Client: handle server responses with Content-Length: 0

- When the client sends `notification/initalized`, servers must respond
  with HTTP 202 and an empty body. We checked for the absence of a
  Content-Type header to verify whether the body was empty.
- However, some servers will send an empty body with a Content-Type
  header, and that header may have an unsupported, default type such as
  `text/html` or `text/plain`.
- Now we we also use the Content-Length header to check for an empty
  body. This header is optional in HTTP/2, so we do not make it our
  primary mechanism for detecting empty bodies.
- Fixes #582

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

Author: Kehrlann

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

@@ -463,11 +463,15 @@ public Mono<Void> sendMessage(McpSchema.JSONRPCMessage sentMessage) {
 						.orElse("")
 						.toLowerCase();
 
-					if (contentType.isBlank()) {
+					String contentLength = responseEvent.responseInfo()
+						.headers()
+						.firstValue("Content-Length")
+						.orElse(null);
+
+					if (contentType.isBlank() || "0".equals(contentLength)) {
 						logger.debug("No content type 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-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