Principal Systems Engineer 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. As AI adoption accelerates inside the enterprise, Harmonic’s ability to safely observe, control, and enforce policy at the endpoint is mission-critical. This role sits at the core of that challenge.

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 hands-on systems engineer with deep expertise in Rust (or C++ with a strong willingness to learn and embrace Rust as your primary language) to help us build and evolve security critical client software. You will work on low level security agents and cross platform components that run on user devices, with a strong emphasis on robustness, performance and security. Your work will power core functionality in how we collect signals, and manage on-device controls for app usage and data movement. You will join a fast-moving team that ships frequently and cares about correctness, observability and maintainability.

What You’ll Do

• Design, implement, and optimise endpoint security agent components in Rust, targeting desktop and/or server platforms (Windows, macOS, Linux).
• Build robust tests, validation tooling and deployment mechanisms for client facing releases that impact our customers in their everyday work.
• Conduct exploratory technical research to understand the art of the possible and how to best approach implementation of new product features.
• Investigate and resolve platform specific issues with precision and speed.
• Work closely with security engineers, backend engineers, and product stakeholders to shape technical requirements.

What Success Looks Like

• You’ve shipped production Rust or C++ code that runs safely and efficiently across platforms.
• Your components are well tested, observable, and resilient to partial failures or bad input.
• You identify performance or architectural bottlenecks before they reach users.
• You actively reduce complexity in critical code paths without compromising clarity or functionality.
• You consistently deliver high-quality changes on a tight feedback loop, including regression-safe updates.

Requirements

What you Bring

• Strong hands-on experience with Rust (or C++ with a strong willingness to learn Rust) in production systems.
• Solid grasp of systems programming concepts: memory safety, lifetimes, threads, channels, error handling, IPC.
• Familiarity with platform APIs and subsystems on Linux, Windows and macOS.
• Experience building CI-tested, reproducible builds for native applications.
• Proficiency with common developer tooling: Git, code review, CI pipelines, structured logging, crash analytics.
• Networking skills and security fundamentals is a plus.

You Might Be a Fit if You…

• Have a strong development background and a rigorous approach to testing and validation.
• Relish the prospect of writing code that will run on millions of devices and needs to be reliable and performant around the clock.
• Enjoy working close to the OS and care deeply about software performance, correctness and safety.
• Previous Kernel or system-level development experience is a great nice-to-have.
• Can build and ship reliable systems fast, with a sharp eye for edge cases and long term impact and maintainability.
• Learn quickly and independently, with great attention to detail and high standards.
• Have made contributions to open-source Rust (or C++) projects or RFCs.
• Believe that good tooling and documentation are part of engineering, not afterthoughts.
• Thrive in fast-paced startup environments where ambiguity is the norm.
• See AI as a tool to help you build smarter, faster and better.

Benefits

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 offsites (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.