Desktop (Electron) sandbox instances

make dev + make start (Electron) uses MUX_ROOT for persisted state (config, sessions, worktrees, etc.). Running multiple Electron instances against the same mux root is noisy and risky during development.

This skill documents the repo workflow for starting multiple desktop dev instances in parallel (including from different git worktrees) by giving each instance its own temporary MUX_ROOT.

Quick start

make dev-desktop-sandbox

What it does

• Creates a fresh temporary MUX_ROOT directory
• Copies these files into the sandbox if present (unless disabled by flags):
  • providers.jsonc (provider config)
  • config.json (project list)
  • Each file is seeded independently from the first root that has it ($MUX_ROOT, then ~/.mux-dev, then ~/.mux), so a root with only config.json doesn't drop provider config
• Provider credential env vars are stripped from the child processes' env when they could silently override or mismatch the intended setup: all of them with --clean-providers (including Bedrock's AWS_REGION and AWS_BEARER_TOKEN_BEDROCK; shared AWS credentials like AWS_PROFILE are kept); otherwise only *_BASE_URL env vars that would shadow a seeded providers.jsonc entry that has an apiKey but no explicit baseUrl (API key env vars are always kept so env-key fallback still works)
• Picks free ports:
  • Vite devserver port (used by the renderer)
  • Electron remote debugging port (optional)
• Disables tutorials by default inside the sandbox (MUX_ENABLE_TUTORIALS_IN_SANDBOX=1 opts back in)
• Runs make dev with:
  • MUX_ROOT=<temp>
  • MUX_VITE_PORT=<free-port>
• Waits for Vite to be reachable, then runs make build-static (Electron expects dist/splash.html)
• Launches Electron (bunx electron .) with:
  • MUX_ROOT=<temp>
  • MUX_DEVSERVER_HOST=127.0.0.1
  • MUX_DEVSERVER_PORT=<vite-port>
  • MUX_SERVER_PORT=0 by default (avoids EADDRINUSE if your config.json pins apiServerPort)
  • CMUX_ALLOW_MULTIPLE_INSTANCES=1 (so you can run alongside another dev instance)

Agent usage with bash.monitor

When launching an Electron sandbox for dogfooding, prefer a monitored background bash so Mux wakes the workspace on Vite/Electron readiness or startup failures without manual polling.

bash({
  script: "make dev-desktop-sandbox",
  display_name: "Desktop Sandbox",
  run_in_background: true,
  timeout_secs: 1800,
  monitor: {
    filter: "Vite|ready|localhost|Electron|ERROR|EADDRINUSE|failed|Failed",
    cooldown_ms: 1000,
    max_events: 5,
  },
});

After a readiness wake, use the sandbox output/ports shown in the matched logs to connect with the Electron or agent-browser workflow. Use task_await only when the wake line is not enough context.

Options

# Start with a clean instance (do not copy providers or projects)
make dev-desktop-sandbox DEV_DESKTOP_SANDBOX_ARGS="--clean-providers --clean-projects"

# Skip copying providers.jsonc
make dev-desktop-sandbox DEV_DESKTOP_SANDBOX_ARGS="--clean-providers"

# Clear projects from config.json (preserves other config)
make dev-desktop-sandbox DEV_DESKTOP_SANDBOX_ARGS="--clean-projects"

# Use a specific root to seed from (default: per-file from $MUX_ROOT, ~/.mux-dev, ~/.mux)
SEED_MUX_ROOT=~/.mux-dev make dev-desktop-sandbox

# Keep the sandbox root directory after exit (useful for debugging)
KEEP_SANDBOX=1 make dev-desktop-sandbox

# Pin Vite port
VITE_PORT=5174 make dev-desktop-sandbox

# Control how long we wait for Vite to come up (ms)
VITE_READY_TIMEOUT_MS=120000 make dev-desktop-sandbox

# Re-enable tutorials for sandbox dogfooding
MUX_ENABLE_TUTORIALS_IN_SANDBOX=1 make dev-desktop-sandbox

# Enable/pin Electron remote debugging port (defaults to an auto-picked free port)
ELECTRON_DEBUG_PORT=9223 make dev-desktop-sandbox

# Disable Electron remote debugging entirely
ELECTRON_DEBUG_PORT=0 make dev-desktop-sandbox

# Override the internal API server port (defaults to 0/random for sandboxes)
MUX_SERVER_PORT=3772 make dev-desktop-sandbox

# Override which make binary to use
MAKE=gmake make dev-desktop-sandbox

Optional: deeper Electron isolation (MUX_E2E=1)

Even with a unique MUX_ROOT, Electron's userData directory (localStorage, window state, single-instance lock, etc.) is not automatically relocated unless MUX_E2E=1 is set.

If you want full isolation (including userData), run:

MUX_E2E=1 make dev-desktop-sandbox

Security notes

• providers.jsonc may contain API keys.
• The sandbox root directory is created on disk (usually under your system temp dir).
• This flow intentionally does not copy secrets.json.