{"slug": "why-reddit-built-its-own-ai-prototyping-tool-blair-metcalf", "title": "Why Reddit built its own AI prototyping tool | Blair Metcalf", "summary": "Reddit's in-house AI prototyping tool, Reddit Proto, has reached 100% adoption across the company's UX and product organization, with more than 700 prototypes live on it, according to Blair Metcalf, Reddit's principal UX engineer and the tool's builder. Reddit Proto pairs a design-system Model Context Protocol server that maps Reddit's product language to production code with a workflow that turns designer annotations into targeted code edits in Cursor, and it packages engineering handoffs as \"feature capsules.\" Metcalf described the system on the How I AI podcast hosted by Claire Vo.", "body_md": "**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.\n\n**Listen or watch on [YouTube](https://youtu.be/zF-J3rMbzZw), [Spotify](https://open.spotify.com/show/4aRP2XSavdtrLG5FZoonOK), or [Apple Podcasts](https://podcasts.apple.com/us/podcast/how-i-ai/id1809663079)**\n\n### What you’ll learn:\n\n1. Why Reddit built its own prototyping harness\n2. How designers share prototypes with leadership safely\n3. Why real Reddit data beats mock data\n4. How one annotation becomes a code edit in Cursor\n5. What a “feature capsule” is\n6. Why Blair built a “Rosetta stone”\n7. Blair’s surprising fix when AI goes sideways\n8. Who turned out to be Reddit’s best prototypers\n\n### This entire episode is brought to you by:\n\n**[Firecrawl](https://firecrawl.dev/howiai)**—Give your AI agents web data and beyond\n\n### In this episode, we cover:\n\n([00:00](https://www.youtube.com/watch?v=zF-J3rMbzZw)) Introduction to Blair\n\n([02:31](https://www.youtube.com/watch?v=zF-J3rMbzZw&t=151s)) Reddit Proto demo\n\n([09:15](https://www.youtube.com/watch?v=zF-J3rMbzZw&t=555s)) Annotations to Cursor: the targeted edit workflow\n\n([11:19](https://www.youtube.com/watch?v=zF-J3rMbzZw&t=679s)) Internal deployment and adoption\n\n([14:14](https://www.youtube.com/watch?v=zF-J3rMbzZw&t=854s)) How leadership reviews and gives feedback\n\n([15:52](https://www.youtube.com/watch?v=zF-J3rMbzZw&t=952s)) Engineering handoff and the feature capsule\n\n([17:50](https://www.youtube.com/watch?v=zF-J3rMbzZw&t=1070s)) How the custom MCP was built\n\n([21:41](https://www.youtube.com/watch?v=zF-J3rMbzZw&t=1301s)) Quick recap\n\n([24:55](https://www.youtube.com/watch?v=zF-J3rMbzZw&t=1495s)) Lightning round and final thoughts\n\n### Tools referenced:\n\n• Cursor: [https://www.cursor.com/](https://www.cursor.com/)\n\n• Figma: [https://www.figma.com/](https://www.figma.com/)\n\n• Figma MCP (Figma’s Model Context Protocol integration): [https://www.figma.com/mcp-catalog/](https://www.figma.com/mcp-catalog/)\n\n• Code Connect (Figma’s code mapping tool): [https://www.figma.com/developers/code-connect](https://www.figma.com/developers/code-connect)\n\n• Gemini: [https://gemini.google.com/](https://gemini.google.com/)\n\n• ChatGPT: [https://chatgpt.com/](https://chatgpt.com/)\n\n• Vercel (referenced as inspiration for internal deploy infra): [https://vercel.com/](https://vercel.com/)\n\n### Other references:\n\n• Reddit: [https://www.reddit.com/](https://www.reddit.com/)\n\n• Kubernetes: [https://kubernetes.io/](https://kubernetes.io/)\n\n### Where to find Blair Metcalf:\n\nLinkedIn: [https://www.linkedin.com/in/jblairmetcalf](https://www.linkedin.com/in/jblairmetcalf)\n\n### Where to find Claire Vo:\n\nChatPRD: [https://www.chatprd.ai/](https://www.chatprd.ai/)\n\nWebsite: [https://clairevo.com/](https://clairevo.com/)\n\nLinkedIn: [https://www.linkedin.com/in/clairevo/](https://www.linkedin.com/in/clairevo/)\n\nProduction and marketing by [https://penname.co/](https://penname.co/). For inquiries about sponsoring the podcast, email [\\[email protected\\]](https://www.lennysnewsletter.com/cdn-cgi/l/email-protection).", "url": "https://wpnews.pro/news/why-reddit-built-its-own-ai-prototyping-tool-blair-metcalf", "canonical_source": "https://www.lennysnewsletter.com/p/why-reddit-built-its-own-ai-prototyping", "published_at": "2026-10-26 12:04:20+00:00", "updated_at": "2026-10-26 12:11:57.154767+00:00", "lang": "en", "topics": ["ai-tools", "ai-agents", "agent-protocols", "developer-tools", "ai-products"], "entities": ["Reddit", "Blair Metcalf", "Reddit Proto", "Cursor", "Figma", "Claire Vo", "How I AI", "Firecrawl"], "also_reported_by": [], "alternates": {"html": "https://wpnews.pro/news/why-reddit-built-its-own-ai-prototyping-tool-blair-metcalf", "markdown": "https://wpnews.pro/news/why-reddit-built-its-own-ai-prototyping-tool-blair-metcalf.md", "text": "https://wpnews.pro/news/why-reddit-built-its-own-ai-prototyping-tool-blair-metcalf.txt", "jsonld": "https://wpnews.pro/news/why-reddit-built-its-own-ai-prototyping-tool-blair-metcalf.jsonld"}}