RPC Handler

Use RPCHandler to communicate with RPC Link and other clients that implement the RPC protocol.

Overview

const handler = new RPCHandler(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()
  ],
})

Supported HTTP Methods

By default, RPCHandler only responds to POST, PUT, PATCH, and DELETE requests. Any other method, such as GET or HEAD, is treated as unmatched, as if no procedure exists at that path.

This is a security default: cross-site, browsers can only send these methods via a CORS preflight or an HTML form, never from a plain link. Safe methods like GET or HEAD are excluded because invoking a procedure can modify data.

Use allowMethods to replace the allowlist: tighten it to POST only, or also accept QUERY, which reads input from the request body and stays preflight-protected:

const handler = new RPCHandler(router, {
  allowMethods: ['POST', 'PUT', 'PATCH', 'DELETE', 'QUERY'],
})

Enabling the GET Method

Add GET to allowMethods to enable it for every procedure:

import { SimpleCsrfProtectionHandlerPlugin } from '@orpc/server/plugins'
import { RPC_DEFAULT_ALLOW_METHODS } from '@orpc/server/standard'

const handler = new RPCHandler(router, {
  allowMethods: ['GET', ...RPC_DEFAULT_ALLOW_METHODS],
  plugins: [
    new SimpleCsrfProtectionHandlerPlugin(), // reject requests triggered by navigations, forms, etc.
  ],
})

Or pass a function to decide per request. For example, only allow GET for procedures that declare it via OpenAPI metadata:

import { getOpenAPIMeta, openapi } from '@orpc/openapi'
import { RPC_DEFAULT_ALLOW_METHODS } from '@orpc/server/standard'

const listPlanets = os
  .meta(openapi({ method: 'GET', path: '/planets' }))
  .handler(() => ['Earth', 'Mars'])

const handler = new RPCHandler({ listPlanets }, {
  allowMethods: (method, procedure, path) => {
    if (method === 'GET' && getOpenAPIMeta(procedure)?.method === 'GET') {
      return true
    }

    return RPC_DEFAULT_ALLOW_METHODS.includes(method)
  },
})

Interceptors

Interceptors let you observe or change different stages of an RPC 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 RPCHandler(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.

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

Adapter Interceptors

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

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 RPCHandler(router, {
  plugins: [
    new CORSHandlerPlugin()
  ],
})

Custom Serializer

RPCHandler uses a built-in serializer that supports many native types. Provide a custom serializer when you need extra types or different encoding behavior. For more details, see RPC Serializer.

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

Filtering Procedures

Use the filter option to exclude procedures from matching:

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

Custom Error Response

By default, RPCHandler uses COMMON_ERROR_STATUS_MAP to determine response status codes. Use errorStatusMap to customize them:

import { COMMON_ERROR_STATUS_MAP } from '@orpc/server'

const handler = new RPCHandler(router, {
  /**
   * The status code should be in the `4xx` or `5xx` range (must be greater than or equal to `400`).
   */
  errorStatusMap: {
    ...COMMON_ERROR_STATUS_MAP,
    CUSTOM_ERROR: 599,
  },
})

Event Stream Options

Configure how an AsyncIteratorObject is streamed to the client. Available options depend on the adapter. For example, the fetch adapter supports:

const handler = new RPCHandler(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 15000
         */
        interval: 15000,
        /**
         * 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