external-pr-review

name	external-pr-review
description	One-shot review of an external GitHub PR. Checks out the PR into an isolated worktree (or temp clone), runs `bramble code-review` against the diff, and produces a calibrated verdict — APPROVE unless there is a blocking correctness issue, with optional improvements listed separately. Read-only on the remote PR, never pushes. User-invoked only via `/external-pr-review`.
argument-hint	<pr-url-or-number> [--backend codex|cursor|gemini] [--model NAME] [--effort low|medium|high]
disable-model-invocation	true

External PR Review

Read-only review of someone else's PR. One bramble code-review pass against the diff, calibrated verdict, optionally posted to GitHub.

Companion to /pr-polish (which iterates fixes on your own branch). This one never edits the PR's files, never pushes.

Calibrating findings

A finding is blocking only when merging the diff as-is would cause something observable to break or regress:

• Real bug introduced by the diff (wrong condition, off-by-one, missing await).
• Security regression introduced by the diff.
• Missing/broken tests for behavior the diff explicitly changes.
• Obvious perf regression (N+1 on a hot path, sync I/O in async hot loop).
• Change to a documented contract without a matching doc/test update.

Everything else — naming, structure, style, optional refactors, pre-existing issues — is Optional, and doesn't gate the verdict.

The blocking test: if this merges as-is, will something observable break or regress? Code-smell answers no. Wrong SQL predicate answers yes.

Arguments

• <pr> (required): full URL https://github.com/owner/repo/pull/N, or bare #N / N when invoked from inside a clone of that repo.
• --backend: codex (default), cursor, or gemini.
• --model: backend-specific override.
• --effort: codex-only. low / medium (default) / high. medium catches cross-call-site coverage gaps that low misses; high for genuinely subtle correctness.

Step 1: Resolve the PR

mkdir -p /tmp/external-pr-review-$PR_NUM
gh pr view "$PR_INPUT" --repo "$OWNER/$REPO" \
  --json number,title,body,headRefOid,headRefName,baseRefName,url,author,isDraft,state \
  > /tmp/external-pr-review-$PR_NUM/pr.json

Closed/merged → ask before continuing. Draft → proceed, note in verdict.

Step 2: Check out the code

Need an isolated worktree at the head SHA. Try in order.

Method A — worktree from local clone (when git config remote.origin.url matches $OWNER/$REPO):

git fetch origin "pull/$PR_NUM/head:refs/remotes/origin/pr-$PR_NUM"
WORKTREE_DIR="/tmp/external-pr-review-$PR_NUM-$(date +%s)"
git worktree add --detach "$WORKTREE_DIR" "refs/remotes/origin/pr-$PR_NUM"

Method B — fresh shallow clone:

WORKTREE_DIR="/tmp/external-pr-review-$OWNER-$REPO-$PR_NUM-$(date +%s)"
gh repo clone "$OWNER/$REPO" "$WORKTREE_DIR" -- --depth 50
cd "$WORKTREE_DIR"
gh pr checkout "$PR_NUM" || \
  git checkout -b "pr-$PR_NUM" "origin/$(jq -r .headRefName /tmp/external-pr-review-$PR_NUM/pr.json)"

gh pr checkout fails on shallow clones with cannot set up tracking information — the fallback works because the fetch already created the remote-tracking ref. If bramble later complains about missing base, git fetch --unshallow.

Verify git rev-parse HEAD matches the head SHA from step 1 before continuing.

If both methods fail, report the error and stop.

Step 3: Build the review goal

--goal is the highest-leverage knob. Without a tight goal, codex/cursor wander into infra/lockfile/cross-repo audits and burn 5–8 minutes on a small PR.

Write this to $WORKTREE_DIR/.external-review-goal.txt:

Review of GitHub PR #<N>: "<title>"
Repo: <owner>/<repo>
Base: <baseRef>  Head: <headRefShort>
Author: <login>

## What the author says this PR does
<verbatim PR body, capped at ~50 lines; truncate with "(truncated)" note>

## Diff stat
<git diff --stat origin/<base>..HEAD, capped at 10 lines>

## Review brief
Calibrate findings against real impact. Mark a finding as blocking only if
merging this diff as-is would cause something OBSERVABLE to break or
regress: real bug, security regression, missing/broken test for changed
behavior, obvious perf regression, or a change to a documented contract
without matching doc/test update.

Style notes, structural preferences, and pre-existing issues belong
under optional improvements, not blockers.

Stay inside the diff. Do not audit Pulumi/infra, lockfiles, generated
code, or cross-repo prompts unless this PR explicitly changes them. If
you find yourself reading a file not in `git diff --name-only`, stop.

One pass. Don't re-verify the same plumbing across multiple turns.

If the PR body is empty or one-liner, substitute title + first commit subject. Don't let the model invent intent from the branch name.

Step 4: Run bramble code-review

export BRAMBLE_BIN="$([ -x "$(pwd)/bazel-bin/bramble/bramble_/bramble" ] \
    && echo "$(pwd)/bazel-bin/bramble/bramble_/bramble" \
    || echo bramble)"

ENVELOPE="$WORKTREE_DIR/.external-review-envelope.json"
LOG_DIR="$WORKTREE_DIR/.external-review-logs"
mkdir -p "$LOG_DIR"

BACKEND="${BACKEND:-codex}"
MODEL_FLAG=""; [ -n "$MODEL" ] && MODEL_FLAG="--model $MODEL"
EFFORT_FLAG=""
[ "$BACKEND" = "codex" ] && EFFORT_FLAG="--effort ${EFFORT:-medium}"

cd "$WORKTREE_DIR" && \
  BRAMBLE_RUN_TAG="external-pr-review:$OWNER/$REPO:$PR_NUM:$BACKEND" \
  "$BRAMBLE_BIN" code-review \
    --backend "$BACKEND" $MODEL_FLAG $EFFORT_FLAG \
    --skip-test-execution --verbose --timeout 10m \
    --goal "$(cat "$WORKTREE_DIR/.external-review-goal.txt")" \
    --envelope-file "$ENVELOPE" \
    2> "$LOG_DIR/stderr.txt"

Run it under Monitor (typical 2–6 min):

Monitor({
  description: "external PR review (bramble code-review)",
  timeout_ms: 720000,
  persistent: false,
  command: "<the bramble invocation above>"
})

Do not also call ScheduleWakeup or any sleep loop while waiting. Monitor's completion event is the only signal needed. ScheduleWakeup with a slash-command prompt will re-fire the whole skill and double-post the review.

If the envelope reports status: "error" but review.raw_text contains a fenced json block, recover by extracting the inner JSON. Otherwise report stderr path and stop.

Step 5: Triage

Each envelope finding has a severity (high/medium/low/nit), path/line, and description. For each:

• Apply the blocking test from "Approval bias" above.
• Cross-reference the cited file:line. If it isn't in the diff, demote to optional (pre-existing) or drop (stale).
• Disagree with the model's severity when warranted — a high style nit is optional; a low real bug is blocking.

Step 6: Print the verdict

# Review of <owner>/<repo>#<N> — "<title>"

**Verdict:** ✅ APPROVE   (or  ⚠️ APPROVE with suggestions  /  🛑 REQUEST CHANGES)

**Summary:** <one sentence>

## Blocking issues
<omit when none>

- **`path:line`** — <one-liner>
  <2–3 sentences: why blocking, what to change>

## Optional improvements
<omit when none>

- **`path:line`** — <one-liner>
  <1–2 sentences, phrased as a suggestion>

Verdict:

• ≥1 blocking → REQUEST CHANGES.
• ≥1 substantive optional (real non-blocking bug, missed migration site, likely-unintended behavior) → APPROVE with suggestions.
• Otherwise → APPROVE. If only pure nits remain, list them under a "Nits (take or leave)" footer and keep the headline plain APPROVE.

Step 7: Ask before posting

First, idempotency check — has the current user already reviewed this SHA?

gh pr view "$PR_NUM" --repo "$OWNER/$REPO" --json reviews,headRefOid \
  --jq '.headRefOid as $sha | .reviews[] | select(.commit.oid == $sha) | {author: .author.login, state}'

If yes (likely a re-fire), offer Update existing, Post anyway, Skip — default Skip.

Otherwise ask via AskUserQuestion:

• Approve on GitHub (only when verdict is APPROVE or APPROVE-with-suggestions): gh pr review --approve --body-file <file>.
• Comment only: gh pr review --comment --body-file <file>.
• Request changes (only when verdict is REQUEST CHANGES): gh pr review --request-changes --body-file <file>.
• Skip (default).

Always --body-file, never --body "..." — long markdown breaks shell quoting.

Step 8: Cleanup

• Worktree → git worktree remove --force <path> then git update-ref -d refs/remotes/origin/pr-<N>.
• Temp clone → rm -rf <path>.

Keep $ENVELOPE and $LOG_DIR on cleanup failure and tell the user where they are.

Edge cases

• Generated code (lockfiles, *.pb.go, snapshots): skip findings on these.
• Revert PR: confirm revert target matches the description; approve unless the revert itself introduces a bug.
• Docs-only PR: read for accuracy and broken links; default APPROVE.
• Stacked PR: verdict is on this diff in isolation; note the dependency.