diff --git a/AGENTS.md b/AGENTS.md
index 016b80205..76eca3bc3 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -297,3 +297,7 @@ This project uses Python 3+. You should not use the `__future__` module.
 
 If you need to worry about feature compatibility between different 3.xx point releases, check the
 closest `pyproject.toml`'s `requires-python` field to see what minimum runtime version is supported.
+
+## Platform Support
+
+Tests and features must support Linux, macOS and Windows unless feature is explicitly OS-specific.