Underpin

Underpin is an AI coding agent for legacy migrations, like Java 8 to 21 or Spring Boot 2 to 3.

From $2,500 · AI developer tools, Series A stage · sold onceBuy Underpin

Who Underpin is for

AI code-migration startups all promise speed and control. Not one shows a file change, pass its tests and merge on the first screen.

Underpin is an AI coding agent for legacy migrations, like Java 8 to 21 or Spring Boot 2 to 3. It renews a codebase file by file as reviewed pull requests with the tests passing, and nothing is switched off. Its buyer is a CTO or VP Engineering sitting on about a million lines of old code.

The numbers behind it

  • 74%of large enterprises started a legacy modernisation project and failed to complete it. Advanced, 2020
  • 10-20%of the tech budget meant for new products goes to fixing tech debt, CIOs say. McKinsey, 2020
  • 23%of organisations still ran Java 8 in production in 2025. Azul State of Java, 2025

How the brand answers

The product draws the repository as a leaded window, one pane per file. Each screen shows one file renewed, checked and merged while the rest of the window, and the product, stays up.

  • The repository as a window of panes: one file opened, checked and merged

    The codebase as a window, renewed pane by pane

    Another big-bang cutover weekend we can't roll back.

    3,410 files are 3,410 panes. Merged files glow marigold, the one in review stands proud in ruby, the rest wait in milky grey. The header keeps the score: 2,286 of 3,410 merged, 67% on Java 21. There's never a cutover, just the next pane.

  • A pull request ready to merge, with tests, build, replayed requests and API all passing

    Four checks before anything merges

    AI-written code will break production on a Friday.

    Each pull request lists its proof: 214 tests pass, 6 of them new; it builds on JDK 21; 1,912 recorded production requests replayed with 0 differences; the public API unchanged. Merge stays greyed out until all four are green.

  • Approving a pull request on a phone: ready for review, merging, merged

    One small PR, reviewable on a phone

    We'll drown in thousands of unreviewable AI pull requests.

    Every pull request is small: one class and its neighbours, like OrderLedger.java with its test, with a one-line summary of what changed and why. It's small enough to approve from a phone, and when it merges its pane in the window turns from ruby to marigold.

What's in the brand

Colours
  • Limestone#E4DED3
  • Lead#2C2B2A
  • Milky glass#B9BDB8
  • Ruby#C8102E
  • Marigold#F4A300
Type
Funnel Display for headlines, Azeret Mono for file names, counts and checks.
Logo
Hand-lettered and traced as drawn, standing on stacked footing lines, the device that underpins it.
Name check
No Underpin in developer tools. An unrelated Underpin works in AI and blockchain in Korea. A web search, not a trademark clearance.
  • Underpin key art: a leaded-glass window being renewed one pane at a time
  • Poster: Java 8 to 21, nothing came down
  • Poster: upgrades your old codebase file by file while the product stays live
  • Poster: a million lines renewed, still standing
  • An engraving of the window's grid
  • A crate of new glass panes beside the stack of old ones

The full case on Alterra Studio

Buy Underpin

Sold once. Checkout holds Underpin for you for 30 minutes while you pay on Stripe.

  • Brand kit

    The Figma file and every asset: logo, colours, type, key art, mockups and the motion board.

    $2,500

Questions about Underpin

What do I get when I buy the Underpin brand? The Figma file and every asset: the Underpin logo and wordmark, the colour and type system, key art, product screens, mockups and the motion board. The brand is sold once, so it comes off Brandbooked when you buy it.

Can I rename Underpin or change its colours? Yes, $300 each for a new name, new colours or new type. We redo the logo and mockups to match.

Does Underpin come with a domain or trademark? No domain is included. No Underpin in developer tools. An unrelated Underpin works in AI and blockchain in Korea. That is a web search, not a trademark clearance.