Response validation errors should return a 500 rather than a 422

[marcalexiei]

What is the problem this feature would solve?

Currently, when the response does not conform to the provided schema,
a 422 HTTP status code (Unprocessable Entity) is returned.

Reproduction

import Elysia from "elysia";
import { z } from "zod";

export const app = new Elysia()
  .get(
    "/api-keys",
    (ctx) => {
      return {
        keys2: [],
      };
    },
    {
      response: {
        200: z.strictObject({
          keys: z.array(z.unknown()),
        }),
      },
    }
  )
  .listen(3000);

1. Start the server

2. Perform a request

   curl -i http://localhost:3000/api-keys

3. Check response status code:

   HTTP/1.1 422 Unprocessable Entity
   Content-Type: application/json
   Date: Tue, 14 Oct 2025 09:49:55 GMT
   Content-Length: 519
   
   # omitted for brevity
   

What is the feature you are proposing to solve the problem?

Since an invalid response typically requires server-side intervention,
I believe a 500 HTTP status code (Internal Server Error) would be more appropriate in this scenario.

What do you think?

If you agree, I can try submitting a PR with this change.

What alternatives have you considered?

N/A

[marcalexiei]

I'm referring to a response validation error, not a request validation error.

In this case, the client made a valid request, but the server returned a response that didn’t match its own declared schema. That’s an error on the server side, not the caller’s.

For example, in my snippet, the handler fails to provide the correct property (keys instead of keys2), so the 422 doesn’t make much sense — it’s not the client’s fault.
A 500 (Internal Server Error) would better reflect that the issue originated in server logic.

---

For clarity: the previous comment that prompted my response has been removed, but the point remains that we're talking about response validation, not request validation.