Refract

Agent-first responsive screenshots. Give a coding agent (or yourself) one primitive: render this URL at N viewports, return the images, and tell me what visually broke. Built on Playwright, shipped as an MCP server + CLI + Node library.

🚧 Pre-release. The engine, CLI, and MCP tool work; npm packages aren't published yet, so install from source (pnpm install && pnpm build).

Install

pnpm add @getrefractjs/core        # library
pnpm add -g @getrefractjs/cli      # CLI
# one-time browser download (deferred until first render):
pnpm exec playwright install chromium

Related MCP server: webdev-mcp

CLI quickstart

refract https://example.com --viewports mobile,tablet,desktop --out ./shots

Outputs ./shots/{preset}.png, one per viewport (full-page, not just the fold), and prints findings under each. Flags: --viewports, --out, --selector, --wait-for, --wait-for-function, --wait-for-network-idle-ms, --freeze, --inject-css, --annotate, --dpr, --concurrency, --storage-state, --engine.

Use --inject-css "#clock,.ad{visibility:hidden}" to hide dynamic or flaky elements before capture — handy for clean, stable diffs (and the hidden elements stop showing up as findings too).

--freeze makes shots deterministic by disabling CSS animations/transitions and eager-loading images; it does not stop canvas, <video>, or JS-driven animation (use --inject-css to hide those). Captures are full-page, so content that only appears more than a few seconds after load should be gated with --wait-for.

Pass --annotate to draw outline boxes over the findings onto the screenshot (errors red, warnings amber) — so the image itself shows what broke. Full-page only.

When the page is ready only after an app-specific signal, gate the capture with --wait-for-function "window.__ready === true"; for slow pages, raise the network-idle cap with --wait-for-network-idle-ms 30000 (it stays best-effort).

Viewports that render identically (e.g. iphone-17-pro and iphone-16-pro are both 402×874 @3) are rendered once and bundled into a single result, with the extra device names listed as aliases — so "render every iPhone" doesn't shoot the same pixels three times.

Library quickstart

import { render } from '@getrefractjs/core';

const shots = await render({ url: 'http://localhost:3000' });
for (const shot of shots) {
  console.log(shot.preset, shot.savedPath);
  for (const f of shot.findings) {
    console.log(`  [${f.severity}] ${f.type} ${f.detail}`);
  }
}

MCP config block

{
  "mcpServers": {
    "refract": {
      "command": "npx",
      "args": ["-y", "@getrefractjs/mcp"]
    }
  }
}

The server exposes two tools, each with a description that tells an agent exactly when and how to call it — no docs required:

• render_responsive — one call returns a text manifest of absolute saved paths, structured findings as JSON (see below), and a downscaled preview image per viewport (≤800px wide, so it won't blow the agent's context window); full-resolution PNGs are written to disk.

• diff_responsive — visual regression in-band: renders, compares against a saved baseline, and returns a per-viewport status (unchanged/changed/…) with the % of pixels changed, a downscaled diff image for each changed viewport, and a report.html path. First run with update: true to save the baseline, then compare after a change.

Load failures come back as teaching errors the agent can act on.

Working in this repo? A committed .mcp.json registers the local server (node packages/mcp/dist/index.js) — run pnpm build first and restart your client so it picks the tool up.

Authenticated pages

Most real apps live behind a login. Refract renders logged-in pages by reusing a Playwright storage-state file (cookies + localStorage) — the standard format, nothing Refract-specific. Generate one once by logging in:

npx playwright codegen --save-storage=auth.json https://your-app.com/login
# log in in the window that opens, then close it

Then point any surface at it:

refract https://your-app.com/dashboard --storage-state ./auth.json

await render({ url: 'https://your-app.com/dashboard', storageState: './auth.json' });

render_responsive({ url: 'https://your-app.com/dashboard', storageState: './auth.json' })

Already have Playwright auth states from your e2e suite (e.g. e2e/.auth/*.json)? Pass one straight through — no regeneration needed. The file's cookies are sent to the URL, so don't pair an auth file from one origin with an untrusted URL.

Findings

Every render returns structured findings per viewport alongside the screenshots — agents act on these instead of eyeballing pixels:

{ preset: "mobile", findings: [
  { type: "horizontal_overflow", severity: "error", detail: "scrollWidth=480 viewport=402",
    selector: "div.card", rect: { x: 0, y: 120, width: 480, height: 90 } },
  { type: "tap_target_small", severity: "warn", selector: "button#tiny-btn", size: "28x24",
    rect: { x: 16, y: 540, width: 28, height: 24 } },
]}

Most findings carry a selector and a rect (the culprit's box in document pixels, so you can zoom straight to what broke); horizontal_overflow names the element that causes it.

The checks descend into open Shadow DOM (web components); closed shadow roots are unreachable. The CLI prints findings under each shot; the MCP tool returns them as JSON keyed by preset.

Token footprint

Findings-first responses and downscaled previews aren't just nice-to-haves — they keep the agent's context window alive. Measured on the demo-site (3 viewports), one render_responsive response costs roughly:

So Refract's default is ~78% smaller than dumping full-res screenshots on Claude, and the structured findings alone are a few hundred tokens. Re-run anytime with pnpm bench; details + per-model formulas in benchmarks/RESULTS.md.

Visual diff

refract diff catches unintended visual change across viewports — the "did my CSS tweak break another page" check. Baselines are just PNGs in a folder, so it's git-agnostic and trivial to wire into CI.

# 1. Save a baseline (once, or to accept new changes)
refract diff https://example.com --update          # writes ./refract-baseline/{preset}.png

# 2. Later, compare a fresh render against it
refract diff https://example.com                   # exits 1 if anything changed

Each viewport is compared with pixelmatch. Output per preset is unchanged, changed (with the % of pixels and a {preset}.diff.png highlighting them), size_changed, or no_baseline. Alongside the pixel diff, each preset reports a findings delta — which findings were fixed (gone since the baseline) or regressed (new) — so you can confirm a fix landed without introducing a new responsive issue. (The --update snapshot stores the findings too, in findings.json; an older baseline without one just omits the delta.) A report.html lands next to the shots with a baseline │ current │ diff grid. The command exits non-zero when anything changed, so CI fails on a regression; re-run with --update to accept the new look as the baseline. Flags: `--baseline

Use in CI

Commit your baselines (./refract-baseline/) and run refract diff against a deployed preview on every PR — it exits non-zero on a regression, failing the job. A ready-to-copy GitHub Actions workflow is in examples/github-actions/visual-diff.yml: it installs the CLI + Chromium, runs the diff, and uploads report.html + the diff PNGs as an artifact when something changed. Refresh baselines by re-running with --update and committing. (Agents working in an MCP client can do the same loop via the diff_responsive tool.)

What this is not

• ❌ A browser extension or live-preview app (that's Responsively's job).

• ❌ A general-purpose browser-control MCP (that's playwright-mcp's job). It renders URLs at viewports; it cannot click, type, or perform a login flow. It can reuse a saved auth state (--storage-state) to render a logged-in page, but it won't log in for you.

• ❌ A visual-regression engine reinvented from scratch (it wraps pixelmatch).

• ❌ A real-device cloud. Playwright emulates viewport, DPR, UA, and touch, and --engine webkit runs the real WebKit engine (close to iOS Safari) — but it's still desktop WebKit, not an actual iOS device or GPU. It does not replace BrowserStack.

Cross-browser

Renders on Chromium by default; pass --engine webkit (or engine: "webkit" in the library / MCP) to render with the real Safari/WebKit engine — the closest local proxy to iOS Safari, where a lot of responsive bugs actually show up.

npx playwright install webkit          # one-time, ~70MB
refract https://example.com --engine webkit

It works everywhere a render does (CLI, library, MCP render_responsive / diff_responsive). For visual diffs, keep a separate baseline dir per engine (--baseline ./baseline-webkit) since engines render slightly differently. Firefox isn't supported yet (it can't emulate isMobile and ignores DPR) — open an issue if you need it.

Roadmap

Where Refract is headed — deeper findings, shareable reports, more device/engine coverage — is in docs/ROADMAP.md. It's directional, not a promise; real usage steers it.

Security

Refract loads any URL you give it — including file:// (local files) and internal/private hosts (e.g. cloud metadata endpoints) — and returns the rendered pixels. Treat it like curl: don't point it, or an agent driving the MCP server, at untrusted or sensitive URLs. There is no URL allow/deny list by default. See SECURITY.md to report a vulnerability.

License

MIT