`preValidation hook`

Runs after body parsing but before schema validation.

Since Fastify 5 Spec ↗

Syntax

app.addHook('preValidation', async (request, reply) => {})

Parameters

Name	Type	Required	Description
`request`	`FastifyRequest`	No	The request with parsed body.
`reply`	`FastifyReply`	No	The reply object.

Returns

void | Promise<void> — Throw or send to short-circuit.

Examples

import Fastify from 'fastify';

const app = Fastify();

app.post('/cats', {
  preValidation: async (req) => {
    const body = req.body as Record<string, unknown>;
    if (typeof body.name === 'string') {
      body.name = body.name.trim();
    }
  },
  schema: {
    body: {
      type: 'object',
      required: ['name'],
      properties: { name: { type: 'string', minLength: 1 } },
    },
  },
}, async (req) => req.body);

Notes

The body is parsed but not yet validated, so this hook can normalize or mutate input before the schema runs. Use route-level preValidation for endpoint-specific logic.