[3.12.x] MCP tool arguments not forwarded to server (all params undefined); works on 3.11.25

Support Bug Reports
1

Describe the Bug

Summary

After upgrading to Cursor 3.12.x, MCP tool calls from Agent reach the MCP server, but all tool arguments are undefined. The same MCP config works on 3.11.25.

Environment

  • OS: Windows 11
  • Cursor broken: 3.12.30 (stable / latest)
  • Cursor working: 3.11.25
  • MCP server: @upstash/context7-mcp@latest (via npx -y)
  • Node: (填你的 node -v)
  • npm: (填你的 npm -v)

MCP config (mcp.json excerpt)

{
  "mcpServers": {
    "context7": {
      "command": "npx",
      "args": ["-y", "@upstash/context7-mcp@latest"],
      "disabled": false
    }
  }
}

Steps to Reproduce

  1. Install / update Cursor to 3.12.30
  2. Enable context7 MCP (or any MCP tool with required string params)
  3. In Agent chat, ask the agent to call a tool, e.g. context7 resolve-library-id with:
    • query: “How to configure multi datasource in Spring Boot”
    • libraryName: “Spring Boot”
  4. Observe MCP / agent tool error

Actual Result (3.12.30)

MCP server responds with validation error — arguments not received:

MCP error -32602: Input validation error: Invalid arguments for tool resolve-library-id: [
  {
    "expected": "string",
    "code": "invalid_type",
    "path": ["query"],
    "message": "Invalid input: expected string, received undefined"
  },
  {
    "expected": "string",
    "code": "invalid_type",
    "path": ["libraryName"],
    "message": "Invalid input: expected string, received undefined"
  }
]

Notes:

  • MCP process does connect (not “Connection closed”)
  • Tool schema on disk has correct properties / required
  • In Agent tool surface, schema sometimes appears as empty properties: {}
  • Running the same MCP via raw JSON-RPC stdio (outside Cursor Agent) works: initialize / tools/list / tools/call all PASS and return real docs

Expected Result

Agent should forward tool arguments to the MCP server exactly as specified (same as 3.11.25).

Working control (3.11.25)

  1. Roll back to Cursor 3.11.25
  2. Same MCP config, same prompts
  3. resolve-library-id and query-docs succeed and return real documentation content

Isolation

Layer 3.12.30 3.11.25
MCP process start (npx) OK OK
Cursor MCP connection OK OK
Agent → MCP argument forwarding BROKEN OK
Direct JSON-RPC to MCP (CLI) OK OK

Conclusion

Regression in Cursor 3.12.x Agent MCP tool invocation: parameters are dropped before/at tool call, not a context7 package install issue.

Additional

  • Please check agent-side MCP tool schema exposure (empty properties) and argument serialization in 3.12.x
  • Happy to provide logs if needed (MCP logs / Developer Tools)

Steps to Reproduce

  1. Upgrade Cursor to 3.12.30 (stable/latest).
  2. Enable context7 MCP:
    command: npx
    args: -y @upstash/context7-mcp@latest
  3. In Agent chat, ask the agent to use context7 resolve-library-id with:
    query = “How to configure multi datasource in Spring Boot”
    libraryName = “Spring Boot”
  4. Observe the error: MCP receives all args as undefined (error -32602).
  5. Roll back to Cursor 3.11.25, repeat steps 2–3 → same MCP works and returns real docs.

Operating System

Windows 10/11

Version Information

cursor ide 3.12.30

Does this stop you from using Cursor

Sometimes - I can sometimes use Cursor

5

Hey, thanks for the detailed report. The isolation table and the working control on 3.11.25 are really helpful.

This looks like a real regression in 3.12.x in the Agent → MCP argument passing layer. The MCP server itself and direct JSON-RPC work fine, it’s only the argument forwarding that breaks. I’ve reported it internally, but I can’t share a fix timeline yet.

As a workaround, please stay on 3.11.25. You already confirmed the same config works correctly there.

Once I have an update on the fix, I’ll reply in this thread.