Senior Backend Engineer - Early-stage Start-up - London

Senior Backend Engineer | Early-Stage AI FinTech | London | Up to £80k + Equity | C#/Python

OB have recently partnered with an early-stage AI fintech start-up that’s building from the ground up, and they’re looking for a Senior Backend Engineer to join the early team and have genuine ownership over the product and platform.

This is a high-agency role for someone who wants to build, ship and own rather than inherit an established codebase.

What you’ll be doing

• Design, build and own core backend services, APIs and infrastructure
• Build event-driven, multi-tenant systems from the ground up
• Ship LLM and agentic workflows into production
• Own data models, failure modes, auditability and system performance
• Build API-first products and infrastructure
• Work directly with the founders and have a major influence on technical direction
• Be comfortable jumping into React / Frontend when a feature needs it

What they’re looking for

• 3–4+ years building, shipping and owning backend systems
• Strong experience with C# and/or Python
• Experience working with AI tooling / LLMs / agentic systems
• Fintech experience is essential, ideally across payments, banking, open finance, ledgers or adjacent financial infrastructure
• Comfortable working across the stack when needed, React experience is a bonus
• Someone who enjoys ambiguity, moves quickly and is comfortable taking ownership

The opportunity

You’ll be joining at a genuinely early stage, meaning you’ll have high agency, direct founder exposure and significant influence over what gets built and how.

You’ll be building core infrastructure for an AI-native fintech from the ground up.

Early team | High ownership | Fast-paced | AI + FinTech

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