diff --git a/CLAUDE.md b/CLAUDE.md
index 8287bfe..b3dde06 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -27,6 +27,11 @@ That means evaluation is **pure**: config can never read files outside the repo
 
 ## Code Evaluation
 
+Before writing or changing any NixOS / Home Manager option, verify it exists and
+has the expected name and type using the `nixos` MCP server tools (`nix` /
+`nix_versions`, configured in `.mcp.json`). Don't rely on memory for option or
+package names — look them up first to avoid invented attributes that fail at eval.
+
 Always validate Nix expressions with `nix eval` before committing. For example:
 
 ```bash