adjust-pr-base

# adjust-pr-base

## Goal

This skill is a fixed procedure:

1. Find the open PR for the current branch.
2. Find the repository default branch.
3. Find the nearest open parent PR by git ancestry.
4. Set the PR base to that parent branch if one exists.
5. If the base changes to a parent PR branch, update the PR description to include that parent PR URL.
6. Otherwise set or keep the PR base as the default branch without adding a base PR link.

Do not add extra policy or heuristics.

## When To Use

- After `git rebase`
- After `gh pr create`
- When working with stacked PRs
- When checking whether `gh pr edit --base ...` is needed
- After a parent PR has been merged or closed, to update this PR's base to the default branch

## Rules

Follow these rules in order:

1. Only inspect the PR for the current branch.
2. Only consider open PRs as parent candidates. Merged or closed PRs are never parent candidates — if the parent PR was merged, it is no longer a valid base and the default branch must be used instead.
3. A parent candidate must be an ancestor of `HEAD`.
4. If multiple parent candidates exist, choose the one with the smallest `git rev-list <candidate>..HEAD --count`.
5. If no parent candidate exists (including when the only candidate was a now-merged/closed PR), use the default branch.
6. If the current base already matches the target base, do nothing.
7. Only update the PR description when the target base comes from an open parent PR and `gh pr edit --base` is run.
8. If the current branch has no open PR, stop and report that nothing was changed.

## Procedure

### 1. Read current branch and PR

```bash
git branch --show-current
gh pr view --json number,title,url,baseRefName,headRefName,state 2>/dev/null
```

If `gh pr view` fails, retry with:

```bash
branch=$(git branch --show-current)
gh pr list --head "$branch" --state open --json number,title,url,baseRefName,headRefName,state
```

If there is no open PR for the current branch, stop.

### 2. Read default branch

```bash
gh repo view --json defaultBranchRef --jq .defaultBranchRef.name
```

### 3. Find the nearest open parent PR

Use git ancestry only. Ignore reflog and subjective reasoning.

```bash
head_oid=$(git rev-parse HEAD)
gh pr list --author "@me" --state open --limit 30 \
  --json number,headRefName,headRefOid,url \
  | jq -r '.[] | "\(.number) \(.headRefName) \(.headRefOid) \(.url)"' \
  | while read num name oid url; do
      [ "$oid" = "$head_oid" ] && continue
      if ! git cat-file -e "$oid" 2>/dev/null; then
        git fetch --quiet origin "$name" 2>/dev/null || continue
      fi
      if git merge-base --is-ancestor "$oid" HEAD 2>/dev/null; then
        dist=$(git rev-list "${oid}..HEAD" --count)
        echo "$dist $num $name $url"
      fi
    done | sort -n | head -n 1
```

Interpretation:

- If this command returns one line, the third column is the target base branch.
- If this command returns one line, the fourth column is the parent PR URL to add to the description when the base changes.
- If this command returns nothing, the target base branch is the default branch and no base PR link is added.

### 4. Update the PR base if needed

```bash
gh pr edit <number> --base <target-branch>
```

Run this only when the current base and target base differ.

### 5. Update the PR description when the base changed to a parent PR

Only do this when step 3 found a parent PR and step 4 changed the base.

Read the current body:

```bash
gh pr view <number> --json body --jq .body
```

Then update the body so it contains exactly one line in this format:

```text
Base PR: <parent-pr-url>
```

If an existing `Base PR:` line is present, replace it. Otherwise append it on its own line near the end of the body.

Apply the body update with `gh pr edit <number>`.
If the target base is the default branch, do not add, replace, or remove any `Base PR:` line.

### 6. Report the result

Always report:

- current branch
- current PR URL
- previous base
- target base
- whether `gh pr edit --base` was run
- whether the PR description was updated with a base PR link

## Output Template

```text
Current branch: <branch>
Current PR: <url>
Previous base: <old-base>
Target base: <target-base>
Action: changed | unchanged | no-open-pr
Reason: nearest open ancestor PR | no open ancestor PR, so default branch | parent PR merged/closed, so default branch
Description: updated-base-pr-link | unchanged
```

## Don'ts

- Do not ask the user to choose between multiple bases.
- Do not inspect closed PRs.
- Do not use reflog as a source of truth.
- Do not keep extra fallback branches in the procedure.
- Do not leave the target base ambiguous.