Gate agent startup on the effective PTY cwd after default fallback

Co-authored-by: Orca <help@stably.ai>

src/main/providers/local-pty-provider.test.ts

@@ -82,6 +82,7 @@ vi.mock('../wsl', () => ({
 }))
 
 import { LocalPtyProvider } from './local-pty-provider'
+import { isRootLikePath } from './pty-path-safety'
 import { POWERLEVEL10K_WIZARD_DISABLE_ENV } from '../pty/powerlevel10k-wizard-env'
 
 describe('LocalPtyProvider', () => {
@@ -228,14 +229,15 @@ describe('LocalPtyProvider', () => {
       )
     })
 
-    it('rejects automatic agent startup without an explicit cwd', async () => {
+    it('falls back to the safe default cwd for automatic agent startup without an explicit cwd', async () => {
       spawnMock.mockClear()
 
-      await expect(provider.spawn({ cols: 80, rows: 24, command: 'codex' })).rejects.toThrow(
-        /requires a non-root workspace/
-      )
+      // Why: an omitted cwd resolves to a guaranteed-safe default home (the
+      // guard only rejects root-like paths), so the agent must still launch.
+      await expect(provider.spawn({ cols: 80, rows: 24, command: 'codex' })).resolves.toBeDefined()
 
-      expect(spawnMock).not.toHaveBeenCalled()
+      const spawnCall = spawnMock.mock.calls.at(-1)!
+      expect(isRootLikePath(spawnCall[2].cwd)).toBe(false)
     })
 
     it('rejects automatic agent startup at POSIX root', async () => {

src/main/providers/local-pty-provider.ts

@@ -343,12 +343,15 @@ export class LocalPtyProvider implements IPtyProvider {
     const startupAgentRecognition = args.command
       ? recognizeAgentProcessFromCommandLine(args.command)
       : null
-    if (args.command && startupAgentRecognition) {
-      assertSafeAgentStartupCwd(args.cwd, args.command)
-    }
 
     const defaultCwd = getDefaultCwd()
     const cwd = args.cwd || defaultCwd
+    // Why: gate on the effective cwd (post default-cwd fallback), not the raw
+    // args.cwd — an omitted cwd resolves to a safe default and must not be
+    // rejected as if it were a root-like path.
+    if (args.command && startupAgentRecognition) {
+      assertSafeAgentStartupCwd(cwd, args.command)
+    }
     const wslInfo = process.platform === 'win32' ? parseWslPath(cwd) : null
     const worktreeWslContext =
       process.platform === 'win32' ? getWslContextFromWorktreeId(args.worktreeId) : undefined

src/main/providers/pty-default-cwd.ts

@@ -22,6 +22,12 @@ export function resolveSafePtyDefaultCwd(env: NodeJS.ProcessEnv = process.env):
   // Why: silently falling back to "/" or a drive root is the exact runaway-CPU
   // bug this module prevents (see runaway-cpu-hidden-usage-pty-design.md) — fail loud.
   if (!selected) {
+    // Log the rejected candidates so the rare "no safe home at all" environment
+    // (e.g. HOME=/ or unset in a minimal container) is diagnosable, not opaque.
+    console.error(
+      '[pty-default-cwd] No safe default working directory for terminal launch; every home candidate resolved to a root-like path.',
+      { platform: process.platform, candidates }
+    )
     throw new Error('No safe default working directory is available for terminal launch.')
   }
   return selected