Avoid opening SSE stream after non-success POST response

markInitialized() and reconnect() ran before the status check, so a 405
from a server without Streamable HTTP still triggered a GET and left a
duplicate SSE session during transport fallback. Only do this on 2xx.
Fixes #773.

Author: rameshreddy-adutla

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

@@ -574,17 +574,18 @@ public Mono<Void> sendMessage(McpSchema.JSONRPCMessage sentMessage) {
 							"Authorization error when sending message", requestSnapshot, responseEvent.responseInfo()));
 				}
 
-				if (transportSession.markInitialized(
-						responseEvent.responseInfo().headers().firstValue("mcp-session-id").orElseGet(() -> null))) {
-					// Once we have a session, we try to open an async stream for
-					// the server to send notifications and requests out-of-band.
-
-					reconnect(null).contextWrite(deliveredSink.contextView()).subscribe();
-				}
-
 				String sessionRepresentation = sessionIdOrPlaceholder(transportSession);
 
 				if (statusCode >= 200 && statusCode < 300) {
+					if (transportSession.markInitialized(responseEvent.responseInfo()
+						.headers()
+						.firstValue("mcp-session-id")
+						.orElseGet(() -> null))) {
+						// Once we have a session, we try to open an async stream
+						// for the server to send notifications and requests
+						// out-of-band.
+						reconnect(null).contextWrite(deliveredSink.contextView()).subscribe();
+					}
 
 					String contentType = responseEvent.responseInfo()
 						.headers()

mcp-test/src/test/java/io/modelcontextprotocol/client/transport/HttpClientStreamableHttpTransportErrorHandlingTest.java

@@ -389,6 +389,22 @@ void test405OnConnectReturnsEmptyFlux() {
 		StepVerifier.create(transport.closeGracefully()).verifyComplete();
 	}
 
+	@Test
+	void test405OnSendMessageDoesNotOpenSseConnection() {
+		serverResponseStatus.set(405);
+		currentServerSessionId.set("ignored-session-id");
+
+		StepVerifier.create(transport.sendMessage(createTestRequestMessage()))
+			.expectErrorMatches(
+					error -> error instanceof McpTransportException && error.getMessage().contains("Status code: 405"))
+			.verify();
+
+		Awaitility.await()
+			.during(Duration.ofMillis(300))
+			.atMost(Duration.ofSeconds(1))
+			.untilAsserted(() -> assertThat(processedSseConnectCount.get()).isZero());
+	}
+
 	@Nested
 	class AuthorizationError {