Software Development Engineer in Test (SDET) in London

Retire the ERP. Light exists to replace factory-era ERPs with software that feels alive. Our Smart Financial Platform gives modern, global companies superpowers—automated accounting, real-time reporting, and financial flows that move at the speed of the business. We build with our customers, ship fast, and obsess over craft. In a short time, Light has gone from idea to the operating core for leading companies like Lovable, Legora, and Keyshot. People don’t just use Light—they enjoy it.

We’re an early team defining a new software category. Think engineers who love debits and credits, designers who care about reconciliation states, and operators who treat finance as a product. If you’re excited to modernise how the world runs money—one workflow at a time—you’re in the right place. Backed by world-class investors and advised by industry titans, we’re building category-defining products with the freedom to ship ambitiously and own outcomes. Come help us make Light the global default for next-gen finance.

About the role: At Light, we are building the Global Financial Platform. To achieve this ambition, we are scaling our engineering organisation rapidly. That growth only works if we get quality right. You will join the Foundation Team, the force multiplier for our entire engineering organisation. Our mandate is to "build the factory that builds the product." As an SDET, you will not just be writing tests for a feature team — you will be architecting the internal quality infrastructure that enables every engineer at Light to ship with confidence.

We are looking for someone who is obsessed with test engineering as a discipline but grounded in the reality of production systems. You will help us move beyond ad-hoc testing to build robust frameworks, define our internal standards for quality, and ensure our teams can ship multiple times per day, safely and with confidence.

What you will do:

• Build the Infrastructure: Design and implement test frameworks, fixtures, and data management patterns that make good testing the path of least resistance across multiple languages and frameworks. Immediate focus is TypeScript, Kotlin, Python, and Terraform.
• Solve Test Data: Build strategies for realistic, safe test data in a fintech context — synthetic data generation, anonymisation, and environment management that lets engineers test confidently without touching production data.
• Define the Standard: Create the "paved road" for product engineers — standardising how every team at Light writes, runs, and maintains tests. Own the testing section of our engineering playbook.
• Solve for Trust: Build quality gates in CI/CD that catch problems before production. Implement test observability so failures are obvious and actionable, not cryptic and ignored.
• Bridge the Gap: Write tests in high-risk areas where coverage is critical — not as permanent ownership, but to model good practice, stabilise production, and demonstrate what good looks like.
• Scale the Culture: Coach engineers across all teams on test design and help them adopt new practices. Your success is measured by how quickly quality ownership distributes to delivery teams.
• Test the Non-Deterministic: Build evaluation frameworks for AI features — prompt regression testing, golden datasets, and reliability metrics that let us deploy AI with the same confidence as traditional code.

What you will need:

• Test Engineering Background: You have significant experience building test infrastructure and tooling, not just writing tests. You understand the difference between unit, integration, and E2E testing — and when each matters.
• Polyglot Capabilities: You are comfortable working across multiple languages. Our stack includes Kotlin, TypeScript, and Python. Mastery of all three is not required, but resistance to learning is a dealbreaker.
• Platform Mindset: You get more satisfaction from unblocking 10 engineers with a great tool than from shipping a single feature yourself. You think in systems, not tasks.
• Focus on Reliability: You hate flaky tests. You understand that trust is the biggest bottleneck to test adoption, and you know how to build the observability and diagnostics necessary to generate that trust.
• Coaching Instinct: You can influence without authority. You will be changing how teams work, and that requires patience, clarity, and credibility earned through doing the work alongside them.
• AI Testing Experience: You have built evaluation frameworks for LLM-based systems. You understand the challenges of testing non-deterministic outputs — golden datasets, semantic similarity, human-in-the-loop evaluation, prompt regression. You know why traditional assertions don’t work and what to use instead.

Useful but not essential:

• Experience in high-growth startups where quality culture had to be built, not inherited.
• Familiarity with GitHub Actions or similar CI/CD platforms.
• Background in fintech or systems where data integrity is non-negotiable.
• Experience with contract testing, property-based testing, or complex integration scenarios.

Level: We are hiring at both Mid and Senior level for this role. Senior candidates will ship high-impact work, drive strategy, and coach across the organisation. Mid-level candidates will execute within that strategy and grow into broader ownership.

The good stuff: In addition to being part of a great team and working in a really fun and innovative environment, we offer:

• A competitive salary + potential stock options.
• 25 days of annual leave + public holidays in the country where you are based.
• Regular socials and company off-sites.
• A huge opportunity for career growth as you’ll help shape a market-defining product.

The famous last words: Working at Light means you’re embarking on an exciting journey to reshape the future of multinational financial operations. Our cheat-code is that we’ve built something like this before - and are now doing it again, only better. Our goal is to make global finance effortless and empower companies to make informed decisions. Join the rocket ship while it’s taking off.