Breaking behavior for .default(...).transform(...) chain when used in a partial object

[JuanoD]

Given the next snippet:

import { z } from "zod/v4";

const arrayFromString = z
  .string()
  .default("")
  .transform((val) => (val ? val.split(",").map((s) => s.trim()) : []));

const partialObj = z.object({ array: arrayFromString }).partial();

console.log(
  arrayFromString.safeParse(undefined).data,
  partialObj.safeParse({}).data?.array,
);

4.3.6 logged [] [] while 4.4.3 logs [] undefined

[pullfrog]

Reproduced. This is a regression introduced in 4.4.3 — 4.4.2 and earlier are fine, and it still reproduces on main.

.partial() just wraps each property in .optional(), so the object in the report isn't required to trigger it:

import { z } from "zod/v4";

const schema = z
  .string()
  .default("")
  .transform((val) => (val ? val.split(",").map((s) => s.trim()) : []))
  .optional();

schema.parse(undefined); // 4.4.2: []   4.4.3: undefined

.prefault() is hit the same way — z.string().prefault("hi").transform((v) => v.length).optional() returns 2 on 4.4.2 and undefined on 4.4.3.

Root cause is in packages/zod/src/v4/core/schemas.ts. $ZodTransform sets payload.fallback = true on every invocation (lines 3436 and 3446), and handleOptionalResult (line 3478) throws the value away whenever the original input was undefined and that flag is set:

function handleOptionalResult(result: ParsePayload, input: unknown) {
  if (input === undefined && (result.issues.length || result.fallback)) {
    return { issues: [], value: undefined };
  }
  return result;
}

The flag is there so a .catch() substitution collapses back to undefined under an outer .optional(), which is reasonable. But marking every transform as a fallback also catches .default() and .prefault(), where the value the transform received was a deliberate value rather than error recovery.

Unaffected on 4.4.3, for reference: the schema on its own (arrayFromString.parse(undefined) still gives []), and .default() without a transform under .partial() (z.object({ array: z.string().default("") }).partial().parse({}) still gives { array: "" }) — which is what makes the current behavior inconsistent.

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