Why Reddit built its own AI prototyping tool | Blair Metcalf

About this episode

Blair Metcalf is the principal UX engineer at Reddit and the builder of Reddit Proto, the company’s in-house vibe-coding harness. With a design system MCP that maps Reddit’s product language to production code, Proto has reached 100% adoption across Reddit’s UX and product org, and more than 700 prototypes are live on it. What you’ll learn: • Why Reddit built its own prototyping harness • How designers share prototypes with leadership safely • Why real Reddit data beats mock data • How one annotation becomes a code edit in Cursor • What a “feature capsule” is • Why Blair built a “Rosetta stone” • Blair’s surprising fix when AI goes sideways • Who turned out to be Reddit’s best prototypers — This entire episode is brought to you by: Firecrawl—Give your AI agents web data and beyond — In this episode, we cover: (00:00) Introduction to Blair (02:31) Reddit Proto demo (09:15) Annotations to Cursor: the targeted edit workflow (11:19) Internal deployment and adoption (14:14) How leadership reviews and gives feedback (15:52) Engineering handoff and the feature capsule (17:50) How the custom MCP was built (21:41) Quick recap (24:55) Lightning round and final thoughts —