
Machine Learning Street Talk (MLST)
Written by a model from the transcript; not yet reviewed.
Alistair Pullen of Cosine AI discusses the company's mandate to build the UK's first sovereign LLM, secured after the US banned Fable under export controls. Cosine, previously focused on coding agents for regulated industries, received compute on the Isambard supercomputer at Bristol through the government's sovereign AI unit. Pullen explains how they plan to compete with US labs spending hundreds of billions by licensing model weights rather than hosting inference, and by making trade-offs in RL scope.
The conversation covers three key differentiators for frontier models: total parameter count, active parameter count, and data. Pullen estimates Sonnet at 1.3–1.5 trillion total parameters with 100+ billion active, and Opus at 1.5–1.8 trillion with 150–180 billion active. He argues open-source models like Mistral's 675B are capped by inference deployment constraints. On RL, he describes credit attribution within trajectories as the key unsolved problem, using an analogy of a teacher grading an essay without explaining what made it a B.
Pullen demonstrates Cosine's swarm system, which decomposed a mechanical watch compiler task into 253 subagents. He argues agentic harnesses are becoming less important as models improve, with bash-only access sufficient for most tasks. The episode closes with Pullen expressing frustration at being a "second-class citizen" in AI access, vowing to make the sovereign model happen.
Cosine's Sovereign LLM Mandatestrategy
Pullen explains that Cosine, a frontier lab in the UK, was inducted into the government's sovereign AI unit about nine to ten weeks prior, receiving compute allocation on the Isambard supercomputer at Bristol.
Licensing Weights Avoids Inference Costsinsight
Pullen explains that Cosine licenses model weights to customers who deploy on their own GPUs or cloud, meaning they don't need to spend on data centers for inference the way US labs do.
Empirical Estimates of Frontier Model Sizesdata
Pullen references a blog post that empirically estimated Sonnet at 1.3–1.5 trillion total parameters with 100+ billion active, and Opus at 1.5–1.8 trillion with 150–180 billion active, which influenced Cosine's architectural decisions.
Credit Attribution in RL Trajectoriesstrategy
Pullen describes the problem that in current RL, all tokens in a 256,000-token rollout are equally weighted toward the final reward, and explains how identifying high-entropy decision points and attributing reward to them would make training significantly more efficient.
The Essay Grading Analogy for RLinsight
Pullen compares current RL to a teacher who gives a B on a 2,500-word essay without explaining why, forcing the student to write hundreds of essays to figure out what works, versus a teacher who circles the specific sentence that was wrong.
Harnesses Losing Importance to Modelsinsight
Pullen argues that agentic harnesses are getting less important over time because models can probably do any task with bash-only access, and that the value is shifting from the harness to the model itself.
Swarm Builds a Watch Compiler in One Shotstory
Pullen describes using Cosine's swarm system with 253 subagents to build a Python SDK for specifying mechanical watches in code with a physics engine and 3D viewer, a task that no single model could accomplish out of the box.
Second-Class Citizen in AI Accessstory
The host expresses frustration that for the first time he feels like a second-class citizen because US users have access to better AI models, and Pullen responds that it boils his blood and they have no choice but to make the sovereign model happen.
This episode is sponsored by Notion. Learn more about Notion's Developer Platform today at https://notion.com/mlst Britain's most capable coding model can't be exported, and that ban is the whole reason Cosine set out to build one from scratch. Alistair Pullen, CEO and co-founder of Cosine, sits down with Tim Scarfe to explain how a frontier system he calls Fable, locked behind US export controls, became the founding case for a UK sovereign model trained on the Isambard supercomputer in Bristol. The bet underneath it is economic. Pullen argues that an inference company, rather than a training-first lab, doesn't need billions to compete: millions, a national compute allocation, and a consortium feedback loop can be enough. From there it gets into the machinery, why open-weight models still trail the frontier on size, active parameters and data, the mixture-of-experts versus dense trade-off and why active params dominate how a model actually feels, and the edge that real coding trajectories confer. The back half is about making agents trustworthy. Pullen makes the case for beating "slop" by rewarding the process instead of the final answer, reframes code review as runtime proof (spin the bug up in a VM and force the agent to actually exploit it), and walks through Swarm, Cosine's system running hundreds of sub-agents in one shot. It ends on why memory is still an unsolved hack, how synthetic graders let you run RL on tasks with no built-in test, and why Pullen reads US export controls as an accidental gift, with a supply-chain sting in the tail. --- TIMESTAMPS: 00:00:00 The sovereign mandate and the Fable ban 00:04:02 Millions vs billions: the inference-company model 00:07:19 The consortium feedback loop 00:07:40 Why open models lag the frontier 00:14:59 MoE vs dense, and why active params matter 00:16:29 Trajectories: the process-data advantage 00:19:48 Beating slop: reward the process, not the answer 00:26:06 Reusable abstractions and the epistemic wall 00:29:56 Code review becomes runtime proof 00:37:32 Do agentic harnesses still matter? 00:40:35 Swarm: orchestrating hundreds of sub-agents 00:45:14 Why memory is still unsolved 00:48:25 Synthetic data and graders for RL 00:53:09 The US export gift and supply-chain risk --- REFERENCES: organization: [00:01:15] Cosine https://cosine.sh [00:04:14] Mistral AI https://mistral.ai [00:05:50] Anthropic https://www.anthropic.com [00:07:42] Cohere https://cohere.com [00:08:36] DeepSeek https://www.deepseek.com tool: [00:02:52] Isambard-AI https://isambard.ac.uk [00:05:56] Colossus (xAI) https://en.wikipedia.org/wiki/Colossus_(supercomputer) [00:07:52] GLM (Z.ai) https://z.ai [00:11:52] NVIDIA B300 https://www.nvidia.com/en-us/data-center/dgx-b300/ [00:15:37] gpt-oss-120b https://huggingface.co/openai/gpt-oss-120b [00:15:52] Devstral 2 https://mistral.ai/news/devstral [00:16:01] Llama 70b https://www.llama.com [00:17:05] Claude Code https://www.anthropic.com/claude-code [00:26:23] ARC-AGI (Francois Chollet) https://arcprize.org [00:40:38] Swarm (Cosine) https://cosine.sh [00:40:50] OpenAI Codex https://github.com/openai/codex [00:41:16] Lumen Outpost (Cosine) https://cosine.sh [00:41:18] Kimi K2 (Moonshot) https://huggingface.co/moonshotai/Kimi-K2-Instruct [00:49:55] SWE-bench https://www.swebench.com [00:52:40] SystemVerilog https://en.wikipedia.org/wiki/SystemVerilog person: [00:23:40] Andrej Karpathy https://karpathy.ai paper: [00:27:10] GRPO (DeepSeekMath) https://arxiv.org/abs/2402.03300 [00:27:13] GSPO https://arxiv.org/abs/2507.18071 Incompressible Knowledge Probes, Bojie Li https://arxiv.org/pdf/2604.24827 Estimating the Size of Claude Opus 4.5/4.6 https://unexcitedneurons.substack.com/p/estimating-the-size-of-claude-opus --- ReScript: https://app.rescript.info/session/5852d2b884c4ce4b?share=10b9799160845bb11779f8ac6cd3124f
11,611 words · 6 speakers
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