Remote MCP servers are shipping fast, and a lot of them are generated straight out of an OpenAPI spec or a no-code builder. That is fine for a demo. It stops being fine the moment a real agent, holding real credentials, is allowed to call real tools against real customer data.
These are the seven failures I built my scanner around, with the quickest way to check each one by hand.
One note before the list, because it trips people up on first contact: MCP protocol revisions are named after dates. 2026-07-28 is a version string, not a deadline. That is the current revision and the one this is written against, and I will only name a revision where the difference changes what you actually do.
1. Tool calls that work without authentication
The most common failure by a distance, and the cheapest to test. Send tools/list without an Authorization header. If you get a tool list back instead of a 401, stop reading and fix this first.
