Your MCP Servers Bill You on Every Request: The Tool-Definition Tax
You connect a few MCP servers and your agent gains dozens of abilities. What the connection also adds is a block of text that rides along in the prompt of every single request, whether the agent uses
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Your "re-measure after every upgrade" step is worth automating, because it pays off three ways. Snapshot each server's tools/list output, hash it, and alert when the hash changes. That catches token creep, a reordered list that quietly drops your cache hit rate, and a description that now tells the model something new. The last one is the security case: the model reads those descriptions as instructions on every turn, so a server upgrade can change what your agent does with no code change on your side. Was the 51% description share on the filesystem server typical of the others you looked at?