Discussion: #4092

Which project does this relate to?

Router (@tanstack/zod-adapter)

Describe the bug

It appears that fallback from @tanstack/zod-adapter isn't ready for Zod v4.
More information about the expected behavior is in my comment below: #4322 (comment)

Steps to Reproduce the Bug or Issue

import { z } from "zod/v4";
import { fallback, zodValidator } from "@tanstack/zod-adapter";

const searchParamSchema = z.object({
  test: fallback(z.string().optional(), null).default(null),
//               ^ ❗ Type Error at z.string().optional()
});

Type Error:

Argument of type 'ZodOptional<ZodString>' is not assignable to parameter of type 'ZodTypeAny'.
  Type 'ZodOptional<ZodString>' is missing the following properties from type 'ZodType<any, any, any>': _type, _parse, _getType, _getOrReturnCtx, and 7 more.ts(2345)

Expected behavior

Zod 4 support