I don't just build products. I find the growth engine and build on top of it.
I'm Robert Costart — I work the loop: spot a real growth opportunity, then build a product that compounds it into the next heartbeat of the company. A million users at Trainline, Vrbo's migration at Expedia, Soldo's LTV model to ~95% accuracy. Now with AI through the whole stack.
Not just building the product. Building the team that ships it.
I run each product like a small studio of AI agents — they source the work, draft and build it, review each other, and hand off what's done. It's how one operator keeps a whole fleet moving, with the judgement calls staying human.
Scan the web for new leads worth writing to
Turn a lecture recording into a study pack
Draft & qualify 40 first-touch contacts
Ship the quote-flow API endpoint
Regression pass across the checkout flow
First-touch outreach drafted & raised
Launch assets exported & handed off
Weekly summary & scale digest sent
A fleet of products, each shipped end to end.
Legal, education, home services, voice — built and grown by one operator with a small trusted bench, and AI through the whole stack.
Also built & earlier
A Chrome extension that reads UK supermarket product pages and flags markers of ultra-processed food — E-numbers, emulsifiers, sweeteners — as you browse. Runs entirely in-browser and collects nothing.
A price-comparison and deal-alert service — co-founded and run for three years. The first startup chapter before Costart Projects.
I ship the thing, and I work out why it grows.
I've spent most of my career being the person who both ships the thing and works out why it's growing. The thread through all of it is a bias for the “how” over the “what”: get close to the work, model it honestly, and let the numbers decide. Today I run Costart Projects as a solo founder, building products with a small trusted bench of designers and developers — and doing it at a scale that used to need a team. This site is both halves of that: the products I'm building, and the operator behind them.
Built the European booking site from a proof of concept into a platform serving a million users.
Led the Vrbo/HomeAway platform migration, turning a projected loss into measurable gains.
Replaced a cost-per-acquisition model with a lifetime-value one, forecasting from 60 days of activity to ~95% accuracy.
What used to need a team, run by one operator.
AI woven through the whole stack — research, build, content, growth — isn't a feature I bolt on. It's how a single founder now operates at team scale, and the same edge I bring to growth teams.
Build end to end
Product, engineering and design in one head — from first commit to live, without hand-off friction.
Own the growth maths
Model acquisition and retention honestly, forecast lifetime value, and let the numbers pick the roadmap.
AI through the stack
Agents and automation compress research, content and ops from days into hours — across a whole fleet.
Building something, or scaling a growth team?
I take on a small number of build and advisory engagements. If AI at the core of your product and growth is where you're headed, let's talk.