Principal Endpoint Agent Engineer in London

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. As an endpoint engineer, you’ll help build and evolve the low-level, security-critical client software that powers Harmonic’s platform. Our software must be fast, resilient, and trustworthy across operating systems and real-world conditions. You’ll have the opportunity to shape foundational agent architecture, influence how we collect and process signals on-device, and set the technical bar for correctness and safety as we define a new category of AI security.

How We Work: AI-First by Design

Harmonic exists to help enterprises adopt AI safely and at scale. We hold ourselves to the same standard. Everyone at Harmonic actively uses AI tools to do their best work, from research and writing to building processes and automating workflows. We expect every new hire to bring curiosity about AI and a willingness to use it to work smarter, faster, and more creatively. For some roles, that means tinkering and staying open to new tools. For others, it means building entirely new systems with AI at the core. We'll be clear about what we expect for each role, and we'll give you the tools and support to get there.

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 software 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.

What you Bring

• Strong hands-on coding 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 maintainability.
• Learn quickly and independently, maintaining great attention to detail and high standards under pressure.
• Have made contributions to open-source Rust (or C++) projects or RFCs.
• Believe that good tooling and documentation are essential parts of engineering, not afterthoughts.
• Thrive in fast-paced startup environments where ambiguity is the norm.
• See AI as a fundamental tool to help you build smarter, faster, and better.
• Actively use AI tools (Claude Code, Cursor, or similar) to accelerate your work across the entire lifecycle—from writing and debugging to researching and prototyping.
• Stay current on how AI is changing the systems engineering domain and have opinions about where it helps and where it doesn't.
• Refuse to default to the playbook you’ve been running for the last decade, choosing instead to automate the mundane and rethink legacy workflows to focus on high-expertise problems.

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 embrace new, unfamiliar situations that require initiative and rapid decision-making. We orient ourselves quickly and deliver results with minimal guidance.
• Never Full: We raise our hands, take on challenges, and assist others whenever possible. We hunger for opportunities to learn and do more.
• Perfect Harmony: We support one another to create cohesion and unity. We collaborate openly, share feedback honestly, and help everyone produce their best work.