diff --git a/nanobot/agent/tools/cron.py b/nanobot/agent/tools/cron.py
index 13b1e12..f8e737b 100644
--- a/nanobot/agent/tools/cron.py
+++ b/nanobot/agent/tools/cron.py
@@ -122,7 +122,10 @@ class CronTool(Tool):
         elif at:
             from datetime import datetime
 
-            dt = datetime.fromisoformat(at)
+            try:
+                dt = datetime.fromisoformat(at)
+            except ValueError:
+                return f"Error: invalid ISO datetime format '{at}'. Expected format: YYYY-MM-DDTHH:MM:SS"
             at_ms = int(dt.timestamp() * 1000)
             schedule = CronSchedule(kind="at", at_ms=at_ms)
             delete_after = True