {
+    return false;
+  }
+}
+
 export async function listFiles(
   baseDir: string,
   options?: CreateGitClientOptions,

packages/git/src/sagas/worktree.ts

@@ -1,14 +1,22 @@
 import * as fs from "node:fs/promises";
 import * as path from "node:path";
 import { GitSaga, type GitSagaInput } from "../git-saga";
-import { addToLocalExclude, branchExists, getDefaultBranch } from "../queries";
+import {
+  addToLocalExclude,
+  branchExists,
+  fetchRef,
+  getDefaultBranch,
+  hasRef,
+} from "../queries";
 import { forceRemove, safeSymlink } from "../utils";
 import { processWorktreeInclude, runPostCheckoutHook } from "../worktree";
 
 export interface CreateWorktreeInput extends GitSagaInput {
   worktreePath: string;
   branchName: string;
   baseBranch?: string;
+  /** Base the worktree on `origin/<baseBranch>` after fetching; falls back to the local ref if the fetch fails. */
+  fetchBeforeCreate?: boolean;
 }
 
 export interface CreateWorktreeOutput {
@@ -26,13 +34,32 @@ export class CreateWorktreeSaga extends GitSaga<
   protected async executeGitOperations(
     input: CreateWorktreeInput,
   ): Promise<CreateWorktreeOutput> {
-    const { baseDir, worktreePath, branchName, baseBranch, signal } = input;
+    const {
+      baseDir,
+      worktreePath,
+      branchName,
+      baseBranch,
+      fetchBeforeCreate,
+      signal,
+    } = input;
 
     const base = await this.readOnlyStep("get-base-branch", async () => {
       if (baseBranch) return baseBranch;
       return getDefaultBranch(baseDir, { abortSignal: signal });
     });
 
+    // Use `this.git` directly to avoid re-entering the write lock the saga already holds.
+    const baseRef = fetchBeforeCreate
+      ? await this.readOnlyStep("resolve-fresh-base-ref", async () => {
+          const remote = "origin";
+          const remoteRef = `${remote}/${base}`;
+          const fetched = await fetchRef(this.git, remote, base);
+          if (!fetched) return base;
+          const exists = await hasRef(this.git, remoteRef);
+          return exists ? remoteRef : base;
+        })
+      : base;
+
     await this.step({
       name: "create-worktree",
       execute: () =>
@@ -44,7 +71,7 @@ export class CreateWorktreeSaga extends GitSaga<
           "-b",
           branchName,
           worktreePath,
-          base,
+          baseRef,
         ]),
       rollback: async () => {
         try {

packages/git/src/worktree.test.ts

@@ -0,0 +1,132 @@
+import { mkdtemp, rm, writeFile } from "node:fs/promises";
+import { tmpdir } from "node:os";
+import * as path from "node:path";
+import { afterEach, beforeEach, describe, expect, it } from "vitest";
+import { createGitClient } from "./client";
+import { WorktreeManager } from "./worktree";
+
+async function initBareRemote(): Promise<string> {
+  const dir = await mkdtemp(path.join(tmpdir(), "posthog-code-remote-"));
+  const git = createGitClient(dir);
+  await git.init(["--bare", "--initial-branch", "main"]);
+  return dir;
+}
+
+async function initLocalClone(remoteDir: string): Promise<string> {
+  const dir = await mkdtemp(path.join(tmpdir(), "posthog-code-local-"));
+  const git = createGitClient(dir);
+  await git.clone(remoteDir, dir);
+  await git.addConfig("user.name", "Test");
+  await git.addConfig("user.email", "test@example.com");
+  await git.addConfig("commit.gpgsign", "false");
+  return dir;
+}
+
+async function commit(repoDir: string, file: string, content: string) {
+  await writeFile(path.join(repoDir, file), content);
+  const git = createGitClient(repoDir);
+  await git.add([file]);
+  await git.commit(`add ${file}`);
+}
+
+async function shaOfBranch(repoDir: string, ref: string): Promise<string> {
+  const git = createGitClient(repoDir);
+  return (await git.revparse([ref])).trim();
+}
+
+describe("WorktreeManager.createWorktree fetchBeforeCreate", () => {
+  let remoteDir: string;
+  let localDir: string;
+  let worktreeBaseDir: string;
+
+  beforeEach(async () => {
+    remoteDir = await initBareRemote();
+
+    // Seed the remote with an initial commit on `main` so other clones can
+    // fetch a real tip.
+    const seedDir = await mkdtemp(path.join(tmpdir(), "posthog-code-seed-"));
+    const seedGit = createGitClient(seedDir);
+    await seedGit.init(["--initial-branch", "main"]);
+    await seedGit.addConfig("user.name", "Test");
+    await seedGit.addConfig("user.email", "test@example.com");
+    await seedGit.addConfig("commit.gpgsign", "false");
+    await commit(seedDir, "initial.txt", "initial\n");
+    await seedGit.addRemote("origin", remoteDir);
+    await seedGit.push(["origin", "main"]);
+    await rm(seedDir, { recursive: true, force: true });
+
+    localDir = await initLocalClone(remoteDir);
+    worktreeBaseDir = await mkdtemp(path.join(tmpdir(), "posthog-code-wts-"));
+  });
+
+  afterEach(async () => {
+    for (const d of [remoteDir, localDir, worktreeBaseDir]) {
+      await rm(d, { recursive: true, force: true });
+    }
+  });
+
+  it.each([
+    {
+      name: "without fetchBeforeCreate, worktree is based on the stale local ref",
+      fetchBeforeCreate: false,
+      expectRemoteTip: false,
+    },
+    {
+      name: "with fetchBeforeCreate, worktree starts at the remote tip",
+      fetchBeforeCreate: true,
+      expectRemoteTip: true,
+    },
+  ])("$name", async ({ fetchBeforeCreate, expectRemoteTip }) => {
+    // Advance the remote: push a new commit from a separate clone.
+    const otherDir = await initLocalClone(remoteDir);
+    await commit(otherDir, "remote-new.txt", "remote-new\n");
+    const otherGit = createGitClient(otherDir);
+    await otherGit.push(["origin", "main"]);
+    const remoteTip = await shaOfBranch(otherDir, "main");
+    await rm(otherDir, { recursive: true, force: true });
+
+    const localTipBefore = await shaOfBranch(localDir, "main");
+    expect(localTipBefore).not.toBe(remoteTip);
+
+    const manager = new WorktreeManager({
+      mainRepoPath: localDir,
+      worktreeBasePath: worktreeBaseDir,
+    });
+    const info = await manager.createWorktree({
+      baseBranch: "main",
+      fetchBeforeCreate,
+    });
+
+    const worktreeHead = await shaOfBranch(info.worktreePath, "HEAD");
+    if (expectRemoteTip) {
+      expect(worktreeHead).toBe(remoteTip);
+    } else {
+      expect(worktreeHead).toBe(localTipBefore);
+      expect(worktreeHead).not.toBe(remoteTip);
+    }
+
+    // Local `main` should never be mutated — only `origin/main` advances on fetch.
+    const localMainAfter = await shaOfBranch(localDir, "main");
+    expect(localMainAfter).toBe(localTipBefore);
+  });
+
+  it("with fetchBeforeCreate and an unreachable remote, falls back to local base", async () => {
+    // Point origin at a directory that doesn't exist so the fetch fails.
+    const git = createGitClient(localDir);
+    await git.remote(["set-url", "origin", "/nonexistent/path/to/remote"]);
+
+    const localTipBefore = await shaOfBranch(localDir, "main");
+
+    const manager = new WorktreeManager({
+      mainRepoPath: localDir,
+      worktreeBasePath: worktreeBaseDir,
+    });
+    const info = await manager.createWorktree({
+      baseBranch: "main",
+      fetchBeforeCreate: true,
+    });
+
+    const worktreeHead = await shaOfBranch(info.worktreePath, "HEAD");
+    expect(worktreeHead).toBe(localTipBefore);
+  });
+});

packages/git/src/worktree.ts

@@ -6,8 +6,10 @@ import { getCleanEnv, getGitOperationManager } from "./operation-manager";
 import {
   addToLocalExclude,
   branchExists,
+  fetchRef,
   getDefaultBranch,
   getHeadSha,
+  hasRef,
   listWorktrees as listWorktreesRaw,
 } from "./queries";
 import { clonePath, forceRemove, safeSymlink } from "./utils";
@@ -125,6 +127,8 @@ export class WorktreeManager {
   async createWorktree(options?: {
     baseBranch?: string;
     onOutput?: (data: string) => void;
+    /** Base the worktree on `origin/<baseBranch>` after fetching; falls back to the local ref if the fetch fails. */
+    fetchBeforeCreate?: boolean;
   }): Promise<WorktreeInfo> {
     const manager = getGitOperationManager();
 
@@ -155,9 +159,13 @@ export class WorktreeManager {
       ? worktreePath
       : `./${WORKTREE_FOLDER_NAME}/${worktreeName}/${this.repoName}`;
 
-    options?.onOutput?.(`Creating worktree from ${baseBranch}...\n`);
+    const baseRef = options?.fetchBeforeCreate
+      ? await this.resolveFreshBaseRef(baseBranch, options?.onOutput)
+      : baseBranch;
+
+    options?.onOutput?.(`Creating worktree from ${baseRef}...\n`);
     const output = await manager.executeWrite(this.mainRepoPath, async () => {
-      return this.spawnWorktreeAdd(["--detach", targetPath, baseBranch], {
+      return this.spawnWorktreeAdd(["--detach", targetPath, baseRef], {
         onOutput: options?.onOutput,
       });
     });
@@ -315,6 +323,44 @@ export class WorktreeManager {
     };
   }
 
+  /**
+   * Returns `origin/<baseBranch>` after fetching, or the local branch name as a fallback.
+   * Bases off `origin/<branch>` rather than fast-forwarding so local refs stay untouched.
+   */
+  private async resolveFreshBaseRef(
+    baseBranch: string,
+    onOutput?: (data: string) => void,
+  ): Promise<string> {
+    const manager = getGitOperationManager();
+    const remote = "origin";
+    const remoteRef = `${remote}/${baseBranch}`;
+
+    onOutput?.(`Fetching ${remoteRef}...\n`);
+    const fetched = await manager.executeWrite(this.mainRepoPath, (git) =>
+      fetchRef(git, remote, baseBranch),
+    );
+
+    if (!fetched) {
+      onOutput?.(
+        `Fetch failed for ${remoteRef}, falling back to local ${baseBranch}.\n`,
+      );
+      return baseBranch;
+    }
+
+    const remoteRefExists = await manager.executeRead(
+      this.mainRepoPath,
+      (git) => hasRef(git, remoteRef),
+    );
+    if (!remoteRefExists) {
+      onOutput?.(
+        `Remote ref ${remoteRef} not found after fetch, falling back to local ${baseBranch}.\n`,
+      );
+      return baseBranch;
+    }
+
+    return remoteRef;
+  }
+
   private spawnWorktreeAdd(
     args: string[],
     options?: { onOutput?: (data: string) => void },