How to customize ServerValidateError with Supabase Auth errors?

[alckordev]

Hi everyone 👋,

I’m integrating @tanstack/react-form v1.15.2 with Supabase Auth inside a Next.js 15 server action.
When credentials are wrong, I want to return the Auth error as form-level and form-level messages.

import { ServerValidateError } from '@tanstack/react-form/nextjs'

// … inside the server action
const { error } = await supabase.auth.signInWithPassword(creds)

if (error) {
  throw new ServerValidateError<Credentials, undefined>({
    formState: {
      // field-level errors
      errorMap: {
        onServer: {
          fields: {
            email:    [{ message: error.message }],
            password: [{ message: error.message }],
          },
        },
      },
      // form-level errors (I’d like to keep these too)
      errors: {
        email:    [{ message: error.message }],
        password: [{ message: error.message }],
      },
    },
  })
}

At runtime this works — the client receives the messages and displays them — but TypeScript complains:

TS2322 – Type '{ fields: { email: { message: string; }[]; password: { message: string; }[]; }; }'
         is not assignable to type 'undefined'.

TS2353 – Object literal may only specify known properties, and 'email' does not exist in type 'undefined[]'.

(types.d.ts:27:5) → property 'onServer' expected 'undefined'
(types.d.ts:248:5) → property 'errors' expected string[]

So both errorMap.onServer.fields and errors trigger type errors.

Relevant versions

  "dependencies": {
    "@supabase/ssr": "^0.6.1",
    "@supabase/supabase-js": "^2.53.0",
    "@tanstack/react-form": "^1.15.2",
    "next": "15.4.5",
    "react": "19.1.0",
    "react-dom": "19.1.0",
    "zod": "^4.0.14"
  },
  "devDependencies": {
    "@eslint/eslintrc": "^3",
    "@types/node": "^20",
    "@types/react": "^19",
    "@types/react-dom": "^19",
    "eslint": "^9",
    "eslint-config-next": "15.4.5",
    "typescript": "^5"
  }

Question

What is the correct, type-safe way to:

1, Attach field-level errors messages coming from Supabase

Thanks a lot for any guidance!