Ultracite

Zero-config linting and formatting for JS/TS projects. Supports three linter backends: Oxlint + Oxfmt (recommended), Biome, and ESLint + Prettier + Stylelint.

Detecting Ultracite

Check if ultracite is in package.json dependencies or devDependencies. Detect the active linter by looking for (searching upward from the current directory):

• biome.json / biome.jsonc → Biome
• eslint.config.* (.mjs, .js, .cjs, .ts, .mts, .cts) → ESLint (with Prettier for formatting)
• oxlint.config.ts, oxlint.config.mts or .oxlintrc.json → Oxlint (with oxfmt.config.ts or oxfmt.config.mts for formatting; init writes the .mts form in a package that isn't an ES module)

CLI Commands

# Check for issues (read-only)
bunx ultracite check

# Auto-fix issues
bunx ultracite fix

# Diagnose setup problems
bunx ultracite doctor

# Initialize in a new project
bunx ultracite init

Replace bunx with npx, pnpm exec, or yarn depending on the package manager, so the project's installed Ultracite runs (pnpm dlx and yarn dlx download the latest release instead, and Yarn 1 has no dlx). Only init in a project without Ultracite needs a download runner such as pnpm dlx.

check and fix accept optional file paths: bunx ultracite check src/index.ts. Unknown options are passed through to the underlying linter (e.g. bunx ultracite check --max-warnings 0).

Initialization

bunx ultracite init runs an interactive setup. For non-interactive (CI) use, pass flags:

bunx ultracite init \
  --pm bun \
  --linter biome \
  --editors universal \
  --agents claude copilot \
  --frameworks react next \
  --integrations husky lint-staged \
  --yes

Flags:

• --pm — npm | yarn | pnpm | bun | deno | nub | aube
• --linter — oxlint (recommended) | biome | eslint
• --editors — universal (writes .vscode/settings.json for every VS Code-based editor) | vscode | cursor | devin | codebuddy | antigravity | bob | kiro | trae | void | zed
• --agents — every choice writes the rules to AGENTS.md. universal writes only that; claude, gemini, aider and replit also add the import, settings or copy that agent needs to read them; IDs like codex, copilot, cline, amp and cursor-cli (40 agents supported) need nothing more
• --frameworks — react | next | solid | vue | svelte | qwik | remix | tanstack | angular | astro | nestjs | jest | vitest | cypress | storybook
• --integrations — husky | lefthook | lint-staged | pre-commit
• --hooks — Enable auto-fix hooks: claude | codex | copilot | cursor | devin | codebuddy | deepagents | grok
• --js-plugins — Opt-in Oxlint JS plugins: @shadcn/lint | anti-slop | eslint-plugin-github | eslint-plugin-jsdoc | eslint-plugin-sonarjs | eslint-plugin-tsdoc | oxlint-plugin-react-doctor
• --type-aware — Enable type-aware linting (ESLint and Biome: extend their type-aware preset; Oxlint: installs oxlint-tsgolint and sets typeAware), and turn on strictNullChecks in tsconfig files
• --install-skill — Install the reusable Ultracite skill after setup
• --skip-install — Skip dependency installation
• --yes, -y — Take the defaults without prompting (automatic when CI=true or there's no terminal); defaults to oxlint when --linter is omitted
• --quiet — Only print warnings and errors

Init creates config that extends Ultracite presets:

// biome.jsonc
{ "extends": ["ultracite/biome/core", "ultracite/biome/react"] }

// eslint.config.mjs — arrays of flat configs, spread together
import core from "ultracite/eslint/core";
import react from "ultracite/eslint/react";
export default [...core, ...react];

// oxlint.config.ts — imports passed to extends
import { defineConfig } from "oxlint";
import core from "ultracite/oxlint/core";
export default defineConfig({
  env: core.env,
  extends: [core],
  ignorePatterns: core.ignorePatterns,
});

Presets available per linter (ultracite/<linter>/<preset>): core, react, next, solid, vue, svelte, qwik, remix, tanstack, angular, astro, nestjs, jest, vitest, cypress, storybook (cypress and storybook only have rules on ESLint; on Oxlint and Biome they're empty). ESLint and Biome also have type-aware, the rules that need type information, and ESLint has a <framework>/type-aware add-on for angular, jest, nestjs, qwik, tanstack and vitest, extended after that framework's preset. Oxlint also has js-plugins (eslint-plugin-github, eslint-plugin-sonarjs and oxlint-plugin-react-doctor run through oxlint's JS plugin support, with next/js-plugins and tanstack/js-plugins adding framework-specific React Doctor rules), anti-slop and shadcn. These are opt-in: init only adds them when picked in its prompt or passed with --js-plugins. Oxlint and ESLint also have gdp, for codebases that use gdp-ts authorization proofs (@gdp-ts/core ships the plugin); init never adds it, so extend it by hand after core.

Migrating an Existing Project

Migrating means moving the project's own ESLint, Prettier, Stylelint, Biome or Oxlint configuration onto Ultracite's presets; the project can keep its linter. Run init (it preselects the linter it finds), carry over rules the presets don't cover after Ultracite's presets, and run ultracite fix to format everything in one commit. Then fix what check still reports, or migrate progressively:

• Turn off each failing rule after Ultracite's presets: in rules for Oxlint (ultracite check -f json lists them as plugin(rule), written plugin/rule), or linter.rules and assist.actions.source for Biome (ultracite check --reporter=summary). On ESLint, ultracite check --suppress-all records today's errors in eslint-suppressions.json instead.
• Turn them back on a few at a time: remove an override (or prune with ultracite check --prune-suppressions), run ultracite fix, fix the rest, one rule per change.

Full steps: https://www.ultracite.ai/docs/migrate.md

Upgrading

npx ultracite@latest upgrade

upgrade installs the latest Ultracite (or the release --version 7 names), reinstalls the linter at the versions that release supports, and runs doctor. Major releases also change how the configuration looks; upgrade lists the guides the project still needs, and with --claude or --codex it has that agent CLI apply them.

To apply them yourself, follow each guide that applies, oldest first. Each one says how to tell whether the project needs it:

• references/upgrades/v5.md: Biome 1's config format to Biome 2's
• references/upgrades/v6.md: the single ultracite preset to ultracite/core plus framework presets
• references/upgrades/v7.md: ultracite/<preset> to ultracite/biome/<preset>, and the lint and format commands to check and fix
• references/upgrades/v8.md: removing prettier/prettier from ESLint and Stylelint configs, the Cypress and Storybook presets, ESLint configs that set type-checked rules, Oxlint's typeAware option, init --yes, and Zed's formatter on ESLint

Code Standards

When writing code in a project with Ultracite, follow these standards. For the full rules reference, see references/code-standards.md.

Key rules at a glance:

Formatting is handled by the project's configured linter/formatter. Respect the repository's existing formatter settings instead of forcing one fixed line width, quote style, or trailing comma policy.

Type safety: Use explicit types when they improve clarity. Prefer unknown over any. Use as const for immutable values and rely on type narrowing over blunt assertions.

Modern JavaScript/TypeScript: Prefer const, destructuring, optional chaining, nullish coalescing, template literals, for...of, and concise arrow functions.

Async and correctness: Always await promises in async functions. Prefer async/await over promise chains. Remove console.log, debugger, and alert from production code.

React and accessibility: Use function components, keep hooks top-level with correct deps, avoid nested component definitions, and use semantic HTML with the right labels, headings, alt text, and keyboard affordances.

Organization, security, performance, and testing: Keep functions focused, prefer early returns, avoid dangerouslySetInnerHTML and eval(), prefer specific imports and top-level regex, and keep tests free of .only and .skip.

Troubleshooting

Run bunx ultracite doctor to diagnose. It checks:

1. Linter and formatter installation (Biome; or ESLint + Prettier + Stylelint; or Oxlint + oxfmt)
2. Config validity (extends the ultracite presets correctly)
3. Ultracite in package.json dependencies
4. Conflicting tools when not using the ESLint backend (legacy .eslintrc.* files, .prettierrc.*/prettier.config.*)

Common fixes:

• Conflicting configs: After moving to Oxlint or Biome, delete leftover .eslintrc.* and .prettierrc.* files. On ESLint, .eslintrc.* files are inert (ESLint 10 only reads eslint.config.*) and .prettierrc.* is the formatter's config, so neither is a conflict
• Missing dependency: Run bunx ultracite init again or manually add ultracite to devDependencies
• Rules not applying: Ensure config file extends the correct presets for your framework