The project's linter and formatter

The project chooses its own lint rules and its own format. This tool is whatever already lints and formats its code, found in the project before anything is run.

Finding it

Look where the project itself checks its code, in this order:

1. CI: the lint and format-check jobs. They are the commands the project trusts.
2. The project's task file: a lint or format script, target or task.
3. The config files: eslint.config.js, ruff.toml, .golangci.yml, .prettierrc and the like.
4. The dependencies: the linter and formatter the project installs, with their versions.

When these disagree, CI wins: it decides whether a change is merged.

Using it

• On changed files while working, fast enough for every edit; on everything in CI.
• The formatter writes locally and only checks in CI: prettier --write against prettier --check, ruff format against ruff format --check, gofmt -w against gofmt -l.
• The pinned version: the same version locally and in CI, or the two disagree about the same code.
• Custom rules carry their fix in the message, so whoever reads the finding knows what to change.

Examples: ESLint (JavaScript), Ruff (Python), Clippy (Rust), golangci-lint (Go); Prettier (JavaScript and more), Black (Python), gofmt (Go).