fix tmux action (#318)

Author: Harold-lkk

lagent/actions/tmux_action.py

@@ -8,9 +8,7 @@
 ``MarkTaskComplete`` emits a sentinel the orchestrator interprets as a
 completion signal.  The orchestrator (``Terminus2Agent``) implements the
 two-step confirmation handshake — the action itself is stateless.
-"""
 
-"""Local tmux session wrapper.
 
 Port of harbor's ``TmuxSession`` that drops the ``BaseEnvironment.exec``
 indirection and runs ``tmux`` as a local subprocess.  Intended for use
@@ -25,7 +23,6 @@
 import subprocess
 import time
 
-from __future__ import annotations
 
 from lagent.actions.base_action import AsyncActionMixin, BaseAction, tool_api
 from lagent.schema import ActionReturn, ActionStatusCode

lagent/schema.py

@@ -51,14 +51,16 @@ class ActionReturn:
     tool_call_id: Optional[str] = None
 
     def format_result(self) -> str:
-        """Concatenate items in result."""
+        """Concatenate items in result, falling back to errmsg when result is empty."""
         result = []
         for item in self.result or []:
             if item['type'] == 'text':
                 result.append(item['content'])
             else:
                 result.append(f"[{item['type']}]({item['content']})")
         result = '\n'.join(result)
+        if not result and self.errmsg:
+            return self.errmsg
         return result