feat: legacy SSE support for MCP server configurations

.gitignore

@@ -5,4 +5,3 @@
 .idea/
 /loki.iml
 /.idea/
-src

src/mcp/mod.rs

@@ -1,3 +1,5 @@
+mod sse_transport;
+
 use crate::config::AppConfig;
 use crate::config::paths;
 use crate::utils::{AbortSignal, abortable_run_with_spinner};
@@ -5,6 +7,7 @@ use crate::vault::Vault;
 use crate::vault::interpolate_secrets;
 use anyhow::{Context, Result, anyhow};
 use futures_util::{StreamExt, TryStreamExt, stream};
+use http::{HeaderName, HeaderValue};
 use indoc::formatdoc;
 use rmcp::service::RunningService;
 use rmcp::transport::StreamableHttpClientTransport;
@@ -12,12 +15,12 @@ use rmcp::transport::TokioChildProcess;
 use rmcp::transport::streamable_http_client::StreamableHttpClientTransportConfig;
 use rmcp::{RoleClient, ServiceExt};
 use serde::{Deserialize, Serialize};
+use sse_transport::LegacySseTransport;
 use std::collections::{HashMap, HashSet};
 use std::fs::OpenOptions;
 use std::path::{Path, PathBuf};
 use std::process::Stdio;
 use std::sync::Arc;
-use http::{HeaderName, HeaderValue};
 use tokio::process::Command;
 
 pub const MCP_INVOKE_META_FUNCTION_NAME_PREFIX: &str = "mcp_invoke";
@@ -328,10 +331,16 @@ pub(crate) async fn spawn_mcp_server(
     spec: &McpServer,
     log_path: Option<&Path>,
 ) -> Result<Arc<ConnectedServer>> {
-    if spec.is_remote() {
+    match spec.transport_type {
-        let url = spec.url.as_deref().expect("validated: remote spec has url");
+        McpTransportType::Http => {
-        spawn_remote_mcp_server(url, spec.headers.as_ref()).await
+            let url = spec.url.as_deref().expect("validated: http spec has url");
-    } else {
+            spawn_http_mcp_server(url, spec.headers.as_ref()).await
+        }
+        McpTransportType::Sse => {
+            let url = spec.url.as_deref().expect("validated: sse spec has url");
+            spawn_sse_mcp_server(url, spec.headers.as_ref()).await
+        }
+        McpTransportType::Stdio => {
             let command = spec
                 .command
                 .as_deref()
@@ -339,8 +348,9 @@ pub(crate) async fn spawn_mcp_server(
             spawn_stdio_mcp_server(command, spec, log_path).await
         }
     }
+}
 
-async fn spawn_remote_mcp_server(
+async fn spawn_http_mcp_server(
     url: &str,
     headers: Option<&HashMap<String, String>>,
 ) -> Result<Arc<ConnectedServer>> {
@@ -365,7 +375,23 @@ async fn spawn_remote_mcp_server(
     let service = Arc::new(
         ().serve(transport)
             .await
-            .with_context(|| format!("Failed to connect to remote MCP server: {url}"))?,
+            .with_context(|| format!("Failed to connect to HTTP MCP server: {url}"))?,
+    );
+    Ok(service)
+}
+
+async fn spawn_sse_mcp_server(
+    url: &str,
+    headers: Option<&HashMap<String, String>>,
+) -> Result<Arc<ConnectedServer>> {
+    let sse = LegacySseTransport::connect(url, headers)
+        .await
+        .with_context(|| format!("Failed to connect to SSE MCP server: {url}"))?;
+    let (sink, stream) = sse.into_parts();
+    let service = Arc::new(
+        ().serve((sink, stream))
+            .await
+            .with_context(|| format!("Failed to initialize SSE MCP server: {url}"))?,
     );
     Ok(service)
 }

src/mcp/sse_transport.rs

@@ -0,0 +1,261 @@
+use anyhow::{Context, Result, anyhow};
+use fmt::{Display, Formatter};
+use futures_util::StreamExt;
+use mpsc::{Receiver, Sender, channel};
+use reqwest::Client;
+use reqwest::header::{HeaderMap, HeaderName, HeaderValue};
+use reqwest_eventsource::{Event, EventSource};
+use rmcp::model::{ClientJsonRpcMessage, ServerJsonRpcMessage};
+use std::collections::HashMap;
+use std::error::Error;
+use std::fmt;
+use std::pin::Pin;
+use std::task::Poll;
+use tokio::sync::mpsc;
+use tokio::time::Duration;
+use url::Url;
+
+const CHANNEL_BUF: usize = 64;
+
+pub struct LegacySseTransport {
+    tx: Sender<ClientJsonRpcMessage>,
+    rx: Receiver<ServerJsonRpcMessage>,
+}
+
+impl LegacySseTransport {
+    pub async fn connect(sse_url: &str, headers: Option<&HashMap<String, String>>) -> Result<Self> {
+        let base_url =
+            Url::parse(sse_url).with_context(|| format!("Invalid SSE URL: {sse_url}"))?;
+
+        let mut client_builder = Client::builder();
+        let mut header_map = HeaderMap::new();
+        if let Some(hdrs) = headers {
+            for (k, v) in hdrs {
+                let name = k
+                    .parse::<HeaderName>()
+                    .with_context(|| format!("Invalid header name: {k}"))?;
+                let value = v
+                    .parse::<HeaderValue>()
+                    .with_context(|| format!("Invalid header value for {k}"))?;
+                header_map.insert(name, value);
+            }
+            client_builder = client_builder.default_headers(header_map);
+        }
+        let client = client_builder
+            .build()
+            .context("Failed to build HTTP client")?;
+
+        let request = client.get(sse_url);
+        let mut es = EventSource::new(request).context("Failed to open SSE connection")?;
+
+        let post_endpoint = wait_for_endpoint_event(&mut es, &base_url).await?;
+
+        let (outgoing_tx, outgoing_rx) = channel::<ClientJsonRpcMessage>(CHANNEL_BUF);
+        let (incoming_tx, incoming_rx) = channel::<ServerJsonRpcMessage>(CHANNEL_BUF);
+
+        tokio::spawn(sse_reader_task(es, incoming_tx));
+        tokio::spawn(post_writer_task(client, post_endpoint, outgoing_rx));
+
+        Ok(Self {
+            tx: outgoing_tx,
+            rx: incoming_rx,
+        })
+    }
+
+    pub fn into_parts(
+        self,
+    ) -> (
+        SseSink<ClientJsonRpcMessage>,
+        SseStream<ServerJsonRpcMessage>,
+    ) {
+        (
+            SseSink {
+                tx: PollSender {
+                    tx: self.tx,
+                    permit: None,
+                },
+            },
+            SseStream { rx: self.rx },
+        )
+    }
+}
+
+async fn wait_for_endpoint_event(es: &mut EventSource, base_url: &Url) -> Result<String> {
+    let timeout = Duration::from_secs(30);
+    tokio::time::timeout(timeout, async {
+        while let Some(event) = es.next().await {
+            match event {
+                Ok(Event::Open) => {}
+                Ok(Event::Message(msg)) if msg.event == "endpoint" => {
+                    let endpoint = msg.data.trim().to_string();
+                    let resolved = resolve_endpoint(&endpoint, base_url)?;
+                    return Ok(resolved);
+                }
+                Ok(Event::Message(_)) => {}
+                Err(e) => {
+                    return Err(anyhow!(
+                        "SSE connection error while waiting for endpoint event: {e}"
+                    ));
+                }
+            }
+        }
+        Err(anyhow!("SSE stream closed before receiving endpoint event"))
+    })
+    .await
+    .map_err(|_| anyhow!("Timed out waiting for endpoint event from SSE server (30s)"))?
+}
+
+fn resolve_endpoint(endpoint: &str, base_url: &Url) -> Result<String> {
+    if endpoint.starts_with("http://") || endpoint.starts_with("https://") {
+        Ok(endpoint.to_string())
+    } else {
+        let mut resolved = base_url.clone();
+        let (path, query) = endpoint.split_once('?').unwrap_or((endpoint, ""));
+        resolved.set_path(path);
+        resolved.set_query(if query.is_empty() { None } else { Some(query) });
+        Ok(resolved.to_string())
+    }
+}
+
+async fn sse_reader_task(mut es: EventSource, tx: Sender<ServerJsonRpcMessage>) {
+    while let Some(event) = es.next().await {
+        match event {
+            Ok(Event::Message(msg)) if msg.event == "message" => {
+                match serde_json::from_str::<ServerJsonRpcMessage>(&msg.data) {
+                    Ok(rpc_msg) => {
+                        if tx.send(rpc_msg).await.is_err() {
+                            break;
+                        }
+                    }
+                    Err(e) => {
+                        warn!("Failed to parse SSE message as JSON-RPC: {e}");
+                    }
+                }
+            }
+            Ok(_) => {}
+            Err(reqwest_eventsource::Error::StreamEnded) => break,
+            Err(e) => {
+                error!("SSE stream error: {e}");
+                break;
+            }
+        }
+    }
+    es.close();
+}
+
+async fn post_writer_task(
+    client: Client,
+    endpoint: String,
+    mut rx: Receiver<ClientJsonRpcMessage>,
+) {
+    while let Some(msg) = rx.recv().await {
+        let body = match serde_json::to_string(&msg) {
+            Ok(b) => b,
+            Err(e) => {
+                error!("Failed to serialize JSON-RPC message: {e}");
+                continue;
+            }
+        };
+        if let Err(e) = client
+            .post(&endpoint)
+            .header("Content-Type", "application/json")
+            .body(body)
+            .send()
+            .await
+        {
+            error!("Failed to POST message to SSE endpoint: {e}");
+        }
+    }
+}
+
+pub struct SseSink<T> {
+    tx: PollSender<T>,
+}
+
+pub struct SseStream<T> {
+    rx: Receiver<T>,
+}
+
+impl<T: Send + 'static> futures_util::Sink<T> for SseSink<T> {
+    type Error = SseSinkError;
+
+    fn poll_ready(
+        mut self: Pin<&mut Self>,
+        cx: &mut std::task::Context<'_>,
+    ) -> Poll<Result<(), Self::Error>> {
+        self.tx.poll_ready(cx)
+    }
+
+    fn start_send(mut self: Pin<&mut Self>, item: T) -> Result<(), Self::Error> {
+        self.tx.start_send(item)
+    }
+
+    fn poll_flush(
+        self: Pin<&mut Self>,
+        _cx: &mut std::task::Context<'_>,
+    ) -> Poll<Result<(), Self::Error>> {
+        Poll::Ready(Ok(()))
+    }
+
+    fn poll_close(
+        self: Pin<&mut Self>,
+        _cx: &mut std::task::Context<'_>,
+    ) -> Poll<Result<(), Self::Error>> {
+        Poll::Ready(Ok(()))
+    }
+}
+
+impl<T: Send + 'static> futures_util::Stream for SseStream<T> {
+    type Item = T;
+
+    fn poll_next(
+        mut self: Pin<&mut Self>,
+        cx: &mut std::task::Context<'_>,
+    ) -> Poll<Option<Self::Item>> {
+        self.rx.poll_recv(cx)
+    }
+}
+
+#[derive(Debug)]
+pub enum SseSinkError {
+    Closed,
+}
+
+impl Display for SseSinkError {
+    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+        match self {
+            SseSinkError::Closed => write!(f, "SSE transport channel closed"),
+        }
+    }
+}
+
+impl Error for SseSinkError {}
+
+struct PollSender<T> {
+    tx: Sender<T>,
+    permit: Option<mpsc::OwnedPermit<T>>,
+}
+
+impl<T: Send + 'static> PollSender<T> {
+    fn poll_ready(&mut self, cx: &mut std::task::Context<'_>) -> Poll<Result<(), SseSinkError>> {
+        if self.permit.is_some() {
+            return Poll::Ready(Ok(()));
+        }
+        let tx = self.tx.clone();
+        let mut fut = Box::pin(tx.reserve_owned());
+        match fut.as_mut().poll(cx) {
+            Poll::Ready(Ok(permit)) => {
+                self.permit = Some(permit);
+                Poll::Ready(Ok(()))
+            }
+            Poll::Ready(Err(_)) => Poll::Ready(Err(SseSinkError::Closed)),
+            Poll::Pending => Poll::Pending,
+        }
+    }
+
+    fn start_send(&mut self, item: T) -> Result<(), SseSinkError> {
+        let permit = self.permit.take().ok_or(SseSinkError::Closed)?;
+        permit.send(item);
+        Ok(())
+    }
+}