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:

1. Why Reddit built its own prototyping harness

2. How designers share prototypes with leadership safely

3. Why real Reddit data beats mock data

4. How one annotation becomes a code edit in Cursor

5. What a “feature capsule” is

6. Why Blair built a “Rosetta stone”

7. Blair’s surprising fix when AI goes sideways

8. Who turned out to be Reddit’s best prototypers

—

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

—

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/

—

Other references:

• Reddit: https://www.reddit.com/

• Kubernetes: https://kubernetes.io/

—

Where to find Blair Metcalf:

LinkedIn: https://www.linkedin.com/in/jblairmetcalf

—

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

—

Production and marketing by https://penname.co/. For inquiries about sponsoring the podcast, email jordan@penname.co.

This is a public episode. If you'd like to discuss this with other subscribers or get access to bonus episodes, visit www.lennysnewsletter.com/subscribe

Why Reddit built its own AI prototyping tool | Blair Metcalf

How I AI (private feed for [email protected])

Why Reddit built its own AI prototyping tool | Blair Metcalf