fastify-lor-zod

Note -- Pre-1.0: minor versions may include breaking changes. Pin your version and check the changelog before upgrading.

A Fastify type provider for Zod v4 with full OpenAPI support. A ground-up rebuild of turkerdev/fastify-type-provider-zod on Zod v4's native APIs — fixes 25+ open issues.

Why fastify-lor-zod?

• Zod v4 native -- uses safeEncode, toJSONSchema, codecs, and registries directly
• Smart serializer -- auto-detects codecs at compile time; falls back to safeParse for ~15% faster non-codec schemas
• Fast validation errors -- lazy ZodError construction on the error path; 1.1--1.5x faster error responses than alternatives, with a safe automatic fallback
• Complete OpenAPI -- all HTTP parts, nullable types, discriminated unions, recursive schemas, content types
• Type-safe end-to-end -- req.body, req.params, req.query, req.headers, and reply.send() fully typed
• 100% test coverage with snapshot parity against fastify-type-provider-zod
• Why "Lor"? -- Son of Zod, here to power your fastify schemas.

Table of Contents

• Install
• Quick Start
• Serializer Compilers
• Benchmarks
• OpenAPI / Swagger
• Typed Plugins
• Error Handling
• Zod v4 Codec Support
• Compatibility
• Contributing
• License

Install

pnpm add fastify-lor-zod
pnpm add -D fastify zod                    # peer dependencies
pnpm add -D @fastify/swagger               # optional, for OpenAPI

Quick Start

import Fastify from 'fastify';
import { z } from 'zod';
import {
  validatorCompiler,
  serializerCompiler,
  type FastifyLorZodTypeProvider,
} from 'fastify-lor-zod';

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

app.withTypeProvider<FastifyLorZodTypeProvider>().get(
  '/user/:id',
  {
    schema: {
      params: z.object({ id: z.coerce.number() }),
      response: {
        200: z.object({ id: z.number(), name: z.string() }),
      },
    },
  },
  (req) => ({ id: req.params.id, name: 'Alice' }),
  //              ^ fully typed as number
);

app.listen({ port: 3000 });

Serializer Compilers

Three strategies for different trade-offs:

Compiler	Validates	Codecs	Speed	Use when
serializerCompiler	Yes	Auto-detect	Fastest validating	Recommended default -- uses safeParse for plain schemas, safeEncode only when codecs are present
parseSerializerCompiler	Yes	No	Same as above	Explicit opt-in to always use safeParse
fastSerializerCompiler	No	Rejected at startup	Fastest overall	Escape hatch for plain schemas — no response validation, the same trade-off vanilla Fastify makes; codec, transform, and preprocess schemas are rejected at route registration

import {
  serializerCompiler,         // default: auto-detects codecs, picks safeParse or safeEncode
  parseSerializerCompiler,    // always z.safeParse + JSON.stringify
  fastSerializerCompiler,     // fast-json-stringify, no validation
} from 'fastify-lor-zod';

app.setSerializerCompiler(serializerCompiler);

createSerializerCompiler and createParseSerializerCompiler each accept a replacer option for JSON.stringify. createFastSerializerCompiler takes no options — fast-json-stringify pre-compiles the serializer at route registration time and does not use JSON.stringify. Because fast-json-stringify cannot execute codec encode functions, transforms, or preprocess steps, the fast compiler rejects such schemas at route registration instead of silently emitting wrong output (there is deliberately no bypass — use serializerCompiler for those routes). Schema .default() values are applied; .catch() fallbacks are not — an unconvertible value throws at serialization time.

Benchmarks

• Fastest on the common shapes — leads the other validating Zod providers on plain objects and discriminated unions (native JSON.stringify beats fast-json-stringify on anyOf); stays competitive on the rest.
• Faster error responses — lazy error construction makes rejected requests ~1.1--1.5x faster than the next-fastest provider (up to ~4.5x vs zod-openapi), where it counts under load.
• On par on the success path — no validation-speed regression to buy the above.
• Does what others can't — serializes one-way transforms in responses that fastify-type-provider-zod throws on and fastify-zod-openapi rejects.

Full tables, methodology, and per-scenario numbers: bench/BENCHMARK.md.

OpenAPI / Swagger

Integrate with @fastify/swagger for automatic OpenAPI spec generation. transform converts Zod schemas per route, transformObject populates components.schemas from a registry (safe to include even without one):

import swagger from '@fastify/swagger';
import swaggerUi from '@fastify/swagger-ui';
import { jsonSchemaTransform, jsonSchemaTransformObject } from 'fastify-lor-zod';

await app.register(swagger, {
  openapi: {
    openapi: '3.0.3',
    info: { title: 'My API', version: '1.0.0' },
  },
  transform: jsonSchemaTransform,
  transformObject: jsonSchemaTransformObject,
});

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

• OAS 3.0 and 3.1 support
• Automatic io: "input" for request schemas, io: "output" for response schemas
• Nullable types, discriminated unions, recursive schemas handled correctly
• Nested content types (application/json, multipart/form-data, etc.)
• Response description preserved from wrapper objects
• zodToJsonConfig passthrough for custom z.toJSONSchema() options

Schema Registry

Register schemas with z.globalRegistry or a custom registry to generate $ref-based components.schemas:

import { z } from 'zod';
import { createJsonSchemaTransforms } from 'fastify-lor-zod';

const registry = z.registry<{ id: string }>();
const UserSchema = z.object({ id: z.number(), name: z.string() });
registry.add(UserSchema, { id: 'User' });

await app.register(swagger, {
  openapi: { openapi: '3.0.3', info: { title: 'My API', version: '1.0.0' } },
  ...createJsonSchemaTransforms({ schemaRegistry: registry }),
});

Schemas whose input and output shapes diverge (e.g. due to .default(), transforms, or codecs) automatically get {Id}Input variants in components.schemas. No configuration needed.

Typed Plugins

import type { FastifyPluginAsyncZod } from 'fastify-lor-zod';

const usersPlugin: FastifyPluginAsyncZod = async (app) => {
  app.get(
    '/users',
    {
      schema: {
        response: { 200: z.array(UserSchema) },
      },
    },
    () => [{ id: 1, name: 'Alice' }],
  );
};

await app.register(usersPlugin);

Typed Handlers

Use RouteHandler to define handlers in separate files while preserving Zod type inference:

import type { RouteHandler } from 'fastify-lor-zod';

const schema = {
  params: z.object({ id: z.coerce.number() }),
  response: { 200: z.object({ name: z.string() }) },
} as const;

const getUser: RouteHandler<typeof schema> = (req) => {
  req.params.id; // number
  return { name: 'Alice' };
};

app.get('/users/:id', { schema }, getUser);

Error Handling

Validation errors are detected with the isRequestValidationError type guard. Serialization errors use instanceof on the ResponseSerializationError class.

Validation errors are built on a fast path that skips stock ZodError construction cost, but they stay fully ZodError-compatible: instanceof z.ZodError, .issues, .message, .format(), and .flatten() all behave as usual. If a future Zod version changes the internals this relies on, the validator falls back to plain schema.safeParse at startup and emits a one-time process warning ([fastify-lor-zod] Zod internals self-check failed ...) -- behavior stays correct, only error construction gets slower.

import {
  isRequestValidationError,
  ResponseSerializationError,
} from 'fastify-lor-zod';

app.setErrorHandler((error, request, reply) => {
  if (isRequestValidationError(error)) {
    // Log input server-side only — may contain sensitive fields
    request.log.error({ input: error.input });
    reply.code(400).send({
      error: 'Validation failed',
      issues: error.validation,          // FastifySchemaValidationError[]
      context: error.validationContext,   // 'body' | 'querystring' | 'params' | 'headers'
    });
    return;
  }

  if (error instanceof ResponseSerializationError) {
    reply.code(500).send({
      error: 'Response serialization failed',
      code: error.code,        // 'ERR_RESPONSE_SERIALIZATION'
      method: error.method,    // 'GET'
      url: error.url,          // '/users/42'
      httpStatus: error.httpStatus, // '200'
    });
    return;
  }

  reply.send(error);
});

Zod v4 Codec Support

Zod v4 codecs encode domain types to wire format. The default serializer handles this automatically:

const dateCodec = z.codec(z.iso.datetime(), z.date(), {
  decode: (iso: string) => new Date(iso),
  encode: (date: Date) => date.toISOString(),
});

app.get(
  '/event',
  {
    schema: {
      response: {
        200: z.object({ startsAt: dateCodec }),
      },
    },
  },
  () => ({ startsAt: new Date() }),
  // Response: { "startsAt": "2025-06-15T10:00:00.000Z" }
);

Compatibility

fastify-lor-zod	Fastify	Zod	@fastify/swagger	fast-json-stringify	Node.js
0.x	>= 5.7.0 < 6	>= 4.4.1 < 5	>= 9.7.0 < 10 (optional)	>= 6.0.0 < 8 (optional, for fastSerializerCompiler)	>= 22

Migrating from fastify-type-provider-zod

See MIGRATION.md for a step-by-step guide.

Contributing

git clone https://github.com/drudolf/fastify-lor-zod.git
cd fastify-lor-zod
pnpm install

Command	Description
pnpm test	Run tests
pnpm test:coverage	Run tests with 100% coverage enforcement
pnpm check	Lint + format (Biome)
pnpm typecheck	Type-check with tsc --noEmit
pnpm knip	Detect unused exports and dependencies
pnpm bench	Run benchmarks against all type providers
pnpm bench:lib <filter>	Run benchmarks for a single library (e.g. lor-zod, type-provider, zod-openapi)
pnpm build	Build the project (ESM and CJS)

Tests follow a spec-first workflow -- see test-spec.md for the full test matrix and CLAUDE.md for project conventions.

License

MIT