A Github Action to detect merge commits in pull requests and to prevent them from being merged.

This action will query GitHub REST API to find all commits in a pull request. After that, this will analyse the response data and check if there are merge commits. If there are any, this action will error and exit.

name: Detect Merge Commits

on:
  pull_request:

permissions:
  contents: read
  pull-requests: read

jobs:
  test:
    name: Check for merge commits
    runs-on: ubuntu-22.04

    steps:
      - name: Checkout
        uses: actions/checkout@v3

      - name: Run test
        uses: NexusPHP/no-merge-commits@v2.1.0
        with:
          token: ${{ secrets.GITHUB_TOKEN }}

You'll also need to add a required status check rule for your action to block merging if it detects merge commits.