diff --git a/.hyperhive/review-classifier.yml b/.hyperhive/review-classifier.yml
index 2070c1ea..8e75b360 100644
--- a/.hyperhive/review-classifier.yml
+++ b/.hyperhive/review-classifier.yml
@@ -34,7 +34,11 @@ classifications:
   - name: tests-with-behaviour
     description: >-
       Every change to how the code behaves comes with a new or updated test
-      that exercises it.
+      that exercises it. A diff whose changed files are all test code
+      (a fixture under `nix/module-eval/`, `nix/checks.nix`, a crate's
+      `tests/` directory, or Rust inside `#[cfg(test)]` or a `#[test]`
+      function) changes no behaviour: the question is not applicable. A PR
+      that changes behaviour and tests together matches.
     polarity: positive
     threshold: 65
   - name: docs-updated