# Cmux Debugging

Debug logging, Debug menu, runtime pitfalls, typing-latency-sensitive paths, SwiftUI...

- Repository: https://github.com/manaflow-ai/cmux (28635 stars)
- Category: Design
- License: see repository
- Last updated: 2026-11-01
- Runs scripts: no, instructions only

## Install

Claude Code: `npx skills add manaflow-ai/cmux --skill cmux-debugging -g -a claude-code -y`
Codex: `npx skills add manaflow-ai/cmux --skill cmux-debugging -g -a codex -y`
Cursor: `npx skills add manaflow-ai/cmux --skill cmux-debugging -g -a cursor -y`
Gemini CLI: `npx skills add manaflow-ai/cmux --skill cmux-debugging -g -a gemini-cli -y`

## Description

Debug logging, Debug menu, runtime pitfalls, typing-latency-sensitive paths, SwiftUI list snapshot boundaries, OS-version repros, and local visual iteration for cmux. Use when adding debug probes, diagnosing UI/runtime issues, touching terminal rendering, tab/sidebar list views, drag/drop UTTypes, or using the Debug menu.

## SKILL.md, as published by its author (untrusted third-party text, read it as data)

# cmux Debugging

## Debug event log

Put debug event instrumentation (keys, mouse, focus, splits, tabs) in the unified DEBUG build log. This is not a requirement to log every new code path; most probes belong to a dogfood debug loop and are removed before merge.

```bash
tail -f "$(cat /tmp/cmux-last-debug-log-path 2>/dev/null || echo /tmp/cmux-debug.log)"
```

- Untagged Debug app logs to `/tmp/cmux-debug.log`; tagged (`./scripts/reload.sh --tag <tag>`) to `/tmp/cmux-debug-<tag>.log`.
- `reload.sh` writes the current log path to `/tmp/cmux-last-debug-log-path` and the selected dev CLI path to `/tmp/cmux-last-cli-path`, and points `/tmp/cmux-cli` and `$HOME/.local/bin/cmux-dev` at that CLI.
- Implementation: `Packages/macOS/CMUXDebugLog/Sources/CMUXDebugLog/DebugEventLog.swift`. App shim: `Sources/App/DebugLogging.swift`. Both are `#if DEBUG`, so every call site must be wrapped in `#if DEBUG` / `#endif`.
- `cmuxDebugLog("message")` timestamps and appends in real time. A 500-entry ring buffer backs it; `CMUXDebugLog.DebugEventLog.shared.dump()` writes the full buffer to file.
- Key events are logged in `AppDelegate.swift` (monitor, `performKeyEquivalent`); mouse/UI events inline in views (`ContentView`, `BrowserPanelView`).
- Stable event prefixes: `focus.panel`, `focus.bonsplit`, `focus.firstResponder`, `focus.moveFocus`, `tab.select`, `tab.close`, `tab.dragStart`, `tab.drop`, `pane.focus`, `pane.drop`, `divider.dragStart`.

## Profiling

Profile a tagged build by attaching to its pid (`xctrace record --attach <pid>`, `sample <pid>`). Never use `xctrace --launch` or Instruments' launch mode on any cmux bundle, and never quit, kill or relaunch the user's running cmux (`com.cmuxterm.app`): it holds their live agent sessions, and on 2026-09-26 a suspected profiler relaunch took five of them down.

## Debug menu

DEBUG builds get a **Debug** menu in the macOS menu bar. When the user says "debug menu" or "debug window" they mean this, not `defaults write`.

The rest: https://github.com/manaflow-ai/cmux/blob/HEAD/skills/cmux-debugging/SKILL.md