Why Reddit built its own AI prototyping tool | Blair Metcalf

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
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This entire episode is brought to you by:
Firecrawl—Give your AI agents web data and beyond
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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
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Tools referenced:
• Cursor: https://www.cursor.com/
• Figma: https://www.figma.com/
• Figma MCP (Figma’s Model Context Protocol integration): https://www.figma.com/mcp-catalog/
• Code Connect (Figma’s code mapping tool): https://www.figma.com/developers/code-connect
• Gemini: https://gemini.google.com/
• ChatGPT: https://chatgpt.com/
• Vercel (referenced as inspiration for internal deploy infra): https://vercel.com/
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Other references:
• Reddit: https://www.reddit.com/
• Kubernetes: https://kubernetes.io/
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Where to find Blair Metcalf:
LinkedIn: https://www.linkedin.com/in/jblairmetcalf
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Where to find Claire Vo:
ChatPRD: https://www.chatprd.ai/
Website: https://clairevo.com/
LinkedIn: https://www.linkedin.com/in/clairevo/
X: https://x.com/clairevo
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Production and marketing by https://penname.co/. For inquiries about sponsoring the podcast, email [email protected].