fix: tool param set to default value when omitted

Author: gregnr

packages/mcp-utils/src/server.test.ts

@@ -1,5 +1,120 @@
+import { Client } from '@modelcontextprotocol/sdk/client/index.js';
+import type { Server } from '@modelcontextprotocol/sdk/server/index.js';
+import {
+  CallToolResultSchema,
+  type CallToolRequest,
+} from '@modelcontextprotocol/sdk/types.js';
 import { describe, expect, test } from 'vitest';
-import { resource, resources, resourceTemplate } from './server.js';
+import { z } from 'zod';
+import {
+  createMcpServer,
+  resource,
+  resources,
+  resourceTemplate,
+  tool,
+} from './server.js';
+import { StreamTransport } from './stream-transport.js';
+
+export const MCP_CLIENT_NAME = 'test-client';
+export const MCP_CLIENT_VERSION = '0.1.0';
+
+type SetupOptions = {
+  server: Server;
+};
+
+/**
+ * Sets up an MCP client and server for testing.
+ */
+async function setup(options: SetupOptions) {
+  const { server } = options;
+  const clientTransport = new StreamTransport();
+  const serverTransport = new StreamTransport();
+
+  clientTransport.readable.pipeTo(serverTransport.writable);
+  serverTransport.readable.pipeTo(clientTransport.writable);
+
+  const client = new Client(
+    {
+      name: MCP_CLIENT_NAME,
+      version: MCP_CLIENT_VERSION,
+    },
+    {
+      capabilities: {},
+    }
+  );
+
+  await server.connect(serverTransport);
+  await client.connect(clientTransport);
+
+  /**
+   * Calls a tool with the given parameters.
+   *
+   * Wrapper around the `client.callTool` method to handle the response and errors.
+   */
+  async function callTool(params: CallToolRequest['params']) {
+    const output = await client.callTool(params);
+    const { content } = CallToolResultSchema.parse(output);
+    const [textContent] = content;
+
+    if (!textContent) {
+      return undefined;
+    }
+
+    if (textContent.type !== 'text') {
+      throw new Error('tool result content is not text');
+    }
+
+    if (textContent.text === '') {
+      throw new Error('tool result content is empty');
+    }
+
+    const result = JSON.parse(textContent.text);
+
+    if (output.isError) {
+      throw new Error(result.error.message);
+    }
+
+    return result;
+  }
+
+  return { client, clientTransport, callTool, server, serverTransport };
+}
+
+describe('tools', () => {
+  test('parameter set to default value when omitted by caller', async () => {
+    const server = createMcpServer({
+      name: 'test-server',
+      version: '0.0.0',
+      tools: {
+        search: tool({
+          description: 'Search text',
+          parameters: z.object({
+            query: z.string(),
+            caseSensitive: z.boolean().default(false),
+          }),
+          execute: async (args) => {
+            return args;
+          },
+        }),
+      },
+    });
+
+    const { callTool } = await setup({ server });
+
+    // Call the tool without the optional parameter
+    const result = await callTool({
+      name: 'search',
+      arguments: {
+        query: 'hello',
+      },
+    });
+
+    expect(result).toEqual({
+      query: 'hello',
+      caseSensitive: false,
+    });
+  });
+});
 
 describe('resources helper', () => {
   test('should add scheme to resource URIs', () => {

packages/mcp-utils/src/server.ts

@@ -340,7 +340,7 @@ export function createMcpServer(options: McpServerOptions) {
         throw new Error('tool not found');
       }
 
-      const args = request.params.arguments as z.infer<Tool['parameters']>;
+      const args = tool.parameters.parse(request.params.arguments);
 
       if (!args) {
         throw new Error('missing arguments');