fastify-zod-openapi

Fastify type provider, validation, serialization and @fastify/swagger support for zod-openapi.

Install

Install via npm, pnpm or pnpm:

``bash
npm install zod fastify-zod-openapi


or

yarn add zod fastify-zod-openapi

or

pnpm install zod fastify-zod-openapi
`

Usage

`ts
import fastify from 'fastify';
import {
type FastifyZodOpenApiSchema,
type FastifyZodOpenApiTypeProvider,
serializerCompiler,
validatorCompiler,
} from 'fastify-zod-openapi';
import * as z from 'zod/v4';

const app = fastify();

app.setValidatorCompiler(validatorCompiler);
app.setSerializerCompiler(serializerCompiler);

app.withTypeProvider().route({
method: 'POST',
url: '/:jobId',
schema: {
body: z.object({
jobId: z.string().meta({
description: 'Job ID',
example: '60002023',
}),
}),
response: {
201: z.object({
jobId: z.string().meta({
description: 'Job ID',
example: '60002023',
}),
}),
},
} satisfies FastifyZodOpenApiSchema,
handler: async (req, res) => {
await res.send({ jobId: req.body.jobId });
},
});

await app.ready();
await app.listen({ port: 5000 });
`

Usage with plugins

`ts
import fastify from 'fastify';
import {
type FastifyPluginAsyncZodOpenApi,
type FastifyZodOpenApiSchema,
serializerCompiler,
validatorCompiler,
} from 'fastify-zod-openapi';
import * as z from 'zod/v4';

const app = fastify();

app.setValidatorCompiler(validatorCompiler);
app.setSerializerCompiler(serializerCompiler);

const plugin: FastifyPluginAsyncZodOpenApi = async (fastify, _opts) => {
fastify.route({
method: 'POST',
url: '/',
// Define your schema
schema: {
body: z.object({
jobId: z.string().meta({
description: 'Job ID',
example: '60002023',
}),
}),
response: {
201: z.object({
jobId: z.string().meta({
description: 'Job ID',
example: '60002023',
}),
}),
},
} satisfies FastifyZodOpenApiSchema,
handler: async (req, res) => {
await res.send({ jobId: req.body.jobId });
},
});
};

app.register(plugin);
`

Usage with @fastify/swagger

`ts
import fastifySwagger from '@fastify/swagger';
import fastifySwaggerUI from '@fastify/swagger-ui';
import fastify from 'fastify';
import {
type FastifyZodOpenApiSchema,
type FastifyZodOpenApiTypeProvider,
fastifyZodOpenApiPlugin,
fastifyZodOpenApiTransformers,
serializerCompiler,
validatorCompiler,
} from 'fastify-zod-openapi';
import * as z from 'zod/v4';

const app = fastify();

app.setValidatorCompiler(validatorCompiler);
app.setSerializerCompiler(serializerCompiler);

await app.register(fastifyZodOpenApiPlugin);
await app.register(fastifySwagger, {
openapi: {
info: {
title: 'hello world',
version: '1.0.0',
},
openapi: '3.1.0',
},
...fastifyZodOpenApiTransformers,
});
await app.register(fastifySwaggerUI, {
routePrefix: '/documentation',
});

app.withTypeProvider().route({
method: 'POST',
url: '/',
schema: {
body: z.object({
jobId: z.string().meta({
description: 'Job ID',
example: '60002023',
}),
}),
response: {
201: {
content: {
'application/json': {
schema: z.object({
jobId: z.string().meta({
description: 'Job ID',
example: '60002023',
}),
}),
},
},
},
},
} satisfies FastifyZodOpenApiSchema,
handler: async (_req, res) =>
res.send({
jobId: '60002023',
}),
});
await app.ready();
await app.listen({ port: 5000 });
`

$3

This library allows you to easily declare components. As an example:

`typescript
const title = z.string().meta({
description: 'Job title',
example: 'My job',
id: 'jobTitle', // <- new field
});
`

Wherever title is used in your request/response schemas across your application, it will instead be created as a reference.

`json
{ "$ref": "#/components/schemas/jobTitle" }
`

For a further dive please follow the documentation here.

If you wish to declare the components manually you will need to do so via the plugin's options. You will also need
to create a custom SerializerCompiler to make use of fast-json-stringify.

`ts
const components: ZodOpenApiComponentsObject = { schemas: { mySchema } };
await app.register(fastifyZodOpenApiPlugin, {
components,
});

const customSerializerCompiler = createSerializerCompiler({
components,
});
`

Alternatively, you can use JSON.stringify instead.

`ts
const customSerializerCompiler = createSerializerCompiler({
stringify: JSON.stringify,
});
`

By default, this library assumes that if a response schema provided is not a Zod Schema, it is a JSON Schema and will naively pass it straight into fast-json-stringify. This will not work in conjunction with Fastify's schema registration.

If you have other routes with response schemas which are not Zod Schemas, you can supply a fallbackSerializer to createSerializerCompiler.

`ts
const customSerializerCompiler = createSerializerCompiler({
fallbackSerializer: ({ schema, url, method }) => customSerializer(schema),
});
`

Please note: the parameters components do not appear to be supported by the @fastify/swagger library.

$3

If you wish to use CreateDocumentOptions, pass it in via the plugin options:

`ts
await app.register(fastifyZodOpenApiPlugin, {
documentOpts: {
unionOneOf: true,
},
});
`

$3

The default response serializer serializerCompiler uses fast-json-stringify. Under the hood, the schema passed to the response is transformed using OpenAPI 3.1.0 and passed to fast-json-stringify as a JSON Schema.

If are running into any compatibility issues or wish to restore the previous JSON.stringify functionality, you can use the createSerializerCompiler function.

`ts
const customSerializerCompiler = createSerializerCompiler({
stringify: JSON.stringify,
});
`

$3

By default, fastify-zod-openapi emits request validation errors in a similar manner to fastify when used in conjunction with it's native JSON Schema error handling.

As an example:

`json
{
"code": "FST_ERR_VALIDATION",
"error": "Bad Request",
"message": "params/jobId Expected number, received string",
"statusCode": 400
}
`

For responses, it will emit a 500 error along with a vague error which will protect your implementation details

`json
{
"code": "FST_ERR_RESPONSE_SERIALIZATION",
"error": "Internal Server Error",
"message": "Response does not match the schema",
"statusCode": 500
}
`

To customise this behaviour, you may follow the fastify error handling guidance.

#### Request Errors

This library throws a RequestValidationError when a request fails to validate against your Zod Schemas

##### setErrorHandler

`ts
fastify.setErrorHandler(function (error, request, reply) {
if (error.validation) {
const zodValidationErrors = error.validation.filter(
(err) => err instanceof RequestValidationError,
);
const zodIssues = zodValidationErrors.map((err) => err.params.issue);
const originalError = zodValidationErrors?.[0]?.params.error;
return reply.status(422).send({
zodIssues
originalError
});
}
});
`

##### setSchemaErrorFormatter

`ts
fastify.setSchemaErrorFormatter(function (errors, dataVar) {
let message = ${dataVar}:;
for (const error of errors) {
if (error instanceof RequestValidationError) {
message += ${error.instancePath} ${error.keyword};
}
}

return new Error(message);
});

// {
// code: 'FST_ERR_VALIDATION',
// error: 'Bad Request',
// message: 'querystring: /jobId invalid_type',
// statusCode: 400,
// }
`

##### attachValidation

`ts
app.withTypeProvider().get(
'/',
{
schema: {
querystring: z.object({
jobId: z.string().meta({
description: 'Job ID',
example: '60002023',
}),
}),
},
attachValidation: true,
},
(req, res) => {
if (req.validationError?.validation) {
const zodValidationErrors = req.validationError.validation.filter(
(err) => err instanceof RequestValidationError,
);
console.error(zodValidationErrors);
}

return res.send(req.query);
},
);
`

#### Response Errors

`ts
app.setErrorHandler((error, _req, res) => {
if (error instanceof ResponseSerializationError) {
return res.status(500).send({
error: 'Bad response',
});
}
});

// {
// error: 'Bad response';
// }
`

Credits

fastify-type-provider-zod: Big kudos to this library for lighting the way with how to create type providers, validators and serializers. fastify-zod-openapi is just an extension to this library whilst adding support for the functionality of zod-openapi.

Development

$3

- Node.js LTS
- pnpm

`shell
pnpm install
pnpm build
`

$3

`shell
pnpm test
`

$3

`shell


Fix issues

pnpm format

Check for issues

pnpm lint
`

$3

To release a new version

1. Create a new GitHub Release
2. Select 🏷️ Choose a tag, enter a version number. eg. v1.2.0 and click + Create new tag: vX.X.X on publish.
3. Click the Generate release notes button and adjust the description.
4. Tick the Set as the latest release box and click Publish release. This will trigger the Release workflow.
5. Check the Pull Requests tab for a PR labelled Release vX.X.X.
6. Click Merge Pull Request on that Pull Request to update main with the new package version.

To release a new beta version

1. Create a new GitHub Release
2. Select 🏷️ Choose a tag, enter a version number with a -beta.X suffix eg. v1.2.0-beta.1 and click + Create new tag: vX.X.X-beta.X on publish.
3. Click the Generate release notes button and adjust the description.
4. Tick the Set as a pre-release box and click Publish release. This will trigger the Prerelease` workflow.