Loki vs Percy

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.

Percy automates visual testing by capturing DOM snapshots and assets, rendering them across browsers, and comparing results with earlier snapshots to flag visual changes. Responsive checks use configurable widths, while snapshot stabilization freezes animations and other dynamic elements to reduce false positives. Percy groups snapshots into the same build when tests run in separate processes or on different machines. Its review workflow includes status updates, notifications, comments, comparisons, and automatic diff matching. Listed integrations include GitHub, GitLab, Slack, Storybook, WebdriverIO, and Jenkins. Documented frameworks include Selenium, Playwright, Cypress, Puppeteer, Ember, Appium, and others. Percy documentation describes testing across real devices and desktop and mobile browsers. The free plan includes 5,000 screenshots per month, unlimited users, and unlimited projects. Desktop costs $199 a month, billed annually, for 10,000 screenshots per month; Desktop & Mobile costs $599 a month, billed annually, for 25,000 screenshots per month. Paid plans have fixed screenshot allowances, and additional usage is charged at the overage rate. Enterprise pricing is custom and unlisted.