Senior Software Engineer (kotlin/Rust)

numi; Greater London; Hybrid; GBP 110,000 - 160,000; Full time

Job summary

numi in London, UK is seeking a Senior Software Engineer to own core product capabilities and infrastructure from scratch in a high-autonomy, greenfield environment. You will drive architecture, set coding standards, and mentor peers, building distributed payment systems at scale.

The role emphasizes strong CS fundamentals, experience with typed languages, and collaboration with product and engineering leads to shape the technical roadmap. Hybrid work model in London.

Qualifications

• Solid experience as a Senior Software Engineer delivering production-grade systems.
• Strong CS fundamentals and system design capabilities.
• Experience with fintech/payments or cloud-native products is preferred.
• Proficient in strongly typed languages and building scalable services.

Responsibilities

• Greenfield delivery: write clean, production-ready code for high-throughput payment systems.
• Drive system design for new domain services focusing on scalability and low latency.
• Establish testing, CI/CD, observability, and code-review standards.

Skills

Senior engineer

Typed languages

Distributed systems

Cloud-native infra

CI/CD & testing

Tools

Go

Rust

Kotlin

Java

TypeScript

Job description

Client: Incubated fintech startup

Location: London, UK (Hybrid)

The Context

A major enterprise payments division is being carved out from a top-tier global banking group to establish an independent, modern technology business. Rather than maintaining legacy banking infrastructure or owning a tiny slice of a massive corporate estate, this project offers the rare chance to build a high-scale tech company from the ground up.

The engineering organisation is scaling rapidly from the foundation up, with the product-facing engineering org ultimately expanding to 100–200 engineers across ~12 teams, alongside a dedicated platform and infrastructure group of 20–30 engineers. With a explicit bias towards hiring experienced engineers in the first 3–6 months, early joiners will have an outsized influence on technical architecture, tooling, coding standards, and engineering culture.

The Role

As a Senior Software Engineer, you will be on the front lines of building core product capabilities and infrastructure from scratch. You will take ownership of complex technical problems, architect production-grade services, and help set the bar for code quality, testing, and system reliability across the business.

This is a high-autonomy, greenfield environment ideal for engineers who enjoy navigating ambiguity, designing distributed systems, and building rather than maintaining legacy systems.

What You’ll Be Doing

• Greenfield Delivery: Write clean, production-ready, testable code to build resilient, high-throughput payment systems from 0 to 1.
• System Design & Architecture: Drive technical design for new domain services, focusing on scalability, security, and low latency.
• Technical Standards: Establish best practices for testing, continuous integration/deployment (CI/CD), observability, and code reviews.
• Product & Domain Collaboration: Work closely with Product Managers and Engineering Leads to break down complex business problems into clear technical roadmaps.
• Cultural Influence: Help shape how teams ship software, evaluate new technologies, and make build-vs-buy decisions early in the organisation's lifecycle.
• Mentorship: Elevate the technical bar of the engineering team through peer code reviews, pair programming, and architectural guidance.

Core Stack:

• Frontend: TypeScript
• Platform / Tooling: Golang, Python

Tech Philosophy: You do not need deep, day‑one expertise in every single language in the stack. What matters most is strong computer science fundamentals, system design capabilities, and a pragmatic "right tool for the job" mindset. The hiring process (including a take-home technical exercise) evaluates engineering judgment, code quality, and problem‑solving over syntax memorization.

What We’re Looking For

• Proven Track Record: Solid experience as a Senior Software Engineer working on production-grade, high-scale software systems (preferably in fintech, payments, or cloud-native digital products).
• Core Engineering Strength: Strong fluency in strongly typed languages (e.g., Kotlin, Java, Rust, Go, C++) with an eagerness to work with Rust and Kotlin in production.
• Distributed Systems Fundamentals: Experience with event-driven architectures, API design, data consistency, high availability, and cloud-native infrastructure.
• 0-to-1 Mindset: Ability to thrive in an environment where foundations are still being laid—bringing structure without adding unnecessary friction.
• Quality Orientation: A strong belief in automated testing, CI/CD automation, and operational ownership.