What happens when an MCP server changes its tool schema?

That “stale catalog” error means the client is still holding tool schemas from an old tools/list call: MCP clients cache those schemas, and a server that changes a tool’s parameters mid-session keeps getting called against the outdated version until the client asks again. The spec gives servers a way to signal the change: a server that declares the listChanged capability can send a notifications/tools/list_changed message, and a client that acts on it re-fetches the list before the next call.

A server that doesn’t declare the capability, or a client that doesn’t act on the notification, leaves the model building arguments for a tool that no longer looks like that, and the call either fails validation as a tool execution error or fails outright as a malformed request. A renamed tool parameter breaks an agent the same way whether the stale definition came from a cached MCP schema or a copy baked into a prompt. Treat a schema change like a deploy: version the tool name or restart the session, since caching gives a client no other trigger to refresh mid-conversation.