Platform Site Reliability Engineer

About Us The last era of AI scaled on a single bet: bigger models, more identical chips, more data. As problems grow more complex and the requirements of intelligence more diverse, that bet is breaking down. Real-world problems are heterogeneous: no single model or chip can solve them alone. The next era of AI requires heterogeneity at the infrastructure level - diverse models on diverse chips, each with distinct strengths, co-evolving into systems of capability that move the Pareto frontier of what is possible. That's what we are building. Callosum is the Intelligent Systems Company. We started from questioning what actually creates intelligence. We believe there is no single answer, but rather a system-level solution. We co-evolve models, workflows, and silicon together to show that intelligence does not come from a single component, but it emerges from the diversity of co-optimised mechanisms working together and aware of each other. Heterogeneity will define the next era of compute, and is a principle that holds in biological, neuronal, and economic systems alike. In early 2026 we launched with results showing orders of magnitude improvements in performance, and this is only the beginning. Agentic AI is the future of how intelligence is deployed: multi-step, long-horizon, and operating in changing environments. These systems are inherently heterogeneous, and can only be as powerful as the infrastructure that runs them. We are engineers and scientists based in London, working together across the full depth of the stack. We are curious, intellectually honest, and building what doesn't exist yet. If you thrive on uncharted territory and are energised by the scale of the challenge, we'd love to hear from you. About The Role Our platform is the production system our customers route real traffic through. When it degrades, their product degrades. You will own its operational health end-to-end: SLOs, observability, incident response, deployment discipline, and capacity planning across heterogeneous compute backends. As the platform scales to millions of concurrent requests across heterogeneous compute, the work shifts from building the operational foundation to defending it under conditions most teams never encounter. You'll define "production-grade" means for a platform at the centre of a fast-growing company, and own it end to end. The reliability practice is yours to build. You will work closely with the platform team, setting the technical direction. You will work closely with the hardware and orchestration teams to expose heterogeneous backends reliably through the platform. Who You'll Build Service-level objectives, monitoring, alerting, and observabilityOn-call and incident response: runbooks, escalation, blameless postmortems, and follow-throughCapacity planning and the operational side of running across heterogeneous compute backends What You Bring Strong SRE or production-engineering background running customer-facing systems at scaleFluency with modern operational tooling: observability stacks, container orchestration, infrastructure-as-code, CI/CDExperience owning incident response and driving reliability improvements.Compliance execution experience What Sets You Apart Open-source contributions to relevant infrastructure, or production systems whose scale and complexity you can speak to in detailEarly-stage and AI-native company experience What We Offer Competitive Salary, determined by skills and experienceEquity & OwnershipPrivate healthcareWe offer Visa sponsorship and relocation benefits to hire the best in the worldWe work in person at our London office. You'll have the tools, space and setup to do your best work, and if you have specific needs, just tell us We're committed to building an inclusive workplace where everyone feels welcome, and believe in equal opportunities for all. Compensation Range: £101K - £192K