fix(agent): close tool-call sequence on interrupt to prevent role alternation violation (#48879)

agent/turn_finalizer.py

@@ -166,6 +166,29 @@ def finalize_turn(
     # same empty-response loop again.
     try:
         agent._drop_trailing_empty_response_scaffolding(messages)
+
+        # When the turn was interrupted and the last message is a tool
+        # result, append a synthetic assistant message to close the
+        # tool-call sequence. Without this, the session persists a
+        # ``tool → user`` alternation that strict providers (Gemini,
+        # Claude) reject, causing them to hallucinate a continuation of
+        # the user's message on the next turn (#48879).
+        #
+        # ``_drop_trailing_empty_response_scaffolding`` only rewinds the
+        # tool tail when an empty-response scaffolding flag is present; a
+        # clean ``/stop`` interrupt after a successful tool sets no such
+        # flag, so the tool result survives as the tail and we close it
+        # here instead. On an interrupt ``final_response`` is typically
+        # empty, so fall back to an explicit placeholder rather than
+        # persisting an empty-content assistant turn.
+        if interrupted and messages and messages[-1].get("role") == "tool":
+            messages.append(
+                {
+                    "role": "assistant",
+                    "content": (final_response or "").strip() or "Operation interrupted.",
+                }
+            )
+
         agent._persist_session(messages, conversation_history)
     except Exception as _persist_err:
         _cleanup_errors.append(f"persist_session: {_persist_err}")