Founding AI Engineer (Enterprise Finance Operations)
What you need
- 2-3 yrs software engineering, shipped products
- Hands-on LLM, agent, tool use experience
- Full-stack ownership through production
- Quick learner, enterprise finance curiosity
What you'll do
- Build end-to-end AI agents for finance workflows
- Own large product areas and new initiatives
- Develop agent infrastructure, evaluations, feedback loops
- Ship to production immediately
- Collaborate with finance experts on agent oversight
AI Transformation for Enterprise Finance
Rex builds AI agents for enterprise finance, starting with order to cash. Our customers delegate a process to us, and we run it with agents while finance experts oversee the work. Our agents operate across email, ERPs and customer portals to resolve payment blockers and help businesses get paid.
We’re building agents that can take responsibility for complex financial workflows: reason over messy data, handle exceptions and improve through feedback from finance experts. The challenge is earning the trust to act autonomously in operations that affect real money.
Who we are
Rex was founded in 2026 by Merlin Kafka (CEO) and Lewis Blackwood (CTO).
Merlin previously served as CPO at Sequence (a16z) and PM at Tessian (Sequoia), and studied industrial engineering at Cambridge. Lewis previously led engineering at Sequence, joined GoCardless early and founded a startup backed by GFC. He studied economics at Cambridge.
Our team brings years of experience building AI and finance products and working with hundreds of finance teams. We know this domain inside-out.
Rex is global from day one and operates across San Francisco and London.
What you’ll do
This is a generalist role and you will be immediately responsible for large product areas and new initiatives. You’ll build products end to end, from understanding a customer’s finance workflow to shipping agents that carry it out. Your work will span the agents, the financial context they reason over, and the tools finance teams use to oversee them. You’ll also build the infrastructure, evaluations and feedback loops that help agents improve from real work. You will ship to production on your first day.
You’ll fit if you
- Have around 2-3 years of software engineering experience and have shipped products people use.
- Have hands-on experience building with LLMs, agents and tool use.
- Can work across the stack and own a problem through to production.
- Learn quickly and want to understand how enterprise finance actually works.
This role is full-time in-person in London.
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About Rex
Rex is an AI-native service for order to cash. We replace people-heavy operations with AI agents that continuously manage invoices, customers and exceptions. Order to cash is a $60B outsourced function because the work is too messy for traditional automation. Every customer has different context, exceptions do not follow clean rules, and the next step is often buried in emails, portals, handoffs, or someone’s memory. Rex agents run continuously in the background and resolve work to SLA standards so cash keeps moving. We're live with pre-IPO tech companies, managing >$500m in receivables from F100 companies, cutting manual follow-up cycles from weeks to hours. We founded Rex after spending four years building Sequence, an a16z-backed finance ops platform, from zero to Series A — where we saw firsthand that at-scale, O2C still runs on manual labour. Rex is the #1 winner of the Vercel AI Accelerator out of 2,500 teams, selected by Guillermo Rauch and leaders from OpenAI, Anthropic, Browserbase and more.
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