test: add Phase 2 streaming SSE passthrough tests

Confirms the existing proxy streams SSE chunks incrementally:
- 5-chunk test with 50ms delays verifies time spread between first
  and last chunk arrival (not buffered)
- Verifies data: [DONE] terminator is forwarded

No src/ changes needed — Body::from_stream(bytes_stream()) already
handles SSE correctly.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/cortex-gateway/tests/common/mod.rs

@@ -1,3 +1,8 @@
+#![allow(dead_code)]
+
+use axum::body::Body;
+use axum::http::header;
+use axum::response::Response;
 use axum::routing::{get, post};
 use axum::{Json, Router};
 use cortex_core::config::{
@@ -5,8 +10,10 @@ use cortex_core::config::{
 };
 use cortex_core::node::{ModelEntry, ModelStatus};
 use cortex_gateway::state::CortexState;
+use futures::{StreamExt, stream};
 use serde_json::{Value, json};
 use std::sync::Arc;
+use std::time::Duration;
 use tokio::net::TcpListener;
 
 /// Spawns a mock mistral.rs backend on a random port.
@@ -63,6 +70,66 @@ async fn mock_list_models() -> Json<Value> {
     }))
 }
 
+/// Spawns a mock mistral.rs backend that returns SSE streaming responses.
+/// Each chunk is delayed by `chunk_delay` to prove the proxy streams incrementally.
+pub async fn spawn_streaming_mock_backend(chunk_count: usize, chunk_delay: Duration) -> String {
+    let app = Router::new()
+        .route(
+            "/v1/chat/completions",
+            post(move |Json(body): Json<Value>| async move {
+                let model = body
+                    .get("model")
+                    .and_then(|v| v.as_str())
+                    .unwrap_or("unknown")
+                    .to_string();
+
+                let chunks: Vec<String> = (0..chunk_count)
+                    .map(|i| {
+                        let content = format!("token{i}");
+                        let chunk = json!({
+                            "id": "chatcmpl-stream-001",
+                            "object": "chat.completion.chunk",
+                            "created": 1700000000_u64,
+                            "model": model,
+                            "choices": [{
+                                "index": 0,
+                                "delta": { "content": content },
+                                "finish_reason": null
+                            }]
+                        });
+                        format!("data: {chunk}\n\n")
+                    })
+                    .collect();
+
+                let delay = chunk_delay;
+                let stream = stream::iter(
+                    chunks
+                        .into_iter()
+                        .chain(std::iter::once("data: [DONE]\n\n".to_string())),
+                )
+                .then(move |chunk| async move {
+                    tokio::time::sleep(delay).await;
+                    Ok::<_, std::convert::Infallible>(chunk)
+                });
+
+                Response::builder()
+                    .header(header::CONTENT_TYPE, "text/event-stream")
+                    .header(header::CACHE_CONTROL, "no-cache")
+                    .body(Body::from_stream(stream))
+                    .unwrap()
+            }),
+        )
+        .route("/v1/models", get(mock_list_models));
+
+    let listener = TcpListener::bind("127.0.0.1:0").await.unwrap();
+    let addr = listener.local_addr().unwrap();
+    tokio::spawn(async move {
+        axum::serve(listener, app).await.unwrap();
+    });
+
+    format!("http://{addr}")
+}
+
 /// Spawns the cortex gateway with a single node pointing at `mock_url`.
 /// The node is pre-seeded as healthy with one loaded model ("test-model").
 /// Returns the gateway's base URL.

crates/cortex-gateway/tests/streaming.rs

@@ -0,0 +1,117 @@
+mod common;
+
+use futures::StreamExt;
+use serde_json::json;
+use std::time::{Duration, Instant};
+
+#[tokio::test]
+async fn test_streaming_sse_passthrough() {
+    let chunk_count = 5;
+    let chunk_delay = Duration::from_millis(50);
+    let mock_url = common::spawn_streaming_mock_backend(chunk_count, chunk_delay).await;
+    let gw_url = common::spawn_gateway(&mock_url).await;
+
+    let client = reqwest::Client::new();
+    let resp = client
+        .post(format!("{gw_url}/v1/chat/completions"))
+        .header("content-type", "application/json")
+        .json(&json!({
+            "model": "test-model",
+            "messages": [{"role": "user", "content": "Hi"}],
+            "stream": true
+        }))
+        .send()
+        .await
+        .expect("request should succeed");
+
+    assert_eq!(resp.status(), 200);
+    assert_eq!(
+        resp.headers()
+            .get("content-type")
+            .and_then(|v| v.to_str().ok())
+            .unwrap_or(""),
+        "text/event-stream"
+    );
+
+    // Collect SSE chunks as they arrive, recording arrival times.
+    let start = Instant::now();
+    let mut chunk_times = Vec::new();
+    let mut chunks = Vec::new();
+    let mut stream = resp.bytes_stream();
+
+    while let Some(result) = stream.next().await {
+        let bytes = result.expect("chunk should be valid");
+        let text = String::from_utf8_lossy(&bytes);
+        for line in text.split("data: ").filter(|s| !s.is_empty()) {
+            let trimmed = line.trim();
+            if !trimmed.is_empty() {
+                chunk_times.push(start.elapsed());
+                chunks.push(trimmed.to_string());
+            }
+        }
+    }
+
+    // Verify we got all content chunks plus [DONE].
+    assert!(
+        chunks.len() >= chunk_count + 1,
+        "expected at least {} chunks (got {}): {:?}",
+        chunk_count + 1,
+        chunks.len(),
+        chunks,
+    );
+
+    // The last chunk should be [DONE].
+    assert_eq!(chunks.last().unwrap(), "[DONE]");
+
+    // Verify the content chunks contain expected tokens.
+    for i in 0..chunk_count {
+        let chunk_json: serde_json::Value =
+            serde_json::from_str(&chunks[i]).expect("chunk should be valid JSON");
+        assert_eq!(
+            chunk_json["choices"][0]["delta"]["content"],
+            format!("token{i}")
+        );
+    }
+
+    // Verify streaming behavior: total time should reflect incremental delivery,
+    // not a single batch. With 5 chunks at 50ms each + [DONE], we expect ~300ms total.
+    // If buffered, all chunks would arrive at once after ~300ms with no spread.
+    // We verify that the last chunk arrived noticeably after the first.
+    let first = chunk_times.first().unwrap();
+    let last = chunk_times.last().unwrap();
+    let spread = *last - *first;
+    assert!(
+        spread >= Duration::from_millis(100),
+        "chunks should arrive incrementally (spread: {spread:?})",
+    );
+}
+
+#[tokio::test]
+async fn test_streaming_done_terminator() {
+    let mock_url = common::spawn_streaming_mock_backend(2, Duration::from_millis(10)).await;
+    let gw_url = common::spawn_gateway(&mock_url).await;
+
+    let client = reqwest::Client::new();
+    let resp = client
+        .post(format!("{gw_url}/v1/chat/completions"))
+        .header("content-type", "application/json")
+        .json(&json!({
+            "model": "test-model",
+            "messages": [{"role": "user", "content": "Hi"}],
+            "stream": true
+        }))
+        .send()
+        .await
+        .expect("request should succeed");
+
+    let body = resp.text().await.unwrap();
+    assert!(
+        body.contains("data: [DONE]"),
+        "response must contain [DONE] terminator"
+    );
+    assert!(body.contains("token0"), "response must contain first token");
+    assert!(
+        body.contains("token1"),
+        "response must contain second token"
+    );
+}