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 (firecrawl.dev/howiai)—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: cursor.com
• Figma: figma.com
• Figma MCP (Figma’s Model Context Protocol integration): figma.com/mcp-catalog
• Code Connect (Figma’s code mapping tool): figma.com/developers/code-connect
• Gemini: gemini.google.com
• ChatGPT: chatgpt.com
• Vercel (referenced as inspiration for internal deploy infra): vercel.com
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Other references:
• Reddit: reddit.com
• Kubernetes: kubernetes.io
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Where to find Blair Metcalf:
LinkedIn: linkedin.com/in/jblairmetcalf
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Where to find Claire Vo:
ChatPRD: chatprd.ai
Website: clairevo.com
LinkedIn: linkedin.com/in/clairevo
X: x.com/clairevo
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