diff --git a/AGENTS.md b/AGENTS.md
index 3b4998065939..6e974d78e991 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -172,5 +172,6 @@ The `api/` module has the strongest stability guarantees — breaking changes ar
 - **Never** break public API without an approved `revapi.yml` exception.
 - **Never** add Hadoop dependencies where `FileIO` abstractions exist.
 - **Never** commit secrets, credentials, or cloud-specific tokens.
+- **Never** use em-dashes (U+2014) or other non-ASCII Unicode characters in source code. The only exception is tests or sample data that specifically exercise Unicode or UTF handling.
 - **Ask first** before adding new third-party dependencies (license compatibility matters).
 - **Ask first** before promoting package-private classes/methods to public.