WebClientStreamableHttpTransport and WebMvcStatelessServerTransport should use McpJsonMapper when handling messages for serialization / deserialization

Author: filiphr

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

@@ -281,6 +281,13 @@ else if (isNotFound(response)) {
 
 	@Override
 	public Mono<Void> sendMessage(McpSchema.JSONRPCMessage message) {
+		String jsonText;
+		try {
+			jsonText = jsonMapper.writeValueAsString(message);
+		}
+		catch (IOException e) {
+			return Mono.error(new RuntimeException("Failed to serialize message", e));
+		}
 		return Mono.create(sink -> {
 			logger.debug("Sending message {}", message);
 			// Here we attempt to initialize the client.
@@ -293,14 +300,15 @@ public Mono<Void> sendMessage(McpSchema.JSONRPCMessage message) {
 
 			Disposable connection = Flux.deferContextual(ctx -> webClient.post()
 				.uri(this.endpoint)
+				.contentType(MediaType.APPLICATION_JSON)
 				.accept(MediaType.APPLICATION_JSON, MediaType.TEXT_EVENT_STREAM)
 				.header(HttpHeaders.PROTOCOL_VERSION,
 						ctx.getOrDefault(McpAsyncClient.NEGOTIATED_PROTOCOL_VERSION,
 								this.latestSupportedProtocolVersion))
 				.headers(httpHeaders -> {
 					transportSession.sessionId().ifPresent(id -> httpHeaders.add(HttpHeaders.MCP_SESSION_ID, id));
 				})
-				.bodyValue(message)
+				.bodyValue(jsonText)
 				.exchangeToFlux(response -> {
 					if (transportSession
 						.markInitialized(response.headers().asHttpHeaders().getFirst(HttpHeaders.MCP_SESSION_ID))) {

mcp-spring/mcp-spring-webmvc/src/main/java/io/modelcontextprotocol/server/transport/WebMvcStatelessServerTransport.java

@@ -118,7 +118,8 @@ private ServerResponse handlePost(ServerRequest request) {
 						.handleRequest(transportContext, jsonrpcRequest)
 						.contextWrite(ctx -> ctx.put(McpTransportContext.KEY, transportContext))
 						.block();
-					return ServerResponse.ok().contentType(MediaType.APPLICATION_JSON).body(jsonrpcResponse);
+					String json = jsonMapper.writeValueAsString(jsonrpcResponse);
+					return ServerResponse.ok().contentType(MediaType.APPLICATION_JSON).body(json);
 				}
 				catch (Exception e) {
 					logger.error("Failed to handle request: {}", e.getMessage());