fix: eliminate uncovered branches in null-id handling

Merge the nested if conditions in the server message router into a
single condition so the false branch is naturally exercised by
non-response messages. Remove isinstance guards in test callbacks
since we control the input.

Author: maxisbey

src/mcp/server/streamable_http.py

@@ -975,13 +975,11 @@ async def message_router():
                         # Determine which request stream(s) should receive this message
                         message = session_message.message
                         target_request_id = None
-                        # Check if this is a response
-                        if isinstance(message, JSONRPCResponse | JSONRPCError):
-                            # Null-id errors (e.g., parse errors) go to the
-                            # GET stream since they can't be correlated to a
-                            # specific request.
-                            if message.id is not None:
-                                target_request_id = str(message.id)
+                        # Check if this is a response with a known request id.
+                        # Null-id errors (e.g., parse errors) fall through to
+                        # the GET stream since they can't be correlated.
+                        if isinstance(message, JSONRPCResponse | JSONRPCError) and message.id is not None:
+                            target_request_id = str(message.id)
                         # Extract related_request_id from meta if it exists
                         elif (  # pragma: no cover
                             session_message.metadata is not None

tests/shared/test_session.py

@@ -313,14 +313,14 @@ async def test_null_id_error_surfaced_via_message_handler():
     request, so they are forwarded to the message handler as MCPError.
     """
     ev_error_received = anyio.Event()
-    error_holder: list[Exception] = []
+    error_holder: list[MCPError] = []
 
     async def capture_errors(
         message: RequestResponder[ServerRequest, ClientResult] | ServerNotification | Exception,
     ) -> None:
-        if isinstance(message, Exception):
-            error_holder.append(message)
-            ev_error_received.set()
+        assert isinstance(message, MCPError)
+        error_holder.append(message)
+        ev_error_received.set()
 
     sent_error = ErrorData(code=PARSE_ERROR, message="Parse error")
 
@@ -347,7 +347,6 @@ async def mock_server():
                 await ev_error_received.wait()
 
     assert len(error_holder) == 1
-    assert isinstance(error_holder[0], MCPError)
     assert error_holder[0].error == sent_error
 
 
@@ -361,15 +360,15 @@ async def test_null_id_error_does_not_affect_pending_request():
     """
     ev_error_received = anyio.Event()
     ev_response_received = anyio.Event()
-    error_holder: list[Exception] = []
+    error_holder: list[MCPError] = []
     result_holder: list[EmptyResult] = []
 
     async def capture_errors(
         message: RequestResponder[ServerRequest, ClientResult] | ServerNotification | Exception,
     ) -> None:
-        if isinstance(message, Exception):
-            error_holder.append(message)
-            ev_error_received.set()
+        assert isinstance(message, MCPError)
+        error_holder.append(message)
+        ev_error_received.set()
 
     sent_error = ErrorData(code=PARSE_ERROR, message="Parse error")
 
@@ -412,7 +411,6 @@ async def make_request(client_session: ClientSession):
 
     # Null-id error reached the message handler
     assert len(error_holder) == 1
-    assert isinstance(error_holder[0], MCPError)
     assert error_holder[0].error == sent_error
 
     # Pending request completed successfully