diff --git a/AGENTS.md b/AGENTS.md
index e8eafe2b..4a809b47 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -5,3 +5,4 @@
 - MUST NOT modify any dune file during development unless explicitly asked
 
 See `.agents/skills/` for platform-specific development guides and `docs/agent-guide/` for design specifications.
+- Avoid O(n²) `List` patterns such as `List.concat` and repeated `List.append` on large sequences; when the project already depends on the `rrbvec` package, use `Rrbvec` vectors instead.