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event_store

Event storage interface for streamable-HTTP resumability.

EventStore and its supporting types are the contract a resumable streamable-HTTP server plugs a store into (see mcp.server.streamable_http, which re-exports every name here). They are defined in this transport-agnostic module - no web framework imported - so servers and application code can name and type against them without loading the HTTP stack.

EventMessage dataclass

A JSONRPCMessage with an optional event ID for stream resumability.

Source code in src/mcp/server/event_store.py 
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@dataclass
class EventMessage:
    """A JSONRPCMessage with an optional event ID for stream resumability."""

    message: JSONRPCMessage
    event_id: str | None = None

EventStore

Bases: ABC

Interface for resumability support via event storage.

Source code in src/mcp/server/event_store.py 
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class EventStore(ABC):
    """Interface for resumability support via event storage."""

    @abstractmethod
    async def store_event(self, stream_id: StreamId, message: JSONRPCMessage | None) -> EventId:
        """Stores an event for later retrieval.

        Args:
            stream_id: ID of the stream the event belongs to
            message: The JSON-RPC message to store, or None for priming events

        Returns:
            The generated event ID for the stored event.
        """
        pass  # pragma: no cover

    @abstractmethod
    async def replay_events_after(
        self,
        last_event_id: EventId,
        send_callback: EventCallback,
    ) -> StreamId | None:
        """Replays events that occurred after the specified event ID.

        Args:
            last_event_id: The ID of the last event the client received
            send_callback: A callback function to send events to the client

        Returns:
            The stream ID of the replayed events, or None if no events were found.
        """
        pass  # pragma: no cover

store_event abstractmethod async 

store_event(
    stream_id: StreamId, message: JSONRPCMessage | None
) -> EventId

Stores an event for later retrieval.

Parameters:

Name Type Description Default
stream_id StreamId

ID of the stream the event belongs to

 required
message JSONRPCMessage | None

The JSON-RPC message to store, or None for priming events

 required

Returns:

Type Description
EventId

The generated event ID for the stored event.
Source code in src/mcp/server/event_store.py 
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@abstractmethod
async def store_event(self, stream_id: StreamId, message: JSONRPCMessage | None) -> EventId:
    """Stores an event for later retrieval.

    Args:
        stream_id: ID of the stream the event belongs to
        message: The JSON-RPC message to store, or None for priming events

    Returns:
        The generated event ID for the stored event.
    """
    pass  # pragma: no cover

replay_events_after abstractmethod async 

replay_events_after(
    last_event_id: EventId, send_callback: EventCallback
) -> StreamId | None

Replays events that occurred after the specified event ID.

Parameters:

Name Type Description Default
last_event_id EventId

The ID of the last event the client received

 required
send_callback EventCallback

A callback function to send events to the client

 required

Returns:

Type Description
StreamId | None

The stream ID of the replayed events, or None if no events were found.
Source code in src/mcp/server/event_store.py 
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@abstractmethod
async def replay_events_after(
    self,
    last_event_id: EventId,
    send_callback: EventCallback,
) -> StreamId | None:
    """Replays events that occurred after the specified event ID.

    Args:
        last_event_id: The ID of the last event the client received
        send_callback: A callback function to send events to the client

    Returns:
        The stream ID of the replayed events, or None if no events were found.
    """
    pass  # pragma: no cover