Senior Software Engineer (kotlin/Rust)

numi; Greater London; Hybrid; GBP 90,000 - 130,000; Full time

Job summary

numi is seeking a Senior Software Engineer in London to build core product capabilities and infrastructure from scratch. You will own complex technical problems, architect production-grade services, and set the bar for code quality, testing, and reliability in a high-growth fintech environment.

This greenfield role offers high autonomy, with a focus on scalable distributed systems, strong CS fundamentals, and pragmatic use of the right tech stack (TypeScript, Go, Python).

Qualifications

• Solid experience as a Senior Software Engineer on production-grade, high-scale systems (fintech/payments or cloud-native products).
• Strong fluency in typed languages (e.g., Kotlin, Java, Rust, Go, C++) with production exposure to Rust/Kotlin.
• Experience with distributed systems, event-driven architectures, API design, data consistency, and cloud-native infra.
• 0-to-1 mindset; able to ship features in a greenfield environment.
• Commitment to automated testing, CI/CD automation, and operational ownership.

Responsibilities

• Greenfield Delivery: write production-ready, testable code to build high-throughput payment systems from 0 to 1.
• System Design & Architecture: drive design for new domain services focusing on scalability, security, and low latency.
• Technical Standards: establish testing, CI/CD, observability, and code review best practices.
• Product & Domain Collaboration: work with PMs and Eng Leads to break down complex problems into tech roadmaps.
• Cultural Influence: shape how teams ship software and evaluate new technologies early in lifecycle.
• Mentorship: elevate engineering standards through peer code reviews and architecture guidance.

Skills

Senior Software Engineer

Distributed Systems

CS Fundamentals

Typed Languages

Automated Testing

CI/CD

Architectural Leadership

Tools

TypeScript

Golang

Python

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.