I am currently trying to use Microsoft Entra ID auth for an MCP server I'm implementing, and I genuinely feel like I must be an idiot.
- I can use the `WWW-Authenticate` header to indicate a resource metadata URL for the client.
- I can use this to indicate an authorization server (Microsoft Entra) and a scope (for the app registration that handle which app roles each user is given to differentiate different capabilities for different users).
- I can NOT indicate a client_id, because that's just something that each client (agent) makes up on its own?
- To initiate a login on the .../authorize URL in Microsoft Entra, you need to pass a known client_id that matches an app registration in Microsoft Entra. Whatever the client makes up will surely not match anything in Microsoft Entra.
- I COULD in theory support dynamic client registration, but of course Microsoft Entra doesn't.
Is it even possible to make this work out of the box? The only way forward I can see is implementing my own dynamic client registration shim in front of Microsoft Entra that just returns the same static client_id to everyone, which matches an actual client_id in Microsoft Entra.
But surely this protocol actually works today for real Enterprises without workarounds? It feels like I must be missing something obvious.