Homebrew Skill

Declare a project's Homebrew dependencies in a Brewfile and drive them with brew bundle. This skill is the single source of truth for Homebrew conventions across every coding agent; harness-specific rules load it rather than repeating it.

When to Use

• A project needs system-level tools that aren't available from its language package manager (pkg-config, ffmpeg, postgresql, a cask)
• A README or setup script tells contributors to run brew install <x>
• Adding install/upgrade automation for system dependencies
• Reviewing an existing Brewfile for convention drift

Core Rule

If a project uses Homebrew, its dependencies belong in a Brewfile — never in loose brew install commands. A brew install foo bar baz line in a README, setup script, CI workflow, or justfile is undeclared state: nobody can diff it, verify it, or remove it later. A Brewfile is a manifest, so it can be reviewed, version-controlled, and checked.

Replace this:

brew install pkg-config ffmpeg
brew install --cask ngrok

With a Brewfile:

brew "pkg-config"
brew "ffmpeg"
cask "ngrok"

Process

Step 1: Detect Homebrew Usage

Look for undeclared brew dependencies before assuming there are none:

ls Brewfile .Brewfile 2>/dev/null
grep -rn "brew install" --include=*.md --include=*.sh --include=*.yml . 2>/dev/null

If brew install appears anywhere but no Brewfile exists, that's the gap this skill closes.

Step 2: Write the Brewfile

Place Brewfile in the repository root. Entry types:

Conventions:

• One dependency per line, grouped by type with tap first, then brew, then cask, then the rest. Keep each group alphabetized so diffs stay readable.
• Comment any entry whose purpose isn't obvious from its name — a bare brew "pkg-config" tells the next reader nothing about which build needs it.
• Declare only what the project needs. A contributor's personal tooling (shell, editor, fonts) belongs in their dotfiles Brewfile, not the repo's.
• Do not pin versions. Homebrew is a rolling-release package manager with no general version-pinning mechanism; brew "node@20" works only because a versioned formula happens to exist. If a project needs pinned tool versions, that's a job for mise, asdf, or the language's own toolchain file — not Homebrew.

Step 3: Wire Up the Commands

Three behaviors that are easy to get wrong:

• brew bundle install upgrades by default. Plain brew bundle install runs brew upgrade on anything outdated, so a command that reads like "install my dependencies" can silently move every version. Pass --no-upgrade whenever the intent is only "make sure these are present", and keep upgrading a separate, deliberate command. brew bundle upgrade is shorthand for brew bundle install --upgrade.
• brew bundle check exits 1 when a dependency is missing and 0 when everything is satisfied. That makes it a real CI gate — a non-zero exit here is a genuine failure, not just information.
• brew bundle cleanup uninstalls. It removes every Homebrew package not listed in the Brewfile, which on a developer's machine means their personal tools. Never run it against a project Brewfile on a shared or personal machine; it is only safe where the Brewfile is the complete intended state (a container, a CI runner, or a dotfiles repo).

Step 4: Scope — Project vs. Global

brew bundle resolves its Brewfile in three ways. State the scope explicitly rather than relying on "whatever is in the current directory":

• In a project repo, target the project Brewfile. It declares what this codebase needs to build and test.
• In a dotfiles repo, --global is correct and intentional: it manages the machine's own baseline toolchain, which every directory then inherits.
• HOMEBREW_BUNDLE_FILE overrides the default path for every brew bundle invocation. It is convenient interactively and a trap in automation — a recipe that works on your machine breaks on a machine where that variable points somewhere else. In automation, pass --file explicitly.

Step 5: Brewfile.lock.json Is Obsolete

Older Homebrew wrote a Brewfile.lock.json pinning resolved versions. Current Homebrew no longer generates it. Verified on Homebrew 6.0.18: brew bundle install produces no lockfile, and neither brew bundle --help nor brew bundle install --help mentions locking at all.

So a Brewfile.lock.json sitting in a repo today is a frozen snapshot from an older Homebrew that will never be refreshed again. It silently drifts from the Brewfile as entries are added and removed, which makes it worse than absent: a reader may reasonably assume it reflects current state.

When you find one:

• Confirm it is actually stale — compare its entry counts against the Brewfile, and check when git last touched each.
• If stale, delete it and gitignore the path. The Brewfile is the manifest; nothing reads the lockfile.
• Never hand-edit it, and never regenerate it by hand to "fix" the drift. There is no supported command that rewrites it.

Do not treat this as a reason to avoid Homebrew for reproducibility — it is a rolling-release package manager and never offered real version pinning. If a project needs reproducible tool versions, use mise, asdf, a container, or Nix instead of trying to freeze Homebrew.

Step 6: Verify

1. brew bundle check --file=Brewfile — confirms the Brewfile parses and reports what's missing (exit 1 if anything is).
2. brew bundle list --file=Brewfile — confirms every entry is recognized.
3. Report what was declared or changed.

Integration with Just

When a project has both a Brewfile and a justfile, wire them together following the justfile skill's lifecycle-recipe conventions:

# Installs system dependencies from the Brewfile
[group('configuration')]
deps-brew:
    brew bundle install --file=Brewfile --no-upgrade

# Upgrades system dependencies declared in the Brewfile
[group('configuration')]
upgrade-brew:
    brew bundle upgrade --file=Brewfile

# Verifies declared system dependencies are installed
[group('checks')]
check-brew:
    brew bundle check --file=Brewfile

Homebrew has no brew bundle outdated subcommand. brew outdated reports every outdated package on the machine, not just the project's, so it does not belong in a project outdated recipe — see the justfile skill's rule against machine-global package managers in project justfiles.

Do NOT

• Leave brew install commands in a README, setup script, or CI workflow when a Brewfile could declare them.
• Run bare brew bundle install where the intent is install-only; it upgrades.
• Run brew bundle cleanup against a project Brewfile on a personal machine.
• Run machine-wide brew upgrade or brew outdated from a project justfile.
• Pin formula versions in a Brewfile; use a real version manager instead.
• Add a contributor's personal tooling to a project Brewfile.
• Hand-edit, regenerate, or trust a Brewfile.lock.json; current Homebrew no longer writes one, so any that exists is stale.