Summary

Fixes #5764

When .default({}) is called on an object schema, the default value is now parsed through the inner schema at definition time so that nested field-level defaults are applied. This ensures the invariant:

schema.default(d).parse(undefined) === schema.parse(d)

holds for object schemas, matching user expectations and Zod v4's own direction on defaults.

Before

const schema = z.object({
  name: z.string().default("untitled"),
  count: z.number().default(0),
}).default({});

schema.parse(undefined); // => {} ❌

After

schema.parse(undefined); // => { name: "untitled", count: 0 } ✅

Approach

In the _default() factory (classic, mini, and core), when the inner type is an object schema and the default is a static plain object (not a function), run it through the inner schema's parse at definition time. This:

• Has zero runtime overhead — the short-circuit optimization in $ZodDefault.parse is preserved
• Handles deeply nested objects recursively (inner defaults are already resolved when outer .default({}) is called)
• Silently falls back to the original default if parsing fails (e.g., invalid default)

Files changed

• packages/zod/src/v4/classic/schemas.ts — classic _default() factory
• packages/zod/src/v4/mini/schemas.ts — mini _default() factory
• packages/zod/src/v4/core/api.ts — core _default() factory
• packages/zod/src/v4/classic/tests/default.test.ts — 4 new test cases

Test plan

• Existing default.test.ts tests still pass (20/20)
• New tests cover: basic cascading, deeply nested objects, partial overrides, shallow clone identity
• Full test suite passes (3583 tests, 0 type errors)
• Format and lint checks pass