In my TRPC app, I have a service that we use that does something like this:

export class MyCustomService {
  public myCustomMethod() {
    throw new MyCustomError('I am a custom error message!')
  }
}

and my TRPC router looks like:

export const appRouter = router({
  methodName: publicProcedure.query(() => {
    myCustomService.myCustomMethod()
  })
})

And I have a function that handles errors:

export const standardTrpcErrorHandler = (error: Error): TRPCError => {
  if (errorinstanceof MyCustomError) {
    return new TRPCError({
      ...error,
      code: 'NOT_FOUND',
      message: error.message,
    })
  }

  return new TRPCError({
    ...error,
    code: 'INTERNAL_SERVER_ERROR',
    message: error.message,
  })
}

And I want to globally apply this logic to error handling. However, from all documentation I can find, the only way to apply logic like this is within the errorFormatter method where TRPC has already translated it to a TRPCError. This would be fine, but I don’t want to manually add 404 to the shape response here, and let that be handled by TRPC normally. I also cannot throw a TRPCError from my service’s method, since that method lives outside the scope of the TRPC router.

Where would I add this logic?