TypeScript Type Inference Issue with createServerFn and beforeLoad using Connect RPC

[EdiAfremovFactify]

Which project does this relate to?

Start

Describe the bug

Problem:
I'm experiencing a TypeScript type inference issue when using createServerFn().handler() and beforeLoad with a Connect RPC function that returns a discriminated union response type.

Code example:

// My Connect RPC response type (discriminated union)
type ConnectRpcResponse<TData extends Message> = 
  | { readonly data: TData; readonly error?: undefined; }
  | { readonly data?: undefined; readonly error: ConnectError; };

// My getCurrentUser function returns this type
export async function getCurrentUser(
  payload: ConnectRpcRequestPayload<GetCurrentUserRequest>
): Promise<ConnectRpcResponse<UserAccountDetails>> {
  // ... implementation
  return { data: response.account.value }; // UserAccountDetails
}

// Trying to use it with createServerFn
const getCurrentUserFn = createServerFn({ method: 'GET' }).handler(async function() {
  const data = await getCurrentUser({});
  return data; // ❌ TypeScript error here 
});

Actual Behavior:
TypeScript recursively infers the entire getCurrentUser response structure, deeply expanding all nested Protobuf Message types within the discriminated union.
This results in a massive inferred type that is not assignable to UserAccountDetails, causing a type error and Cursor IDE to completely hang/freeze, making the file unusable.

Environment:
@tanstack/react-start
Connect RPC with Protobuf messages
TypeScript

Your Example Website or App

N/A

Steps to Reproduce the Bug or Issue

Local

Expected behavior

The handler should infer the return type as ConnectRpcResponse<UserAccountDetails>.

Screenshots or Videos

Platform

• Start Version: start 1.134.10
• OS: macOS
• Bundler: vite
• Bundler Version: 7.1.12

Additional context

No response

[schiller-manuel]

as shown in the typescript error, your data type contains functions (e.g. metadata.append ) which are not serializable and thus the type error occurs.