fix: use the json rpc error from the initialize response and bubble it up to the client (#569)

crates/rmcp/src/service/client.rs

@@ -8,7 +8,7 @@ use crate::{
         ArgumentInfo, CallToolRequest, CallToolRequestParam, CallToolResult, CancelledNotification,
         CancelledNotificationParam, ClientInfo, ClientJsonRpcMessage, ClientNotification,
         ClientRequest, ClientResult, CompleteRequest, CompleteRequestParam, CompleteResult,
-        CompletionContext, CompletionInfo, GetPromptRequest, GetPromptRequestParam,
+        CompletionContext, CompletionInfo, ErrorData, GetPromptRequest, GetPromptRequestParam,
         GetPromptResult, InitializeRequest, InitializedNotification, JsonRpcResponse,
         ListPromptsRequest, ListPromptsResult, ListResourceTemplatesRequest,
         ListResourceTemplatesResult, ListResourcesRequest, ListResourcesResult, ListToolsRequest,
@@ -44,6 +44,9 @@ pub enum ClientInitializeError {
         context: Cow<'static, str>,
     },
 
+    #[error("JSON-RPC error: {0}")]
+    JsonRpcError(ErrorData),
+
     #[error("Cancelled")]
     Cancelled,
 }
@@ -92,6 +95,10 @@ where
             ServerJsonRpcMessage::Response(JsonRpcResponse { id, result, .. }) => {
                 break Ok((result, id));
             }
+            // Handle JSON-RPC error responses
+            ServerJsonRpcMessage::Error(error) => {
+                break Err(ClientInitializeError::JsonRpcError(error.error));
+            }
             // Server could send logging messages before handshake
             ServerJsonRpcMessage::Notification(mut notification) => {
                 let ServerNotification::LoggingMessageNotification(logging) =

crates/rmcp/tests/test_client_initialization.rs

@@ -0,0 +1,51 @@
+// cargo test --features "server client" --package rmcp test_client_initialization
+mod common;
+
+use std::borrow::Cow;
+
+use common::handlers::TestClientHandler;
+use rmcp::{
+    ServiceExt,
+    model::{
+        ErrorCode, ErrorData, JsonRpcError, JsonRpcVersion2_0, RequestId, ServerJsonRpcMessage,
+    },
+    transport::{IntoTransport, Transport},
+};
+
+#[tokio::test]
+async fn test_client_init_handles_jsonrpc_error() {
+    let (server_transport, client_transport) = tokio::io::duplex(1024);
+    let mut server = IntoTransport::<rmcp::RoleServer, _, _>::into_transport(server_transport);
+
+    let client_handle = tokio::spawn(async move {
+        TestClientHandler::new(true, true)
+            .serve(client_transport)
+            .await
+    });
+
+    tokio::spawn(async move {
+        let _init_request = server.receive().await;
+
+        let error_msg = ServerJsonRpcMessage::Error(JsonRpcError {
+            jsonrpc: JsonRpcVersion2_0,
+            id: RequestId::Number(1),
+            error: ErrorData {
+                code: ErrorCode(-32600),
+                message: Cow::Borrowed("Invalid Request"),
+                data: None,
+            },
+        });
+        let _: Result<(), _> = server.send(error_msg).await;
+    });
+
+    let result = client_handle.await.unwrap();
+
+    assert!(result.is_err());
+    match result {
+        Err(rmcp::service::ClientInitializeError::JsonRpcError(error_data)) => {
+            assert_eq!(error_data.code, ErrorCode(-32600));
+            assert_eq!(error_data.message, "Invalid Request");
+        }
+        _ => panic!("Expected ClientInitializeError::JsonRpcError"),
+    }
+}