[CLI][TUI][Linux] SSH disconnect leaves idle TUI holding the thread writer lock; resume fails with "already has an active writer" until the old process is killed

What version of Codex CLI is running?

codex-cli 0.153.4 (npm @openai/codex), reproduced 2026-09-09

What subscription do you have?

API-key authentication through a Responses-compatible provider; no ChatGPT subscription is involved in this reproduction.

Which model were you using?

deepseek-v4-flash

What platform is your computer?

Environment:

• Client: Windows Terminal (Windows 10, built-in OpenSSH ssh.exe), plain SSH session; no ServerAliveInterval configured on the client
• Host: Ubuntu 24.04.4 LTS (x86_64)
• codex-cli 0.153.4 installed via the @openai/codex npm package
• node v22.22.2 / npm 10.9.7
• Remote login shell: bash; no tmux/screen/zellij, no sudo su, no VS Code

What terminal emulator and version are you using (if applicable)?

No response

Codex doctor report

What issue are you seeing?

If the SSH connection drops while the Codex TUI is running (even when it is
idle and the last turn is complete), the TUI process keeps running and keeps
holding the thread's writer lock. The server never notices the half-open TCP
connection (no ClientAliveInterval), so the process can survive for hours.
A later codex resume from a new SSH session fails with:

Error: Failed to resume session from ~/.codex/sessions//rollout-<...>-.jsonl:
thread/resume failed during TUI bootstrap: thread/resume failed:
thread already has an active writer (code -32600)

What steps can reproduce the bug?

1. From Windows Terminal, run ssh <user>@<host> and cd into a project
   directory on the remote host.
2. Run codex, complete one turn, and leave the TUI idle at the prompt.
3. Abruptly kill the SSH client or drop the network.
4. Reconnect and run codex resume (or resume from the TUI picker).
5. Observe the "already has an active writer" failure.

What is the expected behavior?

No response

Additional information

This is the simplest possible repro of the "terminal disconnect leaves an
active writer" lifecycle problem: plain SSH, no editor integration, no wrapper
PTY chain. Configuring sshd ClientAliveInterval is only a server-side
workaround and does not help users who need to take over a thread from another
session while the stale process is still alive.
Related: #39345, #37403, #38297, #12515

Start by reproducing the plain SSH disconnect while the TUI is idle, then trace the codex resume path through thread/resume and TUI bootstrap. The issue names no source files or tests, so locate the lifecycle handling for the disconnected TUI and its thread writer lock. Done means a later resume from a new SSH session no longer fails with "already has an active writer".