diff --git a/.github/workflows/update-pr-stack.yml b/.github/workflows/update-pr-stack.yml
index 87cf4ea..86f030c 100644
--- a/.github/workflows/update-pr-stack.yml
+++ b/.github/workflows/update-pr-stack.yml
@@ -9,6 +9,13 @@ permissions:
   contents: write
   pull-requests: write
 
+# Serialize runs per PR: the push that finishes a conflict resolution fires a
+# synchronize event while the conflict label is still attached, so without a
+# group the follow-up run races the one that triggered it.
+concurrency:
+  group: update-pr-stack-${{ github.event.pull_request.number }}
+  cancel-in-progress: false
+
 jobs:
   update-pr-stack:
     runs-on: ubuntu-latest
diff --git a/README.md b/README.md
index 6e19752..f0737ae 100644
--- a/README.md
+++ b/README.md
@@ -79,6 +79,10 @@ on:
   pull_request:
     types: [closed, synchronize]
 
+concurrency:
+  group: update-pr-stack-${{ github.event.pull_request.number }}
+  cancel-in-progress: false
+
 jobs:
   update-pr-stack:
     runs-on: ubuntu-latest
@@ -110,6 +114,10 @@ permissions:
   contents: write
   pull-requests: write
 
+concurrency:
+  group: update-pr-stack-${{ github.event.pull_request.number }}
+  cancel-in-progress: false
+
 jobs:
   update-pr-stack:
     runs-on: ubuntu-latest
diff --git a/tests/test_e2e.sh b/tests/test_e2e.sh
index 420f2cd..b6e9f19 100755
--- a/tests/test_e2e.sh
+++ b/tests/test_e2e.sh
@@ -558,7 +558,7 @@ wait_for_synchronize_workflow() {
                 log_cmd gh run view "$target_run_id" --repo "$REPO_FULL_NAME" --log || echo >&2 "Could not fetch logs for run $target_run_id"
                 return 1
             fi
-        elif [[ "$run_status" == "queued" || "$run_status" == "in_progress" || "$run_status" == "waiting" ]]; then
+        elif [[ "$run_status" == "queued" || "$run_status" == "in_progress" || "$run_status" == "waiting" || "$run_status" == "pending" ]]; then
             echo >&2 "Workflow $target_run_id is $run_status. Sleeping $sleep_time seconds."
         else
             echo >&2 "Workflow $target_run_id has unexpected status: $run_status. Conclusion: $run_conclusion"
@@ -665,7 +665,7 @@ wait_for_workflow() {
                 log_cmd gh run view "$target_run_id" --repo "$REPO_FULL_NAME" --log || echo >&2 "Could not fetch logs for run $target_run_id"
                 return 1
             fi
-        elif [[ "$run_status" == "queued" || "$run_status" == "in_progress" || "$run_status" == "waiting" ]]; then
+        elif [[ "$run_status" == "queued" || "$run_status" == "in_progress" || "$run_status" == "waiting" || "$run_status" == "pending" ]]; then
             echo >&2 "Workflow $target_run_id is $run_status. Sleeping $sleep_time seconds."
         else
             echo >&2 "Workflow $target_run_id has unexpected status: $run_status. Conclusion: $run_conclusion"