fix: improve SSE event handling to gracefully ignore unrecognized events (#423)

- Replace McpError exceptions with debug/warning logs for unrecognized SSE event types
- Continue processing instead of failing when encountering unknown SSE events
- Update transport implementations:
  - WebClientStreamableHttpTransport: return empty tuple instead of throwing
  - WebFluxSseClientTransport: complete stream instead of erroring
  - HttpClientSseClientTransport: call sink.success() instead of sink.error()
  - HttpClientStreamableHttpTransport: return empty Flux for unknown events

This improves client resilience when servers send non-standard or future SSE event types.

Resolves #272 , #223 , #93, #421

Signed-off-by: Christian Tzolov <[email protected]>

Author: tzolov

mcp-spring/mcp-spring-webflux/src/main/java/io/modelcontextprotocol/client/transport/WebClientStreamableHttpTransport.java

@@ -432,7 +432,8 @@ private Tuple2<Optional<String>, Iterable<McpSchema.JSONRPCMessage>> parse(Serve
 			}
 		}
 		else {
-			throw new McpError("Received unrecognized SSE event type: " + event.event());
+			logger.debug("Received SSE event with type: {}", event);
+			return Tuples.of(Optional.empty(), List.of());
 		}
 	}
 

mcp-spring/mcp-spring-webflux/src/main/java/io/modelcontextprotocol/client/transport/WebFluxSseClientTransport.java

@@ -216,7 +216,8 @@ else if (MESSAGE_EVENT_TYPE.equals(event.event())) {
 				}
 			}
 			else {
-				s.error(new McpError("Received unrecognized SSE event type: " + event.event()));
+				logger.debug("Received unrecognized SSE event type: {}", event);
+				s.complete();
 			}
 		}).transform(handler)).subscribe();
 

mcp-spring/mcp-spring-webflux/src/test/java/io/modelcontextprotocol/client/transport/WebFluxSseClientTransportTests.java

@@ -6,6 +6,7 @@
 
 import java.time.Duration;
 import java.util.Map;
+import java.util.concurrent.CopyOnWriteArrayList;
 import java.util.concurrent.atomic.AtomicInteger;
 import java.util.function.Function;
 
@@ -77,6 +78,11 @@ public int getInboundMessageCount() {
 			return inboundMessageCount.get();
 		}
 
+		public void simulateSseComment(String comment) {
+			events.tryEmitNext(ServerSentEvent.<String>builder().comment(comment).build());
+			inboundMessageCount.incrementAndGet();
+		}
+
 		public void simulateEndpointEvent(String jsonMessage) {
 			events.tryEmitNext(ServerSentEvent.<String>builder().event("endpoint").data(jsonMessage).build());
 			inboundMessageCount.incrementAndGet();
@@ -158,6 +164,27 @@ void testBuilderPattern() {
 		assertThatCode(() -> transport4.closeGracefully().block()).doesNotThrowAnyException();
 	}
 
+	@Test
+	void testCommentSseMessage() {
+		// If the line starts with a character (:) are comment lins and should be ingored
+		// https://html.spec.whatwg.org/multipage/server-sent-events.html#event-stream-interpretation
+
+		CopyOnWriteArrayList<Throwable> droppedErrors = new CopyOnWriteArrayList<>();
+		reactor.core.publisher.Hooks.onErrorDropped(droppedErrors::add);
+
+		try {
+			// Simulate receiving the SSE comment line
+			transport.simulateSseComment("sse comment");
+
+			StepVerifier.create(transport.closeGracefully()).verifyComplete();
+
+			assertThat(droppedErrors).hasSize(0);
+		}
+		finally {
+			reactor.core.publisher.Hooks.resetOnErrorDropped();
+		}
+	}
+
 	@Test
 	void testMessageProcessing() {
 		// Create a test message

mcp/src/main/java/io/modelcontextprotocol/client/transport/HttpClientSseClientTransport.java

@@ -366,10 +366,8 @@ else if (MESSAGE_EVENT_TYPE.equals(responseEvent.sseEvent().event())) {
 								return Flux.just(message);
 							}
 							else {
-								logger.error("Received unrecognized SSE event type: {}",
-										responseEvent.sseEvent().event());
-								sink.error(new McpError(
-										"Received unrecognized SSE event type: " + responseEvent.sseEvent().event()));
+								logger.debug("Received unrecognized SSE event type: {}", responseEvent.sseEvent());
+								sink.success();
 							}
 						}
 						catch (IOException e) {

mcp/src/main/java/io/modelcontextprotocol/client/transport/HttpClientStreamableHttpTransport.java

@@ -264,6 +264,10 @@ private Mono<Disposable> reconnect(McpTransportStream<Disposable> stream) {
 										"Error parsing JSON-RPC message: " + responseEvent.sseEvent().data()));
 							}
 						}
+						else {
+							logger.debug("Received SSE event with type: {}", responseEvent.sseEvent());
+							return Flux.empty();
+						}
 					}
 					else if (statusCode == METHOD_NOT_ALLOWED) { // NotAllowed
 						logger.debug("The server does not support SSE streams, using request-response mode.");