diff --git a/codex-rs/core/src/session/handlers.rs b/codex-rs/core/src/session/handlers.rs
index 066ddbe2f2..3cacd168a7 100644
--- a/codex-rs/core/src/session/handlers.rs
+++ b/codex-rs/core/src/session/handlers.rs
@@ -307,6 +307,8 @@ async fn thread_settings_update(
         collaboration_mode
     } else {
         let state = sess.state.lock().await;
+        // Model and reasoning effort live in CollaborationMode settings today, so
+        // partial thread-settings updates refresh those fields on the active mode.
         state
             .session_configuration
             .collaboration_mode