diff --git a/spec/late-extend.spec.ts b/spec/late-extend.spec.ts
new file mode 100644
index 0000000..ebe6818
--- /dev/null
+++ b/spec/late-extend.spec.ts
@@ -0,0 +1,80 @@
+import { extendZodWithOpenApi } from '../src/zod-extensions';
+import { expectSchema } from './lib/helpers';
+
+// Schemas constructed before `extendZodWithOpenApi` runs must still expose a
+// working `.openapi`. Zod v4 binds prototype keys to instances at construction
+// time and does not link `ZodObject.prototype` (etc.) to `ZodType.prototype`,
+// so a late patch on `ZodType.prototype` alone is unreachable from
+// pre-existing instances.
+describe('Late extendZodWithOpenApi', () => {
+  function requireSeparateZodInstance() {
+    jest.resetModules();
+    delete require.cache[require.resolve('zod')];
+
+    // eslint-disable-next-line @typescript-eslint/no-require-imports
+    return require('zod');
+  }
+
+  const zod = requireSeparateZodInstance();
+
+  const stringBefore = zod.z.string();
+  const numberBefore = zod.z.number();
+  const objectBefore = zod.z.object({ a: zod.z.string() });
+  const arrayBefore = zod.z.array(zod.z.string());
+  const tupleBefore = zod.z.tuple([zod.z.string(), zod.z.number()]);
+  const unionBefore = zod.z.union([zod.z.string(), zod.z.number()]);
+  const optionalBefore = zod.z.string().optional();
+  const nullableBefore = zod.z.number().nullable();
+
+  extendZodWithOpenApi(zod);
+
+  it('attaches .openapi to primitives constructed before the call', () => {
+    expect(typeof stringBefore.openapi).toBe('function');
+    expect(typeof numberBefore.openapi).toBe('function');
+  });
+
+  it('attaches .openapi to composites constructed before the call', () => {
+    expect(typeof objectBefore.openapi).toBe('function');
+    expect(typeof arrayBefore.openapi).toBe('function');
+    expect(typeof tupleBefore.openapi).toBe('function');
+    expect(typeof unionBefore.openapi).toBe('function');
+  });
+
+  it('attaches .openapi to wrappers constructed before the call', () => {
+    expect(typeof optionalBefore.openapi).toBe('function');
+    expect(typeof nullableBefore.openapi).toBe('function');
+  });
+
+  it('keeps .openapi working for schemas constructed after the call', () => {
+    const after = zod.z.string();
+    const afterOptional = zod.z.string().optional();
+    const afterArray = zod.z.array(zod.z.number());
+    expect(typeof after.openapi).toBe('function');
+    expect(typeof afterOptional.openapi).toBe('function');
+    expect(typeof afterArray.openapi).toBe('function');
+  });
+
+  it('produces a usable spec from a pre-existing primitive', () => {
+    expectSchema([stringBefore.openapi('SimpleString')], {
+      SimpleString: { type: 'string' },
+    });
+  });
+
+  it('produces a usable spec from a pre-existing composite', () => {
+    expectSchema([objectBefore.openapi('SimpleObject')], {
+      SimpleObject: {
+        type: 'object',
+        properties: { a: { type: 'string' } },
+        required: ['a'],
+      },
+    });
+  });
+
+  it('does not attach .openapi to ZodError', () => {
+    // ZodError is exported by zod but is not a schema constructor — it
+    // extends Error and has no `_def`. Polluting its prototype with
+    // `.openapi` would change observable behavior in `try { ... } catch (e)`
+    // blocks and would crash with an inscrutable error if anyone called it.
+    expect(typeof zod.ZodError.prototype.openapi).toBe('undefined');
+  });
+});
diff --git a/src/zod-extensions.ts b/src/zod-extensions.ts
index 6498d4e..ec0e45f 100644
--- a/src/zod-extensions.ts
+++ b/src/zod-extensions.ts
@@ -237,6 +237,24 @@ export function extendZodWithOpenApi(zod: typeof z) {
 
     return result;
   };
+
+  // Mirror `.openapi` onto every concrete schema prototype. Zod v4's
+  // `$constructor` binds prototype keys onto each instance at construction
+  // time, and `ZodObject.prototype` (etc.) does NOT chain to
+  // `ZodType.prototype`, so the assignment above is unreachable from any
+  // schema constructed before this function runs. Patching each
+  // `Zod*.prototype` directly makes the extension order-independent.
+  const openapiMethod = zod.ZodType.prototype.openapi;
+  for (const [key, ctor] of Object.entries(zod) as [string, unknown][]) {
+    // `ZodError` is the only `Zod*`-prefixed export that is not a schema
+    // constructor — it extends Error, not $ZodType, and has no `_def` for
+    // `.openapi`'s `new this.constructor(this._def)` clone to consume.
+    if (key === 'ZodError') continue;
+    if (!key.startsWith('Zod') || typeof ctor !== 'function') continue;
+    const proto = (ctor as { prototype?: { openapi?: unknown } }).prototype;
+    if (!proto || typeof proto.openapi !== 'undefined') continue;
+    proto.openapi = openapiMethod;
+  }
 }
 
 function getOpenApiConfiguration(