Clone POJO objects during defaulting/prefaulting

Author: colinhacks

packages/zod/src/v4/classic/schemas.ts

@@ -1688,7 +1688,7 @@ export function _default<T extends core.SomeType>(
     type: "default",
     innerType: innerType as any as core.$ZodType,
     get defaultValue() {
-      return typeof defaultValue === "function" ? (defaultValue as Function)() : defaultValue;
+      return typeof defaultValue === "function" ? (defaultValue as Function)() : util.shallowClone(defaultValue);
     },
   }) as any;
 }
@@ -1716,7 +1716,7 @@ export function prefault<T extends core.SomeType>(
     type: "prefault",
     innerType: innerType as any as core.$ZodType,
     get defaultValue() {
-      return typeof defaultValue === "function" ? (defaultValue as Function)() : defaultValue;
+      return typeof defaultValue === "function" ? (defaultValue as Function)() : util.shallowClone(defaultValue);
     },
   }) as any;
 }

packages/zod/src/v4/classic/tests/default.test.ts

@@ -311,3 +311,15 @@ test("partial should not clobber defaults", () => {
     }
   `);
 });
+
+test("defaulted object schema returns shallow clone", () => {
+  const schema = z
+    .object({
+      a: z.string(),
+    })
+    .default({ a: "x" });
+  const result1 = schema.parse(undefined);
+  const result2 = schema.parse(undefined);
+  expect(result1).not.toBe(result2);
+  expect(result1).toEqual(result2);
+});

packages/zod/src/v4/classic/tests/prefault.test.ts

@@ -35,3 +35,15 @@ test("prefault inside object", () => {
     email: string;
   }>();
 });
+
+test("object schema with prefault should return shallow clone", () => {
+  const schema = z
+    .object({
+      a: z.string(),
+    })
+    .default({ a: "x" });
+  const result1 = schema.parse(undefined);
+  const result2 = schema.parse(undefined);
+  expect(result1).not.toBe(result2);
+  expect(result1).toEqual(result2);
+});

packages/zod/src/v4/core/api.ts

@@ -1321,7 +1321,7 @@ export function _default<T extends schemas.$ZodObject>(
     type: "default",
     innerType,
     get defaultValue() {
-      return typeof defaultValue === "function" ? (defaultValue as Function)() : defaultValue;
+      return typeof defaultValue === "function" ? (defaultValue as Function)() : util.shallowClone(defaultValue);
     },
   }) as any;
 }

packages/zod/src/v4/core/util.ts

@@ -389,6 +389,11 @@ export function isPlainObject(o: any): o is Record<PropertyKey, unknown> {
   return true;
 }
 
+export function shallowClone(o: any): any {
+  if (isPlainObject(o)) return { ...o };
+  return o;
+}
+
 export function numKeys(data: any): number {
   let keyCount = 0;
   for (const key in data) {

packages/zod/src/v4/mini/schemas.ts

@@ -1256,7 +1256,7 @@ export function _default<T extends SomeType>(
     type: "default",
     innerType: innerType as any as core.$ZodType,
     get defaultValue() {
-      return typeof defaultValue === "function" ? (defaultValue as Function)() : defaultValue;
+      return typeof defaultValue === "function" ? (defaultValue as Function)() : util.shallowClone(defaultValue);
     },
   }) as any;
 }
@@ -1281,7 +1281,7 @@ export function prefault<T extends SomeType>(
     type: "prefault",
     innerType: innerType as any as core.$ZodType,
     get defaultValue() {
-      return typeof defaultValue === "function" ? (defaultValue as Function)() : defaultValue;
+      return typeof defaultValue === "function" ? (defaultValue as Function)() : util.shallowClone(defaultValue);
     },
   }) as any;
 }

packages/zod/src/v4/mini/tests/index.test.ts

@@ -869,3 +869,16 @@ test("def typing", () => {
   z.catch(z.string(), "fallback").def.type satisfies "catch";
   z.file().def.type satisfies "file";
 });
+
+test("defaulted object schema returns shallow clone", () => {
+  const schema = z._default(
+    z.object({
+      a: z.string(),
+    }),
+    { a: "x" }
+  );
+  const result1 = schema.parse(undefined);
+  const result2 = schema.parse(undefined);
+  expect(result1).not.toBe(result2);
+  expect(result1).toEqual(result2);
+});