Validation Errors

oRPC provides built-in validation errors that work well by default. However, you might sometimes want to customize them.

Customizing with Client Interceptors

Client Interceptors are preferred because they run before error validation, ensuring that your custom errors are properly validated.

import { onError, ORPCError, ValidationError } from '@orpc/server'
import * as z from 'zod'

const handler = new RPCHandler(router, {
  clientInterceptors: [
    onError((error) => {
      if (
        error instanceof ORPCError
        && error.code === 'BAD_REQUEST'
        && error.cause instanceof ValidationError
      ) {
        // If you only use Zod you can safely cast to ZodIssue[]
        const zodError = new z.ZodError(error.cause.issues as z.core.$ZodIssue[])

        throw new ORPCError('INPUT_VALIDATION_FAILED', {
          status: 422,
          message: z.prettifyError(zodError),
          data: z.flattenError(zodError),
          cause: error.cause,
        })
      }

      if (
        error instanceof ORPCError
        && error.code === 'INTERNAL_SERVER_ERROR'
        && error.cause instanceof ValidationError
      ) {
        throw new ORPCError('OUTPUT_VALIDATION_FAILED', {
          cause: error.cause,
        })
      }
    }),
  ],
})

Customizing with Middleware

import { onError, ORPCError, os, ValidationError } from '@orpc/server'
import * as z from 'zod'

const base = os.use(onError((error) => {
  if (
    error instanceof ORPCError
    && error.code === 'BAD_REQUEST'
    && error.cause instanceof ValidationError
  ) {
    // If you only use Zod you can safely cast to ZodIssue[]
    const zodError = new z.ZodError(error.cause.issues as z.core.$ZodIssue[])

    throw new ORPCError('INPUT_VALIDATION_FAILED', {
      status: 422,
      message: z.prettifyError(zodError),
      data: z.flattenError(zodError),
      cause: error.cause,
    })
  }

  if (
    error instanceof ORPCError
    && error.code === 'INTERNAL_SERVER_ERROR'
    && error.cause instanceof ValidationError
  ) {
    throw new ORPCError('OUTPUT_VALIDATION_FAILED', {
      cause: error.cause,
    })
  }
}))

const getting = base
  .input(z.object({ id: z.uuid() }))
  .output(z.object({ id: z.uuid(), name: z.string() }))
  .handler(async ({ input, context }) => {
    return { id: input.id, name: 'name' }
  })

Every procedure built from base now uses these customized validation errors.

WARNING

Middleware applied before .input/.output catches validation errors by default, but this behavior can be configured.

Type‑Safe Validation Errors

As explained in the error handling guide, when you throw an ORPCError instance, if the code, status and data match with the errors defined in the .errors method, oRPC will treat it exactly as if you had thrown errors.[code] using the type‑safe approach.

import { onError, ORPCError, os, ValidationError } from '@orpc/server'
import * as z from 'zod'

const base = os.errors({
  INPUT_VALIDATION_FAILED: {
    status: 422,
    data: z.object({
      formErrors: z.array(z.string()),
      fieldErrors: z.record(z.string(), z.array(z.string()).optional()),
    }),
  },
})

const example = base
  .input(z.object({ id: z.uuid() }))
  .handler(() => { /** do something */ })

const handler = new RPCHandler({ example }, {
  clientInterceptors: [
    onError((error) => {
      if (
        error instanceof ORPCError
        && error.code === 'BAD_REQUEST'
        && error.cause instanceof ValidationError
      ) {
        // If you only use Zod you can safely cast to ZodIssue[]
        const zodError = new z.ZodError(error.cause.issues as z.core.$ZodIssue[])

        throw new ORPCError('INPUT_VALIDATION_FAILED', {
          status: 422,
          message: z.prettifyError(zodError),
          data: z.flattenError(zodError),
          cause: error.cause,
        })
      }
    }),
  ],
})