core tests: fix compact suite for model-item delta boundary

codex-rs/core/src/context_manager/history.rs

@@ -247,12 +247,13 @@ impl ContextManager {
 
     // These are local items added after the most recent model-emitted item.
     // They are not reflected in `last_token_usage.total_tokens`.
+    // If no model item has been emitted yet, treat the delta as empty.
     fn items_after_last_model_generated_item(&self) -> &[ResponseItem] {
         let start = self
             .items
             .iter()
             .rposition(is_model_generated_item)
-            .map_or(0, |index| index.saturating_add(1));
+            .map_or(self.items.len(), |index| index.saturating_add(1));
         &self.items[start..]
     }
 

codex-rs/core/src/context_manager/history_tests.rs

@@ -249,6 +249,31 @@ fn usage_breakdown_counts_no_items_after_last_model_generated_item() {
     );
 }
 
+#[test]
+fn usage_breakdown_is_zero_without_model_generated_items() {
+    let mut history = create_history_with_items(vec![user_msg("no model output yet")]);
+    history.update_token_info(
+        &TokenUsage {
+            total_tokens: 100,
+            ..Default::default()
+        },
+        None,
+    );
+
+    assert_eq!(
+        history
+            .get_total_token_usage_breakdown()
+            .estimated_tokens_of_items_added_since_last_successful_api_response,
+        0
+    );
+    assert_eq!(
+        history
+            .get_total_token_usage_breakdown()
+            .estimated_bytes_of_items_added_since_last_successful_api_response,
+        0
+    );
+}
+
 #[test]
 fn total_token_usage_includes_all_items_after_last_model_generated_item() {
     let mut history = create_history_with_items(vec![assistant_msg("already counted by API")]);