Callosum
Agent Runtime & Systems - Member of Technical Staff
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About Us
We’re living through a Cambrian explosion of intelligence: new models and new chips, each specialised for different tasks, are arriving all at once. The result is a new era for AI, one of radical heterogeneity.
Callosum is the Intelligent Systems Company. We believe the next generation of AI won't be defined by any single model or chip, but by intelligent systems in which hardware and intelligence co-evolve. We are building the infrastructure that unifies heterogeneous compute across the full stack. This opens a new axis of scaling intelligence: a dynamic system that tailors itself to what each workload actually needs, whether that's speed, cost, precision, or whatever unit comes next.
The last era scaled on a different bet: one bigger model, more of the same chip, more data. That bet is running into structural limits. Frontier models offer extraordinary capability at unsustainable cost, one that today's monolithic infrastructure was never designed to serve.
Our founding principle is that intelligence comes from many specialised systems working together, not from any single component. We build the software orchestration layer that co-evolves models, workflows and silicon into one system, delivering inference tailored to every workload, and demonstrating orders-of-magnitude leaps in capability and cost.
Because our software spans the full stack, our engineering team works directly with heterogeneous accelerators and frontier silicon, including Cerebras, d-Matrix, Intel, NVIDIA, AMD, Normal Computing, Tenstorrent, GreatSky, and Mixx. We are not stopping at today's chips: each new generation of silicon unlocks algorithms that couldn't run before, and we intend to be first to them, every time. If we get it right, it will belong to everyone building on it - not to any single vendor.
In our latest funding round, we raised $100M, led by Atomico with participation from Plural, DCVC and the UK Sovereign AI Fund’s first investment. With this, we are building the infrastructure for the next era of intelligence.
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We are engineers and scientists based in London, working 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
Most agent frameworks are designed for short-lived workflows, best-effort execution, and a narrow set of tools. Callosum is building for a different world: agents that run for weeks, operate across distributed infrastructure, and interact with stateful systems where failures, retries, and side effects must be handled explicitly. Success in this environment requires an agent runtime built entirely from first principles.
Sitting at the heart of our technical mission, this position owns the runtime and programming model for long-running agent systems. Your focus will span durable execution, checkpointing and replay, distributed scheduling, secure tool execution, and end-to-end observability. You will develop the core software that makes agent workflows reliable, debuggable, reproducible, and efficient as we expand our model and tool footprint. This is a high-leverage role tackling complex systems challenges across the entire stack.
Runtime, observability tooling, and framework design are three separate specialties. We are glad to hire someone with real depth in a combination of them.
What You'll Build
- Design and build stateful, asynchronous agent runtimes across distributed environments
- Define event histories, checkpoint and resume semantics, deterministic reconstruction, retries, cancellation, timeouts, and recovery from partial failure
- Build traces and replay, time-travel debugging, and hierarchical tracing across model calls, tools, handoffs, state changes, and runtime decisions
- Develop clear Python APIs, libraries, and DSLs with stable extension points for tools, backends, and new execution models
- Design scheduling, placement, backpressure, resource accounting, sandboxing, permissions, and isolation for secure tool execution at scale
- Develop tooling for trajectory collection and visualisation, performance profiling, experiment comparison, failure attribution, and regression detection across models, prompts, tools, and runtime versions
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What You'll Bring
We are looking for real strength in some of the following rather than coverage of all of it.
- Strong distributed systems background with hands-on experience in durable workflows, actor systems, schedulers, workflow engines, distributed databases, or stream processors
- Experience designing a library, framework, runtime, or DSL that other engineers or researchers adopted
- Deep Python expertise plus proficiency in a systems language such as Rust, C++, or Go - and the instinct to move between abstraction layers when it matters
- Hands-on debugging skills across APIs, runtimes, and distributed infrastructure - with a practical understanding of nondeterminism, side effects, and reproducibility
What Sets You Apart
- Experience building a production-grade runtime, compiler, workflow engine, distributed training system, database, debugger, or orchestration platform
- Meaningful contributions to systems such as Ray, Temporal, Kafka, Erlang/BEAM, PyTorch, JAX, TensorFlow, or comparable projects
- Strong API design judgement, open-source collaboration, and disciplined approaches to testing, versioning, compatibility, and reproducibility
- Experience in ML systems or large-scale inference and training
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.
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