Full-Time
Agent Runtime & Systems
Member of Technical Staff
Callosum
11-50 employees
Orchestrates AI workloads across heterogeneous hardware
Compensation Overview
£101k - £192k/yr
London, UK
In Person
 Software Engineering (1) |
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- 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 domain-specific language adopted by other engineers or researchers.
- Deep Python expertise plus proficiency in a systems language such as Rust, C++, or Go.
- Hands-on debugging skills across APIs, runtimes, and distributed infrastructure, with a practical understanding of nondeterminism, side effects, and reproducibility.
- 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 domain-specific languages 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.
- 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 machine learning systems or large-scale inference and training.
Callosum builds a software platform that manages and coordinates AI workloads across multiple, diverse hardware types. It orchestrates how different AI models run together on chip architectures from Nvidia, AMD, AWS, Cerebras, SambaNova, and more, treating the compute environment as an ecosystem and assigning tasks to the most suitable hardware to improve speed and accuracy while reducing cost. This makes it possible to avoid depending on a single hardware provider and to leverage the strengths of different accelerators in tandem. The platform integrates with major cloud providers (AWS, Google Cloud, Azure) and targets enterprises building complex, multi-agent AI systems as well as chip manufacturers validating new hardware. Callosum’s goal is to advance AI by enabling specialized, cooperative systems—similar to how the human brain works—rather than scaling a single model on one piece of hardware.
Company Size
11-50
Company Stage
Seed
Total Funding
$110.3M
Headquarters
London, United Kingdom
Founded
2025
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Simplify's Take
What believers are saying
- Atomico led the August 2026 round; Plural, DCVC, and UK Sovereign AI followed.
- Bloomberg said the company raised $100 million; one of Europe's largest seed rounds.
- Callosum's June-July 2026 close already showed £57.2 million filed capital and investor board access.
What critics are saying
- Nvidia Dynamo, AMD ROCm, and vendor stacks absorb orchestration before Callosum standardizes.
- Callosum's $100 million seed raised expectations; missed benchmarks or churn hits credibility by 2027.
- If partners like Cerebras internalize routing software, Callosum becomes a replaceable middleware layer.
What makes Callosum unique
- Callosum's August 20, 2026 seed funded heterogeneous routing across models and chips.
- Tailored Inference already ships in cybersecurity and finance, not just slideware.
- Cerebras and Rebellions partnerships give Callosum hardware-neutral credibility across rival silicon.
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↓ -18%Callosum's $100m seed put 24.1% of the holdco into a new preferred class. Callosum's $100m seed maps to a filed £57.20m Series Seed 2 issue for 24.1% of its holdco, while Atomico and Plural-linked directors joined the board. By Hagen Hoferichter · Published 23 August 2026 at 10:02 CEST Callosum's $100 million seed round looks different when the financing company's filings are put beside the announcement. The latest share allotment at Callosum Technologies Ltd records £57.20 million of cash for a new preferred class equal to 24.1% of the resulting issued shares. The same financing period changed the board. Two people publicly associated with Atomico and Plural joined the two founders on a four-seat board. That gives the new capital board-level access, but the filings do not establish investor control or disclose which institution owns which part of the new class. The finding is not that Callosum's $100 million headline is wrong. A round can close in tranches, combine instruments or include commitments that are not yet allotted. The narrower point is more useful for investors: the public record shows a £57.20 million preferred-share close, a 24.1% issued-share block and a new board configuration at the UK holdco that sits above the operating company. The filed close is smaller than the headline, but large enough to reset the denominator. Callosum announced the seed on 20 August 2026, describing a $100 million round led by Atomico with significant participation from Plural, DCVC and the UK Sovereign AI Unit. The company said the capital would support software that matches artificial-intelligence workloads to computing hardware. Bloomberg Law independently reported the round and its headline size. Companies House records for Callosum Technologies Ltd tell a more specific capital story. On 10 August, the holdco reported 4,329,819 Series Seed 2 Preferred shares allotted between 24 June and 8 July at £13.21 cash per share. The arithmetic is straightforward: | Filed position | Shares or cash | What it changes | | Shares before Series Seed 2 | 13,669,818 | Existing ordinary and Series Seed holders' base | | New Series Seed 2 Preferred | 4,329,819 | New class added in the June-July closing window | | Shares after the allotment | 17,999,637 | Post-issue issued-share denominator | | Registered cash for the new class | £57,196,908.99 | Filed-to-date cash, not a claimed full-round total | | New class as a share of issued capital | 24.055% | Simple issued-share dilution for earlier holders | Every earlier holder therefore owns a smaller percentage of the issued-share count after this allotment. The 24.1% figure is a share-count calculation, not a fully diluted ownership percentage. The preferred class has distribution-priority mechanics and one vote per share, while the amended articles contain the complete rights package. That distinction keeps the comparison honest. The filed amount is in pounds and the announcement is in dollars. The public record does not say whether the difference reflects exchange rates, a later tranche, commitments, grants, another instrument or a different measurement date. It does show exactly what was allotted and paid for in the reported close. Callosum built the financing in a new UK parent. The holdco was incorporated on 13 August 2025, shortly before the first major capital steps. A 26 August 2025 allotment issued 8,999,998 ordinary shares for non-cash consideration described as the legal and beneficial ownership of shares in SERNN, Inc. That structure placed the operating business under Callosum Technologies Ltd before the later cash financings. The 9 September 2025 allotment then added 3,043,301 Series Seed shares for £1.36 each and 803,556 Series Seed shares for non-cash consideration described as SAFE conversion. Further cash allotments followed in February and March 2026. By the time the Series Seed 2 class arrived, the holdco had 13,669,818 issued shares. Callosum's operating company record is therefore not the whole financing perimeter. The new preferred class sits in the parent that became the operating company's corporate person with significant control in August 2025. For diligence, the legal-entity choice matters: the round's dilution and governance entry are visible at the holdco, while the product and team are described at the Callosum operating-company level. This is the same analytical problem that appears in Amber's concentrated Series A issuance. The funding announcement supplies the event and participants. The registered allotment shows which legal entity issued the equity and how far the denominator moved. Those are different pieces of the financing map. Two investor-linked directors joined the founders. The capital reset was accompanied by a board change. Companies House filings dated 6 August record the appointments of Ian Hogarth and Alexandr Vidiborschii. Hogarth is publicly listed by Plural, while Vidiborschii appears on Atomico's investment team. Their appointments put two investor-affiliated individuals beside Callosum's two founders, Jascha Achterberg and Danyal Akarca. The board is therefore evenly split by role on the public record: two founders and two people associated with the lead and participating venture firms. That creates board-level access for the new financing group. It does not, on its own, prove a majority of voting directors, reserved-matter rights, a casting vote or unilateral control. The filings identify the people and their appointment dates, not the shareholder agreement behind the appointments. The timing is commercially relevant. The director appointments were filed shortly after the allotment window for the Series Seed 2 shares. The documents do not state whether the board rights were a condition of the round, part of a broader governance reset or an existing contractual right being exercised. They do show that capital and board composition changed in the same financing cycle. For a venture investor, that is a more concrete diligence point than the headline round size. A quarter of issued shares with a preferred distribution position can influence future financings even when the investor allocations are private. A four-person board can make information flow and strategic consent more immediate, while still leaving control questions unresolved. The ownership question is narrower than the headline question. The public announcement names Atomico, Plural, DCVC and the UK Sovereign AI Unit, but the Series Seed 2 SH01 does not allocate the 4,329,819 shares among them. It is therefore safe to describe the class as a 24.1% issued-share block and to connect the board appointments to named people with public ties to Atomico and Plural. It is not safe to assign a percentage to any one investor or to say the UK Sovereign AI Unit owns a particular part of the class. The same boundary applies to the $100 million figure. The registered £57.20 million is a filed amount for one class and closing window. It is not evidence of a funding shortfall, and adding the earlier public $10.25 million announcement to it would manufacture a total without knowing the instruments and dates that each announcement covers. The next decision-changing records are specific: a post-round shareholder list, the amended articles in readable form, and any shareholder or subscription agreement that sets allocation, preference and board rights. Those documents would answer what the 24.1% class means economically, whether further shares were committed but not allotted, and how the two investor-linked directors operate within the four-seat board. Callosum's financing is already legible at one level. A new UK holdco issued a preferred class for £57.20 million, the class represents 24.1% of the resulting issued shares, and two investor-linked directors joined the founders. The unresolved question is no longer whether the seed changed the cap table. It is how the private rights package turns that visible block and board access into economic and governance power.
Callosum raises $100M seed. Callosum raises $100M in a seed round led by Atomico to advance its platform for unifying heterogeneous computing resources for AI. Updated August 20, 2026 Callosum, a company building a global platform to unify heterogeneous compute for AI, has announced the closing of a $100M seed round. The company focuses on the orchestration of AI workloads across diverse silicon and models to optimize cost, energy, and performance. Investors. This round was led by Atomico, with participation from Plural, DCVC, and the UK Sovereign AI Fund. Callosum use of funds. Callosum plans to use the capital to scale its heterogeneous compute platform, expand its global compute partnerships, and further develop its programmable heterogeneity and Tailored Inference technology to deliver performance and cost-efficiency in AI systems. About Callosum. Founded by Danyal Akarca and Jascha Achterberg, Callosum is an AI infrastructure company building a platform for heterogeneous intelligence. Its technology enables AI workloads to run across different models and computing architectures, allowing organizations to optimize inference for specific requirements. Funding details. Company: Callosum Raised: $100M Round: Seed Funding Date: August 20, 2026 Lead Investor: Atomico Additional Investors: Plural, DCVC, UK Sovereign AI Fund Software Category: Artificial Intelligence Source: https://www.callosum.com/blog/seed-round Updated August 20, 2026
Callosum raises a $100 million seed led by Atomico, UK Sovereign AI on the cap table: HedgeCo.Net - Callosum, a London AI-infrastructure startup, raised a $100 million seed round led by Atomico, with participation from Plural, DCVC, and the UK's Sovereign AI fund. Atomico announced the lead on August 20. Bloomberg independently reported the same $100 million seed, the Atomico lead, Plural and DCVC in the round, and a "significant" investment from the UK's £500 million Sovereign AI vehicle. Callosum did not disclose a valuation. It did not disclose how much of the $100 million came from the public fund versus private capital. The company builds software that splits AI workloads and routes each piece to a model and chip suited to the job, rather than assuming a homogeneous GPU grid. Atomico said a first family of tailored-inference APIs is live. The lead's own note, on complex agentic workloads in financial services run with Cerebras, claimed four times the speed, 70% lower compute cost, and a 10% lift in task success versus a single frontier model on conventional infrastructure. Those operating stats are Atomico's and the company's. They are not a third-party benchmark. Bloomberg did not independently verify them. This is a seed, not a growth round. "One of the largest ever raised in Europe" is Atomico's ranking language, not a league-table print. Bloomberg put the UK vehicle at £500 million and called its cheque "significant." It did not print a dollar amount for that ticket, and neither did Atomico. Cheque size from the state remains unpublished. The valuation remains unpublished. For venture allocators the diligence is the cap table, not the orchestration pitch. A £500 million public vehicle taking a "significant" but unquantified slice of a $100 million seed is industrial policy sitting next to Atomico. It is not a priced round. Anyone marking a European AI-infra unicorn off this close is inventing a post-money the company refused to print. The $100 million is the raise. The valuation is blank. The second-order read is how sovereign AI money is attaching to software that sits between models and silicon rather than to a national frontier lab. Allocators should mark $100 million as the seed, Atomico as lead, and Sovereign AI as a disclosed but unquantified participant. Leave the 4x / 70% / 10% claims on the sponsor's page. The scarce object is a valuation and a public-fund ticket size, not another heterogeneous-compute manifesto.
Callosum raises $100M to cut AI costs by routing workloads to specialized chips. 20 August 2026 Key Takeaways * Callosum raises $100 million led by Atomico, one of the largest early-stage rounds in UK history. * The startup routes AI tasks to cheap, specialized chips instead of defaulting to expensive Nvidia GPUs. * Founded by Cambridge neuroscientists, the platform assigns each workload to the most efficient model and chip. The human brain doesn't use one giant processor for everything. It uses small circuits built for specific jobs. AI companies do the opposite, and it's killing their margins. To change that, Callosum raises $100M to build software that routes AI tasks to the cheapest chip that can do the work. The round was led by Atomico, with participation from Plural and DCVC. It also marks the first investment from the UK's new Sovereign AI Fund. At £73.5M, it ranks among the largest seed rounds in UK history. The money comes at a critical time. As AI moves from training massive models to deploying them at scale, companies are discovering that running those models is where the real money burns. The problem Callosum solves. Most companies today run every AI task on expensive, general-purpose Nvidia GPUs. It doesn't matter if the job needs massive compute or just basic processing; it all gets the same premium chip. That brute-force approach is draining budgets fast. For many AI-native companies, inference takes up over 50% of revenue. Training gets the headlines, but inference is what determines if a business is profitable. Callosum routes compute like the human brain. Callosum's answer is to stop treating all compute the same. Founded by Cambridge neuroscientists Danyal Akarca and Jascha Achterberg, the company took inspiration from biology. The human brain doesn't use one giant processor for everything. It uses small, specialized neural circuits tailored for specific actions. Callosum built software that does the same for AI. It acts as a smart routing layer between applications and hardware. It breaks down complex AI tasks and automatically assigns each piece to the cheapest, most efficient model and chip available. Instead of defaulting to Nvidia, Callosum partners with specialized chipmakers including Axelera, Cerebras, and Rebellions. A simple task gets a simple chip. A complex task gets the heavy hardware. 70% savings and 4X speed. The results are already showing up in early deployments. In partnership with chipmaker Cerebras on financial agentic workloads, Callosum reports 70 percent reductions in cost. By matching tasks to specialized silicon instead of general GPUs, it's also delivering 4 times speed gains. The pitch to investors is simple: the next phase of AI isn't about buying more chips. It's about using the hardware you already have as efficiently as possible. "Running generative AI models is wildly expensive," the company notes. Callosum wants to change how software talks to physical microchips so companies stop overpaying. The AI market is shifting. The race to train foundational models is slowing, but the race to deploy them cheaply is accelerating. That makes workload orchestration the next major hardware battleground. With backing from Atomico and the UK government, Callosum is positioning itself at the centre of it. The future of AI compute, the founders argue, isn't running every task on the biggest GPU available. It's picking the right tool for the job, just like the brain does. I'm a crypto writer with 4+ years of experience passionate about turning big, technical ideas into content anyone can understand. From blockchain to stablecoins to everything in between, I enjoy helping readers stay informed in a space that never stops moving. Disclaimer VentureBurn is a media platform covering the latest in cryptocurrency, artificial intelligence, venture capital, and the startup ecosystem. Opinions expressed on VentureBurn are for informational purposes only and do not constitute investment advice. Before making any high-risk investments in digital assets or emerging technologies, readers should conduct their own due diligence. All transactions and financial decisions are made at your own risk, and any losses incurred are solely your responsibility. VentureBurn does not endorse or recommend the buying or selling of any digital assets and is not a licensed investment advisor. Please note that VentureBurn may participate in affiliate marketing programs.