The gate admits one namespace.

Every BCS system sits behind one function. EchoData.Bcs.gate(id, ns) parses an id once and admits it only if its namespace is the one the system declared: a match returns {:ok, snowflake}, another namespace returns {:error, :namespace}, and an unparseable id returns {:error, :invalid}. Two failure modes, named, and nothing admitted by accident.

The gate adds no second parser. Classification beyond the namespace collapses to :invalid, exactly as EchoData.BrandedId.parse/1 reports it, so there is one authority on what an id is and the gate defers to it. The property store, the edges store, the timeline — each crosses this gate before it touches a tier, which is why a malformed write is refused at the door rather than stored and discovered wrong on a read.

Codemojex puts the gate in front of every command. A move names a player and a room, and the room’s gate admits only its own kind on each end before the board is touched; an id from another namespace is turned away with :namespace, never written and never trusted. The boundary the whole course has described is, in the end, this one call.