diff --git a/codex-rs/core/tests/suite/realtime_conversation.rs b/codex-rs/core/tests/suite/realtime_conversation.rs
index 4a9d557c6a..745439ebf3 100644
--- a/codex-rs/core/tests/suite/realtime_conversation.rs
+++ b/codex-rs/core/tests/suite/realtime_conversation.rs
@@ -2110,19 +2110,26 @@ async fn conversation_start_uses_openai_env_key_fallback_with_chatgpt_auth() ->
 
     skip_if_no_network!(Ok(()));
 
-    let server = start_websocket_server(vec![
-        vec![],
-        vec![vec![json!({
-            "type": "session.updated",
-            "session": { "id": "sess_env", "instructions": "backend prompt" }
-        })]],
-    ])
+    // Startup prewarm can reconnect after the empty connection closes. Keep its
+    // Responses requests from consuming the realtime session's scripted reply.
+    let startup_server = start_websocket_server(vec![vec![]]).await;
+    let server = start_websocket_server(vec![vec![vec![json!({
+        "type": "session.updated",
+        "session": { "id": "sess_env", "instructions": "backend prompt" }
+    })]]])
     .await;
 
-    let mut builder = test_codex().with_auth(CodexAuth::create_dummy_chatgpt_auth_for_testing());
-    let test = builder.build_with_websocket_server(&server).await?;
+    let mut builder = test_codex()
+        .with_auth(CodexAuth::create_dummy_chatgpt_auth_for_testing())
+        .with_config({
+            let realtime_base_url = server.uri().to_string();
+            move |config| {
+                config.experimental_realtime_ws_base_url = Some(realtime_base_url);
+            }
+        });
+    let test = builder.build_with_websocket_server(&startup_server).await?;
     assert!(
-        server
+        startup_server
             .wait_for_handshakes(/*expected*/ 1, Duration::from_secs(2))
             .await
     );
@@ -2174,8 +2181,9 @@ async fn conversation_start_uses_openai_env_key_fallback_with_chatgpt_auth() ->
     .await;
     assert_eq!(session_updated, "sess_env");
 
+    assert_eq!(server.handshakes().len(), 1);
     assert_eq!(
-        server.handshakes()[1].header("authorization").as_deref(),
+        server.handshakes()[0].header("authorization").as_deref(),
         Some("Bearer env-realtime-key")
     );
 
@@ -2186,6 +2194,7 @@ async fn conversation_start_uses_openai_env_key_fallback_with_chatgpt_auth() ->
     })
     .await;
 
+    startup_server.shutdown().await;
     server.shutdown().await;
     Ok(())
 }