ESLint @typescript-eslint/no-unsafe-argument with serializerCompiler

[Ephasme]

Bug Report: ESLint @typescript-eslint/no-unsafe-argument with serializerCompiler

Environment:

• fastify-type-provider-zod: 6.0.0
• Fastify: 5.6.0
• TypeScript: 5.x
• ESLint: typescript-eslint strict config

Issue:
When using serializerCompiler from fastify-type-provider-zod, ESLint throws: Unsafe argument of type error typed assigned to a parameter of type FastifySerializerCompiler

Code that triggers the issue:

import { validatorCompiler, serializerCompiler } from 'fastify-type-provider-zod';

function routesPlugin(fastify: FastifyInstance) {
    fastify.setValidatorCompiler(validatorCompiler); // ✅ Works fine
    fastify.setSerializerCompiler(serializerCompiler); // ❌ ESLint error
}

Root cause:
The issue stems from the union type in serializerCompiler:

FastifySerializerCompiler<$ZodType | { properties: $ZodType; }>

While validatorCompiler uses a simple type:

FastifySchemaCompiler<$ZodType>

Workarounds:

• Disable ESLint rule: // eslint-disable-next-line @typescript-eslint/no-unsafe-argument
• Type assertion: serializerCompiler as FastifySerializerCompiler<any>

Expected behavior:
serializerCompiler should work without ESLint warnings, similar to validatorCompiler.

[shaked-hadas]

I'm having this problem as well

[Puetz]

Same problem here

[amitbeck]

Works for me using:

fastify@5.6.2
fastify-type-provider-zod@6.1.0
zod@4.1.13
typescript-eslint@8.48.1