Write SSE comment before closing stream

Author: kabir

reference/jsonrpc/src/main/java/io/a2a/server/apps/quarkus/A2AServerRoutes.java

@@ -824,8 +824,23 @@ public void onComplete() {
                         if (headers.get(CONTENT_TYPE) == null) {
                             headers.set(CONTENT_TYPE, SERVER_SENT_EVENTS);
                         }
+                        response.end();
+                    } else {
+                        // Events were written - send final SSE comment to ensure flush before closing
+                        // Since SSE writes are sequential, when this comment's write completes, we know
+                        // all previous events (including the final event) have been written to the socket.
+                        // This provides proper synchronization without arbitrary delays.
+                        response.write(": stream closing\n\n", ar -> {
+                            if (ar.succeeded()) {
+                                logger.debug("Final SSE comment written, closing stream");
+                                response.end();
+                            } else {
+                                // Write failed (likely client disconnect) - still try to close cleanly
+                                logger.debug("Final SSE comment write failed, closing stream anyway", ar.cause());
+                                response.end();
+                            }
+                        });
                     }
-                    response.end();
                 }
             });
         }

reference/rest/src/main/java/io/a2a/server/rest/quarkus/A2AServerRoutes.java

@@ -951,8 +951,23 @@ public void onComplete() {
                         if (headers.get(CONTENT_TYPE) == null) {
                             headers.set(CONTENT_TYPE, SERVER_SENT_EVENTS);
                         }
+                        response.end();
+                    } else {
+                        // Events were written - send final SSE comment to ensure flush before closing
+                        // Since SSE writes are sequential, when this comment's write completes, we know
+                        // all previous events (including the final event) have been written to the socket.
+                        // This provides proper synchronization without arbitrary delays.
+                        response.write(": stream closing\n\n", ar -> {
+                            if (ar.succeeded()) {
+                                logger.debug("Final SSE comment written, closing stream");
+                                response.end();
+                            } else {
+                                // Write failed (likely client disconnect) - still try to close cleanly
+                                logger.debug("Final SSE comment write failed, closing stream anyway", ar.cause());
+                                response.end();
+                            }
+                        });
                     }
-                    response.end();
                 }
             });
         }

server-common/src/main/java/io/a2a/server/events/EventConsumer.java

@@ -242,12 +242,6 @@ public Flow.Publisher<EventQueueItem> consumeAll() {
                                     boolean writeCompleted = finalEventWriteLatch.await(2, TimeUnit.SECONDS);
                                     if (writeCompleted) {
                                         LOGGER.debug("Final event write confirmed via backpressure signal");
-                                        // Additional safety delay: backpressure callback is shared across all events,
-                                        // so it might fire for a previous event after we set the flag. Also, even
-                                        // with setWriteQueueMaxSize(1), Vert.x may have additional buffering.
-                                        // 50ms ensures the final event is truly transmitted to the network.
-                                        Thread.sleep(50);
-                                        LOGGER.debug("Safety buffer flush delay completed");
                                     } else {
                                         LOGGER.warn("Timeout waiting for final event write confirmation - proceeding anyway");
                                     }