Mcp

“Context engineering” is getting used as a rebrand for prompt writing, and that undersells it into uselessness. The actual discipline is deciding, every single turn, what earns a place in a bounded window and what gets evicted — and living with the fact that every eviction is a bet you’re making about the future of a run you haven’t finished yet. Here’s the physical constraint underneath the buzzword, the packing decision that’s actually hard, what it costs to get it wrong, and how to test whether your packing policy works.

MCP gives you a standard wire format for connecting an agent to tools — it doesn’t make the tools on the other end safe to call. Most MCP servers are thin wrappers around an existing REST API, which quietly imports every assumption that API made about having a human developer as its client. Here’s what MCP actually is, how it differs from the API you already know, and the specific tool-design mistakes that turn a working MCP server into a flaky agent.