London's Callosum raises €85.4M Seed to route AI workloads across heterogeneous chips — Atomico leads, UK Sovereign AI Fund makes its first investment

Callosum (London) has raised €85.4 million ($100 million) in a Seed round led by Atomico, with Plural, DCVC, and the UK Sovereign AI Fund participating. The round is among the largest seed rounds ever raised by a European startup. It marks the first publicly disclosed investment from the British government's £500 million Sovereign AI Fund, launched earlier this year.

The company was founded in 2025 by Danyal Akarca (CEO) and Jascha Achterberg (CTO), two computational neuroscientists who met during their PhDs at the University of Cambridge. Both have published in Nature journals; between them, they hold research and engineering stints at Google DeepMind and Intel. The company's name is taken from the corpus callosum — the brain structure that connects the brain's hemispheres — chosen deliberately to signal that the analogy to biological computation is not decorative. It is the thesis.

From one chip to many

The AI infrastructure status quo is GPU-centric. Most AI workloads run on Nvidia silicon because CUDA, Nvidia's programming layer, has 15 years of software integrations and developer tooling behind it. Callosum's premise is that this monoculture is expensive, fragile, and unnecessary. Different AI tasks have different computational profiles — dense matrix multiplication for training, fast sparse inference for reasoning, low-latency token generation for streaming outputs — and different chip architectures handle each of these differently.

Callosum's platform sits between AI applications and the chips that run them. When a task arrives, the software breaks it into sub-tasks and routes each sub-task to whichever model-and-chip combination handles it most efficiently: one piece might run on a Cerebras wafer-scale processor, another on a conventional Nvidia GPU, another on Arm-based silicon. Callosum calls this heterogeneous intelligence, borrowing from the neuroscience observation that mammalian intelligence does not arise from one repeated circuit type but from the interaction of many specialised ones.

On 20 August — the same day as the funding announcement — Callosum and Cerebras published a joint announcement of a technical partnership to deliver ultra-low-latency heterogeneous agentic inference, confirming that at least one hardware layer of the multi-chip routing vision is already in production.

The government signal

The UK Sovereign AI Fund, a £500 million instrument managed by the British Business Bank and announced as part of the government's AI Opportunities Action Plan, has not disclosed a prior investment. Callosum is its first. The government's choice of infrastructure software rather than an AI application company is deliberate: the thesis is that controlling compute routing is a strategic layer, akin to controlling operating systems or network protocols, where national-level interest is justified.

The $100 million "seed" label is unusual in absolute terms but reflects Callosum's age: the company was founded in 2025, pre-commercial, and structured the round at the stage its age warranted rather than adjusting the label to the amount. Neither lead investor has characterised it otherwise.