Everyone talks about MCP servers as a way to extend Claude Code. Fewer people talk about what they cost.
Every MCP tool you register injects a tool-definition block into your context window on every single turn. That's not a one-time cost — it compounds across your entire session. I wanted to know the actual numbers, so I measured them.
What MCP tool definitions actually look like in your context
When Claude Code loads an MCP server, it reads the server's tool manifest and injects something like this into the system prompt:
<tool>
