Fix isZodRawShape return false on empty object and add test

src/server/mcp.test.ts

@@ -622,6 +622,53 @@ describe("tool()", () => {
     });
   });
 
+  test("should register tool with description, empty params, and annotations", async () => {
+    const mcpServer = new McpServer({
+      name: "test server",
+      version: "1.0",
+    });
+    const client = new Client({
+      name: "test client",
+      version: "1.0",
+    });
+
+    mcpServer.tool(
+      "test", 
+      "A tool with everything but empty params",
+      {},
+      { title: "Complete Test Tool with empty params", readOnlyHint: true, openWorldHint: false },
+      async () => ({
+        content: [{ type: "text", text: "Test response" }]
+      })
+    );
+
+    const [clientTransport, serverTransport] =
+      InMemoryTransport.createLinkedPair();
+
+    await Promise.all([
+      client.connect(clientTransport),
+      mcpServer.server.connect(serverTransport),
+    ]);
+
+    const result = await client.request(
+      { method: "tools/list" },
+      ListToolsResultSchema,
+    );
+
+    expect(result.tools).toHaveLength(1);
+    expect(result.tools[0].name).toBe("test");
+    expect(result.tools[0].description).toBe("A tool with everything but empty params");
+    expect(result.tools[0].inputSchema).toMatchObject({
+      type: "object",
+      properties: {}
+    });
+    expect(result.tools[0].annotations).toEqual({ 
+      title: "Complete Test Tool with empty params", 
+      readOnlyHint: true,
+      openWorldHint: false
+    });
+  });
+
   test("should validate tool args", async () => {
     const mcpServer = new McpServer({
       name: "test server",

src/server/mcp.ts

@@ -663,8 +663,11 @@ export class McpServer {
     // 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;
-      // Check that at least one property is a ZodType instance
-      return Object.values(obj as object).some(v => v instanceof ZodType);
+
+      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;