Webhooks

When a pull request belongs to a stack, GitHub adds a stack property to the pull_request object in webhook event payloads. This lets apps and integrations inspect the stack’s ultimate target branch — not just the direct parent branch of the PR.

The stack object is included in the pull_request webhook payload for all pull request lifecycle events.

The stack Object

The stack object is nested inside the pull_request object and contains information about the stack’s base branch:

{
  "action": "synchronize",
  "pull_request": {
    "number": 42,
    "title": "Add API routes",
    "base": {
      "ref": "feat/auth-layer",
      "sha": "abc123..."
    },
    "stack": {
      "base": {
        "ref": "main",
        "sha": "def456..."
      }
    }
  }
}

Fields

Field	Type	Description
pull_request.stack.base.ref	string	The branch the entire stack ultimately targets (e.g., main).
pull_request.stack.base.sha	string	The HEAD SHA of that target branch at the time of the event.

pull_request.base.ref is the direct parent branch of an individual PR (the branch below it in the stack), while pull_request.stack.base.ref is the ultimate target of the entire stack. These differ for all PRs in the stack except the bottom one.

The stack object is only present when the pull request belongs to a stack. For standalone PRs, the field is null.

GitHub Actions

GitHub Actions automatically evaluates workflow triggers using the stack’s base branch. If a PR is part of a stack targeting main, any workflow configured to run on pull requests targeting main will run for every PR in the stack — no workflow changes are required.

The stack object is also available in GitHub Actions workflow expressions via github.event.pull_request.stack. See How do I access stack metadata in my GitHub Actions workflow? in the FAQ for examples.