fix (ai/mcp): improve handling of zero-argument MCP tools (#5670) (#5683)

Co-authored-by: Grace Yun <74513600+iteratetograceness@users.noreply.github.com>

Author: lgrammel

.changeset/pretty-bikes-appear.md

@@ -0,0 +1,5 @@
+---
+'ai': patch
+---
+
+fix (ai/mcp): better support for zero-argument MCP tools

content/docs/03-ai-sdk-core/15-tools-and-tool-calling.mdx

@@ -768,6 +768,10 @@ const tools = await mcpClient.tools({
         format: z.enum(['json', 'text']).optional(),
       }),
     },
+    // For tools with zero arguments, you should use an empty object:
+    'tool-with-no-args': {
+      parameters: z.object({}),
+    },
   },
 });
 ```

examples/mcp/src/sse/client.ts

@@ -1,7 +1,6 @@
 import { openai } from '@ai-sdk/openai';
 import { experimental_createMCPClient, generateText } from 'ai';
 import 'dotenv/config';
-import { z } from 'zod';
 
 async function main() {
   const mcpClient = await experimental_createMCPClient({
@@ -14,21 +13,17 @@ async function main() {
     },
   });
 
+  const tools = await mcpClient.tools();
+
   const { text: answer } = await generateText({
     model: openai('gpt-4o-mini', { structuredOutputs: true }),
-    tools: await mcpClient.tools({
-      schemas: {
-        'find-product': {
-          parameters: z.object({}),
-        },
-      },
-    }),
+    tools,
     maxSteps: 10,
     onStepFinish: async ({ toolResults }) => {
       console.log(`STEP RESULTS: ${JSON.stringify(toolResults, null, 2)}`);
     },
     system: 'You are a helpful chatbot',
-    prompt: 'Can you find a product called The Product?',
+    prompt: 'List all products, then find availability for Product 1.',
   });
 
   await mcpClient.close();

examples/mcp/src/sse/server.ts

@@ -1,20 +1,38 @@
-import 'dotenv/config';
-import express from 'express';
 import { McpServer } from '@modelcontextprotocol/sdk/server/mcp.js';
 import { SSEServerTransport } from '@modelcontextprotocol/sdk/server/sse.js';
+import 'dotenv/config';
+import express from 'express';
+import { z } from 'zod';
 
 const mcpServer = new McpServer({
   name: 'example-server',
   version: '1.0.0',
 });
 
-mcpServer.tool('find-product', 'Find a product', {}, async () => {
+// Tool with arguments:
+mcpServer.tool(
+  'check-product-stock',
+  'Check if a product is available',
+  {
+    productName: z.string(),
+  },
+  async ({ productName }) => {
+    return {
+      content: [
+        {
+          type: 'text',
+          text: `The product ${productName} is available in stock`,
+        },
+      ],
+    };
+  },
+);
+
+// Tool with zero arguments:
+mcpServer.tool('list-products', 'List all products', async () => {
   return {
     content: [
-      {
-        type: 'text',
-        text: 'The Product is available in stock',
-      },
+      { type: 'text', text: 'Products: Product 1, Product 2, Product 3' },
     ],
   };
 });

packages/ai/core/tool/mcp/mcp-client.test.ts

@@ -276,4 +276,25 @@ describe('MCPClient', () => {
       createMCPClient({ transport: invalidTransport }),
     ).rejects.toThrow();
   });
+
+  it('should support zero-argument tools', async () => {
+    client = await createMCPClient({
+      transport: { type: 'sse', url: 'https://example.com/sse' },
+    });
+    const tools = await client.tools();
+    const tool = tools['mock-tool-no-args'];
+    expect(tool).toHaveProperty('parameters');
+    expect(tool.parameters).toMatchObject({
+      jsonSchema: {
+        type: 'object',
+        properties: {},
+        additionalProperties: false,
+      },
+    });
+
+    const result = await tool.execute({}, { messages: [], toolCallId: '1' });
+    expect(result).toEqual({
+      content: [{ type: 'text', text: 'Mock tool call result' }],
+    });
+  });
 });

packages/ai/core/tool/mcp/mcp-client.ts

@@ -309,7 +309,11 @@ class MCPClient {
 
         const parameters =
           schemas === 'automatic'
-            ? jsonSchema(inputSchema as JSONSchema7)
+            ? jsonSchema({
+                ...inputSchema,
+                properties: inputSchema.properties ?? {},
+                additionalProperties: false,
+              } as JSONSchema7)
             : schemas[name].parameters;
 
         const self = this;

packages/ai/core/tool/mcp/mock-mcp-transport.ts

@@ -14,6 +14,13 @@ const DEFAULT_TOOLS: MCPTool[] = [
       },
     },
   },
+  {
+    name: 'mock-tool-no-args',
+    description: 'A mock tool for testing',
+    inputSchema: {
+      type: 'object',
+    },
+  },
 ];
 
 export class MockMCPTransport implements MCPTransport {