> ## Documentation Index
> Fetch the complete documentation index at: https://docs.diffraction.sh/llms.txt
> Use this file to discover all available pages before exploring further.

# Optional checks

> Enable bounded Pro diagnostics, contextual video review, and base/head comparisons.

## Enable checks

Open **Repos → repository → Checks**. Pro workspace administrators can enable
performance, accessibility, contextual video review, base/head comparison, SEO,
and link health. Performance and accessibility are **on by default** for eligible
repositories without a saved policy. Existing opt-outs are preserved. Other checks
remain off by default. Organization settings can restrict which checks repositories
may use.

Saving affects future reviews. Each run records its effective selection and
policy version. These checks are report-only; they do not independently change
the review score or establish that a PR introduced a defect.

## Performance

Captures navigation/resource timings and bounded top-frame long-task and
long-animation-frame observations where the browser supports them. Script
attribution and expensive resources help explain slow recorded moments. The
web-vitals library also captures available TTFB, FCP, LCP, CLS and INP observations
and timing attribution during the recorded journey.

These are automated lab observations. Unsupported APIs, dropped observations,
and incomplete checkpoints remain explicit. Overlapping task/frame durations
must not be added together. This is not field Core Web Vitals measurement, a
complete CPU profile, or a React profiler. Sensitive flows using secret references
skip source profiling.

Lighthouse also performs one separate page-load audit per review attempt, capped
at 25 seconds, using an eligible Chromium recording's successful app-origin route.
The **Performance** panel shows its lab score, page-load metrics, optimization
findings, estimated savings and affected resources. It runs after recording in a
fresh browser session; it does not reuse the journey's login state. Secret-using
flows, other browser engines and reproduction runs skip this extra audit. The
report records viewport, engine and measurement conditions. Timeouts and partial
results remain visible and do not fail the recorded journey.

## Accessibility

Runs bounded axe checks on captured page states and reports violation rules and
items requiring review, with corrective guidance, target selectors and checkpoints.
Scans include visited pages, selected clicks and a reserved final or failed state,
within a bounded scan budget. Timeouts and scanner failures remain visible. Inspect **Coverage**
for collected results. A passed automated scan is not a conformance assessment;
keyboard, assistive-technology, and unvisited-state coverage remain separate.

## SEO and link health

SEO samples visited pages for title, description, canonical, and robots metadata.
Provide explicit route expectations so preview `noindex` is not mistaken for a
production indexing problem. Optional crawl-document reads inspect only the
reviewed origin's `robots.txt` and `sitemap.xml` within bounds.

Link health samples visible anchors and current-document fragments. Only exact
permitted app-origin paths receive HEAD probes. External links, redirects,
credentials, query strings, and unsafe or state-changing destinations are not
followed. A 404/410 indicates a missing destination; authentication failures,
rate limits, and unsupported HEAD remain uncertain. This is not a site-wide crawler.

```json diffraction.json theme={"theme":{"light":"css-variables","dark":"css-variables"}}
{
  "version": 1,
  "review": {
    "siteHealth": {
      "environment": "preview",
      "productionOrigin": "https://example.com",
      "routes": [{ "path": "/", "indexing": "index", "title": "Example" }],
      "crawlDocuments": true,
      "safeLinkPaths": ["/help"]
    }
  }
}
```

`environment` accepts `preview`, `production`, or `private`. Repository expectations
do not enable checks; save the corresponding switches in **Checks**. Secret-reference
flows skip site-health collection.

## Contextual video review

Adds bounded video analysis when usable, model-eligible video is present and
provider/budget gates pass. Observations retain time intervals, rule references,
and evidence links. A passed final assertion can coexist with a transient issue
in the recording. Missing, skipped, or failed analysis stays visible.

This feature does not guarantee detection of brief defects. Temporal observations
remain outside verified findings and score penalties. Video analysis is distinct
from simply playing a recorded video and from native Playwright video capture.

## Base/head comparisons

Runs the same semantic flow against exact PR base/head revisions in separate
environments. Comparable checkpoints show paired images, changed-pixel counts,
and bounded crops. State mismatches, unstable captures, missing actions, or
incompatible images prevent a truthful pixel comparison.

Enable **Base/head comparison** and supply a synthetic-data contract:

```json diffraction.json theme={"theme":{"light":"css-variables","dark":"css-variables"}}
{
  "version": 1,
  "review": {
    "comparison": {
      "privacy": "dedicated-synthetic-data",
      "state": { "testId": "review-state", "value": "fixture-v1" },
      "fixedTime": "2026-09-07T00:00:00Z",
      "locale": "en-US",
      "timezoneId": "UTC",
      "masks": ["[data-private]"],
      "maximumTargets": 1
    }
  }
}
```

Both revisions must expose `data-testid="review-state"` with the exact text
`fixture-v1` at captured checkpoints. Supply the expected fixture marker in your
application, not just the configuration. The target limit is one by default and
at most four; optional `checkpointStepIds` selects up to eight checkpoints.

Pixel comparison itself makes no AI calls. It still requires execution resources,
and the surrounding review can use AI. Changed pixels are observations, not
verified regressions. Cross-release new/resolved issue tracking and arbitrary
authenticated-state comparison are not established by this feature.