biome

Guide for testing workflows and code generation commands in Biome. Use when running snapshot tests for lint rules, managing insta snapshots, or regenerating analyzer/parser/formatter code after changes.

Compares code formatting and formatting IR between Biome and Prettier to ensure that Biome's formatting is consistent and correct. Use when validating formatter changes, debugging formatting differences, or comparing output for JavaScript, TypeScript, CSS, or other languages.

General development best practices and common gotchas when working on Biome. Use for avoiding common mistakes, understanding Biome-specific patterns (AST, syntax nodes, string extraction, embedded languages), and learning technical tips.

Step-by-step guide for creating and implementing lint rules in Biome's analyzer. Use when implementing rules like noVar, useConst, or any custom lint/assist rule, adding code actions to fix diagnostics, implementing semantic analysis for binding references, or adding configurable options to rules.

Guide for implementing parsers with error recovery for new languages in Biome. Use when adding parsing support for a new language, implementing error recovery in a parser, or writing grammar definitions in .ungram format for JavaScript, CSS, JSON, HTML, GraphQL, or other languages.

Guide for implementing ESLint-to-Biome rule option migrators inside `biome migrate eslint`. Use whenever you add or update a Biome lint rule that has an ESLint source rule with configurable options, need to deserialize plugin-specific ESLint options, or need custom migration logic beyond the auto-generated severity mapping.

Guide for implementing formatting rules using Biome's IR-based formatter infrastructure. Use when implementing formatting for new syntax nodes, handling comments in formatted output, writing or debugging formatter snapshot tests, diagnosing idempotency failures, or comparing Biome's formatting against Prettier for JavaScript, CSS, JSON, HTML, Markdown, or other languages.

Guide for creating high-quality, user-friendly diagnostics in Biome. Use when creating diagnostics for lint rules, adding helpful advice to error messages, implementing code frame displays, or improving diagnostic quality.

Guide for creating and writing proper changesets for Biome PRs. Use when a PR introduces user-visible changes (bug fixes, new features, rule changes, formatter changes, parser changes) that need a changeset entry for the CHANGELOG. Trigger when creating changesets, writing changeset descriptions, or choosing the correct change type.

Guide for creating proper pull requests for the Biome project. Use when opening a PR, writing a PR description, choosing the correct target branch, or filling out the PR template. Trigger when creating PRs, writing PR summaries, or preparing code for review.

Guide for working with Biome's module graph and type inference system. Use when implementing type-aware lint rules, understanding type resolution, working on the module graph infrastructure, or implementing type inference for new features.