OpenAPI Handler

Use OpenAPIHandler to expose HTTP endpoints or communicate with OpenAPI Link and other OpenAPI-compliant clients.

Overview

const handler = new OpenAPIHandler(router, {
  interceptors: [
    async ({ next, path }) => {
      console.time(path.join('.'))

      try {
        return await next()
      }
      catch (err) {
        console.error(`${path.join('.')}:`, err)
        throw err
      }
      finally {
        console.timeEnd(path.join('.'))
      }
    }
  ],
  plugins: [
    new CORSHandlerPlugin()
  ],
})

INFO

The actual usage of OpenAPIHandler depends on the adapter you use. For example, when using the fetch adapter, the handler is used like this:

export async function fetch(request: Request) {
  const { response } = await handler.fetch(request, {
    prefix: '/api',
    context: {} // <- provide initial context if needed
  })

  return response ?? new Response('Not Found', { status: 404 })
}

WARNING

To better support Blob, File, and ReadableStream<Uint8Array> at the root level in cross-origin scenarios, extend your CORS allowlist to allow clients to send and receive the Content-Disposition and Standard-Server headers. Learn more in the Standard Server documentation. If you use the CORS Plugin, include them in allowHeaders and exposeHeaders:

const cors = new CORSHandlerPlugin({
  allowHeaders: ['Content-Disposition', 'Standard-Server'],
  exposeHeaders: ['Content-Disposition', 'Standard-Server'],
})

Interceptors

Interceptors let you observe or change different stages of an OpenAPI request. Common use cases include logging, error handling, and metrics.

Routing Interceptors

Routing interceptors run on every request before routing. Use them when you need to handle all requests, including requests that do not match a procedure.

const handler = new OpenAPIHandler(router, {
  routingInterceptors: [
    async ({ next, request, context }) => {
      if (condition) {
        return { matched: false }
      }

      const { matched, response } = await next()
      return { matched, response }
    },
  ],
})

Interceptors

These interceptors run only for matched requests, after routing and before error handling (but can't use ORPCError for typesafe errors). Use them when you need access to the matched procedure.

TIP

In most cases, interceptors are the best choice. They provide more context, are easier to work with, and run before error handling.

const handler = new OpenAPIHandler(router, {
  interceptors: [
    async ({ next, request, procedure, context }) => {
      try {
        const response = await next()
        return response
      }
      catch (err) {
        if (err instanceof CustomError) {
          throw new ORPCError('CUSTOM_ERROR', { message: err.message, cause: err })
        }

        throw err
      }
    },
    async ({ next, path }) => {
      console.time(path.join('.'))

      try {
        const response = await next()
        return response
      }
      catch (err) {
        console.error(`${path.join('.')}:`, err)
        throw err
      }
      finally {
        console.timeEnd(path.join('.'))
      }
    },
  ],
})

Client Interceptors

Client interceptors run only for matched requests, after input decoding, before output encoding and can use ORPCError for typesafe errors. Use them when you need access to the procedure, input, and output.

const handler = new OpenAPIHandler(router, {
  clientInterceptors: [
    async ({ next, input, context, procedure }) => {
      const output = await next()
      return output
    },
  ],
})

Adapter Interceptors

Some OpenAPIHandler implementations, such as fetch or node adapters, also support adapter interceptors. These run before Routing Interceptors and let you work with the adapter's native request and response objects.

const handler = new OpenAPIHandler(router, {
  fetchInterceptors: [
    async ({ next, request }) => {
      const { matched, response } = await next()
      return { matched, response }
    },
  ],
})

INFO

This example uses the fetch adapter. For other adapters, refer to their JSDoc or adapter-specific documentation.

Plugins

Plugins package reusable interceptors. For example, CORS Plugin adds a routing interceptor to handle preflight requests and adds CORS headers to every response.

const handler = new OpenAPIHandler(router, {
  plugins: [
    new CORSHandlerPlugin()
  ],
})

Custom Serializer

Provide a custom serializer when you need to extend or override the default serialization behavior. For more details, see OpenAPI Serializer.

const handler = new OpenAPIHandler(router, {
  serializer: new OpenAPISerializer({
    handlers: {
      // ...custom handlers
    },
  }),
})

Filtering Procedures

Use the filter option to exclude procedures from matching:

const handler = new OpenAPIHandler(router, {
  filter: (contract, path) => getIsInternalMeta(contract) !== true,
})

Custom Error Response

By default, OpenAPIHandler determines response status codes using COMMON_ERROR_STATUS_MAP and encodes error bodies in the ORPC error format. Use errorStatusMap and customErrorResponseBodyEncoder to customize this behavior:

import { COMMON_ERROR_STATUS_MAP } from '@orpc/openapi'

const handler = new OpenAPIHandler(router, {
  errorStatusMap: {
    ...COMMON_ERROR_STATUS_MAP,
    CUSTOM_ERROR: 599,
  },
  customErrorResponseBodyEncoder: (error) => {
    if (error.code === 'CUSTOM_ERROR') {
      return {
        customMessage: error.message,
        customCode: error.code,
      }
    }

    // fallback to default by returning null or undefined
    return null
  },
})

Common Error Status Map

Error Code	HTTP Status Code
BAD_REQUEST	400
UNAUTHORIZED	401
PAYMENT_REQUIRED	402
FORBIDDEN	403
NOT_FOUND	404
METHOD_NOT_SUPPORTED	405
NOT_ACCEPTABLE	406
TIMEOUT	408
CONFLICT	409
GONE	410
PRECONDITION_FAILED	412
PAYLOAD_TOO_LARGE	413
UNSUPPORTED_MEDIA_TYPE	415
UNPROCESSABLE_CONTENT	422
PRECONDITION_REQUIRED	428
TOO_MANY_REQUESTS	429
CLIENT_CLOSED_REQUEST	499
INTERNAL_SERVER_ERROR	500
NOT_IMPLEMENTED	501
BAD_GATEWAY	502
SERVICE_UNAVAILABLE	503
GATEWAY_TIMEOUT	504

INFO

If you use OpenAPILink with a custom server-side error format, make sure to configure Custom Error Decoding.

Event Stream Options

Configure how event iterators are streamed to the client. Available options depend on the adapter. For example, the fetch adapter supports:

const handler = new OpenAPIHandler(router, {
  toFetchResponse: {
    eventStream: {
      initialComment: {
        /**
         * If true, an initial comment is sent immediately upon stream start to flush headers.
         * This allows the receiving side to establish the connection without waiting for the first event.
         *
         * @default true
         */
        enabled: true,
        /**
         * The content of the initial comment sent upon stream start. Must not include newline characters.
         *
         * @default ''
         */
        comment: '',
      },
      keepAlive: {
        /**
         * If true, a ping comment is sent periodically to keep the connection alive.
         *
         * @default true
         */
        enabled: true,
        /**
         * Interval (in milliseconds) between ping comments sent after the last event.
         *
         * @default 5000
         */
        interval: 5000,
        /**
         * The content of the ping comment. Must not include newline characters.
         *
         * @default ''
         */
        comment: '',
      },
      /**
       * If true, a `close` event is sent even when the iterator completes with `undefined`.
       * When the iterator returns a value, a `close` event is always emitted regardless of this setting.
       *
       * @default true
       */
      emptyCloseEventEnabled: true,
    },
  },
})

Lifecycle

TODO: add lifecycle diagram