fix(session): keep placeholder for tool results filtered to empty

nanobot/agent/loop.py

@@ -1589,7 +1589,11 @@ class AgentLoop:
                 elif isinstance(content, list):
                     filtered = self._sanitize_persisted_blocks(content, should_truncate_text=True)
                     if not filtered:
-                        continue
+                        # Dropping the message would leave its assistant
+                        # tool_call without a result; keep a placeholder.
+                        filtered = [
+                            {"type": "text", "text": "[tool result omitted during persistence]"}
+                        ]
                     entry["content"] = filtered
             elif role == "user":
                 if isinstance(content, str) and ContextBuilder._RUNTIME_CONTEXT_TAG in content:

tests/agent/test_loop_save_turn.py

@@ -1345,3 +1345,32 @@ async def test_turn_after_unanswered_user_keeps_tool_call_pairing(tmp_path: Path
     assert [m["role"] for m in persisted.messages] == [
         "user", "user", "assistant", "tool", "assistant",
     ]
+
+
+def test_save_turn_keeps_placeholder_for_empty_tool_result_blocks() -> None:
+    # Dropping the whole tool message would leave the assistant tool_call
+    # without a result, which strict APIs reject as firmly as orphans.
+    loop = _mk_loop()
+    session = Session(key="test:empty-tool-blocks")
+
+    loop._save_turn(
+        session,
+        [
+            {
+                "role": "assistant",
+                "content": "",
+                "tool_calls": [{
+                    "id": "call_empty",
+                    "type": "function",
+                    "function": {"name": "exec", "arguments": "{}"},
+                }],
+            },
+            {"role": "tool", "tool_call_id": "call_empty", "name": "exec", "content": []},
+        ],
+        skip=0,
+    )
+
+    assert [m["role"] for m in session.messages] == ["assistant", "tool"]
+    assert session.messages[1]["content"] == [
+        {"type": "text", "text": "[tool result omitted during persistence]"}
+    ]