Repository / PR / stack numbers and the ruleset regex below are fabricated/illustrative; the behavior and error strings are verbatim from a real occurrence. (UI-error screenshot to be attached.)

Summary

The bottom PR of a gh stack (v0.0.8) stack cannot be merged into a merge-queue-protected branch by any path:

• API: enablePullRequestAutoMerge and enqueuePullRequest both fail: This pull request is part of a stack and must be merged sequentially using the stack merge API. That API is not in the public GraphQL schema, and gh stack has no merge command.
• UI ("Merge when ready" → "Confirm enqueue pull request"): fails with Repository rule violations found — Commit message must match a given regex pattern. The stack-merge creates a Merge pull request … merge commit despite the queue's rebase method, so it violates a commit_message_pattern rule that only allows Merge branch ….

Non-stacked PRs merge through the same queue fine. Only unstacking (gh stack unstack) unblocks the merge.

Working hypothesis: the trigger is a merge-queue branch whose rules reject merge commits. The stack-merge creates a Merge pull request … commit instead of rebasing (as the queue's rebase method would), and that commit fails commit_message_pattern. Observed: the API rejection and the rule violation. Inferred: that the offending commit is a merge commit (a rebase of the PR's compliant commits would not trip the rule).

Environment

• gh 2.96.0, gh stack v0.0.8
• Trunk main ruleset: merge_queue{merge_method: REBASE}, pull_request{allowed_merge_methods:["rebase"]}, commit_message_pattern = ^((feat|fix|chore|docs|test|refactor): .+|Merge branch .+)
• Stack main ← #101 ← #102; bottom PR #101 approved, checks green, up to date with main.

Repro

1. Trunk with a merge queue (rebase) + a commit_message_pattern rule forbidding Merge pull request ….
2. gh stack init #101 #102 → gh stack submit.
3. Bottom PR green + approved + up to date.
4. Merge it via gh pr merge #101 --auto --rebase, the enqueuePullRequest mutation, or the UI.

Expected

Bottom PR enqueues and merges with the queue's rebase method, like an equivalent non-stacked PR.

Actual

• API: This pull request is part of a stack and must be merged sequentially using the stack merge API. (UNPROCESSABLE, not enqueued).
• UI: Repository rule violations found — Commit message must match a given regex pattern: ….

Likely cause (working hypothesis)

• Enqueue/auto-merge mutations are blocked for any stacked PR, deferring to a "stack merge API" absent from public GraphQL and from gh stack (observed).
• The stack-merge creates a merge commit instead of honoring the branch's rebase merge-queue method, so its Merge pull request … message fails commit_message_pattern (inferred from the rule violation).

Workaround

gh stack unstack (PRs keep bases/approvals) → bottom PR enqueues and merges normally.

Suggested fix

• Have stack-merge honor the target branch's merge-queue method (rebase; no merge commit).
• Expose stack-merge via GraphQL + a gh stack merge command.
• Fail earlier with a stack-specific message instead of a generic rule violation.

Related (no exact duplicate)

• Discussion "Merge stack" on a stacked PR appears to enqueue only the bottom PR #172 — "'Merge stack' appears to enqueue only the bottom PR"
• Stack merge hangs after partial merge invalidates latest-push approval #174 — "Stack merge hangs after partial merge invalidates latest-push approval"
• rebase when lower branch was squash merged #169 — "rebase when lower branch was squash merged"