An MCP server can ship a change that breaks every agent calling it, and nothing in your monitoring will notice. The endpoint still answers 200. The initialize handshake still completes. tools/list still returns a result. Every signal a conventional uptime check knows how to read stays green — and an agent that memorized last week's tool contract starts failing anyway, because the contract underneath it moved. This is drift: a server's tool inventory or a tool's input schema changing between two points in time, with no transport-level symptom at all.
Drift is not a hypothetical. MCP servers are young, most are maintained by small teams, and a tool's inputSchema is just a JSON object in a deploy — there is no compiler stopping someone from renaming a required field, tightening an enum, or dropping a tool nobody remembered an agent still called. The only way to catch it is to have looked at the server before and remember what you saw.
What Actually Counts as Drift
Drift is anything about a server's advertised capability contract that differs from the last time you checked. Concretely:
A tool disappears. It was in yesterday's tools/list, it is not in today's. Any agent that calls it now gets a JSON-RPC error mid-flight, not at startup — the failure shows up wherever the agent happens to reach for that tool.
