Fastify–Ajv Integration: Strict Mode Interaction with Custom Serializers

Answer

Yes. Fastify continues to throw an Ajv validation error for a schema with an unknown keyword when a custom serializer is defined for the same route. The strict-mode check still executes before serialization and cannot be skipped by the serializer. There is no known scenario where a custom serializer by itself suppresses or alters the strict-mode validation outcome.

Confirmed facts

• Ajv validation is performed during request parsing, before the handler runs and before any response serialization.
• The strict: true option is an Ajv compile-time setting that rejects schemas containing unknown keywords or invalid constructs. It is evaluated when the route schema is compiled, not at response time.
• A route serializer or a fast-json custom serializer only affects how a response payload is converted to JSON. It runs in the response phase, after validation and handler execution.
• If validation fails, Fastify returns a 400 error and the response serialization path is not entered for that request.

Likely explanation

Validation and serialization are independent extension points in the request lifecycle. Strict mode is a property of schema compilation and request validation. A custom serializer has no access to the validation step and cannot mask a compile-time Ajv error or a runtime validation failure. The only way validation is bypassed is by explicitly disabling it, e.g., schema: false or overriding Ajv, or by catching and swallowing the error in user code.

Assumptions and uncertainty

This assumes a standard Fastify setup with Ajv as the validator and the default hook ordering where validation precedes the handler and serialization. In older releases hook ordering is less explicit and custom plugin setups can introduce edge cases. Disabling validation entirely is independent of serializer configuration.

Minimal verification for this case

1. Define a route with a schema that triggers strict mode and attach a custom serializer.
2. Send a request that would fail validation.
3. Confirm a 400 validation error is returned and serialization hooks are not invoked.

// example shape
fastify.post('/test', {
  schema: {
    body: { type: 'object', unknownKeyword: true },
    // strict mode enabled via fastify ajv options
  },
  serializer: customSerializer
}, handler)

Add logging in onSend or a response hook to verify serialization is skipped on validation failure. If you can share how strict mode is enabled in your setup — via ajv.customOptions or per-schema — I can tailor the verification to your exact configuration.