Platform Engineer - Member of Technical Staff in London

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

You'll build the platform our customers integrate against, working to the architecture the Lead API Platform Architect sets and alongside the reliability engineer. Your first major project is the metering and billing layer the commercial model runs on. From there, you'll build out the core services that turn the platform's design into something that runs in production. This is a build-heavy, hands-on role. The architect decides how the platform is shaped. You build a large part of it.

What You'll Build

• The metering and billing layer. Usage capture, attribution per workflow, and the systems behind consumption-based and outcome-based pricing.
• Workflow deployment onto the platform. The machinery that registers a workflow, routes it, and gives it the scaling and isolation it needs (some workflows run in their own dedicated container), built to the architect's contract and consuming the execution endpoints each workflow resolves to.
• Core platform services - the request path, rate limiting, quota enforcement, and the rest of the machinery beneath the API.
• Client SDKs and libraries. The packages customers use to integrate, in the main languages they work in.
• Internal platform tooling. The deployment, admin, and operational glue that lets a small team run the platform, shared with the reliability engineer.
• On-call as the platform scales.

What You Bring

• Strong backend or production-engineering background building and operating services at scale.
• Strong distributed-systems depth: API design, concurrency, fault tolerance, and how high-throughput services behave operationally.
• Experience with the infrastructure behind multi-tenant SaaS or API products, such as usage metering, billing, rate limiting, or authentication.
• Fluency in a backend or systems language such as Python, Go, or Rust, and comfort running services in the cloud.
• Strong ownership of reliability and correctness in production.

What Sets You Apart

• Experience building usage metering or billing infrastructure for a consumption-priced product.
• Experience designing and shipping client SDKs that external developers rely on.
• LLM serving, inference APIs, or ML infrastructure experience.
• Familiarity with observability tooling such as Prometheus, Grafana, or OpenTelemetry.
• Early-stage company experience.

What We Offer

• Competitive Salary, determined by skills and experience
• Equity & Ownership
• Private healthcare
• We offer Visa sponsorship and relocation benefits to hire the best in the world
• We 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.