Statusline shows model and context% from other sessions

Summary

The statusline command receives stale cross-session data. When I switch to Sonnet in one session, all other sessions' statuslines update to show "Sonnet" and that session's context%, even though they're still running Opus with different context usage.

Reproduction

1. Open two Claude Code sessions (two terminals)
2. Both start on Opus 4.6 – statuslines show correct model and context%
3. In session B, run /model sonnet
4. Session A's statusline now shows "Sonnet" and session B's context%

Session A never changed models. Its statusline should still show Opus.

Expected

Each session's statusline reflects its own model and context window usage.

Actual

The statusline input JSON (model and context_window.used_percentage) appears to come from whichever session most recently triggered an update, not the session the statusline belongs to.

Environment

• Claude Code 2.1.81
• tmux with multiple panes, each running a separate claude session
• Custom statusline_command in settings.json (Python script reading from stdin)

Notes

The statusline script itself is stateless – it reads JSON from stdin and prints. The cross-contamination happens upstream in whatever feeds the JSON to the command. The used_percentage field is also null (not 0) when a session has no messages yet, which crashes scripts that do int(round(used_percentage)) – but that's a separate issue.