# Ultracite for Replit Agent

Use Ultracite with Replit Agent when you want browser-based app generation to stay aligned with the same standards you expect in a maintained codebase. It is especially useful for full-stack products that start fast, but still need reviewable, team-friendly code as they evolve.

## Setup details

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

### Configuration file

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

No separate hook file

Replit Agent does not use a separate hook configuration in Ultracite.

## Files Ultracite writes for Replit Agent

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

Markdownreplit.md

```
# 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

---

Most formatting and common issues are automatically fixed by Biome. Run `npx ultracite fix` before committing to ensure compliance.
```

## Best for

These are the workflows where Ultracite adds the most leverage to Replit Agent, based on how the agent reads instructions and how teams typically wire it into day-to-day development.

1. **Browser-based full-stack building**  
   Give Replit Agent a durable repo contract before it generates frontend, backend, and configuration code in the same workspace.

2. **Prototypes that need guardrails**  
   Keep early product iterations aligned with your team's typing, accessibility, and architecture expectations.

3. **Shared cloud workspaces**  
   Use one committed `replit.md` file so collaborators get the same coding guidance every time they open the project.

## Why this setup works for Replit Agent

These differentiators come from the way Replit Agent actually handles repo instructions, file updates, and AI-assisted development work.

- **Workspace-native instructions**  
   Replit Agent already expects repo guidance in `replit.md`, so Ultracite can fit naturally into the browser workflow without introducing another config surface.
- **Fast prototype-to-product loop**  
   Replit is strong for building quickly in the browser, and explicit repo rules help generated code stay maintainable once the project grows beyond a demo.
- **Collaborative workspace fit**  
   Because Replit projects are often shared across teammates, a committed rules file helps keep the coding contract stable between sessions and handoffs.

## Install in seconds. Run in milliseconds.

Install Ultracite and start shipping code faster in seconds.