Dev mode: programming MendCode from MendCode

Trying a change to MendCode normally means restarting the daemon, and that cuts the session you are working in and every other one. Dev mode keeps the work in a stable MendCode and tries the change in a second, isolated daemon that restarts alone.

Tier: all (minimalist, balanced, full). It is a command, not a feature switch; nothing of it loads unless you type mendcode dev.

Use

cd path/to/MendCode-checkout        # the stable MendCode runs here, and a session works on the code
mendcode dev start                  # the development daemon: this checkout's source, its own data directory
mendcode dev tui                    # a terminal on it, marked DEV (re-run after a change, or just `dev restart`)
mendcode dev restart                # restart only that daemon; the main daemon and its sessions are untouched
mendcode dev run "say hi"           # a headless run on it, for a session to check a change without a terminal

Also dev stop, dev status and dev keys (below). --dir DIR (before the command) or MENDCODE_DEV_DIR picks the data directory (default ~/.mendcode-dev); --checkout PATH or MENDCODE_DEV_CHECKOUT picks the checkout (default: the git checkout around the current folder, else the one the running program came from).

A session of the stable MendCode may run dev start|restart|stop|status|run|tui|keys from its own shell: they work on another data directory. mendcode dev --dir <main directory> ... and every other way of restarting the main daemon stay refused for a session (permissions).

What runs

dev runs bun apps/cli/src/main.ts of the checkout with --dir DEVDATA, never the installed binary. The daemon it starts is that source's daemon (bun packages/daemon/src/main.ts), so a change shows after dev restart with no build step. A compiled dist/ binary would need a build per try and would drift from the source; Bun is required on PATH (and bun install --frozen-lockfile once in the checkout). dev restart interrupts the turns running in the development daemon (they resume later); it does not ask.

Isolation

• Its own data directory, database, socket (or loopback endpoint) and lock. It never writes the main data directory: dev refuses the main directory, the one a session's shell runs under, and MENDCODE_DIR's.
• No sessions, history, settings or memory are shared. ~/.claude and ~/.codex threads are not offered either (that is already so for any data directory but the default).
• The PR review watch, Dream and loop triggers never run in it, whatever its settings say. (Loops can be created; they do not fire.)
• The terminal shows DEV in the session top bar (also when it is narrow) and next to the version on the home screen. The mark follows the data directory: use mendcode dev tui, not a bare --dir.
• mendcode daemon restart on the main daemon does not touch it, and dev restart does not touch the main one.

Login

• API-key providers: providers.json of the main data directory is copied once, only its providers, with mode 0600 (the directory 0700); nothing is printed. dev keys copies it again after you change a key. Keys in environment variables (OPENAI_API_KEY...) reach the development daemon as they reach any process.
• ChatGPT and GitHub Copilot sign-ins are not copied: their refresh token rotates every time it is used, so two copies would sign each other out. Run /connect once in the development terminal.
• Claude Code as an engine uses your own claude and its own login, as in the main daemon. MendCode never calls the Claude API with a subscription login.
• Settings are not copied: choose a model with /model the first time.

Before this existed

Any version with --dir DATA can do the same by hand:

mendcode --dir ~/.mendcode-dev daemon start                      # a daemon per data directory
bun apps/cli/src/main.ts --dir ~/.mendcode-dev daemon restart    # from the checkout: the source, not the installed binary
bun apps/cli/src/main.ts --dir ~/.mendcode-dev                   # its terminal

Never pass the main directory, and run the checkout's main.ts, not mendcode, so the daemon is the code you changed. Without dev mode there is no DEV mark and the background jobs follow that directory's settings (a new directory has them off).

For contributors

1. Open the stable, installed MendCode in the repository and ask it to make the change.
2. Test it with mendcode dev tui (or let the session use mendcode dev run "...").
3. After each edit, mendcode dev restart. Only the development daemon restarts; your working session keeps going.
4. bun scripts/test-fast.ts --all --jobs 3 and the typecheck still decide whether the change is done (CONTRIBUTING).