diff --git a/src/agent/mod.rs b/src/agent/mod.rs
index 449c2eb..305721a 100644
--- a/src/agent/mod.rs
+++ b/src/agent/mod.rs
@@ -380,6 +380,8 @@ impl Agent {
     /// assistant entry if needed). Called by collect_stream per token batch.
     fn append_streaming(&mut self, text: &str) {
         if let Some(idx) = self.streaming_index() {
+            assert!(!self.context.conversation.entries()[idx].token_ids.is_empty(),
+                "streaming_index returned entry with empty token_ids at {}", idx);
             let mut msg = self.context.conversation.entries()[idx].entry.message().clone();
             msg.append_content(text);
             self.context.conversation.set_message(idx, msg);