feat(streamable-http): add json_response option for stateless server mode (#683)

* feat(streamable-http): add json_response option for stateless server mode

Adds `json_response: bool` field to `StreamableHttpServerConfig`.
When true and `stateful_mode` is false, the server returns
`Content-Type: application/json` directly instead of `text/event-stream`,
eliminating SSE framing overhead for simple request-response patterns.

This completes server-side JSON response support (client-side was added
in #540) and contributes to the stateless server goals of SEP-1442 (#526).

Backwards-compatible: `json_response: false` (default) preserves all
existing SSE behaviour unchanged, and `stateful_mode: true` is unaffected.

Benchmark evidence (50 VUs, 5min, 2 CPUs):
- RPS: 770 → 1139 (+48%)
- get_user_cart latency: 41ms → 0.76ms (-98%)
- checkout latency: 41ms → 0.55ms (-99%)
- Zero regressions, zero errors

* fix(tower): add cancellation awareness and logging to JSON response path

* fix(test): add missing Default to StreamableHttpServerConfig in concurrent streams test

Made-with: Cursor

crates/rmcp/tests/test_sse_concurrent_streams.rs

@@ -84,6 +84,7 @@ async fn start_test_server(ct: CancellationToken, trigger: Arc<Notify>) -> Strin
             sse_keep_alive: Some(Duration::from_secs(15)),
             sse_retry: Some(Duration::from_secs(3)),
             cancellation_token: ct.child_token(),
+            ..Default::default()
         },
     );
 

crates/rmcp/tests/test_streamable_http_json_response.rs

@@ -0,0 +1,155 @@
+use rmcp::transport::streamable_http_server::{
+    StreamableHttpServerConfig, StreamableHttpService, session::local::LocalSessionManager,
+};
+use tokio_util::sync::CancellationToken;
+
+mod common;
+use common::calculator::Calculator;
+
+const INIT_BODY: &str = r#"{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-03-26","capabilities":{},"clientInfo":{"name":"test","version":"1.0"}}}"#;
+
+async fn spawn_server(
+    config: StreamableHttpServerConfig,
+) -> (reqwest::Client, String, CancellationToken) {
+    let ct = config.cancellation_token.clone();
+    let service: StreamableHttpService<Calculator, LocalSessionManager> =
+        StreamableHttpService::new(|| Ok(Calculator::new()), Default::default(), config);
+
+    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();
+    let base_url = format!("http://{addr}/mcp");
+    (client, base_url, ct)
+}
+
+#[tokio::test]
+async fn stateless_json_response_returns_application_json() -> anyhow::Result<()> {
+    let ct = CancellationToken::new();
+    let (client, url, ct) = spawn_server(StreamableHttpServerConfig {
+        stateful_mode: false,
+        json_response: true,
+        sse_keep_alive: None,
+        cancellation_token: ct.child_token(),
+        ..Default::default()
+    })
+    .await;
+
+    let response = client
+        .post(&url)
+        .header("Content-Type", "application/json")
+        .header("Accept", "application/json, text/event-stream")
+        .body(INIT_BODY)
+        .send()
+        .await?;
+
+    assert_eq!(response.status(), 200);
+
+    let content_type = response
+        .headers()
+        .get("content-type")
+        .and_then(|v| v.to_str().ok())
+        .unwrap_or("");
+    assert!(
+        content_type.contains("application/json"),
+        "Expected application/json, got: {content_type}"
+    );
+
+    let body = response.text().await?;
+    let parsed: serde_json::Value = serde_json::from_str(&body)?;
+    assert_eq!(parsed["jsonrpc"], "2.0");
+    assert_eq!(parsed["id"], 1);
+    assert!(parsed["result"].is_object(), "Expected result object");
+
+    ct.cancel();
+    Ok(())
+}
+
+#[tokio::test]
+async fn stateless_sse_mode_default_unchanged() -> anyhow::Result<()> {
+    let ct = CancellationToken::new();
+    let (client, url, ct) = spawn_server(StreamableHttpServerConfig {
+        stateful_mode: false,
+        json_response: false,
+        sse_keep_alive: None,
+        cancellation_token: ct.child_token(),
+        ..Default::default()
+    })
+    .await;
+
+    let response = client
+        .post(&url)
+        .header("Content-Type", "application/json")
+        .header("Accept", "application/json, text/event-stream")
+        .body(INIT_BODY)
+        .send()
+        .await?;
+
+    assert_eq!(response.status(), 200);
+
+    let content_type = response
+        .headers()
+        .get("content-type")
+        .and_then(|v| v.to_str().ok())
+        .unwrap_or("");
+    assert!(
+        content_type.contains("text/event-stream"),
+        "Expected text/event-stream, got: {content_type}"
+    );
+
+    let body = response.text().await?;
+    assert!(
+        body.contains("data:"),
+        "Expected SSE framing (data: prefix), got: {body}"
+    );
+
+    ct.cancel();
+    Ok(())
+}
+
+#[tokio::test]
+async fn json_response_ignored_in_stateful_mode() -> anyhow::Result<()> {
+    let ct = CancellationToken::new();
+    // json_response: true has no effect when stateful_mode: true — server still uses SSE
+    let (client, url, ct) = spawn_server(StreamableHttpServerConfig {
+        stateful_mode: true,
+        json_response: true,
+        sse_keep_alive: None,
+        cancellation_token: ct.child_token(),
+        ..Default::default()
+    })
+    .await;
+
+    let response = client
+        .post(&url)
+        .header("Content-Type", "application/json")
+        .header("Accept", "application/json, text/event-stream")
+        .body(INIT_BODY)
+        .send()
+        .await?;
+
+    assert_eq!(response.status(), 200);
+
+    let content_type = response
+        .headers()
+        .get("content-type")
+        .and_then(|v| v.to_str().ok())
+        .unwrap_or("");
+    assert!(
+        content_type.contains("text/event-stream"),
+        "Stateful mode should always use SSE regardless of json_response, got: {content_type}"
+    );
+
+    ct.cancel();
+    Ok(())
+}