refactor: make BaseContext/Context covariant in their type params

LifespanT and TransportT are only exposed via read-only properties (lifespan,
transport), so covariance is sound. This lets a Context[AppState, HttpTC] be
passed where a Context[object, TransportContext] is expected — needed for
ServerRunner's middleware chain to compose without casts, and for reusable
middleware to be typed Context[object, TransportContext] instead of relying
on Any-slack.

Author: maxisbey

src/mcp/server/context.py

@@ -30,8 +30,8 @@ class ServerRequestContext(RequestContext[ServerSession], Generic[LifespanContex
     close_standalone_sse_stream: CloseSSEStreamCallback | None = None
 
 
-LifespanT = TypeVar("LifespanT", default=Any)
-TransportT = TypeVar("TransportT", bound=TransportContext, default=TransportContext)
+LifespanT = TypeVar("LifespanT", default=Any, covariant=True)
+TransportT = TypeVar("TransportT", bound=TransportContext, default=TransportContext, covariant=True)
 
 
 class Context(BaseContext[TransportT], PeerMixin, TypedServerRequestMixin, Generic[LifespanT, TransportT]):

src/mcp/shared/context.py

@@ -22,7 +22,7 @@
 
 __all__ = ["BaseContext"]
 
-TransportT = TypeVar("TransportT", bound=TransportContext, default=TransportContext)
+TransportT = TypeVar("TransportT", bound=TransportContext, default=TransportContext, covariant=True)
 
 
 class BaseContext(Generic[TransportT]):