Tuning Ajv

Fastify validates with Ajv 8. You can tune compile options, add format support, and customize error messages.

4 min read Level 3/5 #fastify#ajv#validation

What you'll learn

Enable additional formats with ajv-formats
Toggle coerceTypes and useDefaults
Wire custom error messages with ajv-errors

Ajv compiles each JSON Schema into a specialized JavaScript validator the first time it sees the schema, then reuses that validator forever. Fastify exposes Ajv’s options through the factory.

Common Options

import Fastify from 'fastify';
import ajvFormats from 'ajv-formats';
import ajvErrors from 'ajv-errors';

const app = Fastify({
  ajv: {
    customOptions: {
      removeAdditional: 'all',
      coerceTypes: 'array',
      useDefaults: true,
      allErrors: true,
    },
    plugins: [ajvFormats, ajvErrors],
  },
});

removeAdditional — strips unknown body keys instead of throwing
coerceTypes — turns string '42' into number 42 for query/params
useDefaults — fills in default values you declared in the schema
allErrors — collects every problem in one response, not just the first

Add Formats

ajv-formats registers email, uri, uuid, date, date-time, ipv4, regex, and more. Without it, those format keywords are ignored silently.

Custom Error Messages

ajv-errors lets you attach human-friendly messages right inside the schema:

const schema = {
  type: 'object',
  required: ['email'],
  properties: {
    email: { type: 'string', format: 'email' },
  },
  errorMessage: {
    required: { email: 'We need your email to sign you up.' },
    properties: { email: 'That does not look like a valid email.' },
  },
} as const;

Fastify forwards every validation failure to the error handler — see the next lesson for shaping the response.

Error Responses →