diff --git a/loop/agent_system_prompt.txt b/loop/agent_system_prompt.txt
index 0faf441..f2bd7ed 100644
--- a/loop/agent_system_prompt.txt
+++ b/loop/agent_system_prompt.txt
@@ -53,6 +53,7 @@
 Default coding guidelines:
 - Clear is better than clever.
 - Minimal inline comments: non-obvious logic and key decisions only.
+- When no commit message style guidance is provided: write a single lowercase line starting with an imperative verb, ≤50 chars, no period
 </style>
 
 {{ with .Codebase }}