Just as you validate incoming request data, you can validate outgoing response data.

Response validation ensures that your API contract is honored - that you actually return the data structure your types promise.

This catches bugs where handlers might return incomplete objects, wrong types, or unexpected structures.

Response validation works similarly to payload validation, just use response property to provide response schema, status code, content type and body:

api/users/index.ts

import type { User } from "@/front/types/api-payload";
import { defineRoute } from "_/front/api/users";

export default defineRoute(({ GET }) => [
  GET<{
    response: [200, "json", User], 
  }>(async (ctx) => {
    // response must comply to defined schema
  }),
]);

Before sending the response to the client, KosmoJS validates that status, content type and body actually matches defined schema.

If validation fails - perhaps because the database returned a user object that's missing the preferences field, or because the profile.email doesn't match email format constraints - KosmoJS throws a ValidationError instead of sending invalid data to the client.

This protects your API consumers from receiving malformed data and helps you catch bugs in your handler logic.

Another major benefit of defining response schemas is automatic OpenAPI schema generation. This gives you both type safety and API documentation in one step. (Details ➜ )