data type not inferred from validator when method: 'GET' in createServerFn (regression in @tanstack/react-start 1.168.24)

Which project does this relate to?

Start

Describe the bug

When using createServerFn with method: 'GET' and a .validator(), the data parameter inside the .handler() callback is not correctly typed from the validator's output — it falls back to undefined instead of the expected inferred type.

This is a regression introduced in @tanstack/react-start@1.168.24.

Reproduction

import { createServerFn } from '@tanstack/react-start'
import { z } from 'zod'

const schema = z.object({ id: z.string() })

const myFn = createServerFn({ method: 'GET' })
  .validator(schema)
  .handler((ctx) => {
    // ❌ ctx.data is typed as `unknown` instead of `{ id: string }`
    console.log(ctx.data.id)
  })

The same pattern works correctly with method: 'POST', or worked correctly with method: 'GET' prior to version 1.168.24.

Package version

• @tanstack/react-start: 1.168.24

Complete minimal reproducer

SEE DESCRIBE

Steps to Reproduce the Bug

1. Create a server function with method: 'GET'
2. Chain .validator() with any validator (zod schema, function)
3. Access ctx.data inside .handler() — TypeScript reports the type as unknown

Expected behavior

ctx.data should be typed as { id: string } (i.e., the output type inferred from the validator).

Screenshots or Videos

Platform

• Router / Start Version: 1.168.24
• OS: windows
• Browser: Edge
• Browser Version: 149.0.4022.69
• Bundler: vite
• Bundler Version: 8.0.16

Additional context

No response

Start by reproducing the issue in a minimal TypeScript project using createServerFn, method: 'GET', and the validator shown in the report; compare the inferred ctx.data type with the POST case. Trace the createServerFn, validator, and handler type definitions, then verify that the validator output is preserved for GET handlers and that the reproduction type-checks.