feat: DispatchContext.request_id; drop RequestId arg from transport_builder

request_id is the wire-format correlation id (JSON-RPC message id; None for
notifications and for dispatchers without one). Lives on DispatchContext
because it's wire-format-shaped (dispatcher domain), not transport-shaped.
ServerRunner._make_context will read it to populate
ServerRequestContext.request_id.

transport_builder no longer takes RequestId: that arg existed so the builder
could put the id on a TransportContext subclass, which is now redundant with
dctx.request_id. Nothing read it.

Author: maxisbey

src/mcp/shared/direct_dispatcher.py

@@ -26,7 +26,7 @@
 from mcp.shared.exceptions import MCPError, NoBackChannelError
 from mcp.shared.message import MessageMetadata
 from mcp.shared.transport_context import TransportContext
-from mcp.types import INTERNAL_ERROR, REQUEST_TIMEOUT
+from mcp.types import INTERNAL_ERROR, REQUEST_TIMEOUT, RequestId
 
 __all__ = ["DirectDispatcher", "create_direct_dispatcher_pair"]
 
@@ -48,6 +48,8 @@ class _DirectDispatchContext:
     transport: TransportContext
     _back_request: _Request
     _back_notify: _Notify
+    request_id: RequestId | None = None
+    """Always ``None``: direct dispatch has no wire-level request id."""
     message_metadata: MessageMetadata = None  # TODO(maxisbey): remove for Context rework
     """Always ``None``: in-memory dispatch attaches no transport metadata."""
     _on_progress: ProgressFnT | None = None

src/mcp/shared/dispatcher.py

@@ -24,6 +24,7 @@
 
 from mcp.shared.message import MessageMetadata
 from mcp.shared.transport_context import TransportContext
+from mcp.types import RequestId
 
 __all__ = [
     "CallOptions",
@@ -108,6 +109,16 @@ def transport(self) -> TransportT_co:
         """Transport-specific metadata for this inbound message."""
         ...
 
+    @property
+    def request_id(self) -> RequestId | None:
+        """The id of the inbound request, or ``None`` for a notification.
+
+        For JSON-RPC this is the wire ``id`` field. Handlers thread it through
+        as ``related_request_id`` on outbound notifications so HTTP transports
+        can route them onto the originating request's response stream.
+        """
+        ...
+
     @property
     def message_metadata(self) -> MessageMetadata:
         """The metadata the transport attached to this inbound message, if any.

src/mcp/shared/jsonrpc_dispatcher.py

@@ -127,6 +127,10 @@ class _JSONRPCDispatchContext(Generic[TransportT]):
     _closed: bool = False
     cancel_requested: anyio.Event = field(default_factory=anyio.Event)
 
+    @property
+    def request_id(self) -> RequestId | None:
+        return self._request_id
+
     @property
     def can_send_request(self) -> bool:
         return self.transport.can_send_request and not self._closed
@@ -158,7 +162,7 @@ def close(self) -> None:
         self._closed = True
 
 
-def _default_transport_builder(_request_id: RequestId | None, _meta: MessageMetadata) -> TransportContext:
+def _default_transport_builder(_meta: MessageMetadata) -> TransportContext:
     return TransportContext(kind="jsonrpc", can_send_request=True)
 
 
@@ -199,7 +203,7 @@ def __init__(
         read_stream: ReadStream[SessionMessage | Exception],
         write_stream: WriteStream[SessionMessage],
         *,
-        transport_builder: Callable[[RequestId | None, MessageMetadata], TransportT],
+        transport_builder: Callable[[MessageMetadata], TransportT],
         peer_cancel_mode: PeerCancelMode = "interrupt",
         raise_handler_exceptions: bool = False,
     ) -> None: ...
@@ -208,7 +212,7 @@ def __init__(
         read_stream: ReadStream[SessionMessage | Exception],
         write_stream: WriteStream[SessionMessage],
         *,
-        transport_builder: Callable[[RequestId | None, MessageMetadata], TransportT] | None = None,
+        transport_builder: Callable[[MessageMetadata], TransportT] | None = None,
         peer_cancel_mode: PeerCancelMode = "interrupt",
         raise_handler_exceptions: bool = False,
     ) -> None:
@@ -218,7 +222,7 @@ def __init__(
         # `TransportT` is `TransportContext`, so the default is type-correct;
         # pyright can't see across overloads, hence the cast.
         self._transport_builder = cast(
-            "Callable[[RequestId | None, MessageMetadata], TransportT]",
+            "Callable[[MessageMetadata], TransportT]",
             transport_builder or _default_transport_builder,
         )
         self._peer_cancel_mode: PeerCancelMode = peer_cancel_mode
@@ -400,7 +404,7 @@ def _dispatch_request(
                 pass
             case _:
                 progress_token = None
-        transport_ctx = self._transport_builder(req.id, metadata)
+        transport_ctx = self._transport_builder(metadata)
         dctx = _JSONRPCDispatchContext(
             transport=transport_ctx,
             _dispatcher=self,
@@ -446,7 +450,7 @@ def _dispatch_notification(
                 case _:
                     pass
             # fall through: progress is also teed to on_notify
-        transport_ctx = self._transport_builder(None, metadata)
+        transport_ctx = self._transport_builder(metadata)
         dctx = _JSONRPCDispatchContext(
             transport=transport_ctx, _dispatcher=self, _request_id=None, message_metadata=metadata
         )

tests/shared/conftest.py

@@ -35,7 +35,7 @@ def jsonrpc_pair(*, can_send_request: bool = True) -> DispatcherTriple:
     c2s_send, c2s_recv = anyio.create_memory_object_stream[SessionMessage | Exception](32)
     s2c_send, s2c_recv = anyio.create_memory_object_stream[SessionMessage | Exception](32)
 
-    def builder(_rid: object, _meta: object) -> TransportContext:
+    def builder(_meta: object) -> TransportContext:
         return TransportContext(kind="jsonrpc", can_send_request=can_send_request)
 
     client: JSONRPCDispatcher[TransportContext] = JSONRPCDispatcher(s2c_recv, c2s_send, transport_builder=builder)

tests/shared/test_dispatcher.py

@@ -206,6 +206,17 @@ async def test_ctx_message_metadata_is_none_when_transport_attaches_nothing(pair
     assert srec.contexts[0].message_metadata is None
 
 
+@pytest.mark.anyio
+async def test_ctx_request_id_exposes_inbound_id(pair_factory: PairFactory):
+    """JSON-RPC carries the wire id through; direct dispatch has none."""
+    async with running_pair(pair_factory) as (client, _server, _crec, srec):
+        with anyio.fail_after(5):
+            await client.send_raw_request("tools/call", None)
+            await client.send_raw_request("tools/call", None)
+    a, b = (ctx.request_id for ctx in srec.contexts)
+    assert (a is None and b is None) or (isinstance(a, int) and isinstance(b, int) and a != b)
+
+
 @pytest.mark.anyio
 async def test_direct_send_raw_request_wraps_non_mcperror_exception_as_internal_error_with_cause():
     """DirectDispatcher-specific: the original exception is chained via __cause__."""

tests/shared/test_jsonrpc_dispatcher.py

@@ -203,7 +203,7 @@ async def test_raise_handler_exceptions_true_propagates_out_of_run():
     c2s_send, c2s_recv = anyio.create_memory_object_stream[SessionMessage | Exception](32)
     s2c_send, s2c_recv = anyio.create_memory_object_stream[SessionMessage | Exception](32)
 
-    def builder(_rid: object, _meta: object) -> TransportContext:
+    def builder(_meta: object) -> TransportContext:
         return TransportContext(kind="jsonrpc", can_send_request=True)
 
     server: JSONRPCDispatcher[TransportContext] = JSONRPCDispatcher(