What does it mean that "different clients can get different specs"? Different in what dimension? I could imagine this makes creating repeatable and reliable workflows problematic.

Using MCP you can send "notifications" to the server, and the server can send back notifications including the availability of new tools.

So this isn't the same as saying "this user agent gets X, this gets Y". It's more like "this client requested access to X set of tools, so we sent back a notification with the list of those additional tools".

This is why I do think websockets make more sense in a lot of ways here, as there's a lot more two way communication here than you'd expect in a typically API. This communication also is very session based, which is another thing that doesn't make sense for most OpenAPI specs which assume a more REST-like stateless setup.