Shared tooling biome

Biome

Quick Guide: Biome is a unified linter, formatter, and import organizer for JavaScript, TypeScript, JSX, TSX, JSON, CSS, and GraphQL. Single Rust binary replaces ESLint + Prettier with 97% Prettier compatibility. Use biome.json for all configuration. Run biome check --write to lint, format, and organize imports in one pass. Use biome ci in pipelines.

Current stable version: Biome v2.4.x (March 2026). Biome v2 introduced type-aware linting, nested configs, and a revamped import organizer.

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<critical_requirements>

CRITICAL: Before Using This Skill

All code must follow project conventions in CLAUDE.md (kebab-case, named exports, import ordering, import type, named constants)

(You MUST use biome.json or biome.jsonc for ALL configuration — Biome does not use JavaScript config files)

(You MUST pin Biome to an exact version with --save-exact — Biome formatting can change between versions)

(You MUST use biome ci in CI pipelines, NOT biome check — ci is read-only with no --write flag)

(You MUST use biome check --write for local development — runs linter, formatter, and import organizer in one pass)

(You MUST include $schema in biome.json for editor autocompletion and validation)

</critical_requirements>

---

Auto-detection: Biome, biome.json, biome.jsonc, @biomejs/biome, biome check, biome lint, biome format, biome ci, biome-ignore, organizeImports, biome migrate

When to use:

• Setting up a unified linter + formatter for JavaScript/TypeScript projects
• Replacing ESLint + Prettier with a single, faster tool
• Greenfield projects wanting zero-config or minimal-config setup
• Large codebases where linting/formatting speed matters
• Configuring import organizing with custom group ordering
• Migrating from ESLint and/or Prettier to Biome
• Setting up CI pipelines with biome ci
• Configuring pre-commit hooks with Biome's --staged flag

When NOT to use:

• Projects requiring ESLint plugins with no Biome equivalent (e.g., custom framework-specific plugins)
• Projects heavily invested in custom ESLint rules that cannot be replicated
• Projects needing Markdown, YAML, or TOML formatting (Biome does not support these yet)
• Runtime code (this is build-time tooling only)
• Git hooks framework setup (Husky/Lefthook configuration is a separate concern)
• TypeScript compiler configuration (tsconfig.json is a separate concern)

Key patterns covered:

• biome.json configuration with formatter, linter, and assist settings
• Linter rule groups (recommended, all, nursery) and severity levels
• Formatter configuration (indent style, line width, quotes, semicolons)
• Import organizer with custom group ordering
• CLI commands: check, format, lint, ci, migrate
• Migration from ESLint + Prettier
• Git hooks integration (Husky, Lefthook, --staged flag)
• CI integration with GitHub Actions and GitLab CI
• Editor integration (VS Code, JetBrains)
• Suppression comments (biome-ignore, biome-ignore-all, range suppressions)
• Nested configuration for monorepos
• Overrides for file-specific settings

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Examples

• Setup & Configuration — Installation, biome.json, VS Code, monorepos, framework configs
• Linting Rules — Rule groups, domains, suppressions, overrides, import ordering
• Formatting — Formatter options, Prettier compatibility, option mapping
• CI & Git Hooks — GitHub Actions, GitLab CI, Husky, Lefthook, staged files
• Migrating from ESLint/Prettier — Automated migration, rule mapping, package cleanup

Other resources:

• CLI Quick Reference — All CLI commands, flags, and configuration option tables

---

<philosophy>

Philosophy

Biome unifies linting, formatting, and import organizing into a single tool with a single configuration file. Built in Rust, it delivers 20x faster performance than ESLint + Prettier while maintaining 97% Prettier compatibility.

Core principles:

1. One tool, one config — biome.json replaces .eslintrc, prettier.config, and import sorting plugins
2. Sensible defaults — Biome works out of the box with recommended rules enabled; zero-config is a valid setup
3. Speed as a feature — Rust-powered binary processes large codebases in milliseconds, not seconds
4. Unified commands — biome check --write runs everything in one pass (lint + format + organize imports)
5. Safe by default — Safe fixes apply automatically; unsafe fixes require explicit --unsafe flag

</philosophy>

---

<patterns>

Core Patterns

Pattern 1: Basic biome.json Configuration

Every Biome project starts with a biome.json at the project root. Use biome init to scaffold one, then customize.

# Install Biome (always pin exact version)
npm install --save-dev --save-exact @biomejs/biome

# Create default biome.json
npx @biomejs/biome init

// biome.json — minimal recommended setup
{
  "$schema": "https://biomejs.dev/schemas/2.4.7/schema.json",
  "vcs": { "enabled": true, "clientKind": "git", "useIgnoreFile": true },
  "formatter": { "indentStyle": "space", "indentWidth": 2, "lineWidth": 100 },
  "linter": { "rules": { "recommended": true } },
  "assist": {
    "enabled": true,
    "actions": { "source": { "organizeImports": "on" } },
  },
}

Why good: $schema enables editor autocompletion, VCS integration respects .gitignore, explicit indentStyle: "space" avoids Biome's tab default, recommended rules provide a strong baseline

// BAD: No schema, no VCS, relying on defaults that differ from Prettier
{ "linter": { "enabled": true } }

Why bad: Missing $schema loses autocompletion, no VCS means linting node_modules, Biome defaults to tabs which surprises Prettier migrants

Full example: See examples/core.md for a production-ready biome.json with overrides, framework configs, and monorepo setup.

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Pattern 2: Linter Rules Configuration

Biome provides 459+ rules across 8 groups (accessibility, complexity, correctness, nursery, performance, security, style, suspicious). Rules default to recommended — a curated subset of safe, stable rules. Severity levels: "off", "on", "info", "warn", "error". See reference.md for the full rule groups and severity tables.

Domains (Technology-Specific Rules)

Biome v2 introduces domains that group rules by technology. Domains auto-detect from package.json dependencies.

{ "linter": { "domains": { "react": "recommended", "test": "recommended" } } }

Full examples: See examples/linting.md for rule configuration, suppressions, overrides, and import ordering.

---

Pattern 3: Formatter Configuration

Biome's formatter achieves 97% Prettier compatibility. Global settings apply to all languages; language-specific settings override globals. Biome defaults to tabs — set indentStyle: "space" explicitly when migrating from Prettier.

{
  "formatter": { "indentStyle": "space", "indentWidth": 2, "lineWidth": 100 },
  "javascript": {
    "formatter": { "quoteStyle": "double", "semicolons": "always" },
  },
  "json": { "formatter": { "trailingCommas": "none" } },
  "css": { "formatter": { "enabled": true } },
}

Full reference: See examples/formatting.md for all options, Prettier mapping, and language-specific settings.

---

Pattern 4: Import Organizer

Biome v2 revamped the import organizer with custom group ordering. Configure under assist.actions.source.organizeImports.

{
  "assist": {
    "actions": {
      "source": {
        "organizeImports": {
          "level": "on",
          "options": {
            "groups": [
              [":BUN:", ":NODE:"],
              ":PACKAGE:",
              ":BLANK_LINE:",
              ["@company/**"],
              ":BLANK_LINE:",
              ":ALIAS:",
              ":PATH:",
              ":BLANK_LINE:",
              { "type": true },
            ],
            "identifierOrder": "natural",
          },
        },
      },
    },
  },
}

Key options: identifierOrder controls named import sorting — "natural" (default, e.g. var1, var2, var11) or "lexicographic" (strict alphabetical). Groups accept predefined matchers, glob patterns ("@my/lib/**"), object matchers ({ "type": true, "source": ["@my/lib"] }), or arrays combining any of these.

Predefined Groups

Group	Matches
:NODE:	Node.js built-ins and node: protocol
:BUN:	Bun-specific modules
:PACKAGE:	Scoped and bare npm packages
:PACKAGE_WITH_PROTOCOL:	Packages with a protocol prefix
:ALIAS:	Aliased imports (@/, #, ~, $, %)
:PATH:	Absolute and relative path imports
:URL:	HTTP/HTTPS imports
:BLANK_LINE:	Visual separator between groups

Full examples: See examples/linting.md for import ordering with results.

---

Pattern 5: CLI Commands

Use check locally, ci in pipelines. Key commands:

npx biome check --write .          # Lint + format + organize imports (auto-fix)
npx biome ci .                     # CI mode (read-only, optimized for pipelines)
npx biome check --staged --write . # Pre-commit hooks (only staged files)
npx biome check --changed --since=main .  # Changed files only

Full reference: See examples/ci.md for complete CLI usage, filtering, and reporting options.

---

Pattern 6: Suppression Comments

Biome uses biome-ignore comments with required explanations.

// biome-ignore lint/suspicious/noDebugger: needed for local debugging
debugger;

// biome-ignore-all lint/style/noDefaultExport: framework requires default export (top of file only)

// biome-ignore-start lint/suspicious/noDoubleEquals: legacy section
// biome-ignore-end lint/suspicious/noDoubleEquals: legacy section

See reference.md for the full suppression syntax table and specifier levels. See examples/linting.md for inline, file-level, and range suppression examples.

---

Pattern 7: Nested Configuration (Monorepos)

Biome v2 supports nested biome.json files. Each subdirectory can override the root config.

// packages/my-app/biome.json — inherits from root
{ "$schema": "https://biomejs.dev/schemas/2.4.7/schema.json", "extends": "//" }

// packages/legacy-lib/biome.json — relaxes rules
{
  "$schema": "https://biomejs.dev/schemas/2.4.7/schema.json",
  "root": false,
  "linter": { "rules": { "suspicious": { "noExplicitAny": "off" } } },
}

Why good: "extends": "//" shorthand inherits root config, "root": false marks as child, each package can relax or tighten rules independently

Full examples: See examples/core.md for root + child config patterns.

---

Pattern 8: Overrides (File-Specific Rules)

Use overrides for file-pattern-specific configuration without nested config files.

{
  "overrides": [
    {
      "includes": ["**/*.test.ts", "**/*.test.tsx"],
      "linter": { "rules": { "suspicious": { "noExplicitAny": "off" } } },
    },
    {
      "includes": ["**/app/**/page.tsx", "**/app/**/layout.tsx"],
      "linter": { "rules": { "style": { "noDefaultExport": "off" } } },
    },
  ],
}

Why good: Overrides eliminate suppression comments in every file, test files get relaxed rules, framework conventions handled declaratively

Full examples: See examples/linting.md for test, config, and generated file overrides.

</patterns>

---

<performance>

Performance

Biome is written in Rust and processes files in parallel, making it significantly faster than JavaScript-based alternatives.

Tool	Format Time (1000 files)	Lint Time (1000 files)
Biome	~100ms	~200ms
Prettier	~2000ms	N/A
ESLint	N/A	~3000ms
ESLint + Prettier	~5000ms	~5000ms

Approximate benchmarks. Actual performance varies by project size and rule configuration.

Performance Tips:

• Use biome check instead of running biome format and biome lint separately — single pass is faster
• Enable VCS integration to automatically skip ignored files (.gitignore)
• Use --staged or --changed in hooks and CI to process only affected files
• Be selective with nursery rules — some experimental rules may impact performance
• Use --profile-rules (v2.4+) to identify slow lint rules in your configuration

Type-Aware Linting Performance:

Biome v2 introduced type-aware linting without the TypeScript compiler. Enable selectively:

• Default scan discovers nested configs only (fast)
• Project domain scan indexes the full module graph (slower, but still faster than tsc)
• Types domain scan adds type inference (most comprehensive, most expensive)

</performance>

---

<decision_framework>

Decision Framework

Biome vs ESLint + Prettier

Need linting and formatting?
|-- Greenfield project?
|   |-- Want simplest possible setup? -> Biome (one tool, one config)
|   |-- Need niche ESLint plugins? -> ESLint + Prettier
|   +-- Speed is important? -> Biome (20x faster)
|-- Existing ESLint + Prettier project?
|   |-- Happy with current setup? -> Stay with ESLint + Prettier
|   |-- Config complexity is a pain? -> Migrate to Biome
|   |-- Need ESLint plugins without Biome equivalents? -> Stay
|   +-- Want faster CI/pre-commit hooks? -> Biome (or hybrid)
+-- Monorepo?
    |-- Need per-package lint configs? -> Biome v2 nested configs or ESLint 10
    +-- Speed bottleneck in CI? -> Biome

Configuration Complexity

How to configure Biome?
|-- Just starting out? -> Run `biome init`, use defaults with recommended: true
|-- Migrating from ESLint? -> Run `biome migrate eslint --write`
|-- Migrating from Prettier? -> Run `biome migrate prettier --write`
|-- Need per-file rules?
|   |-- Different rules for test files? -> Use `overrides` in biome.json
|   +-- Different rules per package? -> Use nested biome.json with "root": false
+-- Need custom import ordering? -> Configure organizeImports.options.groups

Full migration decision tree: See examples/migration.md.

</decision_framework>

---

<integration>

Integration Guide

Works with:

• JSX/TSX: Full support with react domain for React-specific lint rules
• TypeScript: Type-aware linting without tsc dependency (Biome v2+)
• CSS: Linting enabled by default, formatting opt-in
• JSON/JSONC: Full support including tsconfig.json, package.json
• GraphQL: Linting enabled by default, formatting opt-in
• Vue/Svelte/Astro: Experimental support via html.experimentalFullSupportEnabled (v2.4+)

Replaces / Conflicts with:

• ESLint: Biome replaces ESLint for linting (or use hybrid approach)
• Prettier: Biome replaces Prettier for formatting
• eslint-plugin-import: Biome's import organizer replaces import sorting plugins
• eslint-config-prettier: Not needed — Biome has no formatter/linter conflicts

CI/editor integration: See examples/ci.md and examples/core.md.

</integration>

---

<red_flags>

RED FLAGS

High Priority Issues:

• Using biome check in CI instead of biome ci (check allows --write which is dangerous in pipelines; ci is read-only)
• Not pinning Biome version with --save-exact (formatting can change between minor versions, causing diff noise)
• Missing $schema in biome.json (loses editor autocompletion, validation, and discoverability)
• Using JavaScript config files instead of biome.json (Biome only supports JSON/JSONC configuration)
• Running biome format and biome lint separately when biome check does both (wastes time, parses files twice)

Medium Priority Issues:

• Forgetting to set indentStyle: "space" when migrating from Prettier (Biome defaults to tabs)
• Not enabling VCS integration (without it, Biome may process node_modules and dist)
• Using --unsafe without reviewing changes (unsafe fixes can alter program behavior)
• Not including --no-errors-on-unmatched in git hooks (causes failures when no matching files are staged)
• Enabling all nursery rules (they are experimental and may have bugs or performance issues)

Common Mistakes:

• Using "root": true in a nested config (this is the default; use "root": false for child configs)
• Expecting Markdown/YAML formatting support (Biome does not support these languages yet)
• Not running biome migrate --write when upgrading major versions (config schema changes between v1 and v2)
• Suppression comments without explanations (biome-ignore lint: requires text after the colon)

Gotchas & Edge Cases:

• Biome defaults to tabs, not spaces — always set indentStyle explicitly when migrating from Prettier
• CSS and GraphQL formatting is disabled by default — must opt in with "formatter": { "enabled": true }
• biome-ignore-all must be at the top of the file — placing it mid-file triggers an unused suppression warning
• Import organizer is part of assist, not linter — configure under assist.actions.source.organizeImports
• The --staged flag makes lint-staged unnecessary for Biome-only setups (since Biome v1.7.0+)
• Type-aware linting (project/types domains) triggers file scanning which can slow down first runs on large projects
• biome migrate eslint --write does not migrate inspired rules by default — use --include-inspired to include them
• Range suppressions (biome-ignore-start/biome-ignore-end) must have matching rule specifiers
• Biome treats all JS/TS/JSX/TSX under the javascript config key — there is no separate typescript section
• Configuration files named .biome.json (with leading dot) are also discovered (v2.4+)

</red_flags>

---

<critical_reminders>

CRITICAL REMINDERS

All code must follow project conventions in CLAUDE.md (kebab-case, named exports, import ordering, import type, named constants)

(You MUST use biome.json or biome.jsonc for ALL configuration — Biome does not use JavaScript config files)

(You MUST pin Biome to an exact version with --save-exact — Biome formatting can change between versions)

(You MUST use biome ci in CI pipelines, NOT biome check — ci is read-only with no --write flag)

(You MUST use biome check --write for local development — runs linter, formatter, and import organizer in one pass)

(You MUST include $schema in biome.json for editor autocompletion and validation)

Failure to follow these rules will cause inconsistent formatting, broken CI pipelines, and missed lint errors.

</critical_reminders>