diff --git a/run_agent.py b/run_agent.py
index 7931581f8..fcef30572 100644
--- a/run_agent.py
+++ b/run_agent.py
@@ -108,7 +108,7 @@ HONCHO_TOOL_NAMES = {
 
 
 class _SafeWriter:
-    """Transparent stdio wrapper that catches OSError from broken pipes.
+    """Transparent stdio wrapper that catches OSError/ValueError from broken pipes.
 
     When hermes-agent runs as a systemd service, Docker container, or headless
     daemon, the stdout/stderr pipe can become unavailable (idle timeout, buffer
@@ -117,8 +117,13 @@ class _SafeWriter:
     run_conversation() — especially via double-fault when an except handler
     also tries to print.
 
+    Additionally, when subagents run in ThreadPoolExecutor threads, the shared
+    stdout handle can close between thread teardown and cleanup, raising
+    ``ValueError: I/O operation on closed file`` instead of OSError.
+
     This wrapper delegates all writes to the underlying stream and silently
-    catches OSError. It is transparent when the wrapped stream is healthy.
+    catches both OSError and ValueError. It is transparent when the wrapped
+    stream is healthy.
     """
 
     __slots__ = ("_inner",)
@@ -129,13 +134,13 @@ class _SafeWriter:
     def write(self, data):
         try:
             return self._inner.write(data)
-        except OSError:
+        except (OSError, ValueError):
             return len(data) if isinstance(data, str) else 0
 
     def flush(self):
         try:
             self._inner.flush()
-        except OSError:
+        except (OSError, ValueError):
             pass
 
     def fileno(self):
@@ -144,7 +149,7 @@ class _SafeWriter:
     def isatty(self):
         try:
             return self._inner.isatty()
-        except OSError:
+        except (OSError, ValueError):
             return False
 
     def __getattr__(self, name):