samchungy/fastify-zod-openapi

README

fastify-zod-openapi

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

Install

Install via npm, pnpm or pnpm:

npm install zod fastify-zod-openapi
## or
yarn add zod fastify-zod-openapi
## or
pnpm install zod fastify-zod-openapi

Usage

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<FastifyZodOpenApiTypeProvider>().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

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

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<FastifyZodOpenApiTypeProvider>().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 });

Declaring Components

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

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.

{ "$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.

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

const customSerializerCompiler = createSerializerCompiler({
  components,
});

Alternatively, you can use JSON.stringify instead.

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.

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.

Create Document Options

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

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

Custom Response Serializer

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.

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

Error Handling

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:

{
  "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

{
  "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

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

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

app.withTypeProvider<FastifyZodOpenApiTypeProvider>().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

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

Prerequisites

Node.js LTS
pnpm

pnpm install
pnpm build

Test

pnpm test

Lint

# Fix issues
pnpm format

# Check for issues
pnpm lint

Release

To release a new version

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

To release a new beta version

Create a new GitHub Release
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.
Click the Generate release notes button and adjust the description.
Tick the Set as a pre-release box and click Publish release. This will trigger the Prerelease workflow.