werkrevisor/docs/prs/2026-10-11-PR#85-base-commit-fallback.md

PR#85: The Merge Base Stands In for an Unknown Base Commit

WARNING: This document describes only the change applied in this PR. It may already be outdated once the next PR is merged. Historic PR documentation is not maintained along with new PRs; treat it as a snapshot.

Problem

On 2026-10-06, tools/dry-run with werkrevisor 0.20.6 stopped on hs.hsadmin.ng #283 with the base commit is unknown, so .werkrevisor.toml cannot be read.

The cause: #283 was a stacked PR on the branch mihoe/rbac-grant-reachability-closure, which was deleted after the merge. For such a PR, Gitea returns base.sha = '', but it still knows merge_base.

Since 0.21.0 (werkrevisor PR#39), werkrevisor reads .werkrevisor.toml at the head of the default branch, so the review no longer stops. The base commit stays empty, though: the result comment says against base unknown, and the usage record has "base_sha": "".

Non-Goals

• No change for a PR whose base branch still exists: base.sha stays the base commit shown.
• No change of the diff; Gitea computes it against the merge base anyway.

Specification

Feature: a review names a base commit also after the base branch was deleted

Scenario#85.01: the merge base stands in for an empty base commit

So that the result comment and the usage record name the commit the PR was reviewed against.

• Given a merged PR whose base branch was deleted, so Gitea returns an empty base.sha and a merge_base
• When werkrevisor reviews it without WERKATOR_PR_BASE_SHA
• Then the result comment and the usage record name the merge base as the base commit

Verified by

• test_flow.PublishAndUsageTest.test_the_merge_base_stands_in_for_the_base_of_a_deleted_branch
• test_flow.PublishAndUsageTest.test_the_base_sha_wins_over_the_merge_base

Solution

• cli.base_commit takes base.sha and, if it is empty, merge_base; main and execute use it when WERKATOR_PR_BASE_SHA is not set.
• Both tests fail without the fallback.
• A dry run of hs.hsadmin.ng #283 showed against base unknown with 0.32.10 and its merge base b95fe2e with 0.32.11.
• The guide for dev.hostsharing.net no longer names the bug as open.
• Werkbaum: #ops.basecommit.
• Version 0.32.11.

Open Questions

None.

Additional Changes

• Werkbaum: the idea #model.codex, an optional review by the Codex CLI with a frontier model in a subscription.