fix: improve compatibility with non-compliant MCP servers (#413)

Addresses issues with servers like Shopify that violate MCP/HTTP specs:
- Prioritize application/json in Accept headers to fix content-type issues
- Handle empty notification bodies ({}) that should be bodiless per spec
- Add status code validation and null safety improvements

Resolves #406

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

Author: tzolov

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

@@ -30,6 +30,7 @@
 import io.modelcontextprotocol.spec.McpTransportSessionNotFoundException;
 import io.modelcontextprotocol.spec.McpTransportStream;
 import io.modelcontextprotocol.util.Assert;
+import io.modelcontextprotocol.util.Utils;
 import reactor.core.Disposable;
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
@@ -244,7 +245,7 @@ public Mono<Void> sendMessage(McpSchema.JSONRPCMessage message) {
 
 			Disposable connection = webClient.post()
 				.uri(this.endpoint)
-				.accept(MediaType.TEXT_EVENT_STREAM, MediaType.APPLICATION_JSON)
+				.accept(MediaType.APPLICATION_JSON, MediaType.TEXT_EVENT_STREAM)
 				.headers(httpHeaders -> {
 					transportSession.sessionId().ifPresent(id -> httpHeaders.add("mcp-session-id", id));
 				})
@@ -387,9 +388,14 @@ private static String sessionIdOrPlaceholder(McpTransportSession<?> transportSes
 	private Flux<McpSchema.JSONRPCMessage> responseFlux(ClientResponse response) {
 		return response.bodyToMono(String.class).<Iterable<McpSchema.JSONRPCMessage>>handle((responseMessage, s) -> {
 			try {
-				McpSchema.JSONRPCMessage jsonRpcResponse = McpSchema.deserializeJsonRpcMessage(objectMapper,
-						responseMessage);
-				s.next(List.of(jsonRpcResponse));
+				if (Utils.hasText(responseMessage) && !responseMessage.trim().equals("{}")) {
+					McpSchema.JSONRPCMessage jsonRpcResponse = McpSchema.deserializeJsonRpcMessage(objectMapper,
+							responseMessage);
+					s.next(List.of(jsonRpcResponse));
+				}
+				else {
+					logger.warn("Received empty response message: {}", responseMessage);
+				}
 			}
 			catch (IOException e) {
 				s.error(e);

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

@@ -358,7 +358,7 @@ public Mono<Void> sendMessage(McpSchema.JSONRPCMessage sendMessage) {
 			String jsonBody = this.toString(sendMessage);
 
 			HttpRequest request = requestBuilder.uri(Utils.resolveUri(this.baseUri, this.endpoint))
-				.header("Accept", TEXT_EVENT_STREAM + ", " + APPLICATION_JSON)
+				.header("Accept", APPLICATION_JSON + ", " + TEXT_EVENT_STREAM)
 				.header("Content-Type", APPLICATION_JSON)
 				.header("Cache-Control", "no-cache")
 				.POST(HttpRequest.BodyPublishers.ofString(jsonBody))
@@ -436,11 +436,19 @@ else if (contentType.contains(TEXT_EVENT_STREAM)) {
 					else if (contentType.contains(APPLICATION_JSON)) {
 						messageSink.success();
 						String data = ((ResponseSubscribers.AggregateResponseEvent) responseEvent).data();
-						try {
-							return Mono.just(McpSchema.deserializeJsonRpcMessage(objectMapper, data));
+						if (Utils.hasText(data) && !data.trim().equals("{}")) {
+
+							try {
+								return Mono.just(McpSchema.deserializeJsonRpcMessage(objectMapper, data));
+							}
+							catch (IOException e) {
+								return Mono.error(e);
+							}
 						}
-						catch (IOException e) {
-							return Mono.error(e);
+						else {
+							// No content type means no response body
+							logger.debug("No content type returned for POST in session {}", sessionRepresentation);
+							return Mono.empty();
 						}
 					}
 					logger.warn("Unknown media type {} returned for POST in session {}", contentType,

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

@@ -135,36 +135,46 @@ protected void hookOnSubscribe(Subscription subscription) {
 
 		@Override
 		protected void hookOnNext(String line) {
-			if (line.isEmpty()) {
-				// Empty line means end of event
-				if (this.eventBuilder.length() > 0) {
-					String eventData = this.eventBuilder.toString();
-					SseEvent sseEvent = new SseEvent(currentEventId.get(), currentEventType.get(), eventData.trim());
-
-					this.sink.next(new SseResponseEvent(responseInfo, sseEvent));
-					this.eventBuilder.setLength(0);
-				}
-			}
-			else {
-				if (line.startsWith("data:")) {
-					var matcher = EVENT_DATA_PATTERN.matcher(line);
-					if (matcher.find()) {
-						this.eventBuilder.append(matcher.group(1).trim()).append("\n");
+			if (this.responseInfo.statusCode() >= 200 && this.responseInfo.statusCode() < 300) {
+
+				if (line.isEmpty()) {
+					// Empty line means end of event
+					if (this.eventBuilder.length() > 0) {
+						String eventData = this.eventBuilder.toString();
+						SseEvent sseEvent = new SseEvent(currentEventId.get(), currentEventType.get(),
+								eventData.trim());
+
+						this.sink.next(new SseResponseEvent(responseInfo, sseEvent));
+						this.eventBuilder.setLength(0);
 					}
 				}
-				else if (line.startsWith("id:")) {
-					var matcher = EVENT_ID_PATTERN.matcher(line);
-					if (matcher.find()) {
-						this.currentEventId.set(matcher.group(1).trim());
+				else {
+					if (line.startsWith("data:")) {
+						var matcher = EVENT_DATA_PATTERN.matcher(line);
+						if (matcher.find()) {
+							this.eventBuilder.append(matcher.group(1).trim()).append("\n");
+						}
 					}
-				}
-				else if (line.startsWith("event:")) {
-					var matcher = EVENT_TYPE_PATTERN.matcher(line);
-					if (matcher.find()) {
-						this.currentEventType.set(matcher.group(1).trim());
+					else if (line.startsWith("id:")) {
+						var matcher = EVENT_ID_PATTERN.matcher(line);
+						if (matcher.find()) {
+							this.currentEventId.set(matcher.group(1).trim());
+						}
+					}
+					else if (line.startsWith("event:")) {
+						var matcher = EVENT_TYPE_PATTERN.matcher(line);
+						if (matcher.find()) {
+							this.currentEventType.set(matcher.group(1).trim());
+						}
 					}
 				}
 			}
+			else {
+				// If the response is not successful, emit an error
+				System.out.println("Received non-successful response: " + this.responseInfo.statusCode());
+				SseEvent sseEvent = new SseEvent(null, null, null);
+				this.sink.next(new SseResponseEvent(responseInfo, sseEvent));
+			}
 		}
 
 		@Override

mcp/src/main/java/io/modelcontextprotocol/util/Utils.java

@@ -69,6 +69,9 @@ public static boolean isEmpty(@Nullable Map<?, ?> map) {
 	 * base URL or URI is malformed
 	 */
 	public static URI resolveUri(URI baseUrl, String endpointUrl) {
+		if (!Utils.hasText(endpointUrl)) {
+			return baseUrl;
+		}
 		URI endpointUri = URI.create(endpointUrl);
 		if (endpointUri.isAbsolute() && !isUnderBaseUri(baseUrl, endpointUri)) {
 			throw new IllegalArgumentException("Absolute endpoint URL does not match the base URL.");