Agent Runtime & Systems - Member of Technical Staff

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

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 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.