MCP tool/endpoint works fine and then suddenly starts giving HTTP Bad Request (400) errors

[michelschep]

Pre-submission Checklist

• I have checked that this question would not be more appropriate as an issue in a specific repository
• I have searched existing discussions and documentation for answers

Question Category

• Protocol Specification
• SDK Usage
• Server Implementation
• General Implementation
• Documentation
• Other

Your Question

Hi everyone,

I’m working on an application with:

An Angular front end
A .NET Web API that hosts a Semantic Kernel–powered MCP server (the same endpoint runs inside the Web API)
Workflow:

The user asks a question in the front end.
The question is sent to the Web API.
Semantic Kernel processes the question and calls the appropriate MCP tools via the MCP endpoint.
What works:

For the first few minutes, everything runs smoothly.
The LLM (OpenAI), Semantic Kernel, and the MCP server respond correctly.
The issue:

After a few minutes, all calls to the MCP endpoint start failing with 400 Bad Request, even though neither the endpoint nor the JSON body has changed.
Restarting the Web API restores functionality temporarily.
Has anyone encountered this issue before or have ideas on what might be causing it (and how to fix it)? Any suggestions are greatly appreciated!

Thanks in advance,
Michel