fix(agent): avoid concurrent worktree config writes (#572)

Git writes upstream branch config when a worktree starts from origin/main. Use the already-resolved base SHA as the worktree start point so parallel agent worktrees do not race on shared .git/config.

Tested: bun test src/utils/__tests__/worktree.test.ts

Tested: real gpt-5.5 Sub2API run with four worktree-isolated agents

Not-tested: bun run check:server currently fails unrelated WebSocket restart/timeouts

Confidence: high

Scope-risk: narrow

src/utils/__tests__/worktree.test.ts

@@ -0,0 +1,117 @@
+import { afterEach, beforeEach, describe, expect, test } from 'bun:test'
+import { mkdtempSync, mkdirSync, rmSync, writeFileSync } from 'node:fs'
+import { tmpdir } from 'node:os'
+import { join } from 'node:path'
+import {
+  getCwdState,
+  getOriginalCwd,
+  setCwdState,
+  setOriginalCwd,
+} from '../../bootstrap/state.js'
+import { resetGitFileWatcher } from '../git/gitFilesystem.js'
+import { resetSettingsCache } from '../settings/settingsCache.js'
+import { createAgentWorktree, worktreeBranchName } from '../worktree.js'
+
+let tempDir: string
+let repoDir: string
+let originalCwdState: string
+let originalCwd: string
+let originalConfigDir: string | undefined
+
+function runGit(cwd: string, args: string[], allowFailure = false): {
+  exitCode: number
+  stdout: string
+  stderr: string
+} {
+  const result = Bun.spawnSync(['git', ...args], {
+    cwd,
+    stdout: 'pipe',
+    stderr: 'pipe',
+  })
+  const stdout = new TextDecoder().decode(result.stdout)
+  const stderr = new TextDecoder().decode(result.stderr)
+  if (!allowFailure && result.exitCode !== 0) {
+    throw new Error(stderr || stdout)
+  }
+  return { exitCode: result.exitCode, stdout, stderr }
+}
+
+function writeRepoFile(relativePath: string, content: string): void {
+  const filePath = join(repoDir, relativePath)
+  mkdirSync(join(filePath, '..'), { recursive: true })
+  writeFileSync(filePath, content)
+}
+
+function commit(message: string): void {
+  runGit(repoDir, ['add', '.'])
+  runGit(repoDir, ['commit', '-m', message])
+}
+
+describe('createAgentWorktree', () => {
+  beforeEach(() => {
+    originalCwdState = getCwdState()
+    originalCwd = getOriginalCwd()
+    originalConfigDir = process.env.CLAUDE_CONFIG_DIR
+
+    tempDir = mkdtempSync(join(tmpdir(), 'cc-haha-agent-worktree-'))
+    repoDir = join(tempDir, 'repo')
+    const originDir = join(tempDir, 'origin.git')
+    mkdirSync(repoDir, { recursive: true })
+
+    process.env.CLAUDE_CONFIG_DIR = join(tempDir, 'claude-config')
+    resetSettingsCache()
+    resetGitFileWatcher()
+
+    runGit(repoDir, ['init', '-b', 'main'])
+    runGit(repoDir, ['config', 'user.email', 'worktree-test@example.com'])
+    runGit(repoDir, ['config', 'user.name', 'Worktree Test'])
+    writeRepoFile('README.md', '# worktree test\n')
+    commit('initial')
+
+    runGit(tempDir, ['init', '--bare', originDir])
+    runGit(originDir, ['symbolic-ref', 'HEAD', 'refs/heads/main'])
+    runGit(repoDir, ['remote', 'add', 'origin', originDir])
+    runGit(repoDir, ['push', '-u', 'origin', 'main'])
+
+    setCwdState(repoDir)
+    setOriginalCwd(repoDir)
+    process.chdir(repoDir)
+  })
+
+  afterEach(() => {
+    process.chdir(originalCwd)
+    setCwdState(originalCwdState)
+    setOriginalCwd(originalCwd)
+    resetGitFileWatcher()
+    resetSettingsCache()
+    if (originalConfigDir === undefined) {
+      delete process.env.CLAUDE_CONFIG_DIR
+    } else {
+      process.env.CLAUDE_CONFIG_DIR = originalConfigDir
+    }
+    rmSync(tempDir, { recursive: true, force: true })
+  })
+
+  test('creates agent worktree branches without upstream tracking config', async () => {
+    const slug = 'agent-race-proof'
+    const { worktreeBranch } = await createAgentWorktree(slug)
+
+    expect(worktreeBranch).toBe(worktreeBranchName(slug))
+
+    const upstreamRemote = runGit(
+      repoDir,
+      ['config', '--get', `branch.${worktreeBranch}.remote`],
+      true,
+    )
+    const upstreamMerge = runGit(
+      repoDir,
+      ['config', '--get', `branch.${worktreeBranch}.merge`],
+      true,
+    )
+
+    expect(upstreamRemote.exitCode).not.toBe(0)
+    expect(upstreamRemote.stdout.trim()).toBe('')
+    expect(upstreamMerge.exitCode).not.toBe(0)
+    expect(upstreamMerge.stdout.trim()).toBe('')
+  })
+})

src/utils/worktree.ts

@@ -357,8 +357,10 @@ async function getOrCreateWorktree(
     addArgs.push('--no-checkout')
   }
   // -B (not -b): reset any orphan branch left behind by a removed worktree dir.
-  // Saves a `git branch -D` subprocess (~15ms spawn overhead) on every create.
+  // Use the resolved SHA rather than origin/<branch>; a remote-tracking start
-  addArgs.push('-B', worktreeBranch, worktreePath, baseBranch)
+  // point makes git write branch.<name>.remote/merge into shared .git/config,
+  // which races when multiple agent worktrees start together.
+  addArgs.push('-B', worktreeBranch, worktreePath, baseSha)
 
   const { code: createCode, stderr: createStderr } =
     await execFileNoThrowWithCwd(gitExe(), addArgs, { cwd: repoRoot })