diff --git a/.github/workflows/lean_action_ci.yml b/.github/workflows/lean_action_ci.yml
index 95bc25bd1..af9ebc99f 100644
--- a/.github/workflows/lean_action_ci.yml
+++ b/.github/workflows/lean_action_ci.yml
@@ -9,6 +9,13 @@ on:
   merge_group:
   workflow_dispatch:
 
+# Label each run; only the latest run carrying a given label is kept. Pushes to
+# main and nightly-testing get the run id in their label, making it unique, so
+# those runs are never cancelled.
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}-${{ (github.event_name == 'push' && (github.ref == 'refs/heads/main' || github.ref == 'refs/heads/nightly-testing') && github.run_id) || '' }}
+  cancel-in-progress: true
+
 jobs:
   ci-checks:
     runs-on: ubuntu-latest