diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index d789a94..9668193 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -11,7 +11,9 @@ permissions:
   contents: read
 
 concurrency:
-  group: ci-${{ github.ref }}
+  # Keyed by PR number: a merged PR's events (e.g. release-please's relabel) carry the base branch
+  # as github.ref, and would otherwise share, and cancel, main's push run.
+  group: ci-${{ github.event.pull_request.number || github.ref }}
   cancel-in-progress: ${{ github.event_name == 'pull_request' }}
 
 jobs:
diff --git a/README.md b/README.md
index aa2637f..be13832 100644
--- a/README.md
+++ b/README.md
@@ -434,7 +434,8 @@ on:
 permissions:
   contents: read
 concurrency:
-  group: preview-${{ github.ref }}
+  # PR number, not github.ref: a merged PR's events carry the base branch as github.ref.
+  group: preview-${{ github.event.pull_request.number || github.ref }}
   cancel-in-progress: true
 jobs:
   preview: