fix: Add an exception handling function to WebFluxSseClientTransport

- `sseErrorHandler` is used to handle errors when processing SSE events and situations where an unrecognized event type is received.
- Add unit tests to verify.

Signed-off-by: YunKui Lu <luyunkui95@gmail.com>

Author: YunKuiLu

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

@@ -5,6 +5,7 @@
 
 import java.io.IOException;
 import java.util.function.BiConsumer;
+import java.util.function.BiFunction;
 import java.util.function.Function;
 
 import com.fasterxml.jackson.core.type.TypeReference;
@@ -123,6 +124,16 @@ public class WebFluxSseClientTransport implements McpClientTransport {
 	 */
 	private String sseEndpoint;
 
+	/**
+	 * Handle exceptions that occur during the processing of SSE events to avoid
+	 * connection interruption.
+	 */
+	private BiFunction<Throwable, ServerSentEvent<String>, Mono<? extends JSONRPCMessage>> sseErrorHandler = (error,
+																											  event) -> {
+		logger.warn("Failed to handle SSE event {}", event, error);
+		return Mono.empty();
+	};
+
 	/**
 	 * Constructs a new SseClientTransport with the specified WebClient builder. Uses a
 	 * default ObjectMapper instance for JSON processing.
@@ -215,12 +226,26 @@ else if (MESSAGE_EVENT_TYPE.equals(event.event())) {
 			else {
 				s.error(new McpError("Received unrecognized SSE event type: " + event.event()));
 			}
-		}).transform(handler)).subscribe();
+		}).onErrorResume(e -> sseErrorHandler.apply(e, event)).transform(handler)).subscribe();
 
 		// The connection is established once the server sends the endpoint event
 		return messageEndpointSink.asMono().then();
 	}
 
+	/**
+	 * Sets the handler for processing transport-level errors.
+	 *
+	 * <p>
+	 * The provided handler will be called when errors occur during transport operations,
+	 * such as connection failures or protocol violations.
+	 * </p>
+	 * @param errorHandler a consumer that processes error messages
+	 */
+	public void setSseErrorHandler(
+			BiFunction<Throwable, ServerSentEvent<String>, Mono<? extends JSONRPCMessage>> errorHandler) {
+		this.sseErrorHandler = errorHandler;
+	}
+
 	/**
 	 * Sends a JSON-RPC message to the server using the endpoint provided during
 	 * connection.

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

@@ -6,7 +6,10 @@
 
 import java.time.Duration;
 import java.util.Map;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicInteger;
+import java.util.concurrent.atomic.AtomicReference;
 import java.util.function.Function;
 
 import com.fasterxml.jackson.databind.ObjectMapper;
@@ -86,6 +89,11 @@ public void simulateMessageEvent(String jsonMessage) {
 			inboundMessageCount.incrementAndGet();
 		}
 
+		public void simulateComment(String comment) {
+			events.tryEmitNext(ServerSentEvent.<String>builder().comment(comment).build());
+			inboundMessageCount.incrementAndGet();
+		}
+
 	}
 
 	void startContainer() {
@@ -338,4 +346,26 @@ void testMessageOrderPreservation() {
 		assertThat(transport.getInboundMessageCount()).isEqualTo(3);
 	}
 
+	@Test
+	void customErrorHandlerShouldProcessErrors() throws InterruptedException {
+		AtomicReference<ServerSentEvent<String>> receivedErrorEvent = new AtomicReference<>();
+		AtomicReference<Throwable> handledError = new AtomicReference<>();
+
+		transport.setSseErrorHandler((error, event) -> {
+			receivedErrorEvent.set(event);
+			handledError.set(error);
+			return Mono.empty();
+		});
+
+		// Mock receive a common message `: This is a comment.\n\n`
+		transport.simulateComment("This is a comment.");
+
+		assertThat(receivedErrorEvent.get().comment()).isNotNull().isEqualTo("This is a comment.");
+
+		// Mock receive a common message `:ping - 2025-05-06 08:42:06.508759+00:00\n\n`
+		transport.simulateComment("ping - 2025-05-06 08:42:06.508759+00:00");
+
+		assertThat(receivedErrorEvent.get().comment()).isNotNull().isEqualTo("ping - 2025-05-06 08:42:06.508759+00:00");
+	}
+
 }