gen-commit-msg

gen-commit-msg is a Go CLI that generates Git commit message candidates from staged changes using OpenCode, then lets you pick the best one in a terminal UI (or outputs one message in non-interactive mode).

The tool is built for editor-first commit workflows: generate inside the commit message editor, review, then edit before finalizing the commit.

Why this tool exists

Many AI commit-message tools are either:

• tied to prepare-commit-msg hooks (runs every commit, even when you do not want generation), or
• wrappers that run git commit for you.

This project intentionally does neither.

• No Git hook integration.
• No commit execution.
• You keep your existing flow (git commit, lazygit, editor buffer).

gen-commit-msg only generates message text and writes it to stdout (or a file via --output).

Features

• Collects structured staged-change context via Git before prompting AI.
• Starts and stops opencode serve automatically (with health checks).
• Creates and reuses a dedicated OpenCode agent prompt file.
• Interactive progress view for pipeline steps in TTY mode.
• Subject selection list when multiple candidates are generated.
• Non-interactive mode support (--subject-max 1) for scripts/editor callbacks.
• Config via flags and GCM_* environment variables.
• Optional output file mode (--output / GCM_OUTPUT).

Agent installation and customization

On startup, the tool ensures an OpenCode agent file exists.

• Default agent name: gen-commit-msg
• Default install mode: if-not-exists
• Installed path: ${XDG_CONFIG_HOME:-$HOME/.config}/opencode/agents/gen-commit-msg.md

This file contains the agent configuration and generation instructions used for commit message creation.

You can customize it when needed:

• adjust model settings (for example model name, temperature, steps),
• tune or replace the prompt/instructions for how commit messages are generated.

Important behavior:

• --install-agent if-not-exists (default) keeps your custom file intact.
• --install-agent always overwrites the file with the built-in default prompt.
• --install-agent no skips installation entirely.

Requirements

• Go 1.26+
• git available in PATH
• opencode available in PATH
• Run inside a Git repository
• Have staged changes (git add ...) before running

Install

Build from source:

make build

Binary will be created as ./gen-commit-msg.

Quick start

Generate up to 5 candidates and choose one:

gen-commit-msg

Generate exactly one candidate (no selection list):

gen-commit-msg --subject-max 1

Write result to a file instead of stdout:

gen-commit-msg --output /tmp/commit-msg.txt

Then use your normal commit command:

git commit

Editor workflow (Vim example)

This utility is designed to be called from a commit message buffer.

The snippet below runs gen-commit-msg, captures output to a temp file, and inserts the generated message at the top of the current gitcommit buffer.

let s:gen_commit_msg_height = 5
let s:gen_commit_msg_subject_max = 5

function! <SID>GenCommitMsgCallback(orig_bufnr, term_bufnr, tmpfile, job, status)
    if a:status == 0
        if filereadable(a:tmpfile) && getfsize(a:tmpfile) > 0
            let l:output = readfile(a:tmpfile)
            call appendbufline(a:orig_bufnr, 0, l:output)
        endif
    endif
    call delete(a:tmpfile)
    if a:status == 0 || a:status == 130
        execute 'silent! bwipeout! ' . a:term_bufnr
    else
        echohl WarningMsg | echo 'Command failed with status: ' . a:status | echohl None
    endif
endfunction

function! <SID>GenCommitMsg()
    let l:tmpfile = tempname()
    let l:cmd = 'gen-commit-msg --subject-max ' . shellescape(s:gen_commit_msg_subject_max) . ' --output ' . shellescape(l:tmpfile)
    let l:orig_bufnr = bufnr('%')
    execute 'topleft ' . s:gen_commit_msg_height . 'split | enew'
    let l:Callback = function('<SID>GenCommitMsgCallback', [l:orig_bufnr, bufnr('%'), l:tmpfile])
    call term_start([&shell, &shellcmdflag, l:cmd], {'curwin': 1, 'exit_cb': l:Callback})
endfunction

augroup GitCommitMapping
    autocmd!
    autocmd FileType gitcommit nnoremap <buffer> <leader>O :call <SID>GenCommitMsg()<CR>
augroup END

Press <leader>O in a commit buffer to generate candidates and insert the selected message.

CLI options

Flag precedence is:

CLI flag > environment variable > default

Flag	Env var	Default	Description
--subject-min, -m	GCM_SUBJECT_MIN	1	Minimum subject candidate count
--subject-max, -x	GCM_SUBJECT_MAX	5	Maximum subject candidate count (max 20)
--body	GCM_BODY	true	Generate commit body
--quiet, -q	GCM_QUIET	false	Hide progress view
--agent, -a	GCM_AGENT	gen-commit-msg	OpenCode agent name
--install-agent	GCM_INSTALL_AGENT	if-not-exists	always, if-not-exists, no
--pause	GCM_PAUSE	on-error	Pause before exit: on, off, on-error
--output, -o	GCM_OUTPUT	""	Write selected message to file
--log-level, -l	GCM_LOG_LEVEL	none	trace, debug, info, warn, error, none
--log-file	GCM_LOG_FILE	stderr	Log destination (- for stdout)
--version, -V	—	—	Print version and exit
--help, -h	—	—	Print help and exit

Additional runtime env vars:

• NO_COLOR: disables selection color styling.
• GCM_TUI_SELECTION_COLORS: set to 0 to disable selection color styling.

How it works

Pipeline (interactive mode):

1. Collect staged-change context with multiple Git commands.
2. Ensure OpenCode agent prompt exists.
3. Start OpenCode server and verify readiness.
4. Create OpenCode session.
5. Request structured commit candidates (JSON schema-constrained).
6. Show candidate subjects; user selects one.
7. Cleanup session/server and print (or write) selected message.

The staged context includes summaries (--name-status, --stat, --numstat, --summary, --dirstat) and full staged patch (git diff --cached ...).

Non-interactive behavior

• If stdout is not a TTY and --subject-max > 1: exits with an error and suggests --subject-max 1.
• If stdout is not a TTY and --subject-max == 1: generates silently and prints one message (or writes to --output).

Exit codes

• 0: success (including "no staged files" early exit)
• 1: runtime error
• 2: flag/config parsing error

Development

Common commands:

make fmt
make vet
make lint
make test
make build
make all

Single package tests:

go test -count=1 -race ./internal/config/

Project structure

cmd/gen-commit-msg/main.go    # CLI orchestration
internal/config/              # flags + env parsing
internal/git/                 # repo checks + staged context collection
internal/agent/               # OpenCode agent prompt management
internal/server/              # opencode serve lifecycle
internal/opencode/            # OpenCode session/prompt client
internal/tui/                 # progress + selection Bubble Tea UI
internal/logging/             # slog setup and custom trace level

License

MIT (see LICENSE).