message

Message wrapper with metadata support.

This module defines a wrapper type that combines JSONRPCMessage with metadata to support transport-specific features like resumability.

ClientMessageMetadata dataclass

Metadata specific to client messages.

Source code in src/mcp/shared/message.py

@dataclass
class ClientMessageMetadata:
    """Metadata specific to client messages."""

    resumption_token: ResumptionToken | None = None
    on_resumption_token_update: Callable[[ResumptionToken], Awaitable[None]] | None = None
    # Per-message HTTP headers (e.g. MCP-Protocol-Version, Mcp-Method) the transport should set.
    headers: dict[str, str] | None = None

ServerMessageMetadata dataclass

Metadata specific to server messages.

Source code in src/mcp/shared/message.py

@dataclass
class ServerMessageMetadata:
    """Metadata specific to server messages."""

    related_request_id: RequestId | None = None
    # Transport-specific request context (e.g. starlette Request for HTTP
    # transports, None for stdio). Typed as Any because the server layer is
    # transport-agnostic.
    request_context: Any = None
    # Callback to close SSE stream for the current request without terminating
    close_sse_stream: CloseSSEStreamCallback | None = None
    # Callback to close the standalone GET SSE stream (for unsolicited notifications)
    close_standalone_sse_stream: CloseSSEStreamCallback | None = None

SessionMessage dataclass

A message with specific metadata for transport-specific features.

Source code in src/mcp/shared/message.py

@dataclass
class SessionMessage:
    """A message with specific metadata for transport-specific features."""

    message: JSONRPCMessage
    metadata: MessageMetadata = None