diff --git a/codex-rs/protocol/src/message_history.rs b/codex-rs/protocol/src/message_history.rs
index 02f61111ee..2fe1681976 100644
--- a/codex-rs/protocol/src/message_history.rs
+++ b/codex-rs/protocol/src/message_history.rs
@@ -7,6 +7,7 @@ use ts_rs::TS;
 pub struct HistoryEntry {
     pub conversation_id: String,
     #[serde(default, skip_serializing_if = "Option::is_none")]
+    #[ts(optional)]
     pub agent_id: Option<String>,
     pub ts: u64,
     pub text: String,