fix: accept logging/setLevel and ping before initialized notification (#730)

* fix: accept logging/setLevel and ping before initialized notification

* test: add server initialization tests for pre-init requests

Author: DaleSeo

crates/rmcp/src/service/server.rs

@@ -16,7 +16,7 @@ use crate::{
     model::{
         CancelledNotification, CancelledNotificationParam, ClientInfo, ClientJsonRpcMessage,
         ClientNotification, ClientRequest, ClientResult, CreateMessageRequest,
-        CreateMessageRequestParams, CreateMessageResult, ErrorData, ListRootsRequest,
+        CreateMessageRequestParams, CreateMessageResult, EmptyResult, ErrorData, ListRootsRequest,
         ListRootsResult, LoggingMessageNotification, LoggingMessageNotificationParam,
         ProgressNotification, ProgressNotificationParam, PromptListChangedNotification,
         ProtocolVersion, ResourceListChangedNotification, ResourceUpdatedNotification,
@@ -147,22 +147,6 @@ where
         )))
 }
 
-/// Helper function to expect a notification from the stream
-async fn expect_notification<T>(
-    transport: &mut T,
-    context: &str,
-) -> Result<ClientNotification, ServerInitializeError>
-where
-    T: Transport<RoleServer>,
-{
-    let msg = expect_next_message(transport, context).await?;
-    let msg_clone = msg.clone();
-    msg.into_notification()
-        .ok_or(ServerInitializeError::ExpectedInitializedNotification(
-            Some(msg_clone),
-        ))
-}
-
 pub async fn serve_server_with_ct<S, T, E, A>(
     service: S,
     transport: T,
@@ -246,12 +230,41 @@ where
             ServerInitializeError::transport::<T>(error, "sending initialize response")
         })?;
 
-    // Wait for initialize notification
-    let notification = expect_notification(&mut transport, "initialize notification").await?;
-    let ClientNotification::InitializedNotification(_) = notification else {
-        return Err(ServerInitializeError::ExpectedInitializedNotification(
-            Some(ClientJsonRpcMessage::notification(notification)),
-        ));
+    // Wait for initialized notification. The MCP spec permits logging/setLevel and ping
+    // before initialized; VS Code sends setLevel immediately after the initialize response.
+    let notification = loop {
+        let msg = expect_next_message(&mut transport, "initialize notification").await?;
+        match msg {
+            ClientJsonRpcMessage::Notification(n)
+                if matches!(
+                    n.notification,
+                    ClientNotification::InitializedNotification(_)
+                ) =>
+            {
+                break n.notification;
+            }
+            ClientJsonRpcMessage::Request(req)
+                if matches!(
+                    req.request,
+                    ClientRequest::SetLevelRequest(_) | ClientRequest::PingRequest(_)
+                ) =>
+            {
+                transport
+                    .send(ServerJsonRpcMessage::response(
+                        ServerResult::EmptyResult(EmptyResult {}),
+                        req.id,
+                    ))
+                    .await
+                    .map_err(|error| {
+                        ServerInitializeError::transport::<T>(error, "sending pre-init response")
+                    })?;
+            }
+            other => {
+                return Err(ServerInitializeError::ExpectedInitializedNotification(
+                    Some(other),
+                ));
+            }
+        }
     };
     let context = NotificationContext {
         meta: notification.get_meta().clone(),

crates/rmcp/tests/test_server_initialization.rs

@@ -0,0 +1,160 @@
+// cargo test --features "client" --package rmcp -- server_init
+#![cfg(feature = "client")]
+mod common;
+
+use common::handlers::TestServer;
+use rmcp::{
+    ServiceExt,
+    model::{ClientJsonRpcMessage, ServerJsonRpcMessage, ServerResult},
+    service::ServerInitializeError,
+    transport::{IntoTransport, Transport},
+};
+
+fn msg(raw: &str) -> ClientJsonRpcMessage {
+    serde_json::from_str(raw).expect("invalid test message JSON")
+}
+
+fn init_request() -> ClientJsonRpcMessage {
+    msg(r#"{
+        "jsonrpc": "2.0",
+        "id": 1,
+        "method": "initialize",
+        "params": {
+            "protocolVersion": "2025-11-25",
+            "capabilities": {},
+            "clientInfo": { "name": "test-client", "version": "0.0.1" }
+        }
+    }"#)
+}
+
+fn initialized_notification() -> ClientJsonRpcMessage {
+    msg(r#"{ "jsonrpc": "2.0", "method": "notifications/initialized" }"#)
+}
+
+fn set_level_request(id: u64) -> ClientJsonRpcMessage {
+    msg(&format!(
+        r#"{{ "jsonrpc": "2.0", "id": {id}, "method": "logging/setLevel", "params": {{ "level": "info" }} }}"#
+    ))
+}
+
+fn ping_request(id: u64) -> ClientJsonRpcMessage {
+    msg(&format!(
+        r#"{{ "jsonrpc": "2.0", "id": {id}, "method": "ping" }}"#
+    ))
+}
+
+fn list_tools_request(id: u64) -> ClientJsonRpcMessage {
+    msg(&format!(
+        r#"{{ "jsonrpc": "2.0", "id": {id}, "method": "tools/list" }}"#
+    ))
+}
+
+async fn do_initialize(client: &mut impl Transport<rmcp::RoleClient>) {
+    client.send(init_request()).await.unwrap();
+    let _response = client.receive().await.unwrap();
+}
+
+// Server responds with EmptyResult to setLevel received before initialized.
+#[tokio::test]
+async fn server_init_set_level_response_is_empty_result() {
+    let (server_transport, client_transport) = tokio::io::duplex(4096);
+    let _server = tokio::spawn(async move { TestServer::new().serve(server_transport).await });
+    let mut client = IntoTransport::<rmcp::RoleClient, _, _>::into_transport(client_transport);
+
+    do_initialize(&mut client).await;
+    client.send(set_level_request(2)).await.unwrap();
+
+    let response = client.receive().await.unwrap();
+    assert!(
+        matches!(
+            response,
+            ServerJsonRpcMessage::Response(ref r)
+                if matches!(r.result, ServerResult::EmptyResult(_))
+        ),
+        "expected EmptyResult for setLevel, got: {response:?}"
+    );
+}
+
+// Server initializes successfully when setLevel is sent before the initialized notification.
+#[tokio::test]
+async fn server_init_succeeds_after_set_level_before_initialized() {
+    let (server_transport, client_transport) = tokio::io::duplex(4096);
+    let server_handle =
+        tokio::spawn(async move { TestServer::new().serve(server_transport).await });
+    let mut client = IntoTransport::<rmcp::RoleClient, _, _>::into_transport(client_transport);
+
+    do_initialize(&mut client).await;
+    client.send(set_level_request(2)).await.unwrap();
+    let _response = client.receive().await.unwrap();
+    client.send(initialized_notification()).await.unwrap();
+
+    let result = server_handle.await.unwrap();
+    assert!(
+        result.is_ok(),
+        "server should initialize successfully after setLevel"
+    );
+    result.unwrap().cancel().await.unwrap();
+}
+
+// Server responds with EmptyResult to ping received before initialized.
+#[tokio::test]
+async fn server_init_ping_response_is_empty_result() {
+    let (server_transport, client_transport) = tokio::io::duplex(4096);
+    let _server = tokio::spawn(async move { TestServer::new().serve(server_transport).await });
+    let mut client = IntoTransport::<rmcp::RoleClient, _, _>::into_transport(client_transport);
+
+    do_initialize(&mut client).await;
+    client.send(ping_request(2)).await.unwrap();
+
+    let response = client.receive().await.unwrap();
+    assert!(
+        matches!(
+            response,
+            ServerJsonRpcMessage::Response(ref r)
+                if matches!(r.result, ServerResult::EmptyResult(_))
+        ),
+        "expected EmptyResult for ping, got: {response:?}"
+    );
+}
+
+// Server initializes successfully when ping is sent before the initialized notification.
+#[tokio::test]
+async fn server_init_succeeds_after_ping_before_initialized() {
+    let (server_transport, client_transport) = tokio::io::duplex(4096);
+    let server_handle =
+        tokio::spawn(async move { TestServer::new().serve(server_transport).await });
+    let mut client = IntoTransport::<rmcp::RoleClient, _, _>::into_transport(client_transport);
+
+    do_initialize(&mut client).await;
+    client.send(ping_request(2)).await.unwrap();
+    let _response = client.receive().await.unwrap();
+    client.send(initialized_notification()).await.unwrap();
+
+    let result = server_handle.await.unwrap();
+    assert!(
+        result.is_ok(),
+        "server should initialize successfully after ping"
+    );
+    result.unwrap().cancel().await.unwrap();
+}
+
+// Server returns ExpectedInitializedNotification for any other message before initialized.
+#[tokio::test]
+async fn server_init_rejects_unexpected_message_before_initialized() {
+    let (server_transport, client_transport) = tokio::io::duplex(4096);
+    let server_handle =
+        tokio::spawn(async move { TestServer::new().serve(server_transport).await });
+    let mut client = IntoTransport::<rmcp::RoleClient, _, _>::into_transport(client_transport);
+
+    do_initialize(&mut client).await;
+    client.send(list_tools_request(2)).await.unwrap();
+
+    let result = server_handle.await.unwrap();
+    assert!(
+        matches!(
+            result,
+            Err(ServerInitializeError::ExpectedInitializedNotification(_))
+        ),
+        "expected ExpectedInitializedNotification error"
+    );
+}