diff

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SKILL.md

---
name: diff
description: "Diff a live page's accessibility violations against a baseline — by default compares uncommitted changes (stash-based), or pass --branch [<name>] to diff against a branch. Reports only new violations introduced, violations fixed, and pre-existing count. Use `scan` for a full audit with no diffing."
argument-hint: "[--branch [<name>]] [target|url]"
allowed-tools: Bash, Read, Glob, Grep, Skill, Task
---

Default branch: !`git symbolic-ref refs/remotes/origin/HEAD --short 2>/dev/null | sed 's|.*/||' || echo main`

Report only what changed. Locate; don't fix.

Parse `$ARGUMENTS`: strip `--branch <name>` if present → branch mode (no value → the default branch above). The remainder is the **target** — a URL, a config target name, or empty for the default target from `accesslint.config.json`; if empty and no config, ask for a URL.

## 1. Audit

```bash
PORT=$(npx -y @accesslint/chrome@latest ensure | node -e 'process.stdin.on("data",d=>process.stdout.write(""+JSON.parse(d).port))')
```

**Stash mode** (default — uncommitted changes). Tell the user first: _"Running in diff mode — stashing your changes to capture a baseline, then restoring. Your working tree will be fully restored."_ If `git stash push` fails, warn and exit.

```bash
git stash push -u -m "accesslint-diff-baseline"
npx -y @accesslint/cli@latest scan <target> --port "$PORT" --snapshot accesslint-diff --snapshot-dir /tmp --update-snapshot
git stash pop && sleep 2
npx -y @accesslint/cli@latest scan <target> --port "$PORT" --snapshot accesslint-diff --snapshot-dir /tmp --format json
```

**Branch mode** (`--branch <name>`). Tell the user first: _"Diffing against `<name>` — checking out that branch to capture a baseline, then restoring. Your working tree will be fully restored."_

Branch switching triggers a rebuild but not a browser reload — the CLI opens a fresh tab each time so it always reads the current build. Use `--wait-for "<selector>"` to gate the audit until the rebuild is ready; without it, warn the user that a slow build may yield a stale baseline.

```bash
git diff --quiet && git diff --cached --quiet || git stash push -u -m "accesslint-diff-branch"
git checkout <branch>
npx -y @accesslint/cli@latest scan <target> --port "$PORT" --snapshot accesslint-diff --snapshot-dir /tmp --update-snapshot [--wait-for "<selector>"]
git checkout - && git stash pop 2>/dev/null
npx -y @accesslint/cli@latest scan <target> --port "$PORT" --snapshot accesslint-diff --snapshot-dir /tmp --format json [--wait-for "<selector>"]
```

Pass `--selector`, `--include-aaa` to **both** runs.

## 2. Report

```
Accessibility diff — http://localhost:3000/ vs main (94 rules, live DOM)
2 new · 1 fixed · 4 pre-existing hidden

New — Critical
- color-contrast — 2.1:1 (needs 4.5:1), #bbb on #fff
    where: main > p.subtitle   fix: darken to #767676
Fixed
- img-alt — <img src="old.jpg"> (no longer present)
```

Each new violation: **where** (selector verbatim + `file:line (symbol)` if `source` present — never fabricate), **evidence**, **fix** (mechanical change or `NEEDS HUMAN`).

Don't edit. For fixes: apply mechanical ones then re-run `accesslint:diff` to verify; for bulk work hand off to `accesslint:audit`.

## 3. Tear down

```bash
npx -y @accesslint/chrome@latest stop --all  # skip if ensure reported "managed":false
```

## Gotchas

- `ensure` always determines the port — never hardcode 9222.
- CLI exit 2 = bad URL/target or page never loaded; check the dev server.
- A `<target>` name resolves the same in both runs only if `accesslint.config.json` is stable across the stash/checkout — if your changes touch the config, pass an explicit URL instead.
- Stash mode: `sleep 2` covers most HMR cases; if baseline looks identical to current, add `--wait-for "<selector>"`.
- Branch mode: no HMR — CLI opens a fresh tab each run. `--wait-for` is the rebuild gate.
- Heavy DOM changes between runs cause selector drift — re-run with `accesslint:scan` for the full picture.