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fastify-type-provider-zod

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# Fastify Type Provider Zod

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## Zod compatibility

| fastify-type-provider-zod | zod |
|---------------------------|-------|
| <=4.x | v3 |
| >=5.x <7.x | v4 |
| >=7.x | v4.2+ |

> **Important (v7+)**
>
> Starting from **v7**, this library uses Zod’s `.encode()` / `.decode()` APIs introduced in **Zod 4.2**.
> Because of this change, **response serialization is now based on `z.output<T>` instead of `z.input<T>`**.
>
> This means Fastify serializers always expect the **post-transformation output type** of your Zod schemas.

## How to use?

```ts
import type { ZodTypeProvider } from 'fastify-type-provider-zod';
import { serializerCompiler, validatorCompiler } from 'fastify-type-provider-zod';
import { z } from 'zod/v4';

const app = Fastify();

// Add schema validator and serializer
app.setValidatorCompiler(validatorCompiler);
app.setSerializerCompiler(serializerCompiler);

app.withTypeProvider<ZodTypeProvider>().route({
  method: 'GET',
  url: '/',
  // Define your schema
  schema: {
    querystring: z.object({
      name: z.string().min(4),
    }),
    response: {
      200: z.string(),
    },
  },
  handler: (req, res) => {
    res.send(req.query.name);
  },
});

app.listen({ port: 4949 });
```

You can also pass options to the `serializerCompiler` function:

```ts
type ZodSerializerCompilerOptions = {
  replacer?: ReplacerFunction;
};
```

```ts
import Fastify from 'fastify';
import { createSerializerCompiler, validatorCompiler } from 'fastify-type-provider-zod';

const app = Fastify();

const replacer = function (key, value) {
  if (this[key] instanceof Date) {
    return { _date: value.toISOString() };
  }
  return value;
};

// Create a custom serializer compiler
const customSerializerCompiler = createSerializerCompiler({ replacer });

// Add schema validator and serializer
app.setValidatorCompiler(validatorCompiler);
app.setSerializerCompiler(customSerializerCompiler);

// ...

app.listen({ port: 4949 });
```

## How to use together with @fastify/swagger

```ts
import fastify from 'fastify';
import fastifySwagger from '@fastify/swagger';
import fastifySwaggerUI from '@fastify/swagger-ui';
import { z } from 'zod/v4';
import type { ZodTypeProvider } from 'fastify-type-provider-zod';
import {
  jsonSchemaTransform,
  createJsonSchemaTransform,
  serializerCompiler,
  validatorCompiler,
} from 'fastify-type-provider-zod';

const app = fastify();
app.setValidatorCompiler(validatorCompiler);
app.setSerializerCompiler(serializerCompiler);

app.register(fastifySwagger, {
  openapi: {
    info: {
      title: 'SampleApi',
      description: 'Sample backend service',
      version: '1.0.0',
    },
    servers: [],
  },
  transform: jsonSchemaTransform,

// You can also create transform with custom skiplist of endpoints that should not be included in the specification:
  //
  // transform: createJsonSchemaTransform({
  //   skipList: [ '/documentation/static/*' ]
  // })
});

app.register(fastifySwaggerUI, {
  routePrefix: '/documentation',
});

const LOGIN_SCHEMA = z.object({
  username: z.string().max(32).describe('Some description for username'),
  password: z.string().max(32),
});

app.after(() => {
  app.withTypeProvider<ZodTypeProvider>().route({
    method: 'POST',
    url: '/login',
    schema: { body: LOGIN_SCHEMA },
    handler: (req, res) => {
      res.send('ok');
    },
  });
});

async function run() {
  await app.ready();

await app.listen({
    port: 4949,
  });

console.log(`Documentation running at http://localhost:4949/documentation`);
}

run();
```

## Customizing error responses

You can add custom handling of request and response validation errors to your fastify error handler like this:

```ts
import { hasZodFastifySchemaValidationErrors } from 'fastify-type-provider-zod';

fastifyApp.setErrorHandler((err, req, reply) => {
  if (hasZodFastifySchemaValidationErrors(err)) {
    return reply.code(400).send({
      error: 'Response Validation Error',
      message: "Request doesn't match the schema",
      statusCode: 400,
      details: {
        issues: err.validation,
        method: req.method,
        url: req.url,
      },
    });
  }

if (isResponseSerializationError(err)) {
    return reply.code(500).send({
      error: 'Internal Server Error',
      message: "Response doesn't match the schema",
      statusCode: 500,
      details: {
        issues: err.cause.issues,
        method: err.method,
        url: err.url,
      },
    });
  }

// the rest of the error handler
});
```

## How to create refs to the schemas?

When provided, this package will automatically create refs using the `jsonSchemaTransformObject` function. You register the schemas with the global Zod registry and assign them an `id`. `fastifySwagger` will then create an OpenAPI document that references the schemas.

The following example creates a ref to the `User` schema and will include the `User` schema in the OpenAPI document.

```ts
import fastifySwagger from '@fastify/swagger';
import fastifySwaggerUI from '@fastify/swagger-ui';
import fastify from 'fastify';
import { z } from 'zod/v4';
import type { ZodTypeProvider } from 'fastify-type-provider-zod';
import {
  jsonSchemaTransformObject,
  jsonSchemaTransform,
  serializerCompiler,
  validatorCompiler,
} from 'fastify-type-provider-zod';

const USER_SCHEMA = z.object({
  id: z.number().int().positive(),
  name: z.string().describe('The name of the user'),
});

z.globalRegistry.add(USER_SCHEMA, { id: 'User' });

const app = fastify();
app.setValidatorCompiler(validatorCompiler);
app.setSerializerCompiler(serializerCompiler);

app.register(fastifySwaggerUI, {
  routePrefix: '/documentation',
});

app.after(() => {
  app.withTypeProvider<ZodTypeProvider>().route({
    method: 'GET',
    url: '/users',
    schema: {
      response: {
        200: USER_SCHEMA.array(),
      },
    },
    handler: (req, res) => {
      res.send([]);
    },
  });
});

async function run() {
  await app.ready();

await app.listen({
    port: 4949,
  });

console.log(`Documentation running at http://localhost:4949/documentation`);
}

run();
```

## How to define an empty body response

Use `z.undefined()` to define a response with no body. This enforces correct types on `res.send()` and generates an accurate OpenAPI schema:

```ts
import { z } from 'zod/v4';
import type { ZodTypeProvider } from 'fastify-type-provider-zod';

app.withTypeProvider<ZodTypeProvider>().route({
  method: 'DELETE',
  url: '/resource',
  schema: {
    response: {
      204: z.undefined().describe('Resource deleted'),
    },
  },
  handler: (_req, res) => {
    res.status(204).send();
  },
});
```

## How to define multiple response content types

You can define per-content-type response schemas following the OpenAPI 3.x response object format. The `jsonSchemaTransform` will include them correctly in the generated spec, and `res.send()` will be typed as the union of all defined schemas.

```ts
import { z } from 'zod/v4';
import type { ZodTypeProvider } from 'fastify-type-provider-zod';

app.withTypeProvider<ZodTypeProvider>().route({
  method: 'GET',
  url: '/items',
  schema: {
    response: {
      200: {
        description: 'Successful response',
        content: {
          'application/json': { schema: z.object({ id: z.number(), name: z.string() }) },
          'application/vnd.v1+json': { schema: z.array(z.object({ id: z.number(), name: z.string() })) },
        },
      },
      default: {
        content: {
          '*/*': { schema: z.object({ message: z.string() }) },
        },
      },
    },
  },
  handler: (req, res) => {
    res.send({ id: 1, name: 'item' });
  },
});
```

Fastify dispatches serialization per content type — set the response `Content-Type` header in the handler to select which schema validates the response body:

```ts
handler: (req, res) => {
  res.header('Content-Type', 'application/vnd.v1+json');
  res.send([{ id: 1, name: 'item' }]);
},
```

## How to create a plugin?

```ts
import { z } from 'zod/v4';
import type { FastifyPluginAsyncZod } from 'fastify-type-provider-zod';

const plugin: FastifyPluginAsyncZod = async function (fastify, _opts) {
  fastify.route({
    method: 'GET',
    url: '/',
    // Define your schema
    schema: {
      querystring: z.object({
        name: z.string().min(4),
      }),
      response: {
        200: z.string(),
      },
    },
    handler: (req, res) => {
      res.send(req.query.name);
    },
  });
};
```

## How to specify different OpenAPI targets

You can specify different JSON Schema targets for OpenAPI compatibility using the `createJsonSchemaTransform` function with the `zodToJsonConfig.target` option.

By default target "openapi-3.0" is used for documents with "openapi" field set to "3.0.x", and target "draft-2020-12" is used for documents with "openapi" field set to "3.1.x".

### Usage

```typescript
import { createJsonSchemaTransform } from "fastify-type-provider-zod";

// For OpenAPI 3.0.x compatibility
const transform = createJsonSchemaTransform({
  zodToJsonConfig: { target: "openapi-3.0" },
});

// For OpenAPI 3.1+
const transform = createJsonSchemaTransform({
  zodToJsonConfig: { target: "draft-2020-12" },
});
```

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