Machine attach local detach ends without a framed outcome

[schickling-assistant]

pty attach --attach-stream-fd-v1 currently closes the inherited machine stream without a terminal protocol outcome when its local user intentionally detaches with Ctrl+\. The command exits successfully, but a consumer sees the same EOF shape as a truncated stream.

An embedded terminal therefore cannot make the correct downstream decision: intentional detach should preserve the session and close the view cleanly, while transport truncation must fail or reconnect because terminal state may be incomplete. Treating both as the same EOF either reports normal user actions as corruption or silently accepts real data loss.

Reproduction

https://github.com/schickling-repros/2026-07-pty-machine-detach-outcome

./repro.sh

The repro uses a real detached session and the shipped bin/pty launcher at exact revision d5fabc3917407aeb937a012bd97679c303e18033.

Expected

The ordered machine stream ends with exactly one framed outcome:

• empty existing DETACH (type 2) for intentional local detach, including detach before the initial baseline; or
• EXIT (type 4) when the session process ends.

EOF after either outcome is clean. EOF without either outcome remains truncation.

Actual

The command exits 0 but fd 3 contains only GEOMETRY, SCREEN (types 10, 5) before EOF.

Proposed fix: #146

Posted on behalf of @schickling

field	value
agent_name	co1-sage
agent_session_id	ed878dac-3735-4276-b3e0-ea1b1cd65291
agent_tool	Codex CLI
agent_tool_version	0.145.0
agent_runtime	Codex CLI 0.145.0
agent_model	unknown
runtime_profile	/nix/store/mnx8agbdq3wiyb6vz63lhgscgazkrn98-coding-agent-runtime-profile/share/coding-agents/profile.json
skills_manifest	/nix/store/5r69m9k2llmri3na81518zx0a7y0d3cn-agent-skills-corpus/share/agent-skills/manifest.json
worktree	pty/schickling-assistant/2026-07-31-machine-detach-outcome
machine	dev3
tooling_profile	dotfiles@0fb7e03