Zod 4 - Default Date values in nested objects

[arnolicious]

Description
When defining a nested schema with, which includes dates, and the default values for those dates are set on the parent object, the default values are strings instead dates.
This is a regression compared to zod v3

export const schema = z.object({
	name: z.string().min(2),
	email: z.email(),
	dateRange: z.object({
		start: z.date(),
		end: z.date()
	}).default({
		start: new Date(),
		end: new Date()
	}),
	dateRangeIndividual: z.object({
		start: z.date().default(new Date()),
		end: z.date().default(new Date())
	}),
	date: z.date().default(new Date())
});

Resulting $form object values:

{
  name: "",
  email: "",
  dateRange: {
    start: "2025-10-29T10:47:03.068Z",   // <- string
    end: "2025-10-29T10:47:03.068Z"     // <- string
  },
  dateRangeIndividual: {
    start: 2025-10-29T10:52:25.537Z,      // <- date
    end: 2025-10-29T10:52:25.537Z        // <- date
  },
  date: 2025-10-29T10:47:03.068Z         // <- date
}

If applicable, a MRE
https://www.sveltelab.dev/kata9i0q30dg6vk?files=.%2Fsrc%2Froutes%2Fschema.ts

[arnolicious]

I tried letting an LLM have a go at fixing it, this patch actually solved the issue for me, and so far hasn't seemed to cause any other issues, so if that might be of any help to fix or work around this issue:

diff --git a/node_modules/sveltekit-superforms/dist/jsonSchema/schemaDefaults.js b/node_modules/sveltekit-superforms/dist/jsonSchema/schemaDefaults.js
index aa1285a..966e619 100644
--- a/node_modules/sveltekit-superforms/dist/jsonSchema/schemaDefaults.js
+++ b/node_modules/sveltekit-superforms/dist/jsonSchema/schemaDefaults.js
@@ -95,9 +96,42 @@ function _defaultValues(schema, isOptional, path) {
     if (info.properties) {
         for (const [key, objSchema] of Object.entries(info.properties)) {
             assertSchema(objSchema, [...path, key]);
-            const def = objectDefaults && objectDefaults[key] !== undefined
-                ? objectDefaults[key]
-                : _defaultValues(objSchema, !info.required?.includes(key), [...path, key]);
+            // Determine the default value for this property. If an object-level default
+            // was provided, attempt to convert it to the correct typed form using the
+            // property's schema information (e.g. convert ISO strings back to Date).
+            let def;
+            if (objectDefaults && objectDefaults[key] !== undefined) {
+                try {
+                    const propInfo = schemaInfo(objSchema, !info.required?.includes(key), [...path, key]);
+                    if (propInfo) {
+                        // Use the first resolved type to format simple values (dates, sets, maps, bigint, symbol)
+                        const propType = propInfo.types[0];
+                        // If property is an object and the provided default is an object,
+                        // merge the provided default with recursively computed defaults so
+                        // nested schema defaults (including conversions) are applied.
+                        if (propType === 'object' &&
+                            typeof objectDefaults[key] === 'object' &&
+                            objectDefaults[key] !== null &&
+                            !Array.isArray(objectDefaults[key])) {
+                            const nested = _defaultValues(objSchema, !info.required?.includes(key), [...path, key]);
+                            def = merge.withOptions({ allowUndefinedOverrides: true }, nested ?? {}, objectDefaults[key]);
+                        }
+                        else {
+                            def = formatDefaultValue(propType, objectDefaults[key]);
+                        }
+                    }
+                    else {
+                        def = objectDefaults[key];
+                    }
+                }
+                catch (e) {
+                    // If anything goes wrong determining/formatting, fall back to provided value
+                    def = objectDefaults[key];
+                }
+            }
+            else {
+                def = _defaultValues(objSchema, !info.required?.includes(key), [...path, key]);
+            }
             //if (def !== undefined) output[key] = def;
             if (output === undefined)
                 output = {};

[ciscoheat]

Fix Applied ✅

I've successfully applied the fix suggested in the comments and added comprehensive tests to verify it works correctly.

Changes Made

Fixed file: src/lib/jsonSchema/schemaDefaults.ts

The issue was that when an object schema has a default value containing nested dates, those dates get serialized to ISO strings in the JSON Schema. The fix ensures these strings are properly converted back to Date objects when processing defaults.

Key improvements:

1. When object-level defaults are provided, we now check the property's schema type
2. For primitive types (dates, bigint, symbol, etc.), we use formatDefaultValue() to convert strings back to their proper types
3. For nested objects, we recursively process them by creating a temporary schema with the parent's default, ensuring deep conversion of nested dates

Tests Added

Created src/tests/issue-650.test.ts with 4 comprehensive test cases:

1. ✅ Default dates in nested objects - The exact scenario from the issue report
2. ✅ Nested objects with default dates at multiple levels - Deep nesting with defaults at each level
3. ✅ Mixed default values in nested objects - Objects with dates, strings, and numbers
4. ✅ Individual field defaults - Ensures existing behavior still works when defaults are on individual fields

Test Results

All new tests pass, and the full test suite (408 tests) passes without any regressions.

The fix handles:

• ✅ Dates in nested objects with object-level defaults
• ✅ Multiple levels of nesting with defaults
• ✅ Mixed types (dates, strings, numbers) in default objects
• ✅ Backwards compatibility with field-level defaults

This resolves the regression from Zod v3 and ensures all date values are properly typed as Date objects rather than strings.

[ciscoheat]

Added now in 2.28.1, can you confirm that it works?