diff --git a/tools/cronjob_tools.py b/tools/cronjob_tools.py
index 53e778a7d..d0ef994ae 100644
--- a/tools/cronjob_tools.py
+++ b/tools/cronjob_tools.py
@@ -513,7 +513,7 @@ Important safety rule: cron-run sessions should not recursively schedule more cr
                 "properties": {
                     "provider": {
                         "type": "string",
-                        "description": "Provider name (e.g. 'openrouter', 'anthropic'). Omit to use and pin the current provider."
+                        "description": "Provider name (e.g. 'openrouter', 'anthropic', or 'custom:<name>' for a provider defined in custom_providers config — always include the ':<name>' suffix, never pass the bare 'custom'). Omit to use and pin the current provider."
                     },
                     "model": {
                         "type": "string",