fix(sse-client): consume control frames; refresh message endpoint

crates/rmcp/src/lib.rs

@@ -102,7 +102,7 @@
 //! start a MCP server in Python and then list the tools and call `git status`
 //! as follows:
 //!
-//! ```rust
+//! ```rust,ignore
 //! use anyhow::Result;
 //! use rmcp::{model::CallToolRequestParam, service::ServiceExt, transport::{TokioChildProcess, ConfigureCommandExt}};
 //! use tokio::process::Command;

crates/rmcp/src/transport.rs

@@ -39,7 +39,7 @@
 //!
 //! ## Examples
 //!
-//! ```rust
+//! ```rust,ignore
 //! # use rmcp::{
 //! #     ServiceExt, serve_client, serve_server,
 //! # };

crates/rmcp/src/transport/common/client_side_sse.rs

@@ -98,10 +98,29 @@ impl<E: std::error::Error + Send> SseStreamReconnect for NeverReconnect<E> {
     }
 }
 
+/// Abstraction for SSE reconnection logic. Implementors can hook into
+/// [`handle_control_event`](Self::handle_control_event) to consume control
+/// frames (e.g. `event: endpoint`) that arrive when a server restarts an SSE
+/// stream. The default implementation is a no-op, keeping existing behaviour
+/// intact.
 pub(crate) trait SseStreamReconnect {
     type Error: std::error::Error;
     type Future: Future<Output = Result<BoxedSseResponse, Self::Error>> + Send;
     fn retry_connection(&mut self, last_event_id: Option<&str>) -> Self::Future;
+    fn handle_control_event(&mut self, _event: &Sse) -> Result<(), Self::Error> {
+        Ok(())
+    }
+    fn handle_stream_error(
+        &mut self,
+        error: &(dyn std::error::Error + 'static),
+        last_event_id: Option<&str>,
+    ) {
+        if let Some(id) = last_event_id {
+            tracing::warn!(%id, "sse stream error: {error}");
+        } else {
+            tracing::warn!("sse stream error: {error}");
+        }
+    }
 }
 
 pin_project_lite::pin_project! {
@@ -189,14 +208,31 @@ where
                             *this.server_retry_interval =
                                 Some(Duration::from_millis(new_server_retry));
                         }
-                        if let Some(event_id) = sse.id {
-                            *this.last_event_id = Some(event_id);
+                        if let Some(ref event_id) = sse.id {
+                            *this.last_event_id = Some(event_id.clone());
+                        }
+                        // Only treat blank/`message` events as JSON-RPC payloads.
+                        // Other control frames (endpoint, ping, etc.) are passed to
+                        // the reconnection handler.
+                        let is_message_event =
+                            matches!(sse.event.as_deref(), None | Some("") | Some("message"));
+                        if !is_message_event {
+                            match this.connector.handle_control_event(&sse) {
+                                Ok(()) => return self.poll_next(cx),
+                                Err(e) => {
+                                    this.state.set(SseAutoReconnectStreamState::Terminated);
+                                    return Poll::Ready(Some(Err(e)));
+                                }
+                            }
                         }
                         if let Some(data) = sse.data {
                             match serde_json::from_str::<ServerJsonRpcMessage>(&data) {
                                 Err(e) => {
-                                    // not sure should this be a hard error
-                                    tracing::warn!("failed to deserialize server message: {e}");
+                                    // Downgrade to debug to avoid noisy logs when servers emit
+                                    // non-JSON payloads as message frames. Include last_event_id
+                                    // to aid troubleshooting while keeping default behaviour.
+                                    let last_id = this.last_event_id.as_deref().unwrap_or("");
+                                    tracing::debug!(last_event_id=%last_id, "failed to deserialize server message: {e}");
                                     return self.poll_next(cx);
                                 }
                                 Ok(message) => {
@@ -208,7 +244,8 @@ where
                         }
                     }
                     Some(Err(e)) => {
-                        tracing::warn!("sse stream error: {e}");
+                        this.connector
+                            .handle_stream_error(&e, this.last_event_id.as_deref());
                         let retrying = this
                             .connector
                             .retry_connection(this.last_event_id.as_deref());

crates/rmcp/src/transport/sse_client.rs

@@ -1,9 +1,12 @@
-//！ reference: https://html.spec.whatwg.org/multipage/server-sent-events.html
-use std::{pin::Pin, sync::Arc};
+//! Reference: <https://html.spec.whatwg.org/multipage/server-sent-events.html>
+use std::{
+    pin::Pin,
+    sync::{Arc, RwLock},
+};
 
 use futures::{StreamExt, future::BoxFuture};
 use http::Uri;
-use sse_stream::Error as SseError;
+use sse_stream::{Error as SseError, Sse};
 use thiserror::Error;
 
 use super::{
@@ -54,9 +57,13 @@ pub trait SseClient: Clone + Send + Sync + 'static {
     ) -> impl Future<Output = Result<BoxedSseResponse, SseTransportError<Self::Error>>> + Send + '_;
 }
 
+/// Helper that refreshes the POST endpoint whenever the server emits
+/// control frames during SSE reconnect; used together with
+/// [`SseAutoReconnectStream`].
 struct SseClientReconnect<C> {
     pub client: C,
     pub uri: Uri,
+    pub message_endpoint: Arc<RwLock<Uri>>,
 }
 
 impl<C: SseClient> SseStreamReconnect for SseClientReconnect<C> {
@@ -68,6 +75,37 @@ impl<C: SseClient> SseStreamReconnect for SseClientReconnect<C> {
         let last_event_id = last_event_id.map(|s| s.to_owned());
         Box::pin(async move { client.get_stream(uri, last_event_id, None).await })
     }
+
+    fn handle_control_event(&mut self, event: &Sse) -> Result<(), Self::Error> {
+        if event.event.as_deref() != Some("endpoint") {
+            return Ok(());
+        }
+        let Some(data) = event.data.as_ref() else {
+            return Ok(());
+        };
+        // Servers typically resend the message POST endpoint (often with a new
+        // sessionId) when a stream reconnects. Reuse `message_endpoint` helper
+        // to resolve it and update the shared URI.
+        let new_endpoint = message_endpoint(self.uri.clone(), data.clone())
+            .map_err(SseTransportError::InvalidUri)?;
+        *self
+            .message_endpoint
+            .write()
+            .expect("message endpoint lock poisoned") = new_endpoint;
+        Ok(())
+    }
+
+    fn handle_stream_error(
+        &mut self,
+        error: &(dyn std::error::Error + 'static),
+        last_event_id: Option<&str>,
+    ) {
+        tracing::warn!(
+            uri = %self.uri,
+            last_event_id = last_event_id.unwrap_or(""),
+            "sse stream error: {error}"
+        );
+    }
 }
 type ServerMessageStream<C> = Pin<Box<SseAutoReconnectStream<SseClientReconnect<C>>>>;
 
@@ -81,7 +119,7 @@ type ServerMessageStream<C> = Pin<Box<SseAutoReconnectStream<SseClientReconnect<
 ///
 /// ## Using reqwest
 ///
-/// ```rust
+/// ```rust,ignore
 /// use rmcp::transport::SseClientTransport;
 ///
 /// // Enable the reqwest feature in Cargo.toml:
@@ -95,7 +133,7 @@ type ServerMessageStream<C> = Pin<Box<SseAutoReconnectStream<SseClientReconnect<
 ///
 /// ## Using a custom HTTP client
 ///
-/// ```rust
+/// ```rust,ignore
 /// use rmcp::transport::sse_client::{SseClient, SseClientTransport, SseClientConfig};
 /// use std::sync::Arc;
 /// use futures::stream::BoxStream;
@@ -154,7 +192,9 @@ type ServerMessageStream<C> = Pin<Box<SseAutoReconnectStream<SseClientReconnect<
 pub struct SseClientTransport<C: SseClient> {
     client: C,
     config: SseClientConfig,
-    message_endpoint: Uri,
+    /// Current POST endpoint; refreshed when the server sends new endpoint
+    /// control frames.
+    message_endpoint: Arc<RwLock<Uri>>,
     stream: Option<ServerMessageStream<C>>,
 }
 
@@ -168,8 +208,16 @@ impl<C: SseClient> Transport<RoleClient> for SseClientTransport<C> {
         item: crate::service::TxJsonRpcMessage<RoleClient>,
     ) -> impl Future<Output = Result<(), Self::Error>> + Send + 'static {
         let client = self.client.clone();
-        let uri = self.message_endpoint.clone();
-        async move { client.post_message(uri, item, None).await }
+        let message_endpoint = self.message_endpoint.clone();
+        async move {
+            let uri = {
+                let guard = message_endpoint
+                    .read()
+                    .expect("message endpoint lock poisoned");
+                guard.clone()
+            };
+            client.post_message(uri, item, None).await
+        }
     }
     async fn close(&mut self) -> Result<(), Self::Error> {
         self.stream.take();
@@ -194,7 +242,7 @@ impl<C: SseClient> SseClientTransport<C> {
         let sse_endpoint = config.sse_endpoint.as_ref().parse::<http::Uri>()?;
 
         let mut sse_stream = client.get_stream(sse_endpoint.clone(), None, None).await?;
-        let message_endpoint = if let Some(endpoint) = config.use_message_endpoint.clone() {
+        let initial_message_endpoint = if let Some(endpoint) = config.use_message_endpoint.clone() {
             let ep = endpoint.parse::<http::Uri>()?;
             let mut sse_endpoint_parts = sse_endpoint.clone().into_parts();
             sse_endpoint_parts.path_and_query = ep.into_parts().path_and_query;
@@ -214,12 +262,14 @@ impl<C: SseClient> SseClientTransport<C> {
                 break message_endpoint(sse_endpoint.clone(), ep)?;
             }
         };
+        let message_endpoint = Arc::new(RwLock::new(initial_message_endpoint));
 
         let stream = Box::pin(SseAutoReconnectStream::new(
             sse_stream,
             SseClientReconnect {
                 client: client.clone(),
                 uri: sse_endpoint.clone(),
+                message_endpoint: message_endpoint.clone(),
             },
             config.retry_policy.clone(),
         ));
@@ -274,7 +324,7 @@ pub struct SseClientConfig {
     /// and the server send the message endpoint event as `message?session_id=123`,
     /// then the message endpoint will be `http://example.com/message`.
     ///
-    /// This follow the rules of JavaScript's [`new URL(url, base)`](https://developer.mozilla.org/zh-CN/docs/Web/API/URL/URL)
+    /// This follows the rules of JavaScript's [`new URL(url, base)`](https://developer.mozilla.org/en-US/docs/Web/API/URL/URL)
     pub sse_endpoint: Arc<str>,
     pub retry_policy: Arc<dyn SseRetryPolicy>,
     /// if this is settled, the client will use this endpoint to send message and skip get the endpoint event
@@ -293,8 +343,40 @@ impl Default for SseClientConfig {
 
 #[cfg(test)]
 mod tests {
+    use futures::StreamExt;
+    use serde_json::{Value, json};
+
     use super::*;
 
+    #[derive(Clone)]
+    struct DummyClient;
+
+    #[derive(Debug, thiserror::Error)]
+    #[error("dummy error")]
+    struct DummyError;
+
+    impl SseClient for DummyClient {
+        type Error = DummyError;
+
+        async fn post_message(
+            &self,
+            _uri: Uri,
+            _message: ClientJsonRpcMessage,
+            _auth_token: Option<String>,
+        ) -> Result<(), SseTransportError<Self::Error>> {
+            Ok(())
+        }
+
+        async fn get_stream(
+            &self,
+            _uri: Uri,
+            _last_event_id: Option<String>,
+            _auth_token: Option<String>,
+        ) -> Result<BoxedSseResponse, SseTransportError<Self::Error>> {
+            unreachable!("get_stream should not be called in this test")
+        }
+    }
+
     #[test]
     fn test_message_endpoint() {
         let base_url = "https://localhost/sse".parse::<http::Uri>().unwrap();
@@ -319,4 +401,58 @@ mod tests {
         .unwrap();
         assert_eq!(result.to_string(), "http://example.com/xxx?sessionId=x");
     }
+
+    #[test]
+    fn handle_endpoint_control_event_updates_uri() {
+        let initial_endpoint = "https://example.com/message?sessionId=old"
+            .parse::<Uri>()
+            .unwrap();
+        let shared_endpoint = Arc::new(RwLock::new(initial_endpoint));
+        let mut reconnect = SseClientReconnect {
+            client: DummyClient,
+            uri: "https://example.com/sse".parse::<Uri>().unwrap(),
+            message_endpoint: shared_endpoint.clone(),
+        };
+
+        let control_event = Sse::default()
+            .event("endpoint")
+            .data("/message?sessionId=new");
+
+        reconnect.handle_control_event(&control_event).unwrap();
+
+        let guard = shared_endpoint.read().expect("lock poisoned");
+        assert_eq!(
+            guard.to_string(),
+            "https://example.com/message?sessionId=new"
+        );
+    }
+
+    #[tokio::test]
+    async fn control_event_frames_are_skipped() {
+        let payload = json!({
+            "jsonrpc": "2.0",
+            "id": 1,
+            "result": {"ok": true}
+        })
+        .to_string();
+
+        let events = vec![
+            Ok(Sse::default()
+                .event("endpoint")
+                .data("/message?sessionId=reconnect")),
+            Ok(Sse::default().event("message").data(payload.clone())),
+        ];
+
+        let sse_src: BoxedSseResponse = futures::stream::iter(events).boxed();
+        let reconn_stream = SseAutoReconnectStream::never_reconnect(sse_src, DummyError);
+        futures::pin_mut!(reconn_stream);
+
+        let message = reconn_stream.next().await.expect("stream item").unwrap();
+        let actual: Value = serde_json::to_value(message).expect("serialize actual message");
+        // We only need to assert that a valid JSON-RPC response came through after
+        // skipping control frames. The exact `result` shape depends on the SDK's
+        // typed result enums and is not asserted here.
+        assert_eq!(actual.get("jsonrpc"), Some(&Value::String("2.0".into())));
+        assert_eq!(actual.get("id"), Some(&Value::Number(1u64.into())));
+    }
 }

target_ci_mirror/CACHEDIR.TAG

@@ -0,0 +1,3 @@
+Signature: 8a477f597d28d172789f06886806bc55
+# This file is a cache directory tag created by cargo.
+# For information about cache directory tags see https://bford.info/cachedir/