diff --git a/AGENTS.md b/AGENTS.md
index 8ce3ee5..620a8f8 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -26,6 +26,10 @@
   where they are accepted and we need to cast to a string, we should branch on
   `\is_finite($value)`, using `(string) $value` for the finite case and
   `\is_nan($value) ? 'NAN' : ($value > 0 ? 'INF' : '-INF')` otherwise.
+- Never call `strtolower()`, `strtoupper()`, `strcasecmp()`, `stripos()`, or
+  other locale-sensitive case functions; use the locale-independent
+  `GuzzleHttp\Psr7\Utils::asciiToLower()`, `asciiToUpper()`,
+  `caselessEquals()`, and `caselessContains()` helpers instead.
 - Changes in behavior need a `CHANGELOG.md` entry in the unreleased section of
   the target branch and an `UPGRADING.md` note when the behavior differs between
   major versions.