Ultracite Rules for Claude Code (as of 2025-08-04)

CLAUDE.md

Project Context

Ultracite enforces strict type safety, accessibility standards, and consistent code quality for JavaScript/TypeScript projects using Biome's lightning-fast formatter and linter.

Key Principles

• Zero configuration required
• Subsecond performance
• Maximum type safety
• AI-friendly code generation

Before Writing Code

1. Analyze existing patterns in the codebase
2. Consider edge cases and error scenarios
3. Follow the rules below strictly
4. Validate accessibility requirements

Rules

Accessibility (a11y)

• Don't use accessKey attribute on any HTML element.
• Don't set aria-hidden="true" on focusable elements.
• Don't add ARIA roles, states, and properties to elements that don't support them.
• Don't use distracting elements like <marquee> or <blink>.
• Only use the scope prop on <th> elements.
• Don't assign non-interactive ARIA roles to interactive HTML elements.
• Make sure label elements have text content and are associated with an input.
• Don't assign interactive ARIA roles to non-interactive HTML elements.
• Don't assign tabIndex to non-interactive HTML elements.
• Don't use positive integers for tabIndex property.
• Don't include "image", "picture", or "photo" in img alt prop.
• Don't use explicit role property that's the same as the implicit/default role.
• Make static elements with click handlers use a valid role attribute.
• Always include a title element for SVG elements.
• Give all elements requiring alt text meaningful information for screen readers.
• Make sure anchors have content that's accessible to screen readers.
• Assign tabIndex to non-interactive HTML elements with aria-activedescendant.
• Include all required ARIA attributes for elements with ARIA roles.
• Make sure ARIA properties are valid for the element's supported roles.
• Always include a type attribute for button elements.
• Make elements with interactive roles and handlers focusable.
• Give heading elements content that's accessible to screen readers (not hidden with aria-hidden).
• Always include a lang attribute on the html element.
• Always include a title attribute for iframe elements.
• Accompany onClick with at least one of: onKeyUp, onKeyDown, or onKeyPress.
• Accompany onMouseOver/onMouseOut with onFocus/onBlur.
• Include caption tracks for audio and video elements.
• Use semantic elements instead of role attributes in JSX.
• Make sure all anchors are valid and navigable.
• Ensure all ARIA properties (aria-*) are valid.
• Use valid, non-abstract ARIA roles for elements with ARIA roles.
• Use valid ARIA state and property values.
• Use valid values for the autocomplete attribute on input elements.
• Use correct ISO language/country codes for the lang attribute.

Code Complexity and Quality

• Don't use consecutive spaces in regular expression literals.
• Don't use the arguments object.
• Don't use primitive type aliases or misleading types.
• Don't use the comma operator.
• Don't use empty type parameters in type aliases and interfaces.
• Don't write functions that exceed a given Cognitive Complexity score.
• Don't nest describe() blocks too deeply in test files.
• Don't use unnecessary boolean casts.
• Don't use unnecessary callbacks with flatMap.
• Use for...of statements instead of Array.forEach.
• Don't create classes that only have static members (like a static namespace).
• Don't use this and super in static contexts.
• Don't use unnecessary catch clauses.
• Don't use unnecessary constructors.
• Don't use unnecessary continue statements.
• Don't export empty modules that don't change anything.
• Don't use unnecessary escape sequences in regular expression literals.
• Don't use unnecessary fragments.
• Don't use unnecessary labels.
• Don't use unnecessary nested block statements.
• Don't rename imports, exports, and destructured assignments to the same name.
• Don't use unnecessary string or template literal concatenation.
• Don't use String.raw in template literals when there are no escape sequences.
• Don't use useless case statements in switch statements.
• Don't use ternary operators when simpler alternatives exist.
• Don't use useless this aliasing.
• Don't use any or unknown as type constraints.
• Don't initialize variables to undefined.
• Don't use the void operators (they're not familiar).
• Use arrow functions instead of function expressions.
• Use Date.now() to get milliseconds since the Unix Epoch.
• Use .flatMap() instead of map().flat() when possible.
• Use literal property access instead of computed property access.
• Don't use parseInt() or Number.parseInt() when binary, octal, or hexadecimal literals work.
• Use concise optional chaining instead of chained logical expressions.
• Use regular expression literals instead of the RegExp constructor when possible.
• Don't use number literal object member names that aren't base 10 or use underscore separators.
• Remove redundant terms from logical expressions.
• Use while loops instead of for loops when you don't need initializer and update expressions.
• Don't pass children as props.
• Don't reassign const variables.
• Don't use constant expressions in conditions.
• Don't use Math.min and Math.max to clamp values when the result is constant.
• Don't return a value from a constructor.
• Don't use empty character classes in regular expression literals.
• Don't use empty destructuring patterns.
• Don't call global object properties as functions.
• Don't declare functions and vars that are accessible outside their block.
• Make sure builtins are correctly instantiated.
• Don't use super() incorrectly inside classes. Also check that super() is called in classes that extend other constructors.
• Don't use variables and function parameters before they're declared.
• Don't use 8 and 9 escape sequences in string literals.
• Don't use literal numbers that lose precision.

React and JSX Best Practices

• Don't use the return value of React.render.
• Make sure all dependencies are correctly specified in React hooks.
• Make sure all React hooks are called from the top level of component functions.
• Don't forget key props in iterators and collection literals.
• Don't destructure props inside JSX components in Solid projects.
• Don't define React components inside other components.
• Don't use event handlers on non-interactive elements.
• Don't assign to React component props.
• Don't use both children and dangerouslySetInnerHTML props on the same element.
• Don't use dangerous JSX props.
• Don't use Array index in keys.
• Don't insert comments as text nodes.
• Don't assign JSX properties multiple times.
• Don't add extra closing tags for components without children.
• Use <>...</> instead of <Fragment>...</Fragment>.
• Watch out for possible "wrong" semicolons inside JSX elements.

Correctness and Safety

• Don't assign a value to itself.
• Don't return a value from a setter.
• Don't compare expressions that modify string case with non-compliant values.
• Don't use lexical declarations in switch clauses.
• Don't use variables that haven't been declared in the document.
• Don't write unreachable code.
• Make sure super() is called exactly once on every code path in a class constructor before this is accessed if the class has a superclass.
• Don't use control flow statements in finally blocks.
• Don't use optional chaining where undefined values aren't allowed.
• Don't have unused function parameters.
• Don't have unused imports.
• Don't have unused labels.
• Don't have unused private class members.
• Don't have unused variables.
• Make sure void (self-closing) elements don't have children.
• Don't return a value from a function with the return type 'void'
• Use isNaN() when checking for NaN.
• Make sure "for" loop update clauses move the counter in the right direction.
• Make sure typeof expressions are compared to valid values.
• Make sure generator functions contain yield.
• Don't use await inside loops.
• Don't use bitwise operators.
• Don't use expressions where the operation doesn't change the value.
• Make sure Promise-like statements are handled appropriately.
• Don't use __dirname and __filename in the global scope.
• Prevent import cycles.
• Don't use configured elements.
• Don't hardcode sensitive data like API keys and tokens.
• Don't let variable declarations shadow variables from outer scopes.
• Don't use the TypeScript directive @ts-ignore.
• Prevent duplicate polyfills from Polyfill.io.
• Don't use useless backreferences in regular expressions that always match empty strings.
• Don't use unnecessary escapes in string literals.
• Don't use useless undefined.
• Make sure getters and setters for the same property are next to each other in class and object definitions.
• Make sure object literals are declared consistently (defaults to explicit definitions).
• Use static Response methods instead of new Response() constructor when possible.
• Make sure switch-case statements are exhaustive.
• Make sure the preconnect attribute is used when using Google Fonts.
• Use Array#{indexOf,lastIndexOf}() instead of Array#{findIndex,findLastIndex}() when looking for the index of an item.
• Make sure iterable callbacks return consistent values.
• Use with { type: "json" } for JSON module imports.
• Use numeric separators in numeric literals.
• Use object spread instead of Object.assign() when constructing new objects.
• Always use the radix argument when using parseInt().
• Make sure JSDoc comment lines start with a single asterisk, except for the first one.
• Include a description parameter for Symbol().
• Don't use spread (...) syntax on accumulators.
• Don't use the delete operator.
• Don't access namespace imports dynamically.
• Don't use namespace imports.
• Declare regex literals at the top level.
• Don't use target="_blank" without rel="noopener".

TypeScript Best Practices

• Don't use TypeScript enums.
• Don't export imported variables.
• Don't add type annotations to variables, parameters, and class properties that are initialized with literal expressions.
• Don't use TypeScript namespaces.
• Don't use non-null assertions with the ! postfix operator.
• Don't use parameter properties in class constructors.
• Don't use user-defined types.
• Use as const instead of literal types and type annotations.
• Use either T[] or Array<T> consistently.
• Initialize each enum member value explicitly.
• Use export type for types.
• Use import type for types.
• Make sure all enum members are literal values.
• Don't use TypeScript const enum.
• Don't declare empty interfaces.
• Don't let variables evolve into any type through reassignments.
• Don't use the any type.
• Don't misuse the non-null assertion operator (!) in TypeScript files.
• Don't use implicit any type on variable declarations.
• Don't merge interfaces and classes unsafely.
• Don't use overload signatures that aren't next to each other.
• Use the namespace keyword instead of the module keyword to declare TypeScript namespaces.

Style and Consistency

• Don't use global eval().
• Don't use callbacks in asynchronous tests and hooks.
• Don't use negation in if statements that have else clauses.
• Don't use nested ternary expressions.
• Don't reassign function parameters.
• This rule lets you specify global variable names you don't want to use in your application.
• Don't use specified modules when loaded by import or require.
• Don't use constants whose value is the upper-case version of their name.
• Use String.slice() instead of String.substr() and String.substring().
• Don't use template literals if you don't need interpolation or special-character handling.
• Don't use else blocks when the if block breaks early.
• Don't use yoda expressions.
• Don't use Array constructors.
• Use at() instead of integer index access.
• Follow curly brace conventions.
• Use else if instead of nested if statements in else clauses.
• Use single if statements instead of nested if clauses.
• Use new for all builtins except String, Number, and Boolean.
• Use consistent accessibility modifiers on class properties and methods.
• Use const declarations for variables that are only assigned once.
• Put default function parameters and optional function parameters last.
• Include a default clause in switch statements.
• Use the ** operator instead of Math.pow.
• Use for-of loops when you need the index to extract an item from the iterated array.
• Use node:assert/strict over node:assert.
• Use the node: protocol for Node.js builtin modules.
• Use Number properties instead of global ones.
• Use assignment operator shorthand where possible.
• Use function types instead of object types with call signatures.
• Use template literals over string concatenation.
• Use new when throwing an error.
• Don't throw non-Error values.
• Use String.trimStart() and String.trimEnd() over String.trimLeft() and String.trimRight().
• Use standard constants instead of approximated literals.
• Don't assign values in expressions.
• Don't use async functions as Promise executors.
• Don't reassign exceptions in catch clauses.
• Don't reassign class members.
• Don't compare against -0.
• Don't use labeled statements that aren't loops.
• Don't use void type outside of generic or return types.
• Don't use console.
• Don't use control characters and escape sequences that match control characters in regular expression literals.
• Don't use debugger.
• Don't assign directly to document.cookie.
• Use === and !==.
• Don't use duplicate case labels.
• Don't use duplicate class members.
• Don't use duplicate conditions in if-else-if chains.
• Don't use two keys with the same name inside objects.
• Don't use duplicate function parameter names.
• Don't have duplicate hooks in describe blocks.
• Don't use empty block statements and static blocks.
• Don't let switch clauses fall through.
• Don't reassign function declarations.
• Don't allow assignments to native objects and read-only global variables.
• Use Number.isFinite instead of global isFinite.
• Use Number.isNaN instead of global isNaN.
• Don't assign to imported bindings.
• Don't use irregular whitespace characters.
• Don't use labels that share a name with a variable.
• Don't use characters made with multiple code points in character class syntax.
• Make sure to use new and constructor properly.
• Don't use shorthand assign when the variable appears on both sides.
• Don't use octal escape sequences in string literals.
• Don't use Object.prototype builtins directly.
• Don't redeclare variables, functions, classes, and types in the same scope.
• Don't have redundant "use strict".
• Don't compare things where both sides are exactly the same.
• Don't let identifiers shadow restricted names.
• Don't use sparse arrays (arrays with holes).
• Don't use template literal placeholder syntax in regular strings.
• Don't use the then property.
• Don't use unsafe negation.
• Don't use var.
• Don't use with statements in non-strict contexts.
• Make sure async functions actually use await.
• Make sure default clauses in switch statements come last.
• Make sure to pass a message value when creating a built-in error.
• Make sure get methods always return a value.
• Use a recommended display strategy with Google Fonts.
• Make sure for-in loops include an if statement.
• Use Array.isArray() instead of instanceof Array.
• Make sure to use the digits argument with Number#toFixed().
• Make sure to use the "use strict" directive in script files.

Testing Best Practices

• Don't use export or module.exports in test files.
• Don't use focused tests.
• Make sure the assertion function, like expect, is placed inside an it() function call.
• Don't use disabled tests.

Common Tasks

• npx ultracite init - Initialize Ultracite in your project
• npx ultracite format - Format and fix code automatically
• npx ultracite lint - Check for issues without fixing