fix(transport): downgrade idle timeout log from error to debug

Idle keep-alive timeout is normal zombie-session cleanup, not a transport failure.

Route it through a dedicated WorkerQuitReason::IdleTimeout variant.

Log it at debug level instead of treating it as a fatal error.

Remove the unused LocalSessionWorkerError::KeepAliveTimeout variant.

Closes #817

Author: lutz-grex

crates/rmcp/src/transport/streamable_http_server/session/local.rs

@@ -928,8 +928,6 @@ pub enum LocalSessionWorkerError {
     FailToSendInitializeRequest(SessionError),
     #[error("fail to handle message: {0}")]
     FailToHandleMessage(SessionError),
-    #[error("keep alive timeout after {}ms", _0.as_millis())]
-    KeepAliveTimeout(Duration),
     #[error("Transport closed")]
     TransportClosed,
     #[error("Tokio join error {0}")]
@@ -1008,7 +1006,7 @@ impl Worker for LocalSessionWorker {
                     return Err(WorkerQuitReason::Cancelled)
                 }
                 _ = keep_alive_timeout => {
-                    return Err(WorkerQuitReason::fatal(LocalSessionWorkerError::KeepAliveTimeout(keep_alive), "poll next session event"))
+                    return Err(WorkerQuitReason::IdleTimeout(keep_alive))
                 }
             };
             match event {

crates/rmcp/src/transport/worker.rs

@@ -1,4 +1,4 @@
-use std::borrow::Cow;
+use std::{borrow::Cow, time::Duration};
 
 use tokio_util::sync::CancellationToken;
 use tracing::{Instrument, Level};
@@ -22,6 +22,8 @@ pub enum WorkerQuitReason<E> {
     TransportClosed,
     #[error("Handler terminated")]
     HandlerTerminated,
+    #[error("Worker idle timeout ({}ms)", _0.as_millis())]
+    IdleTimeout(Duration),
 }
 
 impl<E: std::error::Error + Send + 'static> WorkerQuitReason<E> {
@@ -122,7 +124,8 @@ impl<W: Worker> WorkerTransport<W> {
                 .inspect_err(|e| match e {
                     WorkerQuitReason::Cancelled
                     | WorkerQuitReason::TransportClosed
-                    | WorkerQuitReason::HandlerTerminated => {
+                    | WorkerQuitReason::HandlerTerminated
+                    | WorkerQuitReason::IdleTimeout(_) => {
                         tracing::debug!("worker quit with reason: {:?}", e);
                     }
                     WorkerQuitReason::Join(e) => {

crates/rmcp/tests/test_streamable_http_idle_timeout_log.rs

@@ -0,0 +1,149 @@
+#![cfg(all(
+    feature = "transport-streamable-http-server",
+    feature = "transport-streamable-http-client-reqwest",
+    not(feature = "local")
+))]
+
+use std::{
+    sync::{Arc, Mutex},
+    time::Duration,
+};
+
+use rmcp::transport::streamable_http_server::{
+    StreamableHttpServerConfig, StreamableHttpService, session::local::LocalSessionManager,
+};
+use tokio_util::sync::CancellationToken;
+use tracing_subscriber::layer::SubscriberExt;
+
+mod common;
+use common::calculator::Calculator;
+
+// Issue #817: keep-alive timeout emits tracing::error! for normal idle reaping.
+
+struct CapturedEvent {
+    level: tracing::Level,
+    message: String,
+}
+
+struct CapturingLayer {
+    events: Arc<Mutex<Vec<CapturedEvent>>>,
+}
+
+impl<S: tracing::Subscriber> tracing_subscriber::Layer<S> for CapturingLayer {
+    fn on_event(
+        &self,
+        event: &tracing::Event<'_>,
+        _ctx: tracing_subscriber::layer::Context<'_, S>,
+    ) {
+        let mut visitor = MessageVisitor(String::new());
+        event.record(&mut visitor);
+        self.events.lock().unwrap().push(CapturedEvent {
+            level: *event.metadata().level(),
+            message: visitor.0,
+        });
+    }
+}
+
+struct MessageVisitor(String);
+
+impl tracing::field::Visit for MessageVisitor {
+    fn record_debug(&mut self, field: &tracing::field::Field, value: &dyn std::fmt::Debug) {
+        if field.name() == "message" {
+            self.0 = format!("{:?}", value);
+        }
+    }
+}
+
+#[tokio::test(flavor = "current_thread")]
+async fn test_keep_alive_timeout_does_not_emit_error_log() {
+    let events = Arc::new(Mutex::new(Vec::<CapturedEvent>::new()));
+
+    let subscriber = tracing_subscriber::registry().with(CapturingLayer {
+        events: events.clone(),
+    });
+
+    let _guard = tracing::subscriber::set_default(subscriber);
+
+    let ct = CancellationToken::new();
+    let mut session_manager = LocalSessionManager::default();
+    session_manager.session_config.keep_alive = Some(Duration::from_millis(200));
+    let session_manager = Arc::new(session_manager);
+
+    let service = StreamableHttpService::new(
+        || Ok(Calculator::new()),
+        session_manager.clone(),
+        StreamableHttpServerConfig::default()
+            .with_sse_keep_alive(None)
+            .with_cancellation_token(ct.child_token()),
+    );
+
+    let router = axum::Router::new().nest_service("/mcp", service);
+    let tcp_listener = tokio::net::TcpListener::bind("127.0.0.1:0").await.unwrap();
+    let addr = tcp_listener.local_addr().unwrap();
+
+    tokio::spawn({
+        let ct = ct.clone();
+        async move {
+            let _ = axum::serve(tcp_listener, router)
+                .with_graceful_shutdown(async move { ct.cancelled_owned().await })
+                .await;
+        }
+    });
+
+    let client = reqwest::Client::new();
+
+    // Initialize session
+    let response = client
+        .post(format!("http://{addr}/mcp"))
+        .header("Content-Type", "application/json")
+        .header("Accept", "application/json, text/event-stream")
+        .body(r#"{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-06-18","capabilities":{},"clientInfo":{"name":"test","version":"1.0"}}}"#)
+        .send()
+        .await
+        .unwrap();
+    assert_eq!(response.status(), 200);
+    let session_id = response.headers()["mcp-session-id"]
+        .to_str()
+        .unwrap()
+        .to_string();
+
+    // Complete handshake
+    client
+        .post(format!("http://{addr}/mcp"))
+        .header("Content-Type", "application/json")
+        .header("Accept", "application/json, text/event-stream")
+        .header("mcp-session-id", &session_id)
+        .header("Mcp-Protocol-Version", "2025-06-18")
+        .body(r#"{"jsonrpc":"2.0","method":"notifications/initialized"}"#)
+        .send()
+        .await
+        .unwrap();
+
+    // Wait for keep_alive timeout (200ms) plus margin
+    tokio::time::sleep(Duration::from_millis(400)).await;
+
+    let captured = events.lock().unwrap();
+
+    let error_events: Vec<_> = captured
+        .iter()
+        .filter(|e| e.level == tracing::Level::ERROR)
+        .filter(|e| e.message.contains("keep alive timeout") || e.message.contains("IdleTimeout"))
+        .collect();
+    assert!(
+        error_events.is_empty(),
+        "keep-alive timeout should not produce ERROR logs, found {}: {:?}",
+        error_events.len(),
+        error_events.iter().map(|e| &e.message).collect::<Vec<_>>()
+    );
+
+    let debug_events: Vec<_> = captured
+        .iter()
+        .filter(|e| e.level == tracing::Level::DEBUG && e.message.contains("IdleTimeout"))
+        .collect();
+    assert!(
+        !debug_events.is_empty(),
+        "expected a DEBUG log with IdleTimeout, but found none"
+    );
+
+    ct.cancel();
+}