Founding Research Engineer

Built Different; Greater London; Hybrid; GBP 117,000 - 143,000; Full time

Benefits offered by this job

Founding equity

Hybrid work

Job summary

Built Different is seeking top AI engineers to design RL environments that leverage real human behavioral data. The role offers a hybrid London/Paris/Amsterdam setup with some remote options and a base salary of £130,000 plus founding equity.

You will work at the frontier of AI labs, building end-to-end evaluation-to-training loops, and deliver significant gains in model learning signals with real-world impact.

Job description

Hybrid | Amsterdam / London / Paris | £130,000 + Founding Equity

The best AI engineers aren't waiting for a job ad. They're waiting for the right moment. This might be it.

Anthropic leadership has discussed spending over $1 billion on RL environments in the next year alone. The market is moving - fast and at scale. One pre-seed company just closed a round at a valuation most Series A companies would envy, with frontier AI labs already as paying clients and acquisition interest already on the table (x2).

The reason? They cracked something the rest of the market hasn't.

Most teams build RL environments from synthetic data - easy to demo, easy to commoditise, brittle. This team mines real human behavioural data - how domain experts actually reason, decide, and solve complex tasks over long-horizon workflows. 10-100+ step environments. Closed-loop systems where environments, data, training, and evaluation are tightly integrated. Not proxies. Not short cuts.

25-30% uplift in model task success rates. 50-65% more training signals. Evals that reflect how humans actually work.

The funding just landed. Second time founders. The founding team is being built right now. There are very few seats. Are you going to be in one of them?

£130,000 + founding equity at a ~$40M seed valuation

Frontier AI labs as paying clients from day one

Environment design treated as a first-class problem - not an after thought

London / Paris hybrid with some remote options

Get in now - before this role disappears and you're watching from the outside

You'll love this if…

• You've built the evaluation-to-training loop - environments, rollouts, reward signals feeding post-training - at a serious RL or AI company
• You've fine-tuned foundation models, and done it on smaller open-weight models rather than just throwing scale at the problem
• You think turning real human expertise into training signal is one of the most underrated problems in AI right now
• Your RL is the genuinely hard kind - non-trivial reward design, long-horizon, multi-agent - not moving data from A to B
• You move fast, think in first principles, and thrive without a playbook
• You want to work at the forefront of innovation with frontier labs already paying to use what you build

Roles like this don't stay open.

Because great teams are Built Different.