<!-- Append this section to AGENTS.md. Do not replace the existing file. This is project guidance, not a custom agent. -->

Express Codex Instructions: Middleware, Routing and Validation

Section to append to AGENTS.md for Express. This is project guidance, not a custom agent.

Updated for Express 5.3.0, Nov 2026

Do not upgrade dependencies or the framework version unless the task asks.

Project shape

Middleware is an ordered list. Security headers, body parsing, and auth run before the route that needs them. A router mounts on a path. Do not register a catch-all before a more specific route and then wonder why the specific route never runs.

An async handler either uses the project's wrapper or relies on Express 5's rejection forwarding. It does not do both. Do not catch a rejection, send a response, and also call next(err).

Validation and bodies

Validate params, query, and body with the project's schema. Reject unknown fields on a write. Cap the JSON body size in the parser. A route that reads req.body with no schema is a review.

Auth and errors

A route that changes data has auth middleware. Do not check auth only in the UI. After auth, check that the resource belongs to the caller. A client-supplied user id is not identity.

The error middleware has four arguments and sits last. It logs the error and sends a status and a short body. It does not send a stack in production. Headers are not written twice.

Security and tests

helmet or the project's header middleware stays enabled. CORS lists origins. Do not reflect any Origin with credentials. Rate-limit the login route the project already protects.

A test hits the route with supertest or the project's client. Cover no token, a bad body, and a body larger than the limit if you changed the limit. Run the test script. Record the output.

Mistakes to avoid

• An Express 4 wildcard left in an Express 5 router.
• next(err) after the response was already sent.
• A write route with no auth and no schema.
• An unbounded express.json() body.
• A second error handler that runs after the first already sent the body.

What changed in this major

Express 5 forwards rejected promises. Replace Express 4 /* wildcards. Do not call next(err) after res.send.

Deprecated APIs to never generate

Express 5 uses path-to-regexp v8. app.get('/*') throws at startup because the * has no name. Write app.get('/*splat', ...).

Commands

Run the package test script.

Example

app.use(express.json({ limit: "100kb" }))