Model fails to find Mcp Server tool when name contains double underscores

Bug Reports
1

Describe the Bug

When a tool name in Mcp Server contains double underscores __ (e.g., example__tool), it appears correctly in Cursor’s Mcp Tools UI, but the model fails to locate it during actual requests.

Steps to Reproduce

  1. Register a tool with a name containing __ (e.g., test__tool) in Mcp Server.
  2. Verify the tool is visible in Cursor’s Mcp Tools interface.
  3. Attempt to call the tool via model inference (API or interactive request).
  4. Observe whether the tool is missing or returns an error.

Expected Behavior

The model should successfully locate and use the tool regardless of underscores in the name.

Operating System

MacOS

Current Cursor Version (Menu → About Cursor → Copy)

• Cursor version: 1.3.9 (Universal)

• Model/API endpoint: claude-4-sonet

2

hi @pythonton does this still occur on 1.4.2?

3

yes @condor

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4

Thank you @pythonton , will pass that to the team.

Is that your own MCP?

5

@condor

Yes, this is an internal mcp server, protocol type sse

In order to verify the impact of double underscores (__), we conducted experiments and added 2 tools to a new mcp server, with the following names

v3_dataflow_flow_flows_create
v3_dataflow_flow_flows__fid__start_post

You will find that Cursor only recognizes v3_dataflow_flow_flows_create and cannot recognize v3_dataflow_flow_flows__fid__start_post

6

Thank you. While we are looking into it and checking how to allow for double underscores, is it possible for now to replace _ _ with _ so you can keep using it in the meantime?

7

@condor

We publish MCP Tool based on API.

The API has been used by many users.

If we change the API name, it will affect the production environment.

So, we can only wait for Cursor to repair it

1 Like
8

Thank you for the feedback. I have notified the team already.

1 Like
9

Thank you, condor

If you fix this problem, we will be able to better use Cursor to solve our needs.

Because we have too many APIs that use __ double underline

1 Like