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.cursor/rules/security.mdc

Cursor security rule for T3

A focused Cursor `.mdc` rule for auth, secrets, and untrusted input. It attaches when those files change, instead of repeating security policy in every chat. This copy is written for a T3 project. This repository is a T3 app: Next.js, tRPC, and the database client the repo already uses. Inputs are validated with Zod. The database is never imported into a client component.

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 T3-style Next.js app with tRPC. It does not assume Prisma versus Drizzle, or a particular auth library. Use the one in the repo.

Customize in the generatorBrowse the library

Example file

Balanced strictness, readable style, pragmatic tests. Download name: security.mdc

---
description: "Security rules for a T3 repository"
globs: "**/*{auth,Auth,secret,config,Config}*"
alwaysApply: false
---

# Security rules

Attach these rules when editing auth, config, or secret-handling code in this T3 repository.
This repository is a T3 app: Next.js, tRPC, and the database client the repo already uses. Inputs are validated with Zod. The database is never imported into a client component.

## Secrets and trust

- Protected procedures check the session. A hidden button is not auth.
- Zod-parse every input, including ids.
- Keep database URLs and auth secrets in server env vars.

## Errors

- Fail at the boundary. Do not swallow errors.
- Use TRPCError with a code for expected failures. Unexpected errors can propagate.

## Done when

- Untrusted input is validated before a side effect.
- No secret was added to source, logs, or a client bundle.
- `npm run typecheck` passes and the procedure rejects an invalid input.

Add it to the project

  1. 01

    Copy a starting file

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

  2. 02

    Save it at .cursor/rules/security.mdc

    Save it at `.cursor/rules/security.mdc` in the T3 repo. Narrow the globs if your auth and config files live in a known folder.

  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 T3 agent often gets wrong, such as a tRPC procedure that validates input with Zod and rejects a bad id. If it ignores the file, confirm the path and start a new session.

Questions about this file

Where do I put Cursor security rule in a T3 repo?

Save it at `.cursor/rules/security.mdc` in the T3 repo. Narrow the globs if your auth and config files live in a known folder.

Which tools read this Cursor security rule?

Cursor loads `.cursor/rules/security.mdc` for files that match its globs. Pair it with the always-on project rule. It is not a substitute for code review.

What does the T3 example assume?

The example assumes a T3-style Next.js app with tRPC. It does not assume Prisma versus Drizzle, or a particular auth library. Use the one in the repo.

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 T3

Cursor security rule for other stacks