About StackOne

StackOne is the AI Integration Gateway powering the next generation of SaaS and AI Agents. Backed by GV, Workday Ventures, and angels/advisors from DeepMind, OpenAI, GitHub & Mistral, we’ve raised $24M to enable developers to orchestrate thousands of secure, scalable, and accurate actions in AI Agents.

With an AI-native integration toolkit that delivers real-time execution, managed authentication, granular permissions, and full observability, all built with safety at its core, we’re now doubling down on AI R&D & creating our own lab to push the boundaries of tool calling for agents: training specialized LLMs designed to outperform general-purpose models in what matters most: precision, reliability, and safety in agentic execution.

About the role

You'll help build a world where users of any agents can integrate with the tool of their choice in one click thanks to StackOne.

We’re looking for an AI Research Engineer to secure the next generation of tool-calling for LLMs. You’ll research and prototype defences that limit the attack surface of emerging standards such as MCP servers, and make tool execution safer against prompt injection and malicious calls.

You’ll work with our researchers on state-of-the-art tool-calling models, and with our engineers on the execution engine that runs them.

You will own , design and run experiments on cutting-edge architectures, manage distributed training clusters, and help curate & generate high-quality datasets. This role sits closer to the research/ML infra side than product engineering, but with a strong mandate for applied, production-ready results.

In this role, you will work with wider AI team of StackOne (comprising of other researchers and engineers) and report directly to the CTO.

Responsibilities

• Investigate vulnerabilities in tool-calling protocols (MCP, OpenAPI, custom plug-ins).

• Design and test mechanisms to restrict malicious or unsafe tool calls.

• Collaborate on the architecture of a secure tool execution engine.

• Partner with the core research team to align safety work with model quality.

• Design and manage synthetic data generation workflows to augment real-world datasets.

• Develop tools for dataset curation, labeling, filtering, and augmentation.

• Conduct benchmarking, and evaluations to measure fine-tuning impact.

• Collaborate with the rest of the engineering team to integrate fine-tuned models into production stacks.

• Stay ahead of research in parameter-efficient fine-tuning, synthetic data, and LLM training.

What we're looking for

• Knowledge of prompt injection, sandboxing, and least-privilege patterns.

• Experience building or auditing APIs, plug-ins, or protocol runtimes.

• Ability to design and evaluate defences for real-time agent systems.

• Background in Gen AI, with experience with Tool Calling

• Interest in security engineering, or applied cryptography.

• Desire to work in a fast-paced startup, taking ownership of projects e2e and bias towards shipping.

Benefits

• 25 days holiday + 1 additional day holiday per year of tenure

• Participation in the company’s employee share options plan

• Private health insurance (including dental & optical)

• Health, fitness and gift card discounts

• £1,000 for your home office set up + £500/year top-up

• Paid lunch in the office

• Annual team offsite to sunny spots (last ones were in Spain and Portugal ☀️)

• Join one of Europe’s fastest-growing startups

• Work with a veteran team of ex-employees of Google, Microsoft, Oracle, Coinbase, JP Morgan and more

• Cycle2Work and Electric Cars scheme

• Hybrid work set up - typically 2d in the office

Ready to help us change the game for AI Agents? Get in touch and let's chat!

We believe diversity drives innovation. We encourage individuals from all backgrounds to apply. As an equal-opportunity employer, we celebrate diversity and are committed to creating an inclusive environment for all employees.