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Cursor rules for Rails

Cursor rules are a markdown file with a short frontmatter header. Cursor attaches them to the agent so it stops guessing how this repo builds, tests, and styles code. This copy is written for a Rails project. This repository is a Ruby on Rails app. Follow the conventions already in the repo: MVC, migrations, and strong parameters. Do not introduce a second frontend stack.

The example uses a balanced, readable style, pragmatic tests, and comments only where the intent is not obvious. Open the generator to make the agent stricter, switch to test-first, quiet the comments, or pick a different file.

The example assumes a Rails app that uses migrations and Active Record. It does not assume Hotwire, React, or a particular auth gem. If the repo has chosen one, stay with it.

Customize in the generatorBrowse the library

Example file

Balanced strictness, readable style, pragmatic tests. Download name: project.mdc. Legacy path: .cursorrules

---
description: "Working rules for a Rails repository"
globs: "**/*"
alwaysApply: true
---

# Project rules

Follow these rules on every task in this Rails repository.
If the project still uses a root `.cursorrules` file, keep the same rules there.

## Stack

This repository is a Ruby on Rails app. Follow the conventions already in the repo: MVC, migrations, and strong parameters. Do not introduce a second frontend stack.

## Layout

- Controllers stay thin. Business rules live in models or plain objects the project already uses.
- Views and Turbo or Hotwire frames stay with the stack the app already renders.
- Schema changes are migrations. Do not edit `schema.rb` by hand.

## Commands

- Dev server: `bin/rails server`
- Tests: `bin/rails test`
- Migrate: `bin/rails db:migrate`

Use the script the repo already defines when it differs from the command above.

## Conventions

- Use strong parameters. Do not pass `params` straight into `update`.
- Name files the way Zeitwerk expects. Do not fight the autoloader with a custom require.
- Add an index when you query a new column in a hot path.
- Use the existing test style, whether that is Minitest or RSpec.
- Keep credentials in the encrypted credentials store or the env. Do not commit secrets.
- Prefer framework generators' layout over a new folder taxonomy.

## Testing

Add or update tests when behavior changes.

Add a model or request test for the behavior you changed. Cover the forbidden parameter case.

## Style

- Optimize for the next reader over the shortest diff.
- Comment only when the why is non-obvious.
- Name things the way neighboring files already do.
- Do not reformat code you did not change.

## Errors

- Fail at the boundary. Do not swallow errors.
- Use model validations and a 4xx response for bad input. Unexpected errors belong in the Rails logger.

## Scope

- Do the requested task. Do not refactor unrelated code.
- Match the rigor already in the repo. Fix what you touch.
- Ask one clarifying question only when the task cannot be finished without it. Otherwise state the assumption and continue.

## Avoid

- Do not call `update_attribute` to skip validations unless the existing code does that for the same reason.
- Do not introduce callbacks that hide a side effect the caller cannot see.
- Do not query in a loop. Use `includes` when the view walks an association.
- Do not add a SPA framework to a Hotwire app without being asked.

## Done when

- The requested behavior works.
- `bin/rails test` passes, and `db:migrate` applies cleanly when the schema changed.
- The diff stays inside the task.
- You list anything you could not run.

Add it to the project

  1. 01

    Copy a starting file

    Copy the example Cursor rules on this page, or open the generator if you want different testing, strictness, or comment rules.

  2. 02

    Save it at .cursor/rules/project.mdc

    Save it at `.cursor/rules/project.mdc` in the Rails repo and create the folders if they are missing. If the project still uses one root rules file, save the body as `.cursorrules`.

  3. 03

    Start a fresh agent session

    Rules files are read when a session starts. A chat that is already open may still be using the old instructions.

  4. 04

    Give it one small task

    Ask for one change a Rails agent often gets wrong, such as a controller action that permits only the intended parameters. If it ignores the file, confirm the path and start a new session.

Questions about this file

Where do I put Cursor rules in a Rails repo?

Save it at `.cursor/rules/project.mdc` in the Rails repo and create the folders if they are missing. If the project still uses one root rules file, save the body as `.cursorrules`.

Which tools read this Cursor rules?

Cursor reads `.cursor/rules/*.mdc` and, on older setups, a root `.cursorrules` file. Other tools ignore the frontmatter, so use AGENTS.md when more than one agent edits the repo.

What does the Rails example assume?

The example assumes a Rails app that uses migrations and Active Record. It does not assume Hotwire, React, or a particular auth gem. If the repo has chosen one, stay with it.

Does VibeSpell upload my repository?

No. Recipe pages are static, and the generator builds the file in your browser from the options you pick. Your source code never leaves the machine.

Other files for Rails

Cursor rules for other stacks