Reddit builds internal AI prototyping harness for non-engineers

Reddit has developed an internal tool called Reddit Proto, a custom harness that allows designers and product managers to create production-like prototypes without managing Git repositories or local environments. The tool uses Reddit's design system, components, and UX patterns to keep users on rails, enabling them to generate and review multiple ideas before selecting one for production. A key feature is the ability to authenticate with real production data, allowing stakeholders to test features like search with personalized, real-time data rather than mocked Figma designs. The system deploys prototypes on internal Kubernetes infrastructure as PWAs, currently hosting approximately 700 prototypes. Adoption is 100% among UX and product teams, with over 150 users, including visual designers and animators.

The tool integrates with Cursor for vibe-coding and uses a custom MCP, referred to as Proto MCP Ultra, to map Figma designs to production code tokens and components. This MCP maps every token, component, and surface in the Reddit Product Language design system across Figma, iOS, and GitHub repositories. It incorporates CI review feedback and engineer feedback to refine its mappings, ensuring that generated code is production-shaped. The workflow involves using Figma MCP to create a 'feature capsule' from a Figma frame, which analyzes design frames and components to generate a Product Requirements Document. This document includes user goals, acceptance criteria, and links to Figma assets. The generated code is written to a sibling repository to keep production clean, then submitted as a draft PR for review. Blair Metcalf argued that using real data and design system constraints in prototypes creates instant alignment with leadership and accelerates product velocity by avoiding detailed rat-holes on mockups.