diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 36fdddd..c4b8a6c 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -3,8 +3,9 @@ name: CI
 on:
   push:
     branches: [main]
+  # Every pull request, whatever its base: a stacked PR needs build, typecheck
+  # and test results of its own before it lands.
   pull_request:
-    branches: [main]
 
 concurrency:
   group: ci-${{ github.ref }}