If you're using Cursor and you don't have a .cursorrules file, you're leaving performance on the table. If you have one and it's a copy-paste from the internet, you might be making things worse.
What .cursorrules does
.cursorrules is a markdown file in your project root that Cursor reads at the start of every session. It tells the AI how to work with your specific codebase — your tech stack, coding conventions, testing requirements, architectural patterns.
Without it, Cursor uses generic knowledge from its training data. With it, Cursor knows your project's rules before you start typing.
Common mistakes
1. Too vague
"Write clean code. Follow best practices. Use TypeScript."
This tells Cursor nothing it doesn't already know. Be specific: "Use Zod for runtime validation at API boundaries. Use TypeScript strict mode. Prefer Map over Record for dynamic keys."
2. Too long
A 5,000-word .cursorrules file is counterproductive. The AI has limited context. Every token spent on instructions is a token not spent on understanding your actual code. Aim for 500-1500 words. Be concise.
3. Contradictory rules
"Always add comprehensive error handling" + "Keep functions simple and short" creates a tension the AI resolves unpredictably. Prioritize: "Add error handling at API boundaries and database calls. Internal functions can trust their inputs."
4. Copy-pasted from someone else's project
A React frontend's .cursorrules doesn't help a Go backend. Rules should reflect YOUR codebase, YOUR conventions, YOUR patterns. Generic rules add noise.
5. Never updated
Your .cursorrules should evolve with your codebase. If you added a new pattern three months ago and your rules still describe the old one, Cursor is getting stale instructions.
How to structure a good .cursorrules
Start with these sections:
Tech stack — What frameworks, libraries, and tools you use. Be specific about versions if they matter.
Code style — Naming conventions, file organization, import ordering. Things that make a diff look clean.
Architecture patterns — How you structure features, where business logic lives, how data flows.
Testing — What to test, how to test it, what test framework you use, what mocking patterns to follow.
What to avoid — Anti-patterns specific to your codebase. "Don't use class components." "Don't import from the internal package directly."
When to ask — Tell Cursor when to stop and ask you instead of guessing. "If an architectural decision is ambiguous, ask before proceeding."
The optimization gap
Here's what most developers don't realize: you can measure whether your .cursorrules is actually improving Cursor's output.
Take a set of common tasks in your codebase. Run them with your current .cursorrules. Run them without. Score the results on specific criteria: Does the code follow your conventions? Does it handle errors correctly? Would you merge this PR without changes?
If the difference isn't significant, your rules aren't working. If you can't tell which output used the rules, they're noise.
At Presient Labs, we do this systematically. Submit your .cursorrules, we run it through our pipeline — evolutionary optimization, 3 blind judges, binary pass/fail criteria. You get back an optimized version with a report card showing the exact before/after scores.
$25. One time. Works on any .cursorrules file, including premium ones you bought somewhere else. If the optimized version doesn't beat your original by at least 10 percentage points under blind testing, automatic refund.
We're not selling you rules. We're making your rules better — and proving it.