`pty attach` needs an explicit attach-only/no-restart mode

[schickling-assistant]

pty attach currently has two behaviors for retained dead-session metadata:

• --auto-restart restarts immediately.
• without that flag, it prompts Restart? [Y/n], where every response except exactly n restarts the stored command.

That leaves no explicit attach-only CLI mode for a relay or supervisor that must connect only while the existing daemon is alive and must never execute stored launch metadata. A TTY-backed relay can remain at the prompt after its target dies; later input intended for the target is then consumed as the answer and can restart it.

Minimal reproduction

https://github.com/schickling-repros/2026-07-pty-attach-only-mode/tree/schickling-assistant/2026-07-30-pty-attach-only-mode

PTY_BIN="$(nix build --no-link --print-out-paths \
  github:compoundingtech/pty/face607b443b3ff35e934434a28f575822392aa8)/bin/pty" \
  ./repro.sh

The repro uses a synthetic command and a fresh temporary PTY_ROOT. It shows the command execution count changing from 1 to 2 after future relay input reaches the dead-session prompt, despite --auto-restart being absent.

Desired contract

Add an explicit attach-only mode, for example:

pty attach --no-restart <session>

Under that mode:

• a currently running daemon is attached normally;
• a missing, exited, or vanished session exits nonzero without prompting;
• if the attached daemon exits, the client exits and never evaluates stored command metadata.

The library-level attach() API already has the desired after-connect behavior. The missing boundary is in the CLI's pre-attach dead-session path.

Related but distinct work

• Bind client effects to an immutable daemon incarnation #121 concerns binding effects to an immutable daemon incarnation. That would strengthen selection against replacement races, but does not provide a no-restart CLI policy.
• feat(restart): refuse to restart a stateful agent session unless --force #73 guards explicit pty restart for stateful agents. This gap occurs through pty attach before any explicit restart command.

Version

• PTY main: face607b443b3ff35e934434a28f575822392aa8 (2026-07-27)
• Linux

Posted on behalf of @schickling

field	value
agent_name	co2-bear
agent_session_id	a078daee-6f98-4916-91a8-d21291407789
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	2026-07-pty-attach-only-mode/main
machine	dev3
tooling_profile	dotfiles@0fb7e03