[BUG] Claude desktop MCP "searched available tools" causes possible regression

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Since today (Feb 13), Claude Desktop has not been playing nicely with our local MCP server that has been working for the past 3-4 months. The model is still the same, Sonnet 4.5. This is tested on a simple query that has always worked before.

• Doesn't understand it should use the MCP server unless explicitly stated to do so, then Searched available tools is done and it finally loads the available tools. This step has never happened before.

<img width="248" height="41" alt="Image" src="https://github.com/user-attachments/assets/3c73a930-9290-4503-9de1-d41f414325ef" />

• Doesn't seem to read the full tool instruction, because it doesn't provide mandatory arguments into a tool call. It also provides the wrong format into an argument (eg, it provides a list[str] into an argument that expects list[list[str]] type).

Note that this same query is still working on our langchain agent that uses langchain-anthropic.

What Should Happen?

As described in the problem, it should understand tool descriptions and arguments because the same MCP server has always worked with the same test query on Claude Desktop until today.

Error Messages/Logs

Steps to Reproduce

Our MCP server code is proprietary.
Claude Desktop version is Claude 1.1.3189 (1b7b58)
Model is Sonnet 4.5

Since the Searched available tools step seems to be new, I'm assuming it's not able to grab the full context of tool descriptions or something?

Claude Model

Sonnet (default)

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

1.1.3189

Platform

Other

Operating System

Windows

Terminal/Shell

Other

Additional Information

_No response_