Auto-format all changed files with one command, for your AI agent

The problem: AI agents waste tokens on formatting

As I principle, I auto-format everything possible. When AI agents (Codex/Cursor/Claude Code/Goose) write code, I want their code to also be autoformatted. If it isn’t git pre-commit hooks will fail and the LLM’s will waste time and tokens.

For the last few months, I’ve had an AGENTS.md file with explicit instructions on how to auto-format code.

Auto-format code using the following commands:

- Kotlin: `ktlint -F ...`
- Ruby: `bin/run bundle exec rubocop -A ...`
- JavaScript: `npx standard --fix ...`
- ERB: `bin/run bundle exec erb_lint -a --lint-all ...`
- CSS, Markdown, JSON: `npx prettier --write`
- I18n: `npm run i18n:fix`

But I don’t want to teach my AI agent which linter to use for which file type. And I want all my linters to run in parallel.

The solution: ffx

fast-format-x (ffx) is a single command that auto-formats every changed file using the correct formatter for that file type. Written in Rust for speed. All formatters run in parallel.

Replace all those formatting instructions in your AGENTS.md with one line:

Run `ffx` to auto-format every changed file (it runs the correct formatter for each file)

How it works

Install ffx and then run ffx init to get a config in your repo, .fast-format-x.yaml:

version: 1

tools:
  - name: prettier
    include: ['**/*.md', '**/*.yml', '**/*.yaml', '**/*.js', '**/*.ts']
    cmd: npx
    args: [prettier, --write]
    check_args: [prettier, --check]

  - name: rubocop
    include: ['**/*.rb', '**/*.rake']
    cmd: bundle
    args: [exec, rubocop, -A]
    check_args: [exec, rubocop]

  - name: gofmt
    include: ['**/*.go']
    cmd: gofmt
    args: [-w]
    check_args: [-l]

Then run ffx. It finds changed files, matches them to tools, and runs the right formatter on each file. All in parallel.

Installation

From fast-format-x:

curl -LsSf https://ffx.bfoos.net/install.sh | bash
ffx init  # Install pre-commit hook and create starter config

Usage

ffx              # Format changed files
ffx --staged     # Format staged files only
ffx --all        # Format all matching files
ffx --all --check  # CI mode: check without modifying

Why this matters for AI

AI agents are great at writing code. They shouldn’t need to think about which formatter to call for which file type. That’s a solved problem.

One command. One line in your AGENTS.md. No wasted time or tokens.