The unexpected merge commit SHA

Post date: Aug 01, 2025

Quick question - let's say I show you a GitHub API response listing some pull requests. Among other things, the listed objects have a property called merge_commit_sha - it's set to null on some and it's a valid SHA string on others. What would you assume about those PRs based on that property?

If your answer is that the ones with merge_commit_sha set to null are not yet merged while the ones with merge_commit_sha are merged and the merge_commit_sha is a SHA of the merge commit - you're wrong. I assumed that too and caused a bug.

The actual answer is that:

1. PRs that are not yet merged and have conflicts with their target branch have merge_commit_sha set to null.
2. PRs that are already merged have merge_commit_sha set to the SHA of their merge commit.
3. Here's the kicker - PRs that are not yet merged, but don't have any conflicts with their target branch have merge_commit_sha set to a SHA of a secret internal commit that GitHub creates behind the scenes to test the mergeability of your branch.

To be fair, they say so in the documentation, but it still threw me off.