MCP Tools Parameter Schema Not Displaying Correctly - Shows Generic "random_string" Instead of Actual Schema

unknown_unknown · June 26, 2025, 8:39pm

Describe the Bug

I have implemented a custom MCP (Model Context Protocol) server for Zephyr Scale API integration. The tools are successfully registered and appear in Cursor’s MCP tools list, but the parameter schemas are not displaying correctly. Instead of showing the actual Joi schema parameters I’ve defined, all tools show a generic “random_string: Dummy parameter for non-parameter tools” parameter.
Expected Behavior:
Tools should display the actual schema parameters defined in my Joi schemas. For example, listTestCases should show parameters like:
projectKey (string, required)
folderId (number, optional)
maxResults (number, optional)
startAt (number, optional)
Actual Behavior:
All tools show only: random_string: Dummy parameter for no-parameter tools
Technical Details:
MCP Server Configuration: Located in ~/.cursor/mcp.json
Server Type: Custom TypeScript MCP server using @modelcontextprotocol/sdk
Schema System: Using Joi schemas converted to JSON Schema format
Tool Registration: Using server.setRequestHandler(ListToolsRequestSchema, …) with proper inputSchema parameter
What I’ve Verified:
API calls work correctly when tools are invoked
Schemas are properly detected and converted from Joi to JSON Schema format
CLI mode works perfectly with correct parameter validation
MCP server starts without errors and tools are listed
Debug output shows proper schema structure being returned

Code Sample - Tool Registration:
server.setRequestHandler(ListToolsRequestSchema, async () => ({
tools: tools.map(tool => ({
name: tool.name,
description: tool.description,
inputSchema: tool.schema // This contains proper JSON Schema
}))
}));

MCP Configuration:
“Zephyr Scale Tools”: {
“command”: “npx”,
“args”: [“ts-node”, “/Users/[USER NAME]/Documents/Projects/custom-model-contexts/src/index.ts”],
“cwd”: “/Users/[USER NAME]/Documents/Projects/custom-model-contexts”
}

Environment:
OS: macOS 14.5.0 (Darwin 24.5.0)
Cursor Version: [Please include your current Cursor version]
Node.js: [Your Node.js version]
MCP SDK Version: Latest (@modelcontextprotocol/sdk)

This issue has been started after I upgrade my Cursor IDE to
Version: 1.1.6 (Universal)
VSCode Version: 1.96.2
Commit: 5b19bac7a947f54e4caa3eb7e4c5fbf832389850
Date: 2025-06-25T02:16:57.571Z
Electron: 34.5.1
Chromium: 132.0.6834.210
Node.js: 20.19.0
V8: 13.2.152.41-electron.0
OS: Darwin x64 24.5.0

Steps to Reproduce

Here are the detailed steps to reproduce the MCP parameter schema bug for Cursor IDE support:

Bug Report: MCP Tools Show Generic “random_string” Parameter Instead of Actual Schema

Steps to Reproduce

Prerequisites

macOS with Node.js installed
Cursor IDE installed
Terminal access

Step 1: Create Test MCP Server

Create a new directory:

mkdir ~/test-mcp-server
cd ~/test-mcp-server

Initialize Node.js project:

npm init -y
npm install @modelcontextprotocol/sdk joi joi-to-json-schema
npm install -D typescript ts-node @types/node

Create server.ts with this minimal test case:

#!/usr/bin/env node
import { Server } from '@modelcontextprotocol/sdk/server/index.js';
import { StdioServerTransport } from '@modelcontextprotocol/sdk/server/stdio.js';
import { ListToolsRequestSchema, CallToolRequestSchema } from '@modelcontextprotocol/sdk/types.js';
import Joi from 'joi';
import convert from 'joi-to-json-schema';

// Define a simple schema
const testSchema = Joi.object({
  projectKey: Joi.string().required().description('Project key identifier'),
  maxResults: Joi.number().integer().min(1).max(100).default(50).description('Maximum results to return'),
  startAt: Joi.number().integer().min(0).default(0).description('Starting index for pagination')
});

const server = new Server(
  { name: 'test-mcp-server', version: '1.0.0' },
  { capabilities: { tools: {} } }
);

// Register tools with proper schema
server.setRequestHandler(ListToolsRequestSchema, async () => {
  const jsonSchema = convert(testSchema);
  console.error('DEBUG: Returning schema:', JSON.stringify(jsonSchema, null, 2));
  
  return {
    tools: [{
      name: 'test_tool',
      description: 'A test tool with proper schema',
      inputSchema: jsonSchema
    }]
  };
});

// Handle tool calls
server.setRequestHandler(CallToolRequestSchema, async (request) => {
  if (request.params.name === 'test_tool') {
    return {
      content: [{ type: 'text', text: `Called with args: ${JSON.stringify(request.params.arguments)}` }]
    };
  }
  throw new Error(`Unknown tool: ${request.params.name}`);
});

async function main() {
  const transport = new StdioServerTransport();
  await server.connect(transport);
}

main().catch(console.error);

Step 2: Configure MCP in Cursor

Open ~/.cursor/mcp.json (create if doesn’t exist)

Add the test server configuration:

{
  "mcpServers": {
    "test-mcp": {
      "command": "npx",
      "args": [
        "ts-node",
        "/Users/[YOUR_USERNAME]/test-mcp-server/server.ts"
      ],
      "cwd": "/Users/[YOUR_USERNAME]/test-mcp-server"
    }
  }
}

Save the file

Step 3: Test Server Works Standalone

Verify the server can start:

cd ~/test-mcp-server
npx ts-node server.ts

It should wait for input (MCP protocol). Press Ctrl+C to exit.

Step 4: Restart Cursor

Completely quit Cursor IDE
Relaunch Cursor IDE
Wait 10-15 seconds for MCP servers to initialize

Step 5: Reproduce the Bug

In Cursor, access the MCP tools interface (usually via command palette or tools panel)
Look for the test_tool from the “test-mcp” server
Click on the tool to view its parameters

Expected Result:
Should show parameters:

projectKey (string, required) - “Project key identifier”
maxResults (number, optional, default: 50) - “Maximum results to return”
startAt (number, optional, default: 0) - “Starting index for pagination”

Actual Result:
Shows only:

random_string (string, required) - “Dummy parameter for no-parameter tools”

Step 6: Verify Schema is Actually Correct

Check the debug output in Cursor’s MCP logs (if available) or run:

cd ~/test-mcp-server
echo '{"jsonrpc":"2.0","id":1,"method":"tools/list","params":{}}' | npx ts-node server.ts

You should see the proper JSON schema being output in the DEBUG line

Step 7: Additional Verification

Try with an even simpler schema:

const simpleSchema = {
  type: 'object',
  properties: {
    name: { type: 'string', description: 'A simple name parameter' }
  },
  required: ['name']
};

Replace the convert(testSchema) with simpleSchema and test again
The issue should persist even with hand-crafted JSON Schema

Environment Details

OS: macOS [version]
Cursor Version: [your version]
Node.js Version: [your version]
MCP SDK Version: Latest from npm

Expected vs Actual Behavior

Expected: MCP tools should display the actual schema parameters defined in inputSchema
Actual: All MCP tools show a generic random_string parameter regardless of their actual schema

Impact

This prevents users from properly using custom MCP tools as they cannot see what parameters are actually required or available.

Additional Notes

The tools themselves work correctly when called with proper parameters
The issue appears to be in Cursor’s parameter schema parsing/display logic
Other MCP implementations (like built-in servers) may or may not exhibit this issue

These steps create a minimal reproducible test case that should clearly demonstrate the bug to the Cursor support team.

Expected Behavior

MCP Client should recognise the Tool expected parameters. Should not return random_string

Operating System

MacOS

Current Cursor Version (Menu → About Cursor → Copy)

Version: 1.1.6 (Universal)
VSCode Version: 1.96.2
Commit: 5b19bac7a947f54e4caa3eb7e4c5fbf832389850
Date: 2025-06-25T02:16:57.571Z
Electron: 34.5.1
Chromium: 132.0.6834.210
Node.js: 20.19.0
V8: 13.2.152.41-electron.0
OS: Darwin x64 24.5.0

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MCP Tools Parameter Schema Not Displaying Correctly - Shows Generic "random_string" Instead of Actual Schema

Describe the Bug

Steps to Reproduce

Steps to Reproduce

Prerequisites

Step 1: Create Test MCP Server

Step 2: Configure MCP in Cursor

Step 3: Test Server Works Standalone

Step 4: Restart Cursor

Step 5: Reproduce the Bug

Step 6: Verify Schema is Actually Correct

Step 7: Additional Verification

Environment Details

Expected vs Actual Behavior

Impact

Additional Notes

Expected Behavior

Operating System

Current Cursor Version (Menu → About Cursor → Copy)

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