Configure GitHub Actions concurrency groups to cancel superseded workflow runs

Steps

1. Add a concurrency block at the workflow level to group runs by branch: concurrency: { group: '${{ github.workflow }}-${{ github.ref }}', cancel-in-progress: true }
2. For pull request workflows, use the PR number in the group key to prevent runs on different PRs from cancelling each other: group: 'pr-${{ github.event.pull_request.number }}'
3. For deployment workflows where you want to queue rather than cancel, set cancel-in-progress: false — the new run will wait for the in-progress run to finish before starting
4. Apply concurrency at the job level (not just workflow level) when only specific jobs should be serialized: add a concurrency block inside the job definition with a more specific group key
5. Use a conditional expression for cancel-in-progress to allow main branch runs to complete while cancelling PR branch runs: cancel-in-progress: ${{ github.ref != 'refs/heads/main' }}
6. Verify behavior in the Actions UI: a pending run shows a yellow dot with 'Waiting for a pending job in queue'; cancelled runs show as 'Cancelled' in the workflow history