I'm using Claude Code - with some MCP installed - so you would assume that whole MCP thing would work with an agentic product from the makers of this standard. In 9 out of 10 cases where a MCP would make sense to use - it doesn't know when to call the MCP. And yes, i've done all the claude.md crap. There is no transparency in this protocol about how AI would know when to call an MCP (besides direct prompting). To cut short - it's not reliable.
This is an issue with the prompt and/or the tool descriptions and/or the model.
A MCP server is really just a collection of functions the model can call, and a list of those functions and their description/input params. In theory the MCP connector is calling /get-tools on the MCP servers and injecting that into the prompt, so the model knows which tools are available, their description and input params. It's then up to the model to pick a specific tool to use.
I don't know where specifically the MCP tools are injected into the prompt or what the original system prompt is (ie: maybe it's saying "always give the internal tools priority and only use MCPs if nothing else fits").
It could be the MCP server has poor descriptions of the tool. That is what the model uses to decide to use it or not.
It could also be the model just sucks. Claude Opus/Sonnet seem to be some of the best at tool calling, but it's still up to the model to pick which tools to use. Some models are better than other. Some models start to regress in their tool calling abilities as the context window fills up.
My instinct is the MCP tools have bad descriptions. I've done a bit of reverse engineering of Claude Code and most of the tool descriptions are very detailed. "Use this tool to call a bash command" would be a bad tool description. The Claude Code bash tool description is 110 lines long containing detailed usage information, when to use it, when not to use it, example usage and etc. It also has a summary at the bottom of very important things (that were just written above in the same description) for the model not to forget (they use the word IMPORTANT and YOU MUST a lot in the prompts/descriptions)
