Context pack

Eight cold-start agents re-reading the same seven files cost 39.5M cache-read tokens in a traced run — 94% of that run's price. The pack replaces the re-reading with one shared page.

Input

• The repo and the base ref (the same base the review uses).
• The plan folder, if there is one: .plans/<task>/plan.md.

Build it

1. The diff, complete for the changed lines:

   git diff "$BASE"...HEAD

   Drop lockfiles, generated output and vendored directories. If the diff alone is over the budget, split it by file group and say which group is where.

2. The files it touches, one line each: path, what the file is, and its role in the change. No file bodies.
3. The API contracts the change crosses: endpoint, method, request shape, response shape, and where the shape is declared (the type or interface file, with a line number).
4. The test entry points: the test files that cover the changed source files, and the command that runs them.
5. The plan's acceptance criteria, verbatim, one per line, with any criterion marked manual: flagged as outside automated verification.
6. What not to re-derive: the facts a reviewer would otherwise spend turns rediscovering — which field decides the behaviour, which component owns the call, what the previous behaviour was, and the known-broken things that are not part of this change.

Write it

Write .plans/<task>/context-pack.md:

# Context pack

BASE: origin/master
CHANGED: 7 files, +283/-32

## What this change does
<two or three lines>

## Files
- `components/button/StartClassButton.vue` — start button; emits `started` on a successful start
- ...

## API contracts
- `POST /api/program-batch/start/{batchCode}` → 204; success is what emits `started`
  declared in `types/program-batch-student.d.ts:41`

## Test entry points
- `components/button/StartClassButton.test.ts` — `npx vitest run components/button/StartClassButton.test.ts`
- `pages/classes/_components/ClassesCard.test.ts` — `npx vitest run pages/classes/_components/ClassesCard.test.ts`

## Acceptance criteria
1. ...
2. manual: the list stops reporting NOT ATTEMPTED after a start (needs a real backend)

## Do not re-derive
- `batchStatus === "NOT ATTEMPTED"` is the signal; `courseStatus` is not.
- The modal's query key is built once at setup, so it does not follow a changed prop.

## Diff
```diff
<the diff>

## Rules

- **Budget: 3,000 tokens** (about 2,200 words). If it does not fit, cut prose before cutting the diff, and cut the diff by file group before dropping anything.
- Every claim carries a `file:line` anchor, so a reviewer can open exactly one place.
- No file bodies. If a reviewer needs more, they open that file — the pack's job is to say where, not to replace the file.
- The pack states the diff, the change's intent and the criteria. It does not argue that the change is correct; that is the reviewer's job.
- Write it once per change. If the branch moves, rewrite it rather than patching it.

Report one line: `Context pack: <path> (<n> words)`.