Software Development Engineer in Test

AI is being run everywhere, by humans and agents. Harmonic Security governs all of it. The AI Governance and Control (AIGC) platform understands user / agent intent and data context in real time, giving security teams visibility and control across every tool and workflow so companies can move faster with AI while minimizing risk.
We operate in one of the fastest-moving spaces in tech: the emerging intersection of AI and cybersecurity. It's a market still being defined, and we're among the first to build natively for it - protecting sensitive data in real time, at the endpoint, with minimal friction for the teams using it. Enterprises get full visibility and control. Their people get to innovate without guardrails slowing them down.
Named to the Rising in Cyber 2026 list, Harmonic is gaining serious recognition and serious traction. We're led by deep cybersecurity expertise and backed by investors who know the space: N47, Ten Eleven Ventures, and In-Q-Tel.
As AI adoption inside the enterprise accelerates, the ability to safely observe, control, and enforce policy in real time isn't a nice-to-have. It's mission-critical infrastructure. That's what we build.

How We Work: AI-First by Design
Harmonic exists to help enterprises adopt AI safely and at scale. We hold ourselves to that same standard. Everyone here actively leverages AI tools to perform their best work, from deep research and writing to building robust processes and automating complex workflows. We expect every new hire to bring a genuine curiosity for AI and a commitment to using it to work smarter, faster, and with greater creativity.
For some, this involves tinkering and remaining open to emerging tools; for others, it means architecting entirely new systems with AI at the core. We will be transparent about expectations for every role and provide the tools and support needed for you to thrive.

About the Team
Our Product Delivery team is the engine that turns vision into impact. We ship early and often, getting valuable features into the hands of customers quickly and iterating from there. We work in the open by default, sharing progress and ideas, and we trust each other to own outcomes. We’re a small but mighty crew where every person plays a critical role and we’re committed to using AI to work smarter and faster.

About the Role
We're looking for a Software Development Engineer in Test (SDET) to build and maintain automated testing infrastructure across our products. You'll design test frameworks, set up cross-platform test environments, hunt down bugs, integrate tests into CI/CD pipelines, and work closely with engineers to keep our quality bar high. This is a hands-on, adaptable role where you'll need to apply yourself wherever testing is needed most.
Our product protects enterprises as they adopt AI tools. We build everything from customer-facing portals and browser extensions that monitor AI interactions in real-time, to endpoint agents and MCP gateways that enforce security policies. Rigorous, scalable test coverage across this diverse stack is critical to our success. AI-tool fluency is expected. The team uses AI daily, and the product itself secures enterprise AI adoption, so familiarity cuts both ways.

What You'll Do

• Design, implement, and maintain test frameworks for our UI, browser extensions, endpoint agent, or backend services - you'll need to apply yourself wherever QA is needed

• Build and run cross-platform tests on Windows, macOS, and Linux - set up test environments to validate our endpoint agent doesn't break user systems

• Expand, run, and investigate load tests to validate system behaviour at scale and ensure we can handle growth

• Investigate bugs, write failing tests, get them fixed - file detailed tickets, write tests that expose the issue, fix it yourself if you can, or partner with engineers to verify the fix

• Integrate tests into CI/CD - add tests to GitLab CI pipelines, set up quality gates and detailed test reports, keep tests fast and reliable

• Monitor, triage, and improve - file detailed bugs in Jira, escalate critical issues quickly, track test coverage, shape the QA roadmap

What You Bring

• 5+ years as an SDET building and maintaining automated test infrastructure

• Strong programming skills in at least one of TypeScript, Java, or Rust

• Experience with modern test frameworks and willingness to learn new ones (the stack includes Jest, Playwright, Bun test, Vitest, JUnit5, Selenium, and Rust's native testing via cargo test)

• Cross-platform testing experience, comfortable setting up environments, debugging platform-specific issues, and working with VMs or real machines

• CI/CD pipeline knowledge, integrating tests into GitLab CI or similar, setting up quality gates, and making reports useful

• Demonstrable experience using AI tools to speed up workflows and boost productivity

• Solid debugging skills and proficiency with Git, code review, and CI/CD systems

You Might Be a Fit if You

• Take proactive ownership of quality, seeking out risks before they reach production

• Bring a thorough, creative approach to complex and often custom testing challenges

• Write test code with the same rigour, readability, and discipline as production code

• Balance broad coverage with observability, reliability, and fast, meaningful feedback

• Thrive in a fast-paced startup environment where ambiguity and continuous improvement are the norm…

Source: Arbeitnow (https://www.arbeitnow.co.uk/jobs/companies/harmonic-security-inc/software-development-engineer-in-test-london-325382)