About StackOne: StackOne is the AI Integration Gateway for SaaS products and AI Agents. Backed by GV and Workday Ventures ($24M raised), we help builders of SaaS platforms and AI Agents orchestrate hundreds of scalable, accurate, and enterprise-grade integrations. Our platform combines 25,000 pre-mapped actions on 200 connectors, an AI-powered integration development toolkit, plus security by design: a real-time architecture, managed authentication and permissions, and end-to-end observability. Join us on our fast trajectory to build the future of agentic integrations. 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 We’re looking for full-stack engineers who can ship fast, think in systems, and obsess over scalability, performance & usability. You’ll build core APIs, scale infra, and ship dev tools & UIs that makes tool execution easy, safe and predictable. It’s a broad role: backend-heavy, product-aware, and deeply technical. Expect to work across APIs, auth, infra, and frontend, building the foundation that lets agents take real action. Responsibilities Act as a full-stack web application engineering: ideate, build, and ship features across frontend, backend, data, and infrastructure. Model data & schemas across SQL, document/columnar stores, and OLAP ; design for correctness, performance, and cost. Design and evolve public APIs and standards: REST contracts, OpenAPI, pagination, auth, error models, plus MCP/tool-calling conventions. Engineer for scale and resilience : rate limiting, retries, idempotency, back-pressure, caching, SLIs/SLOs, and safe fallbacks. Build and operate event systems with Kafka-class streaming patterns: producers/consumers, partitioning, DLQs, reprocessing, schema evolution. Raise observability & reliability : tracing, metrics, logs, alerting, zero-downtime deploys, runbooks. Leverage LLMs/agents to accelerate development while maintaining reliability, testing, and code quality. What we're looking for Proven experience as a full-stack engineer shipping user-facing, backend-heavy systems. Expertise in TypeScript and solid experience with React ; comfortable building clean UIs for complex backend capabilities. Strong background designing and operating public APIs and multi-tenant systems. Experience with distributed systems concepts: streams, queues, idempotency, consistency, and performance trade-offs. Ability to think in systems (throughput, tail latency, failure modes) and explain trade-offs clearly. Curiosity and pragmatism with LLMs/agents : know when they help, and how to guardrail/evaluate them. Desire to work in a fast-paced startup, take ownership end-to-end, and bias toward shipping. (Preferred) Experience publishing CLIs & SDKs (internal or open-source). Our Stack Y ou will have a lot of freedom and agency to chose and work with the technologies you love. Our tech stack includes: Infra & monitoring: AWS (ECS, RDS, Elasticache, Lambda etc.), Cloudflare, Tinybird, Modal, Dagster, Langsmith, Datadog, Sentry, Incident.io Languages & Frameworks : Typescript (Node.js), Python, NestJS, React, Svelte, Other day-to-day Tools: Postman, Notion, Slack, Jira, Github and more Benefits Meaningful share options (EMI) - share in the company's success as we grow 25 days holiday 1 additional day per year of tenure Private health insurance - including dental & optical £15/day lunch budget when working from our London office, up to £120/month £1,000 for your home office set up £500/year top-up 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 Health, fitness and gift card discounts Cycle2Work and Electric Cars scheme Hybrid work set up - typically 2 days in the office Ready to help us change the game for SaaS integrations? 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.