diff --git a/AGENTS.md b/AGENTS.md
index f01901e..85287b3 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -104,6 +104,7 @@ the custom scripts instead of system-installed packages.
 - **East const** (e.g. `char const*` not `const char*`)
 - **Public members first** in class declarations, private members at the
   bottom
+- **Private data members** start with `m_` (e.g. `m_state`)
 - `<>` includes only for system headers (std, OS, etc.)
 - `""` includes for third-party dependencies
 - **Naming:** `snake_case` for variables, functions, and classes