Loki vs SnapDiff

Loki is a free, MIT-licensed tool for checking Storybook projects for visual regressions. It captures story screenshots and compares them with reference images, producing visual differences for review. The workflow is to create reference screenshots, run comparisons, review the differences, and approve updated references. Loki is designed for continuous integration, where tests can fail if reference images are missing. Capture targets include Chrome in Docker or locally, Chrome in AWS Lambda, the iOS simulator, and the Android emulator; an AWS Lambda renderer is available for large test suites. Node 16 or later is required. Recent Storybook versions need no extra integration for non-React Native projects, while React Native setup uses an import in `storybook/storybook.js`. Projects can configure Loki in a `package.json` section or supported Loki configuration files. Loki does not start Storybook or simulator and emulator servers, so they need to be running before tests. Documented animation limitations include looping requestAnimationFrame animations, GIFs, SVG animations, native Lottie animations, and React Native Animated.

SnapDiff captures web pages and compares screenshots pixel by pixel to identify visual regressions. Its HTTP API can compare two URLs without a test suite or CI/CD pipeline, and each diff returns a percentage, a highlighted image, and bounding boxes around changed regions. A GitHub Action runs visual checks on pull requests and can discover Vercel preview URLs. For scheduled monitoring, the watch API checks URLs on a cron schedule and sends webhooks when changes exceed a threshold. Comparisons use pixelmatch with antialiasing detection, configurable thresholds, ignore regions, and CSS selector masking. SnapDiff accepts HTTPS preview, staging, Storybook, and deployment URLs, including those on Vercel, Netlify, and Cloudflare Pages. Its MCP server supports Claude Code, Cursor, Windsurf, and other MCP clients for verifying UI changes. The service says it hides more than 40 cookie banners and 20 chat widgets and blocks more than 30 ad and tracker domains during capture. Free includes 200 diffs and 2,000 screenshots per month; paid plans start with Dev at $19 a month. Screenshots and diff images are retained during a subscription and for 30 days afterward.