diff --git a/nix/module-eval.nix b/nix/module-eval.nix
index 3cd84707..82585bcc 100644
--- a/nix/module-eval.nix
+++ b/nix/module-eval.nix
@@ -571,6 +571,13 @@ in
 # The results are embedded in the builder text on purpose: that is what
 # makes this derivation's hash depend on them, so a nix-only change that
 # flips a case cannot be answered from cache.
+#
+# It also means the verdict can be read without building anything, which is
+# what makes mutation-testing a case affordable — mutate, re-read, restore,
+# with nothing queued on the shared builder:
+#
+#     nix derivation show .#checks.<system>.module-eval \
+#       | jq -r '..|.buildCommand? // empty'
 pkgs.runCommand "hyperhive-module-eval" { } ''
   ${report}
   ${