feat: add server-side SSE priming events and retry field (SEP-1699)

Add support for sending priming events with retry field when establishing
SSE streams. When retryInterval is configured, the server sends an initial
event with id, retry field, and empty data to prime the client for
reconnection timing.

Changes:
- Add retryInterval option to StreamableHTTPServerTransportOptions
- Send priming event after GET SSE stream headers when configured
- Include retry field to control client reconnection timing

Author: felixweinberger

src/server/streamableHttp.ts

@@ -108,6 +108,13 @@ export interface StreamableHTTPServerTransportOptions {
      * Default is false for backwards compatibility.
      */
     enableDnsRebindingProtection?: boolean;
+
+    /**
+     * Retry interval in milliseconds to suggest to clients in SSE retry field.
+     * When set, the server will send a retry field in SSE events to control
+     * client reconnection timing (SEP-1699).
+     */
+    retryInterval?: number;
 }
 
 /**
@@ -160,6 +167,7 @@ export class StreamableHTTPServerTransport implements Transport {
     private _allowedHosts?: string[];
     private _allowedOrigins?: string[];
     private _enableDnsRebindingProtection: boolean;
+    private _retryInterval?: number;
 
     sessionId?: string;
     onclose?: () => void;
@@ -175,6 +183,7 @@ export class StreamableHTTPServerTransport implements Transport {
         this._allowedHosts = options.allowedHosts;
         this._allowedOrigins = options.allowedOrigins;
         this._enableDnsRebindingProtection = options.enableDnsRebindingProtection ?? false;
+        this._retryInterval = options.retryInterval;
     }
 
     /**
@@ -320,6 +329,18 @@ export class StreamableHTTPServerTransport implements Transport {
         // otherwise the client will just wait for the first message
         res.writeHead(200, headers).flushHeaders();
 
+        // 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(this._standaloneSseStreamId, {} as JSONRPCMessage)
+                : `priming-${Date.now()}`;
+
+            const primingEvent = `id: ${primingEventId}\nretry: ${this._retryInterval}\ndata: \n\n`;
+            res.write(primingEvent);
+        }
+
         // Assign the response to the standalone SSE stream
         this._streamMapping.set(this._standaloneSseStreamId, res);
         // Set up close handler for client disconnects