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fix: notify session when SSE stream closes without event ID #1940

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fix: notify session when SSE stream closes without event ID #1940

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19 changes: 18 additions & 1 deletion src/mcp/client/streamable_http.py
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Original file line number Diff line number Diff line change
Expand Up @@ -17,6 +17,7 @@
from mcp.shared._httpx_utils import create_mcp_http_client
from mcp.shared.message import ClientMessageMetadata, SessionMessage
from mcp.types import (
CONNECTION_CLOSED,
ErrorData,
InitializeResult,
JSONRPCError,
Expand Down Expand Up @@ -337,9 +338,25 @@ async def _handle_sse_response(
logger.debug(f"SSE stream ended: {e}")

# Stream ended without response - reconnect if we received an event with ID
if last_event_id is not None: # pragma: no branch
if last_event_id is not None:
logger.info("SSE stream disconnected, reconnecting...")
await self._handle_reconnection(ctx, last_event_id, retry_interval_ms)
else:
# No event ID received - cannot reconnect, notify session of failure
request_id: RequestId | None = None
if isinstance(ctx.session_message.message, JSONRPCRequest):
request_id = ctx.session_message.message.id

if request_id is not None:
jsonrpc_error = JSONRPCError(
jsonrpc="2.0",
id=request_id,
error=ErrorData(
code=CONNECTION_CLOSED,
message="SSE stream closed unexpectedly without response",
),
)
await ctx.read_stream_writer.send(SessionMessage(jsonrpc_error))

async def _handle_reconnection(
self,
Expand Down
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