feat: add subscription listen streams (SEP-2575) (#1000)

* feat: add subscription listen streams

* refactor: reuse legacy request helper

examples/servers/src/subscriptions_streamhttp.rs

@@ -0,0 +1,83 @@
+use std::{borrow::Cow, time::Duration};
+
+use rmcp::{
+    ErrorData, ServerHandler,
+    model::{ProtocolVersion, ServerCapabilities, ServerInfo, SubscriptionFilter},
+    service::SubscriptionContext,
+    transport::streamable_http_server::{
+        StreamableHttpServerConfig, StreamableHttpService, session::local::LocalSessionManager,
+    },
+};
+use tokio_util::sync::CancellationToken;
+
+#[derive(Clone)]
+struct SubscriptionServer;
+
+impl ServerHandler for SubscriptionServer {
+    fn supported_protocol_versions(&self) -> Cow<'static, [ProtocolVersion]> {
+        Cow::Borrowed(&[ProtocolVersion::V_2026_07_28])
+    }
+
+    fn get_info(&self) -> ServerInfo {
+        ServerInfo::new(
+            ServerCapabilities::builder()
+                .enable_tools()
+                .enable_tool_list_changed()
+                .build(),
+        )
+    }
+
+    fn accepted_subscription_filter(
+        &self,
+        requested: &SubscriptionFilter,
+    ) -> Option<SubscriptionFilter> {
+        Some(requested.supported_by(&self.get_info().capabilities))
+    }
+
+    async fn listen(&self, context: SubscriptionContext) -> Result<(), ErrorData> {
+        loop {
+            tokio::select! {
+                () = context.cancelled() => return Ok(()),
+                () = tokio::time::sleep(Duration::from_secs(2)) => {
+                    context
+                        .sink()
+                        .notify_tool_list_changed()
+                        .await
+                        .map_err(|error| {
+                            ErrorData::internal_error(error.to_string(), None)
+                        })?;
+                }
+            }
+        }
+    }
+}
+
+#[tokio::main]
+async fn main() -> anyhow::Result<()> {
+    tracing_subscriber::fmt()
+        .with_env_filter(
+            tracing_subscriber::EnvFilter::try_from_default_env().unwrap_or_else(|_| "info".into()),
+        )
+        .init();
+
+    let cancellation_token = CancellationToken::new();
+    let service: StreamableHttpService<SubscriptionServer, LocalSessionManager> =
+        StreamableHttpService::new(
+            || Ok(SubscriptionServer),
+            Default::default(),
+            StreamableHttpServerConfig::default()
+                .with_legacy_session_mode(false)
+                .with_sse_keep_alive(Some(Duration::from_secs(10)))
+                .with_cancellation_token(cancellation_token.child_token()),
+        );
+    let router = axum::Router::new().nest_service("/mcp", service);
+    let listener = tokio::net::TcpListener::bind("127.0.0.1:8000").await?;
+
+    axum::serve(listener, router)
+        .with_graceful_shutdown(async move {
+            let _ = tokio::signal::ctrl_c().await;
+            cancellation_token.cancel();
+        })
+        .await?;
+    Ok(())
+}