Senior SDET in London

About Harmonic Security

Harmonic Security lets teams adopt AI tools safely by protecting sensitive data in real time with minimal effort. It gives enterprises full control and stops leaks so that their teams can innovate confidently. We are led by cybersecurity experts and backed by top investors including N47, Ten Eleven Ventures, and In-Q-Tel. We’ve achieved early traction and strong product-market fit with a world-class team, and we’re now focused on scaling quality, reliability, and confidence as our product footprint expands. This is an opportunity to join early, build the automated testing foundations that keep a complex, cross-platform security product dependable, and help define the quality bar for a new category of AI security.

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 security agents and (Model Context Protocol) MCP gateways that enforce security policies. Rigorous, scalable test coverage across this diverse stack is critical to our success.

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 - 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, elevate critical issues quickly, track test coverage, shape the QA roadmap.

What Success Looks Like

• You’ve improved test infrastructure: You’ve enhanced and maintained test frameworks wherever needed - UI, browser extensions, endpoint agents, or backend services.
• Cross‐platform confidence: Our products ship to Windows, macOS, and Linux with confidence.
• Load testing validates scale: The team knows the system can handle growth before deploying to more users.
• Bugs get caught and fixed fast: Your failing tests expose issues quickly, and you work with engineers to get them resolved.
• CI/CD is reliable: Tests are integrated smoothly, reports are clear, pipelines stay green, and engineers know why things break.
• Coverage is tracked and improving: You monitor what’s tested and what’s not, continuously filling gaps.

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 - at Harmonic we use Jest, Playwright, Bun test, Vitest, JUnit5, Selenium, and Rust's native testing framework (cargo test).
• Cross‐platform testing experience on Windows, macOS, and Linux - comfortable setting up test environments, debugging platform‐specific issues, and working with VMs or real machines.
• CI/CD pipeline knowledge - you know how to integrate tests into GitLab CI (or similar), set up quality gates, and make test reports useful.
• Solid debugging skills - comfortable investigating bugs, reproducing issues, and working with engineers to get them fixed.
• Proficiency with standard tooling - Git, code review, and CI/CD systems.

Desirable

• Experience with Jira, Notion, GitLab, OpenSearch, Swagger, Grafana, AWS, Figma, or Sentry (or similar tools).
• Comfortable using AI tools to speed up workflows and boost productivity.
• You don’t need prior experience with every tool or framework listed - we value adaptability and the ability to learn quickly.

You Might Be a Fit if You...

• Have a proactive approach to work - you seek out problems before they reach production and value continuous improvement.
• Are a thorough and creative thinker who understands how to build complex, often custom, automated tests.
• Write test code with the same rigour, readability, and discipline as production code.
• Think like both a developer and a tester: you care about behaviour, performance, edge cases, and long‐term maintenance.
• Are comfortable working across platforms - whether that means setting up VMs, debugging on unfamiliar operating systems, or figuring out how to test platform‐specific behaviour.
• Collaborate well with engineers - you help troubleshoot test failures, partner on bug fixes, and make testing easier for the whole team.
• Value observability and test reliability as much as test breadth - fast meaningful feedback is your gold standard.
• Thrive in fast‐paced startup environments where ambiguity is the norm.
• See AI as a tool to help you build smarter, faster and better.

Why Join Us

This isn’t just a job; it’s an opportunity to be part of a team that is redefining cybersecurity. We believe today’s talent is tomorrow’s success, and we’re committed to creating an environment where you can do the best work of your life.

• Competitive pay and meaningful equity with a direct stake in Harmonic’s success.
• Comprehensive benefits, pension plan, generous PTO, and flexible hybrid work.
• A small, passionate team that values transparency, creativity, and learning.
• Thoughtful leadership that cares deeply about growth, impact, and people.
• Annual global off‐sites (past trips include Lisbon and Nashville).
• The chance to directly shape both our product and our culture as we build a category‐defining company.

Harmonic’s Core Values

• Flourish in the Unknown: We relish being thrown into new, unfamiliar situations that require initiative and rapid decision‐making. We orient ourselves quickly and deliver results with minimal guidance.
• Never Full: We never hesitate to raise our hands and take on challenges to assist those in need. We hunger for opportunities to learn and do more.
• Perfect Harmony: We have a genuine willingness to assist and support one another to create cohesion and unity. We foster success through collaboration and honest sharing of feedback and ideas, enabling everyone to grow and produce their best work.