diff --git a/AGENTS.md b/AGENTS.md
index 2de54d0f..6e4d7db8 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -62,6 +62,9 @@ Demo traces for local runs are bundled in `tests/traces`.
   next to its first caller, and extend an existing helper over adding a parallel one.
 - **Derive over hardcode.**  drop references to a
   concern from agents that don't own it, and template sections the downstream consumer never reads.
+- **Don't reinvent what already exists.** Before writing a helper, loader, or parser, check
+  whether one already exists and import it. If new code is genuinely needed, place it in the
+  module whose responsibility matches its purpose.
 
 ### Correctness and honesty