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fix: made the diff-target fallbacks run and anchored the issue key #10

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Merged
andyhorn merged 2 commits into from
Oct 5, 2026
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fix: made the diff-target fallbacks run and anchored the issue key #10

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04043df
made the default-branch and pull request base fallbacks actually run
andyhorn Oct 5, 2026
7048f6d
required the issue label to start a word in the branch name
andyhorn Oct 5, 2026
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47 changes: 34 additions & 13 deletions skills/explain-diff-html/SKILL.md
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Original file line number Diff line number Diff line change
Expand Up @@ -167,7 +167,15 @@ needs no network:

```bash
base="$(git symbolic-ref --short refs/remotes/origin/HEAD 2>/dev/null)"
base="${base:-origin/$(git remote show origin | sed -n 's/.*HEAD branch: //p')}"
if [ -z "$base" ]; then
head="$(git remote show origin 2>/dev/null | sed -n 's/.*HEAD branch: //p')"
base="${head:+origin/$head}" # empty, not "origin/", when the call fails
fi
for b in origin/main origin/master; do
[ -n "$base" ] && break
git rev-parse --verify -q "$b" >/dev/null && base="$b"
done
echo "${base:?no default branch found}"
```

Keep the `origin/` prefix on both paths. The first form already returns
Expand All @@ -176,9 +184,11 @@ branch. A local branch is routinely behind the remote and missing entirely in a
single-branch clone, and the diff then silently covers the wrong range.

`refs/remotes/origin/HEAD` is missing in some clones, which is why the network
call is the fallback rather than the first attempt. Fall back again to whichever
of `origin/main` or `origin/master` exists, and fetch the base fresh before
diffing.
call is the fallback rather than the first attempt. The snippet adds `origin/`
only to a non-empty answer, because a failed network call would otherwise leave
the non-empty string `origin/` and skip the last fallback. That last fallback
takes whichever of `origin/main` or `origin/master` exists. Fetch the base fresh
before diffing.

Peel a tag to its commit before you use it anywhere. For an annotated tag,
`git rev-parse v4.6.6` returns the tag object, not the commit it points at, and
Expand Down Expand Up @@ -209,9 +219,13 @@ work="$(mktemp -d)"
echo "$work" # write this path out in later steps
git fetch origin "refs/pull/<n>/head:refs/explain/pr-<n>" # pull request only
pr_base="$(gh pr view <n> --json baseRefOid -q .baseRefOid)" # the commit it opened against
git fetch origin "$pr_base" ||
git fetch origin "$(gh pr view <n> --json baseRefName -q .baseRefName)"
if [ -z "$pr_base" ] || ! git fetch origin "$pr_base"; then
# The sha is unreachable or refused: diff from where the head left the base branch.
git fetch origin "$(gh pr view <n> --json baseRefName -q .baseRefName)" &&
pr_base="$(git merge-base FETCH_HEAD refs/explain/pr-<n>)"
fi
git diff "$pr_base" refs/explain/pr-<n> > "$work/diff.txt" # base first, then head
[ -s "$work/diff.txt" ] || echo "STOP: the diff is empty; resolve the base before drafting"
```

Each tool call starts a new shell, so `$work` is empty in every later command.
Expand All @@ -231,15 +245,20 @@ blank page, not an error. Evidence: uPortal PR 2983 gives 0 files that way and
17 the correct way, while fastapi PR 15800 gives the right answer both ways
because it was squashed.

`gh pr diff <n>` returns the same diff and needs no fetch, so use it when `gh`
is available and keep the git form for when it is not.
`gh pr diff <n>` returns the same diff and needs no fetch, so use it when it
works. The git form above still asks `gh` for the base commit, so it does not
replace `gh`. Without `gh`, ask the user which commit the pull request is based
on rather than guessing one.

Fetching a bare sha is not always allowed. The server decides whether to serve
an object that no ref advertises. GitHub.com serves one that is reachable, and a
GitHub Enterprise install may refuse. A base branch that was force-pushed or
deleted can also leave `baseRefOid` unreachable. So fall back to fetching the
base branch, as above, because without that fallback the failure is another empty
diff and another blank page.
base branch, as above, and diff from its merge base with the head, because
without that fallback the failure is another empty diff and another blank page.
The empty-diff check is the backstop. A pull request merged with a merge commit
is already on its base branch, so the fallback's merge base is the head itself
and the diff is empty. Stop there rather than draft a blank page.

For a branch or the current checkout there is no recorded base, so use the
three-dot form against the default branch:
Expand Down Expand Up @@ -284,9 +303,11 @@ The output contract gives the filename key precedence. These are the patterns
behind it.

A tracker-style issue key matches `[A-Z][A-Z0-9]+-[0-9]+`, such as `PROJ-1234`.
A labeled issue number matches `(issue|gh|#)[-_]?[0-9]+`, such as `issue-456` or
`gh-456`. Require the label: a bare number also matches a date or a version in a
branch name such as `cleanup-2024-q1`, which produces a meaningless filename.
A labeled issue number matches `(^|[/_-])(issue|gh|#)[-_]?[0-9]+`, such as
`issue-456` or `gh-456`; drop the leading separator from the match. Require the
label: a bare number also matches a date or a version in a branch name such as
`cleanup-2024-q1`, which produces a meaningless filename. Require it to start the
name or follow a separator, too, or `fix-high-5` yields `gh-5` from inside "high".

Both read the branch name, so both apply only to a pull request, a named branch,
or the current checkout. A commit range and a single commit have no branch, which
Expand Down
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