What happened?

When in Plan Mode, calling write_file to create the plan file fails with a sandbox error:

Error checking existing file: Sandbox Error: read_file failed for '...plans/cross-project-session-cleanup.md'. Exit code 71. Details: sandbox-exec: execvp() of '__read' failed: No such file or directory

The issue is that write_file apparently tries to read_file the target path first (to check if it exists), and this fails when the parent directory doesn't exist yet. In Plan Mode, run_shell_command is blocked (read-only tools only), so you can't mkdir -p to create the parent directory either.

This creates a deadlock: you can't exit Plan Mode without creating the plan file, but you can't create the plan file because its parent directory doesn't exist and you can't create it.

Reproduction steps:

1. Enter Plan Mode via enter_plan_mode
2. Attempt to write a plan file to a path where the parent directory doesn't yet exist (e.g., the session's plans/ subdirectory hasn't been created)
3. write_file fails with the sandbox error above
4. run_shell_command to mkdir -p is blocked: "You are in Plan Mode with access to read-only tools"
5. exit_plan_mode requires the plan file to exist first
6. Agent is stuck — user must manually exit plan mode

What did you expect to happen?

Either:

• write_file should handle the case where the parent directory doesn't exist (it claims to "automatically create missing parent directories")
• Or Plan Mode should allow creating the plans directory structure
• Or exit_plan_mode should not require the plan file to exist (e.g., allow exiting with user-approved plans from the chat context)

Client information

Anything else we need to know?

The write_file tool description says it "automatically creates missing parent directories," but the sandbox read_file pre-check fails before it gets to the mkdir step. The error code 71 (EPROTOTYPE / sandbox violation) suggests the sandbox policy doesn't allow the read check on a non-existent path in that directory tree.

The workaround is for the user to manually type "I exited plan mode for you" to unblock the agent.