Context

On GitLab, a CI job can block an unsquashed MR by reading CI_MERGE_REQUEST_SQUASH_ON_MERGE.
On GitHub, the same approach is not possible: there is no "squash" checkbox on the PR.
The merge method (Merge commit, Squash, Rebase) is chosen when clicking the button, after CI has run.

There are two approaches.

Option 1 — Ruleset: squash only on develop

  1. Settings → General → Pull Requests: keep Allow squash merging and Allow merge commits enabled (the repository settings are the master switch; a ruleset can only restrict further).
  2. Settings → Rules → Rulesets → New branch ruleset:
    • Target: develop
    • Enable Require a pull request before merging
    • Allowed merge methods: Squash only
  3. main / master: no restriction (or a separate ruleset) to keep merge commits for release PRs.
  • ✅ No code, no effort for developers; applies to both the UI and the API.
  • ⚠️ Some users report that the rule is not always enforced when the repository allows several methods. Test it with a PR into develop before relying on it.

Option 2 — GitHub Actions: a single commit per PR

The PR must contain a single commit: the developer squashes it themselves before merging.

.github/workflows/squash-required.yml:

name: squash-required on: pull_request: branches: [develop] types: [opened, synchronize, reopened, edited] jobs: single-commit: runs-on: ubuntu-latest steps: - name: Check the PR has a single commit env: COMMITS: ${{ github.event.pull_request.commits }} run: | if [ "$COMMITS" -ne 1 ]; then echo "::error::PRs into develop must contain a single commit (found $COMMITS). Squash it: git reset --soft origin/develop && git commit && git push --force-with-lease" exit 1 fi

For the check to block the merge: in the develop ruleset, enable
Require status checks to pass and add single-commit.

  • ✅ Reliable: runs again on every push, cannot be bypassed afterwards (unlike the GitLab checkbox).
  • ⚠️ Developers must squash locally, then run git push --force-with-lease.

Comparison

Ruleset (option 1) Action (option 2)
What is enforced Merge method = squash PR = a single commit
Code to maintain None One workflow
Developer effort None Squash before merging
Reliability To be tested on the repository Guaranteed if the check is required

Recommendation

Start with option 1. If it does not behave as expected, add option 2
as a safety net.

Sources