Fastify schema validation

Fastify attaches JSON Schema to routes for validation, serialization, and OpenAPI generation. Invalid requests fail before the handler runs; response schemas enable fast serialization via `fast-json-stringify`.

					const createUser = {
  schema: {
    body: {
      type: 'object',
      required: ['email'],
      properties: { email: { type: 'string', format: 'email' } },
      additionalProperties: false,
    },
    response: {
      201: {
        type: 'object',
        properties: { id: { type: 'string' } },
      },
    },
  },
};

fastify.post('/users', createUser, async (req) => {
  return { id: await service.create(req.body) };
});
				

Schemas cover `body`, `querystring`, `params`, and `headers`. `addSchema` registers reusable $ref fragments. Type providers (`@fastify/type-provider-typebox`) connect schemas to TypeScript inference.

On interviews: validation plus serialization performance, `additionalProperties: false` for strict contracts, and schema-driven documentation.

Common pitfalls: response schemas that do not match actual payloads (runtime errors), no shared $ref leading to drift, and validating only body while ignoring query params.

The trade-off is balancing simplicity, performance, safety, and operability — name which axis you optimized and what cost you accepted.

Checklist:

• Define route schemas for input and output.
• Reuse fragments with addSchema and $ref.
• Reject unknown properties on write endpoints.
• Align schemas with TypeScript via type providers.