---
id: 06-sandcastle-reviewer-prompt
epic: 2026-05-14-library-evaluation-policy
title: Sandcastle reviewer prompt — Library-trace check section
type: technical-story
status: todo
feature: tooling
depends-on: [02-pre-commit-check-script]
blocks: []
---

## Goal

Append a "Library-trace check" section to `.sandcastle/reviewer.prompt.md` instructing the reviewer agent to run `node scripts/library-decisions/check.mjs --staged-against <base>` before issuing its verdict, and add the `--staged-against <base>` flag to `check.mjs` so it can compare against a given base ref rather than only the git index.

## Why

The sandcastle reviewer runs in a clean sandbox where `git diff --cached` may not reflect the full branch diff. The `--staged-against <base>` flag allows the reviewer to pass the PR's base branch as the comparison point, giving the same check a CI-compatible code path. Without this, the fourth enforcement layer is advisory only — it has no mechanical check to back it up.

## Done when

- `scripts/library-decisions/check.mjs` accepts a `--staged-against <base>` flag; when present, compares `git diff <base>...HEAD -- '**/package.json'` instead of `git diff --cached`.
- `check.test.mjs` has a new test case: `--staged-against main` mode with a new feature-tier dep and no trace → exit 1.
- `.sandcastle/reviewer.prompt.md` contains a "Library-trace check" section (appended, not replacing existing content) instructing the reviewer to run `node scripts/library-decisions/check.mjs --staged-against <base-branch>` and reject the slice if it exits non-zero.
- `pnpm lint && pnpm test && pnpm fallow:audit && pnpm coverage:diff` all pass.

## In scope

- `scripts/library-decisions/check.mjs` — add `--staged-against` flag.
- `scripts/library-decisions/check.test.mjs` — add test for the new flag.
- `.sandcastle/reviewer.prompt.md` — append the Library-trace check section.

## Out of scope

- Changing the reviewer prompt's existing sections.
- The pre-commit (index-mode) check behavior — already in Story 02.
- CI integration (GitHub Actions) — deferred.

## Tasks

- [ ] Add `--staged-against <base>` flag to `check.mjs` (switches from `git diff --cached` to `git diff <base>...HEAD`); add a test covering `--staged-against` mode (temp git repo fixture, new feature-tier dep, no trace → exit 1); append "Library-trace check" section to `.sandcastle/reviewer.prompt.md` with the `node scripts/library-decisions/check.mjs --staged-against <base>` invocation and reject instruction; all gates pass on this single commit.