Add SEP-1699 priming event support for POST SSE responses

Send priming event with id and empty data when retryInterval is configured.
This enables clients to establish resumption capability before any actual
messages are sent, allowing proper reconnection behavior.

Author: felixweinberger

src/server/streamableHttp.ts

@@ -568,6 +568,18 @@ export class StreamableHTTPServerTransport implements Transport {
                     }
 
                     res.writeHead(200, headers);
+
+                    // SEP-1699: Send priming event with id and empty data to establish resumption capability
+                    // This primes the client's Last-Event-ID for reconnection and sets retry interval
+                    // Only send when retryInterval is configured (enabling polling behavior)
+                    if (this._retryInterval !== undefined) {
+                        const primingEventId = this._eventStore
+                            ? await this._eventStore.storeEvent(streamId, {} as JSONRPCMessage)
+                            : `priming-${Date.now()}`;
+
+                        const primingEvent = `id: ${primingEventId}\nretry: ${this._retryInterval}\ndata: \n\n`;
+                        res.write(primingEvent);
+                    }
                 }
                 // Store the response for this request to send messages back through this connection
                 // We need to track by request ID to maintain the connection