Description
When using Java 21 McpSyncClient to consume an SSE endpoint, the client only receives the first ~83 events, then stops receiving any further data.
Wireshark confirms that the server continues sending data and TCP transmission is normal, but the Java application layer no longer triggers onNext.

Environment

• Java: 21.0.8
• Spring Boot: 3.5.4
• Server: mcp-spring-webflux 0.11.2
• Client: mcp-spring-webmvc 0.11.2

Problematic Code Snippet
io.modelcontextprotocol.client.transport.ResponseSubscribers$SseLineSubscriber

		@Override
		protected void hookOnSubscribe(Subscription subscription) {

			sink.onRequest(n -> {
				subscription.request(n);  // Only passively forwards demand
			});

			sink.onDispose(() -> {
				subscription.cancel();
			});
		}

Root Cause

• The HttpClient uses reactive stream backpressure: data is only pushed when subscription.request(n) is called.
• The current implementation relies on FluxSink.onRequest to passively trigger demand, which is only invoked
• when downstream subscribers request data (e.g., default request(256)).
• Once the initial demand is exhausted (~256 lines), HttpClient stops pushing new data.
  Since each my SSE event typically spans 3–4 lines, this results in ~83 events being received before the stream stalls.

Possible Fix
Actively request unlimited demand in hookOnSubscribe:

@Override
protected void hookOnSubscribe(Subscription subscription) {
    subscription.request(Long.MAX_VALUE); //  Actively drive the stream
    sink.onDispose(() -> subscription.cancel());
}