feat: improve error handling by extracting root cause messages

- Add findRootCause() utility method utility to traverse exception chains
- Update McpServerSession and McpStreamableServerSession to use root cause messages
- Ensures meaningful error messages are returned instead of wrapped exception messages

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

Author: tzolov

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

@@ -78,4 +78,13 @@ public McpError build() {
 
 	}
 
+	public static Throwable findRootCause(Throwable throwable) {
+		Assert.notNull(throwable, "throwable must not be null");
+		Throwable rootCause = throwable;
+		while (rootCause.getCause() != null && rootCause.getCause() != rootCause) {
+			rootCause = rootCause.getCause();
+		}
+		return rootCause;
+	}
+
 }

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

@@ -224,9 +224,12 @@ public Mono<Void> handle(McpSchema.JSONRPCMessage message) {
 			else if (message instanceof McpSchema.JSONRPCRequest request) {
 				logger.debug("Received request: {}", request);
 				return handleIncomingRequest(request, transportContext).onErrorResume(error -> {
+					McpSchema.JSONRPCResponse.JSONRPCError jsonRpcError = (error instanceof McpError mcpError
+							&& mcpError.getJsonRpcError() != null) ? mcpError.getJsonRpcError()
+									: new McpSchema.JSONRPCResponse.JSONRPCError(McpSchema.ErrorCodes.INTERNAL_ERROR,
+											McpError.findRootCause(error).getMessage(), null);
 					var errorResponse = new McpSchema.JSONRPCResponse(McpSchema.JSONRPC_VERSION, request.id(), null,
-							new McpSchema.JSONRPCResponse.JSONRPCError(McpSchema.ErrorCodes.INTERNAL_ERROR,
-									error.getMessage(), null));
+							jsonRpcError);
 					// TODO: Should the error go to SSE or back as POST return?
 					return this.transport.sendMessage(errorResponse).then(Mono.empty());
 				}).flatMap(this.transport::sendMessage);
@@ -287,7 +290,7 @@ private Mono<McpSchema.JSONRPCResponse> handleIncomingRequest(McpSchema.JSONRPCR
 							&& mcpError.getJsonRpcError() != null) ? mcpError.getJsonRpcError()
 									// TODO: add error message through the data field
 									: new McpSchema.JSONRPCResponse.JSONRPCError(McpSchema.ErrorCodes.INTERNAL_ERROR,
-											error.getMessage(), null);
+											McpError.findRootCause(error).getMessage(), null);
 					return Mono.just(
 							new McpSchema.JSONRPCResponse(McpSchema.JSONRPC_VERSION, request.id(), null, jsonRpcError));
 				});

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

@@ -181,9 +181,8 @@ public Mono<Void> responseStream(McpSchema.JSONRPCRequest jsonrpcRequest, McpStr
 				.onErrorResume(e -> {
 					McpSchema.JSONRPCResponse.JSONRPCError jsonRpcError = (e instanceof McpError mcpError
 							&& mcpError.getJsonRpcError() != null) ? mcpError.getJsonRpcError()
-									// TODO: add error message through the data field
 									: new McpSchema.JSONRPCResponse.JSONRPCError(McpSchema.ErrorCodes.INTERNAL_ERROR,
-											e.getMessage(), null);
+											McpError.findRootCause(e).getMessage(), null);
 
 					var errorResponse = new McpSchema.JSONRPCResponse(McpSchema.JSONRPC_VERSION, jsonrpcRequest.id(),
 							null, jsonRpcError);