Wrap JSONRPC messages with SessionMessage for metadata support (#590)

src/mcp/shared/message.py

@@ -0,0 +1,35 @@
+"""
+Message wrapper with metadata support.
+
+This module defines a wrapper type that combines JSONRPCMessage with metadata
+to support transport-specific features like resumability.
+"""
+
+from dataclasses import dataclass
+
+from mcp.types import JSONRPCMessage, RequestId
+
+
+@dataclass
+class ClientMessageMetadata:
+    """Metadata specific to client messages."""
+
+    resumption_token: str | None = None
+
+
+@dataclass
+class ServerMessageMetadata:
+    """Metadata specific to server messages."""
+
+    related_request_id: RequestId | None = None
+
+
+MessageMetadata = ClientMessageMetadata | ServerMessageMetadata | None
+
+
+@dataclass
+class SessionMessage:
+    """A message with specific metadata for transport-specific features."""
+
+    message: JSONRPCMessage
+    metadata: MessageMetadata = None