Type Your API Responses, Not Just Your Requests

The mistake:

// ✗ Validates input carefully
const res = await fetch("https://api.example.com/users", {
  method: "POST",
  body: JSON.stringify(validated_user)
});

// ✗ Just casts as any
const data = (await res.json()) as User;

API changes its schema. Your type is now wrong. Crash in production.

The fix: Parse responses with zod.

const UserSchema = z.object({
  id: z.string(),
  email: z.string().email(),
  name: z.string()
});

const res = await fetch("https://api.example.com/users");
const data = UserSchema.parse(await res.json());  // Throws if invalid

If the API returns a field you don’t expect, or removes one you do, the error is immediate and loud.

Trust nothing from external systems. Validate responses the same way you validate inputs.

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