Merge API returns "New changes require approval from someone other than the last pusher" when the real block is an out-of-date branch

[akshayrangnekar]

🏷️ Discussion Type

Bug

Body

What happened: On a protected main that requires the branch to be up to date before merging, plus "require approval of the most recent push", merging a PR whose branch was behind main failed with:

▎ New changes require approval from someone other than the last pusher. 2 of 2 required status checks are expected.

The same message came back from PUT /repos/{owner}/{repo}/pulls/{n}/merge and from gh pr merge --squash --admin (GraphQL mergePullRequest). The real cause was the out-of-date branch: after "Update branch" (rebase), the PR merged normally.

Expected: an error that names the actual blocker, for example "This branch is out of date with the base branch", as the web UI does. The message sent us looking at approval rules and an admin bypass, neither of which was the problem.

Repro: protected base branch with strict status checks and last-push approval enabled → open a PR → advance the base branch → merge through the API or gh pr merge without updating the branch.

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[ronitgupta138]

This is a known quirk in how GitHub evaluates branch protection rules during API merge operations.

Why This Happens

When a repository enables both:

1. Require branches to be up to date before merging (strict status checks)
2. Require approval of the most recent push (dismiss_stale_reviews or require review after push)

The API merge endpoint (PUT /repos/.../pulls/{n}/merge and GraphQL mergePullRequest) evaluates rules in a rigid validation pipeline. When your PR branch is behind main, the merge ref (refs/pull/:id/merge) is either missing or stale.

Because strict status checks require that checks have passed against the latest integrated base, the server-side validator detects that the newest push onto main hasn't been approved in the context of this pull request branch. The validation pipeline checks the review state of the unintegrated head commit before it checks whether the branch simply needs a rebase/update, returning the misleading message:

New changes require approval from someone other than the last pusher. 2 of 2 required status checks are expected.

Workaround & Best Practice

Before attempting an API or CLI merge (gh pr merge) on branches with strict protection rules, always verify branch sync or trigger an update via the Update Branch API:

# Update PR branch with latest main
gh api -X PUT repos/{owner}/{repo}/pulls/{n}/update-branch

Once updated, the merge validation pipeline will pass and execute the merge without false approval rejections.

[RizgarOzan]

fwiw the merge endpoint won't tell you which rule blocked it, but the PR object will. Read mergeable_state (REST) or mergeStateStatus (GraphQL) before you call merge: an out-of-date branch shows up as behind, a rule-blocked one as blocked.

gh pr view 123 --json mergeStateStatus,reviewDecision

The enum values are in the GraphQL docs: https://docs.github.com/en/graphql/reference/enums#mergestatestatus (BEHIND = "The head ref is out of date", BLOCKED = "The merge is blocked"). REST returns the same thing lowercased in mergeable_state on GET /repos/{owner}/{repo}/pulls/{n}.

If it says behind, update the branch from the API too and then merge:

gh pr update-branch 123        # add --rebase if you want a rebase instead of a merge commit

REST equivalent: PUT /repos/{owner}/{repo}/pulls/{n}/update-branch, https://docs.github.com/en/rest/pulls/pulls#update-a-pull-request-branch. Then wait for the required checks to pass on the new head and call merge.

Agree the error text is wrong for this case, that part is on GitHub. Checking the state first at least stops the script from chasing the approval rule.