Streamable HTTP: drain SSE frames until the initialize response, igno…

crates/rmcp/src/transport/streamable_http_client.rs

@@ -5,6 +5,7 @@ pub use sse_stream::Error as SseError;
 use sse_stream::Sse;
 use thiserror::Error;
 use tokio_util::sync::CancellationToken;
+use tracing::debug;
 
 use super::common::client_side_sse::{ExponentialBackoff, SseRetryPolicy, SseStreamReconnect};
 use crate::{
@@ -90,16 +91,28 @@ impl StreamableHttpPostResponse {
         match self {
             Self::Json(message, session_id) => Ok((message, session_id)),
             Self::Sse(mut stream, session_id) => {
-                let event =
-                    stream
-                        .next()
-                        .await
-                        .ok_or(StreamableHttpError::UnexpectedServerResponse(
-                            "empty sse stream".into(),
-                        ))??;
-                let message: ServerJsonRpcMessage =
-                    serde_json::from_str(&event.data.unwrap_or_default())?;
-                Ok((message, session_id))
+                while let Some(event) = stream.next().await {
+                    let event = event?;
+                    let payload = event.data.unwrap_or_default();
+                    if payload.trim().is_empty() {
+                        continue;
+                    }
+
+                    let message: ServerJsonRpcMessage = serde_json::from_str(&payload)?;
+
+                    if matches!(message, ServerJsonRpcMessage::Response(_)) {
+                        return Ok((message, session_id));
+                    }
+
+                    debug!(
+                        ?message,
+                        "received message before initialize response; continuing to drain stream"
+                    );
+                }
+
+                Err(StreamableHttpError::UnexpectedServerResponse(
+                    "empty sse stream".into(),
+                ))
             }
             _ => Err(StreamableHttpError::UnexpectedServerResponse(
                 "expect initialized, accepted".into(),