Not Every Verb Is the Product
MCP stdio, cloud remember, and client verbs the engine marks optional are attachments. They pass the same schema gate when they run. They are not boot. A missing required verb refuses the client/engine pair at hello and names the verb. An optional verb is retained from the hello reply; each feature gates itself, so an older engine renders that surface disabled instead of failing when the operator reaches for it.
Capability negotiation is one declaration. The protocol schema classifies every engine-directed verb as protocol, required, or optional. The engine's advertised list and the native manifests are generated from it. A command the app can send that the classification does not cover fails a shadow test. Optional is a recorded fact, not a shrug.
MCP Does Not Teach the Loop a New Soul
An MCP server is a schema source and a runner. It receives its own environment keys and no other server's, and no leg's. The loop still does not learn what the tool is. A server that needs a credential the operator did not declare for it does not inherit one from the room.