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.

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/mcp.test.ts

@@ -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.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.
  */