Callosum
Platform Engineer - Member of Technical Staff
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Platform Engineer - Member of Technical Staff
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 by 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 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; 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.
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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
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The metering and billing layer:
- Usage capture
- Attribution per workflow
- Systems behind consumption-based and outcome-based pricing
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Workflow deployment onto the platform:
- Machinery that registers a workflow
- Routes it and gives it the scaling and isolation it needs (some workflows run in their own dedicated container)
- Designed according to the architect’s contracts and consuming execution endpoints
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Core platform services:
- Request path
- Rate limiting
- Quota enforcement
- Rest of the machinery beneath the API
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Client SDKs and libraries:
- Packages customers use to integrate
- Optimised for the languages they work in
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Internal platform tooling:
- Deployment, admin, and operational glue
- Lets a small team run the platform
- Shared with the reliability engineer
Get help with your application
Your very own career expert that helps elevate your application to the next level.
- On-call support as the platform scales.
What You Bring
-
Strong backend or production-engineering background
- Building and operating services at scale
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Deep distributed-systems expertise:
- API design
- Concurrency
- Fault tolerance
- Operational behaviours of high-throughput services
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Experience with multi-tenant SaaS or API products:
- Usage metering
- Billing
- Rate limiting
- Authentication
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Fluency in a backend or systems language, e.g., Python, Go, or Rust
- Comfort running services in the cloud
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Strong ownership of reliability and correctness in production environments.
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 (e.g., Prometheus, Grafana, OpenTelemetry)
- Early-stage company experience
What We Offer
- Competitive salary, determined by skills and experience
- Equity & ownership
- Private healthcare
- Visa sponsorship and relocation benefits to attract the best talent globally
- In-person work at our London office with dedicated tools, space, and flexibility (just tell us your needs)
- Commitment to an inclusive and welcome workspace
- Belief in equal opportunities for all
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