Ignore unsupported SSE event

Signed-off-by: Daniel Garnier-Moiroux <git@garnier.wf>

Author: Kehrlann

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

@@ -356,7 +356,8 @@ private int indexOfLineTerminator(int from) {
 	 * fields until a blank line dispatches the event. Per the SSE spec, {@code id} and
 	 * {@code event} persist across events until re-set; {@code data} is reset after each
 	 * dispatch, and a blank line dispatches only when a {@code data:} field was seen,
-	 * whether or not it carried a value.
+	 * whether or not it carried a value. Comments and fields the parser does not handle,
+	 * such as {@code retry:}, are ignored as the spec requires.
 	 */
 	static final class SseEventParser {
 
@@ -421,7 +422,9 @@ else if (line.startsWith(":")) {
 				logger.debug("Ignoring comment line: {}", line);
 			}
 			else {
-				throw new McpTransportException("Invalid SSE response line: " + line);
+				// The SSE spec mandates that fields the client does not know about, such
+				// as `retry:`, are ignored rather than treated as a protocol error.
+				logger.debug("Ignoring unknown SSE field line: {}", line);
 			}
 			return Optional.empty();
 		}

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

@@ -10,10 +10,8 @@
 
 import io.modelcontextprotocol.client.transport.ResponseSubscribers.SseEvent;
 import io.modelcontextprotocol.client.transport.ResponseSubscribers.SseEventParser;
-import io.modelcontextprotocol.spec.McpTransportException;
 
 import static org.assertj.core.api.Assertions.assertThat;
-import static org.assertj.core.api.Assertions.assertThatThrownBy;
 
 class SseEventParserTests {
 
@@ -100,11 +98,16 @@ void blankLineWithNoPendingDataIsEmpty() {
 	}
 
 	@Test
-	void unknownFieldThrowsMcpTransportException() {
+	void unknownFieldsAreIgnored() {
+		// The SSE spec mandates that unknown fields are ignored, so neither a standard
+		// field the parser does not act on nor a malformed line may fail the stream.
 		SseEventParser p = new SseEventParser(Integer.MAX_VALUE);
-		assertThatThrownBy(() -> p.feed("bogus line")).isInstanceOf(McpTransportException.class)
-			.hasMessageContaining("Invalid SSE response line")
-			.hasMessageContaining("bogus line");
+		assertThat(p.feed("retry: 3000")).isEmpty();
+		assertThat(p.feed("bogus line")).isEmpty();
+		assertThat(p.feed("data: hello")).isEmpty();
+		Optional<SseEvent> event = p.feed("");
+		assertThat(event).isPresent();
+		assertThat(event.get().data()).isEqualTo("hello");
 	}
 
 	@Test

mcp-core/src/test/java/io/modelcontextprotocol/client/transport/Utf8LineDecoderTests.java

@@ -178,8 +178,9 @@ void loneCrTerminatesLine() {
 		// SSE takes its line endings from HTML, which terminates on CRLF, CR and LF
 		// alike, and HttpResponse.BodySubscribers#fromLineSubscriber -- the path this
 		// decoder replaces -- splits on all three. Splitting on LF alone leaves a
-		// CR-framed stream as one unterminated run: downstream an "Invalid SSE response
-		// line", or past BoundedLineBodySubscriber's bound an aborted response.
+		// CR-framed stream as one unterminated run: downstream a single unparseable line
+		// whose SSE fields are silently dropped, leaving the request the stream answers
+		// hanging, or past BoundedLineBodySubscriber's bound an aborted response.
 		Utf8LineDecoder dec = new Utf8LineDecoder();
 		assertThat(dec.decode(chunk("one\rtwo\rthree\r"))).containsExactly("one", "two", "three");
 		assertThat(dec.flush()).isEmpty();