The pane name that vanished on reattach — why tmux -e is a trap for identity

The symptom

Fresh build from main. The founder asks his Codex and Claude panes: "너 세션명 뭐니?" (what's your session name?). The pane badge says nova-mole-4900. But Codex runs echo $DALKKAK_SESSION_NAME and gets nothing; Claude answers "unknown". The UI knows the name; the agent inside the pane doesn't.

The cause — and the tell

The name reached panes one way: tmux new-session -A -D -e DALKKAK_SESSION_NAME=.... tmux's -e sets the environment only when a session is created. But Talkak runs a persistent -L dalkkak tmux server so panes survive app restarts — and new-session -A reattaches an existing session without re-applying -e. Those panes were created before the change, so reattaching them never delivered the new variable.

The tell was that DALKKAK_PANE_ID did work (Claude's wrapper fired, which is guarded on it). Same -e line — but DALKKAK_PANE_ID was added to it long ago, so old sessions already carried it. One var present, its sibling on the very same line absent: that's the fingerprint of an env added after the session was born.

This is the nastiest shape of bug: it doesn't fail loudly, it fails half-configured. The badge works, one env works, one doesn't. It looks like a typo, not an architecture problem.

The fix

Stop injecting identity through a create-only channel.

• pty::spawn now writes ~/Library/Application Support/DalkkakAI/sessions/<pane_id> = the name on every spawn.
• The Claude wrapper and the Codex ~/.codex/AGENTS.md block read the name from that file via the stable $DALKKAK_PANE_ID (the env stays only as a fallback).
• And ensure_codex_agents now replaces its marked block instead of skip-if-present — otherwise existing users keep the old instruction forever and never get the file-read one.

The pane id is stable and reliably present; the file is rewritten every spawn; so the name is correct whether the session is brand new or reattached from last week.

Verified — including in the real app

Code: cargo check 0, inline tests 5/5, and a wrapper dry-run that reads the name from the file with no DALKKAK_SESSION_NAME set (file → the name; file missing → env fallback; both gone → "unknown").

Then the proof that actually mattered: the founder's Codex pane ran cat "$HOME/Library/Application Support/DalkkakAI/sessions/$DALKKAK_PANE_ID" and answered nova-mole-4900 — matching its badge. The mechanism I could only dry-run, his machine confirmed end to end.

The lesson

An identity injected through a create-only channel — a tmux -e, a one-shot env, a first-run write — silently rots on any long-lived or reattached session. Bind identity to a stable key (the pane id), read from a source that is rewritten every spawn (a file), not to a value that only lands once. And idempotent installers must replace, not skip: skip-if-present is how existing users get stranded on stale instructions across updates.