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Blair Metcalf · Design engineer

I build the harness, not just the thing.

Documentation, component systems and agent guidance that let one person do what used to take a team. Principal UX Engineer at Reddit, 23 years at the seam between design and engineering.

See the workHow this site was built

Principal UX Engineer. Builds the layer other people build on.

Role
Principal UX Engineer, Reddit
Focus
Agentic tools: the interfaces between people, models and the systems they build with.
Based in
Seattle
  • 23 years between design and engineering. Principal UX Engineer at Reddit since 2021. UX Lead at Google Cloud from 2015 to 2021.
  • Built Reddit Proto, the AI harness that became the company's de facto way to decide. 100% of UX and Product use it, with 800 prototypes deployed.
  • Led design for posting and commenting at Reddit. GIFs in comments drew about 60% more engagement than text and was covered by more than 10 publications.
  • Recruited and led a 22-person UX team at Google, where the prototyping framework was critical to 8 launches. Comfortable leading or building. Happiest doing both.
  • Daily stack: TypeScript, React, Next.js, Tailwind, Storybook and Playwright.

Selected work

  • A drawn diagram. The harness sits on top: component specs, interaction patterns and agent guidance. Below it, a designer describes an idea, an agent builds it on the harness, a person judges it, and out comes a tested prototype.A drawn diagram. The harness sits on top: component specs, interaction patterns and agent guidance. Below it, a designer describes an idea, an agent builds it on the harness, a person judges it, and out comes a tested prototype.
    01

    Reddit Proto

    An AI harness that became the company's de facto way to decide. 100% of UX and Product use it, and they've deployed 800 prototypes.

    Proves: It changed how decisions get made

  • The home page of this site: the line "I build the harness, not just the thing" beside a field of small pink and cyan lightning bolts.The home page of this site: the line "I build the harness, not just the thing" beside a field of small pink and cyan lightning bolts.
    02

    This site

    A portfolio built by one person directing Claude Code agents, with the plan, the rules and the gates left on show.

    Proves: The harness, applied to itself

  • Google Codesign's home screen: a Create button above a grid of saved prototype pages.
    03

    Google Codesign

    A no-code tool that let designers assemble working prototypes from real Google Cloud components, built with a 22-person UX team.

    Proves: The pattern predates the hype

  • Three phone screens: the comment composer with GIFs to pick from, a comment thread with photos inline, and the post composer showing a crochet dress.Three phone screens: the comment composer with GIFs to pick from, a comment thread with photos inline, and the post composer showing a crochet dress.
    04

    Reddit Contribution

    Images, video and GIFs in comments, and a posting flow rebuilt from building blocks. I led the design team and made the case for giving GIFs away.

    Proves: Why the harness had to exist

Earlier work

  • Two face-shaped pillows propped side by side on a pale green sofa: a woman with blue-streaked hair sticking her tongue out, and a grinning man in glasses.

    PillowMob

    2011–2014

    Co-founder. Design, front end, identity, photography.

Twenty-three years, one move.

  1. Agencies, 2003–201490 projects
  2. PillowMob, 2011–2014Co-founder
  3. Halo 4, 2012–2013UI developer
  4. Motorola, 2014–2015iOS
  5. Google Cloud, 2015–2021UX Lead
  6. Reddit, 2021–2024Contribution
  7. Reddit Proto, 2024–nowLead

Open to a good conversation

If you're building the tools agents work through, let's talk.

The best version of this work can't go on a public page. Email me and I'll show you the rest in person.

me@jblairmetcalf.com