Fix Zod object detection logic

Previously, this check was `value instanceof ZodType`, which breaks if the user is using a different version of Zod than the library. Instead, do the check using the structure of the input object rather than its prototype chain. Additionally, this adds a vendored version of Zod to use for tests, which ensures that regressions like this would be caught next time.

Author: geelen

src/server/__tests__/vendor/zod@3.24.4.umd.d.ts

@@ -0,0 +1,5 @@
+import * as _zod from 'zod';
+
+// This makes the vendored zod use the same types as the actual zod package
+export declare const z: typeof _zod.z;
+export default _zod.default;

src/server/__tests__/vendor/zod@3.24.4.umd.js

@@ -1,7 +1,6 @@
 import { McpServer } from "./mcp.js";
 import { Client } from "../client/index.js";
 import { InMemoryTransport } from "../inMemory.js";
-import { z } from "zod";
 import {
  ListToolsResultSchema,
  CallToolResultSchema,
@@ -17,6 +16,9 @@ import {
 import { ResourceTemplate } from "./mcp.js";
 import { completable } from "./completable.js";
 import { UriTemplate } from "../shared/uriTemplate.js";
+// Note: deliberately using a different Zod version to the one bundled
+// with the SDK as that's what our users are likely to be doing
+import { z } from "./__tests__/vendor/zod@3.24.4.umd.js";
 
 describe("McpServer", () => {
  test("should expose underlying Server instance", () => {

src/server/mcp.test.ts

@@ -659,16 +659,6 @@ export class McpServer {
  if (this._registeredTools[name]) {
  throw new Error(`Tool ${name} is already registered`);
  }
- 
- // Helper to check if an object is a Zod schema (ZodRawShape)
- const isZodRawShape = (obj: unknown): obj is ZodRawShape => {
- if (typeof obj !== "object" || obj === null) return false;
-
- const isEmptyObject = z.object({}).strict().safeParse(obj).success;
-
- // Check if object is empty or at least one property is a ZodType instance
- return isEmptyObject || Object.values(obj as object).some(v => v instanceof ZodType);
- };
 
  let description: string | undefined;
  if (typeof rest[0] === "string") {
@@ -931,6 +921,25 @@ const EMPTY_OBJECT_JSON_SCHEMA = {
  type: "object" as const,
 };
 
+// Helper to check if an object is a Zod schema (ZodRawShape)
+function isZodRawShape(obj: unknown): obj is ZodRawShape {
+ if (typeof obj !== "object" || obj === null) return false;
+
+ const isEmptyObject = z.object({}).strict().safeParse(obj).success;
+
+ // Check if object is empty or at least one property is a ZodType instance
+ return isEmptyObject || Object.values(obj as object).some(isZodType);
+}
+
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+function isZodType(value: any): value is ZodType {
+ return value !== null &&
+ typeof value === 'object' &&
+ typeof value.parse === 'function' &&
+ typeof value.safeParse === 'function' &&
+ typeof value._def === 'object';
+}
+
 /**
  * Additional, optional information for annotating a resource.
  */