Fix SSE event classification to follow spec for missing event field (#913)

Per the SSE specification (WHATWG HTML Living Standard 9.2.6), an event with no explicit event field MUST be dispatched as a message event. HttpClientStreamableHttpTransport previously used strict equality and silently dropped such frames in the reconnect/GET stream path, causing server-initiated notifications to never reach the handler. Extract classification into a package-private isMessageEvent helper and cover with parameterized unit tests.

Closes gh-885

Author: iaJingda

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

@@ -114,6 +114,29 @@ public class HttpClientStreamableHttpTransport implements McpClientTransport {
 
 	public static int BAD_REQUEST = 400;
 
+	/**
+	 * Determines whether an SSE event should be treated as a "message" event carrying a
+	 * JSON-RPC payload.
+	 *
+	 * <p>
+	 * Per the <a href=
+	 * "https://html.spec.whatwg.org/multipage/server-sent-events.html#event-stream-interpretation">
+	 * SSE specification (WHATWG HTML Living Standard §9.2.6)</a>, an event with no
+	 * explicit {@code event:} field MUST be dispatched as a {@code message} event by
+	 * default. This method applies that rule by treating {@code null} or empty event
+	 * names as equivalent to {@link #MESSAGE_EVENT_TYPE}.
+	 *
+	 * <p>
+	 * This alignment ensures interoperability with MCP servers that emit bare
+	 * {@code data:} frames without an accompanying {@code event:} line, which are valid
+	 * per the SSE spec.
+	 * @param eventName the SSE event name, which may be {@code null} or empty
+	 * @return {@code true} if the event should be parsed as a JSON-RPC message
+	 */
+	static boolean isMessageEvent(String eventName) {
+		return eventName == null || eventName.isEmpty() || MESSAGE_EVENT_TYPE.equals(eventName);
+	}
+
 	private final McpJsonMapper jsonMapper;
 
 	private final URI baseUri;
@@ -323,7 +346,7 @@ else if (statusCode == METHOD_NOT_ALLOWED) {
 											+ statusCode));
 						}
 						else if (statusCode >= 200 && statusCode < 300) {
-							if (MESSAGE_EVENT_TYPE.equals(sseResponseEvent.sseEvent().event())) {
+							if (isMessageEvent(sseResponseEvent.sseEvent().event())) {
 								String data = sseResponseEvent.sseEvent().data();
 								// Per 2025-11-25 spec (SEP-1699), servers may
 								// send SSE events

mcp-core/src/test/java/io/modelcontextprotocol/client/transport/HttpClientStreamableHttpTransportSseEventTypeTest.java

@@ -0,0 +1,51 @@
+/*
+ * Copyright 2024-2026 the original author or authors.
+ */
+
+package io.modelcontextprotocol.client.transport;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.NullAndEmptySource;
+import org.junit.jupiter.params.provider.ValueSource;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Unit tests for {@link HttpClientStreamableHttpTransport#isMessageEvent(String)}.
+ *
+ * <p>
+ * Verifies that SSE event classification follows the <a href=
+ * "https://html.spec.whatwg.org/multipage/server-sent-events.html#event-stream-interpretation">
+ * WHATWG HTML Living Standard §9.2.6</a>: an event without an explicit {@code event:}
+ * field must be dispatched as a {@code message} event.
+ *
+ * @author jiajingda
+ * @see <a href="https://github.com/modelcontextprotocol/java-sdk/issues/885">#885</a>
+ */
+class HttpClientStreamableHttpTransportSseEventTypeTest {
+
+	@ParameterizedTest
+	@NullAndEmptySource
+	void shouldTreatNullOrEmptyEventAsMessage(String eventName) {
+		assertThat(HttpClientStreamableHttpTransport.isMessageEvent(eventName))
+			.as("SSE frame with null/empty event field must be treated as a 'message' event per SSE spec")
+			.isTrue();
+	}
+
+	@Test
+	void shouldTreatExplicitMessageEventAsMessage() {
+		assertThat(HttpClientStreamableHttpTransport.isMessageEvent("message"))
+			.as("Explicit 'message' event must be parsed as a JSON-RPC message")
+			.isTrue();
+	}
+
+	@ParameterizedTest
+	@ValueSource(strings = { "ping", "error", "notification", "MESSAGE", "Message", "custom-event" })
+	void shouldNotTreatOtherEventsAsMessage(String eventName) {
+		assertThat(HttpClientStreamableHttpTransport.isMessageEvent(eventName))
+			.as("Non-'message' SSE event '%s' must not be parsed as a JSON-RPC message", eventName)
+			.isFalse();
+	}
+
+}