Client: handle server responses with Content-Length: 0

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

@@ -18,14 +18,13 @@
 
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
-
-import io.modelcontextprotocol.json.McpJsonMapper;
-import io.modelcontextprotocol.json.TypeRef;
-
+import io.modelcontextprotocol.client.transport.ResponseSubscribers.ResponseEvent;
 import io.modelcontextprotocol.client.transport.customizer.McpAsyncHttpClientRequestCustomizer;
 import io.modelcontextprotocol.client.transport.customizer.McpSyncHttpClientRequestCustomizer;
-import io.modelcontextprotocol.client.transport.ResponseSubscribers.ResponseEvent;
 import io.modelcontextprotocol.common.McpTransportContext;
+import io.modelcontextprotocol.json.McpJsonMapper;
+import io.modelcontextprotocol.json.TypeRef;
+import io.modelcontextprotocol.spec.HttpHeaders;
 import io.modelcontextprotocol.spec.McpClientTransport;
 import io.modelcontextprotocol.spec.McpSchema;
 import io.modelcontextprotocol.spec.McpSchema.JSONRPCMessage;
@@ -469,7 +468,7 @@ private Mono<HttpResponse<String>> sendHttpPost(final String endpoint, final Str
 		return Mono.deferContextual(ctx -> {
 			var builder = this.requestBuilder.copy()
 				.uri(requestUri)
-				.header("Content-Type", "application/json")
+				.header(HttpHeaders.CONTENT_TYPE, "application/json")
 				.header(MCP_PROTOCOL_VERSION_HEADER_NAME, MCP_PROTOCOL_VERSION)
 				.POST(HttpRequest.BodyPublishers.ofString(body));
 			var transportContext = ctx.getOrDefault(McpTransportContext.KEY, McpTransportContext.EMPTY);

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

@@ -246,7 +246,7 @@ private Mono<Disposable> reconnect(McpTransportStream<Disposable> stream) {
 				}
 
 				var builder = requestBuilder.uri(uri)
-					.header("Accept", TEXT_EVENT_STREAM)
+					.header(HttpHeaders.ACCEPT, TEXT_EVENT_STREAM)
 					.header("Cache-Control", "no-cache")
 					.header(HttpHeaders.PROTOCOL_VERSION, MCP_PROTOCOL_VERSION)
 					.GET();
@@ -371,7 +371,7 @@ private BodyHandler<Void> toSendMessageBodySubscriber(FluxSink<ResponseEvent> si
 
 		BodyHandler<Void> responseBodyHandler = responseInfo -> {
 
-			String contentType = responseInfo.headers().firstValue("Content-Type").orElse("").toLowerCase();
+			String contentType = responseInfo.headers().firstValue(HttpHeaders.CONTENT_TYPE).orElse("").toLowerCase();
 
 			if (contentType.contains(TEXT_EVENT_STREAM)) {
 				// For SSE streams, use line subscriber that returns Void
@@ -420,9 +420,9 @@ public Mono<Void> sendMessage(McpSchema.JSONRPCMessage sentMessage) {
 				}
 
 				var builder = requestBuilder.uri(uri)
-					.header("Accept", APPLICATION_JSON + ", " + TEXT_EVENT_STREAM)
-					.header("Content-Type", APPLICATION_JSON)
-					.header("Cache-Control", "no-cache")
+					.header(HttpHeaders.ACCEPT, APPLICATION_JSON + ", " + TEXT_EVENT_STREAM)
+					.header(HttpHeaders.CONTENT_TYPE, APPLICATION_JSON)
+					.header(HttpHeaders.CACHE_CONTROL, "no-cache")
 					.header(HttpHeaders.PROTOCOL_VERSION, MCP_PROTOCOL_VERSION)
 					.POST(HttpRequest.BodyPublishers.ofString(jsonBody));
 				var transportContext = ctx.getOrDefault(McpTransportContext.KEY, McpTransportContext.EMPTY);
@@ -459,15 +459,19 @@ public Mono<Void> sendMessage(McpSchema.JSONRPCMessage sentMessage) {
 
 					String contentType = responseEvent.responseInfo()
 						.headers()
-						.firstValue("Content-Type")
+						.firstValue(HttpHeaders.CONTENT_TYPE)
 						.orElse("")
 						.toLowerCase();
 
-					if (contentType.isBlank()) {
-						logger.debug("No content type returned for POST in session {}", sessionRepresentation);
+					String contentLength = responseEvent.responseInfo()
+						.headers()
+						.firstValue(HttpHeaders.CONTENT_LENGTH)
+						.orElse(null);
+
+					if (contentType.isBlank() || "0".equals(contentLength)) {
+						logger.debug("No body returned for POST in session {}", sessionRepresentation);
 						// No content type means no response body, so we can just
-						// return
-						// an empty stream
+						// return an empty stream
 						deliveredSink.success();
 						return Flux.empty();
 					}

mcp-core/src/main/java/io/modelcontextprotocol/spec/HttpHeaders.java

@@ -26,4 +26,31 @@ public interface HttpHeaders {
 	 */
 	String PROTOCOL_VERSION = "MCP-Protocol-Version";
 
+	/**
+	 * The HTTP Content-Length header.
+	 * @see <a href=
+	 * "https://httpwg.org/specs/rfc9110.html#field.content-length">RFC9110</a>
+	 */
+	String CONTENT_LENGTH = "Content-Length";
+
+	/**
+	 * The HTTP Content-Type header.
+	 * @see <a href=
+	 * "https://httpwg.org/specs/rfc9110.html#field.content-type">RFC9110</a>
+	 */
+	String CONTENT_TYPE = "Content-Type";
+
+	/**
+	 * The HTTP Accept header.
+	 * @see <a href= "https://httpwg.org/specs/rfc9110.html#field.accept">RFC9110</a>
+	 */
+	String ACCEPT = "Accept";
+
+	/**
+	 * The HTTP Cache-Control header.
+	 * @see <a href=
+	 * "https://httpwg.org/specs/rfc9111.html#field.cache-control">RFC9111</a>
+	 */
+	String CACHE_CONTROL = "Cache-Control";
+
 }

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

@@ -293,9 +293,10 @@ public Mono<Void> sendMessage(McpSchema.JSONRPCMessage message) {
 					// 200 OK for notifications
 					if (response.statusCode().is2xxSuccessful()) {
 						Optional<MediaType> contentType = response.headers().contentType();
+						long contentLength = response.headers().contentLength().orElse(-1);
 						// Existing SDKs consume notifications with no response body nor
 						// content type
-						if (contentType.isEmpty()) {
+						if (contentType.isEmpty() || contentLength == 0) {
 							logger.trace("Message was successfully sent via POST for session {}",
 									sessionRepresentation);
 							// signal the caller that the message was successfully