Ultracite for Claude Code

Use Ultracite with Claude Code when you want an agentic terminal workflow to respect the same code standards your team expects from human contributors. It is a strong fit for repo-wide edits, review loops, and autonomous tasks where consistency matters as much as speed.

Setup details

These values come from the same data Ultracite uses when it creates or updates Claude Code instructions, so the page matches what actually gets written into the repo.

Configuration file

. claude/CLAUDE.md

Ultracite writes to this exact file when you initialize or update the agent integration.

Write mode

Append to existing instructions

Existing instructions stay in place and Ultracite appends its rules where the agent expects them.

Header handling

Writes rules directly

The generated file starts directly with Ultracite rules and skips an extra heading block.

Hook support

Ships with a post-edit hook

.claude/settings.json runs npm run fix -- --skip=correctness/noUnusedImports after supported file edits.

Files Ultracite writes for Claude Code

This preview shows the exact default file content Ultracite writes for Claude Code when you use the standard Biome setup.

Ultracite Code Standards

This project uses Ultracite, a zero-config preset that enforces strict code quality standards through automated formatting and linting.

Quick Reference

  • Format code: npx ultracite fix
  • Check for issues: npx ultracite check
  • Diagnose setup: npx ultracite doctor

Biome (the underlying engine) provides robust linting and formatting. Most issues are automatically fixable.

Core Principles

Write code that is accessible, performant, type-safe, and maintainable. Focus on clarity and explicit intent over brevity.

Type Safety & Explicitness

  • Use explicit types for function parameters and return values when they enhance clarity
  • Prefer unknown over any when the type is genuinely unknown
  • Use const assertions (as const) for immutable values and literal types
  • Leverage TypeScript's type narrowing instead of type assertions
  • Use meaningful variable names instead of magic numbers - extract constants with descriptive names

Modern JavaScript/TypeScript

  • Use arrow functions for callbacks and short functions
  • Prefer for...of loops over .forEach() and indexed for loops
  • Use optional chaining (?.) and nullish coalescing (??) for safer property access
  • Prefer template literals over string concatenation
  • Use destructuring for object and array assignments
  • Use const by default, let only when reassignment is needed, never var

Async & Promises

  • Always await promises in async functions - don't forget to use the return value
  • Use async/await syntax instead of promise chains for better readability
  • Handle errors appropriately in async code with try-catch blocks
  • Don't use async functions as Promise executors

React & JSX

  • Use function components over class components
  • Call hooks at the top level only, never conditionally
  • Specify all dependencies in hook dependency arrays correctly
  • Use the key prop for elements in iterables (prefer unique IDs over array indices)
  • Nest children between opening and closing tags instead of passing as props
  • Don't define components inside other components
  • Use semantic HTML and ARIA attributes for accessibility:
    • Provide meaningful alt text for images
    • Use proper heading hierarchy
    • Add labels for form inputs
    • Include keyboard event handlers alongside mouse events
    • Use semantic elements (<button>, <nav>, etc.) instead of divs with roles

Error Handling & Debugging

  • Remove console.log, debugger, and alert statements from production code
  • Throw Error objects with descriptive messages, not strings or other values
  • Use try-catch blocks meaningfully - don't catch errors just to rethrow them
  • Prefer early returns over nested conditionals for error cases

Code Organization

  • Keep functions focused and under reasonable cognitive complexity limits
  • Extract complex conditions into well-named boolean variables
  • Use early returns to reduce nesting
  • Prefer simple conditionals over nested ternary operators
  • Group related code together and separate concerns

Security

  • Add rel="noopener" when using target="_blank" on links
  • Avoid dangerouslySetInnerHTML unless absolutely necessary
  • Don't use eval() or assign directly to document.cookie
  • Validate and sanitize user input

Performance

  • Avoid spread syntax in accumulators within loops
  • Use top-level regex literals instead of creating them in loops
  • Prefer specific imports over namespace imports
  • Avoid barrel files (index files that re-export everything)
  • Use proper image components (e.g., Next.js <Image>) over <img> tags

Framework-Specific Guidance

Next.js:

  • Use Next.js <Image> component for images
  • Use next/head or App Router metadata API for head elements
  • Use Server Components for async data fetching instead of async Client Components

React 19+:

  • Use ref as a prop instead of React.forwardRef

Solid/Svelte/Vue/Qwik:

  • Use class and for attributes (not className or htmlFor)

Testing

  • Write assertions inside it() or test() blocks
  • Avoid done callbacks in async tests - use async/await instead
  • Don't use .only or .skip in committed code
  • Keep test suites reasonably flat - avoid excessive describe nesting

When Biome Can't Help

Biome's linter will catch most issues automatically. Focus your attention on:

  1. Business logic correctness - Biome can't validate your algorithms
  2. Meaningful naming - Use descriptive names for functions, variables, and types
  3. Architecture decisions - Component structure, data flow, and API design
  4. Edge cases - Handle boundary conditions and error states
  5. User experience - Accessibility, performance, and usability considerations
  6. Documentation - Add comments for complex logic, but prefer self-documenting code

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