Do unused MCP tools cost you money?

A short case study from my "building and testing MCP agents" series — it stands on its own, but the method behind it is laid out in https://dev.to/langensjonathan/the-parameters-that-actually-matter-when-youre-tuning-an-ai-agent-2agd . TL;DR: I benchmarked two agents that are identical except for one thing — how many MCP servers they're connected to — on the exact same question. Both got the right answer, both called the same single tool. The one with more MCP servers attached still cost 28% more per question , purely from the extra tool schemas the model has to be told about on every single call, whether it uses them or not. The setup MAVERIK is my open-source MCP test bench: define a suite of questions with pass criteria, run it against one or more agent configurations, and compare the results on hard numbers. This post is one deliberately tiny experiment with it: change exactly one thing about an agent, hold everything else fixed, and see what the numbers attribute to that one change. I have a small "GitHub summarizer" agent: one system prompt, one job — answer questions about my GitHub account by calling the GitHub MCP server . I duplicated its configuration (MAVERIK supports this directly — same model, same prompt, same everything) and changed one field on the copy: the set of attached MCP servers, adding deepwiki , microsoft-learn , and context7 . Neither agent needs any of those three for the question I was about to ask; they were attached because that's what the "kitchen sink" version of this agent had accumulated over a few sessions of general-purpose use. Then I wrote the simplest possible test suite — one question: "How many repositories do I have?" with a contains criterion checking the answer includes the correct count. No judge model, no subjectivity — it either says the right number or it doesn't. I ran both agents against it, 2 repetitions each, same model ( claude-haiku ) for both, and pulled up MAVERIK's Agent Comparison report. Agent A — github on