isDefinedError return never

[FLchs]

Hello everyone !

I'm using a contract first oRPC setup, with my errors defined this way :

const create = oc
  .route({
    method: "POST",
    path: "/",
  })
  .errors(errors)
  .input(createVehicleSchema)
  .output(successSchema);

My errors :

import { type ErrorMap } from "@orpc/contract";
import * as z from "zod/v4";

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

In the front I'm using tanstack query but the error is typed "never" :

  const createVehicleMutation = useMutation(
    openapi.vehicles.create.mutationOptions({
      onError: (error) => {
        if (isDefinedError(error) && error.code === "INPUT_VALIDATION_FAILED") { //  typescript: Property 'code' does not exist on type 'never'. 
          form.setErrorMap({
            onSubmit: {
              fields: error.data.fieldErrors,
            },
          });
        }
      });

I'm in a Bun monorepo and somehow I feel it might comes from import quirk...
isDefinedError is typed:

isDefinedError<ErrorFromErrorMap<MergedErrorMap<Record<never, never>, MergedErrorMap<Record<never, never>, ErrorMap>>>>(error: ErrorFromErrorMap<MergedErrorMap<Record<never, never>, MergedErrorMap<Record<never, never>, ErrorMap>>>): error is never

Any help would be greatly appreciated.

[dosubot[bot]]

Hi @FLchs! I'm Dosu and I’m helping the orpc team.

Your error is typed as never because the error map isn't properly propagated to the client—TypeScript only sees an empty error map, so ErrorFromErrorMap collapses to never. This usually happens if the contract/router used to generate your OpenAPI client doesn't include your defined errors, or if there's an import quirk in your monorepo setup that causes type mismatches or dependency duplication (especially with Bun) [source].

To fix this:

• Make sure the contract/router you expose to the client (and use for OpenAPI generation) includes your .errors(errors) call, so the error map is populated at build time [source].

• In your onError handler, you can cast the error to ErrorFromErrorMap<InferContractRouterErrorMap> before using isDefinedError. This restores type safety and lets you access error.code, error.data, etc. Example:

  import { InferContractRouterErrorMap, ErrorFromErrorMap } from '@orpc/contract';
  
  onError: (e) => {
    const error = e as ErrorFromErrorMap<InferContractRouterErrorMap<typeof router>>;
    if (isDefinedError(error) && error.code === "INPUT_VALIDATION_FAILED") {
      // ...
    }
  }

  [source]

• If you suspect import quirks or multiple ORPCError definitions (common in monorepos), try deduping dependencies or use a custom isDefinedError that checks object properties instead of instanceof [source].

• Avoid using lazy routers in your contract if you want type inference to work for errors [source].

This isn't a bug in oRPC, but a result of how TypeScript infers types from your contract and how your monorepo/bundler resolves modules. Once the client sees the correct error map, the error type will be inferred correctly and isDefinedError will work as expected.

To reply, just mention @dosu.

---

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[FLchs]

const error = e as ErrorFromErrorMap<InferContractRouterErrorMap<typeof contract>>;
// type of error: Error

I guess that's where the issue comes from.

[unnoq]

Can you help me create a minimal repro?

[FLchs]

I'm going to try but I suspect it will works in a minimal repro 🥲
I'm doing it asap.

[FLchs]

Ok here it is : https://github.com/FLchs/orpc-typed-error-repro

I should have added that I used the concept described here : https://orpc.dev/docs/advanced/validation-errors#type%E2%80%90safe-validation-errors but anyhow, I'm inclined to think that any error provided in .errors() should work anyway.

I'm sorry it's quick and dirty but if you look at ./apps/web/src/App.tsx the type error of error is present.

If you run the app (bun dev) and click on the error generating button you can see the error is defined :

{
  "defined": true,
  "code": "INPUT_VALIDATION_FAILED",
  "status": 422,
  "message": "✖ Invalid input: expected number, received string\n  → at id",
  "data": {
    "fieldErrors": {
      "id": [
        "Invalid input: expected number, received string"
      ]
    },
    "formErrors": []
  }
}

I'm positive it is an error on my part and that I probably mixed instruction from the documentation, but somehow I can't put my finger on it.

[unnoq]

You should follow this: https://orpc.dev/docs/best-practices/monorepo-setup

[unnoq]

Here how you can define standalone ErrorMap correctly:

-export const errors: ErrorMap = {
+export const errors = {
  INPUT_VALIDATION_FAILED: {
    status: 422,
    data: z.object({
      fieldErrors: z.record(z.string(), z.array(z.string()).optional()),
      formErrors: z.array(z.string()),
    }),
  },
-};
+} satisfies ErrorMap;

[FLchs]

You should follow this: https://orpc.dev/docs/best-practices/monorepo-setup

Adding references didn't change a thing.

Here how you can define standalone ErrorMap correctly:

-export const errors: ErrorMap = {
+export const errors = {
  INPUT_VALIDATION_FAILED: {
    status: 422,
    data: z.object({
      fieldErrors: z.record(z.string(), z.array(z.string()).optional()),
      formErrors: z.array(z.string()),
    }),
  },
-};
+} satisfies ErrorMap;

This actually fixed it in both the repro and my code :)
It was definitely a skill issue on my part.

Really appreciate your help @unnoq, thank you so much.