The most expensive part of running AI may soon be the decision about how to run it. Callosum, a London startup founded in 2025 by two Cambridge University neuroscientists, has banked $100M in seed money to build software that makes that call.
Atomico led the round, with Plural, DCVC and the UK Sovereign AI Fund joining in. The deal marks the first disclosed check from the Sovereign AI Fund, the £500M vehicle the British government launched in April, and ranks among the largest seed rounds Europe has seen.
The pitch is that workloads should not automatically land on the priciest GPU. Callosum’s systems software routes simple requests to cheaper models and silicon, while reserving frontier models for hard reasoning tasks.
The company frames its mission as heterogeneous intelligence: matching combinations of models and silicon to the cost and speed constraints of each problem, rather than treating AI as one universal solution.
Partnerships are already in place with Cerebras Systems for low-latency, multi-agent inference and with chipmaker Rebellion. Callosum says its Tailored Inference API family is running in production for cybersecurity and finance customers.
The raise follows an €8.76M pre-seed led by Plural in February. Callosum also appears in the UK government’s £1.1B AI hardware plan, and AI minister Kanishka Narayan has argued that Britain’s edge will come from using chips efficiently.
