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Luke Fryer
Luke Fryer

Posted on • Originally published at aipromptarchitect.co.uk

The Ultimate .cursorrules Blueprint: Stop Fighting Your AI Code Editor

#nextjs #ai #webdev #developer

If you're using Cursor, Windsurf, or any AI code editor without a .cursorrules file, you're leaving 80% of the value on the table.

I've spent the last 6 months refining my .cursorrules for a production Next.js/React/Firebase app. Here's the complete blueprint.

Why .cursorrules Matters

Without configuration, AI code editors:

  • Generate code that doesn't match your architecture
  • Use outdated patterns (class components, var declarations)
  • Ignore your project's conventions
  • Produce inconsistent code across sessions

A .cursorrules file is a system prompt for your entire codebase. It tells the AI who you are, what patterns to follow, and what to avoid.

The 7-Section Blueprint

1. Project Identity 

You are an expert developer on a Next.js 14 app with TypeScript, 
Firebase (Firestore + Auth + Functions), and Stripe.
The app is a SaaS platform for prompt engineering.
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2. Architecture Rules 

# Architecture
- Use App Router exclusively (no Pages Router)
- Server Components by default, 'use client' only when needed
- All API calls go through server actions or API routes
- Firebase Admin SDK for server-side, Client SDK for client-side
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3. Code Style 

# Code Style
- TypeScript strict mode, no 'any' types
- Functional components with arrow syntax
- Custom hooks for shared logic (prefix: use)
- Barrel exports from feature directories
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4. Naming Conventions 

# Naming
- Components: PascalCase (UserProfile.tsx)
- Hooks: camelCase with 'use' prefix (useAuth.ts)
- Utils: camelCase (formatDate.ts)
- Types: PascalCase with 'I' prefix for interfaces
- Constants: SCREAMING_SNAKE_CASE
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5. Critical Constraints 

# NEVER
- Never use 'any' type — use 'unknown' + type guards
- Never deploy all Firebase functions at once
- Never commit API keys or secrets
- Never use default exports (except pages)
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6. Error Handling 

# Error Handling
- All async functions must have try/catch
- Use custom error classes (AppError, ValidationError)
- Log errors with structured metadata
- Return user-friendly error messages, log technical details
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7. Testing & Quality 

# Testing
- Unit tests with Vitest for utils and hooks
- Component tests with Testing Library
- E2E tests with Playwright for critical paths
- Minimum 80% coverage for new code
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Real-World Example: Before & After

Before .cursorrules (what Cursor generates):

export default function Page() {
  const [data, setData] = useState<any>(null)

  useEffect(() => {
    fetch('/api/data').then(res => res.json()).then(setData)
  }, [])

  return <div>{data?.name}</div>
}
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After .cursorrules (what Cursor generates):

'use client'

import { useQuery } from '@tanstack/react-query'
import { fetchUserData } from '@/services/userService'
import { UserData } from '@/types/user'
import { LoadingSpinner } from '@/components/ui/LoadingSpinner'
import { ErrorBoundary } from '@/components/ui/ErrorBoundary'

export const UserProfilePage: React.FC = () => {
  const { data, isLoading, error } = useQuery<UserData>({
    queryKey: ['userData'],
    queryFn: fetchUserData,
  })

  if (isLoading) return <LoadingSpinner />
  if (error) return <ErrorBoundary error={error} />

  return (
    <section aria-label="User Profile">
      <h1>{data.displayName}</h1>
    </section>
  )
}
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Night and day difference. Same editor, same model, completely different output quality.

Pro Tips

  1. Version control your .cursorrules — treat it like code, iterate on it
  2. Include negative examples — "NEVER do X" is more effective than "always do Y"
  3. Add project-specific context — mention your actual libraries, not generic ones
  4. Review AI output weekly — update rules based on recurring mistakes
  5. Keep it under 500 lines — too long and the AI loses focus

Resources

The complete guide with a downloadable template: The Ultimate .cursorrules Blueprint for Next.js

For the underlying framework these rules are built on: Best Prompt Engineering Practices 2026

If you want the STCO framework that structures these system prompts: STCO Framework Complete Guide

Drop your best .cursorrules tips in the comments — what rules made the biggest difference for your project? 👇

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