[2.59.0] Cannot submit form with unchecked checkbox

[henrykrinkle01]

Describe the bug

When a form has an unchecked checkbox, and the checkbox schema is defined with z.coerce.boolean() (like suggested by the docs), the form can not be submitted. It stops working when I upgrade kit to 2.59.0.

Reproduction

import { form} from '$app/server';
import z from 'zod';

const schema = z.object({
  canSubmitUnchecked: z.boolean().optional().transform((v) => v ?? false),
  cannotSubmitUnchecked: z.coerce.boolean<boolean>(),
});

export const testForm = form(schema, async (data) => {
  console.log(data);
});

https://stackblitz.com/edit/ji42wvw5?file=src%2Froutes%2F%2Bpage.svelte,src%2Froutes%2Ftest.remote.ts

Logs

System Info

svelte@5.55.5
kit@2.59.0

Severity

serious, but I can work around it

Additional Information

No response

[dimfeld]

This changed with Zod 4.4 which contained colinhacks/zod#5661. Earlier versions would coerce an absent key (e.g. undefined coerces to false boolean) but as of 4.4 it does not.

You can check https://svelte.dev/playground/b5dcb68f81294756bd622f8506ade336?version=latest and change the import version to 4.3 to see the difference.