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 is a visual testing tool that captures DOM snapshots and assets, renders them, and checks them against earlier snapshots to identify visual changes. It can render pages across browsers and platforms, including desktop and mobile, and compare responsive layouts at configurable widths. Percy groups visual changes while filtering noise and freezes animations and other dynamic elements to reduce false positives. Snapshots from tests running on different processes or machines can be grouped into a single build. Its review workflow includes status updates, notifications, comments, baseline management, branch logic, and approval carryforwards. Percy supports web applications, component libraries, end-to-end testing frameworks, static sites, custom SDK integrations, and native mobile applications. The CLI can capture and upload snapshots without an automation script. A free plan includes 5,000 screenshots per month; paid plans have fixed screenshot allocations, with extra screenshots charged at an overage rate. Desktop costs $199 a month billed annually, and Desktop & Mobile costs $599 a month billed annually.