ML Research Engineer in London

TLDR: We are looking for several ML Engineers to train, post-train, and evaluate the LLMs at the core of our platform.

This is hands-on modern model training work: large-scale data pipelines, SFT/RLHF/DPO-style alignment, reward models, distributed multi-GPU training, and evaluation.

About us

White Circle is an AI Safety company building the safety, reliability, and optimization layer for AI systems.

At the core of our platform are policies – simple natural-language rules that define what an AI model should and shouldn’t do.

We automatically test, enforce, and continuously improve these policies at scale.

• We’ve raised $11M from top funds, founders, and senior leaders at Open AI, Anthropic, Hugging Face, Mistral, Deep Mind, Datadog, Sentry, and others
• We process over 100M+ API calls every month
• We fine-tune and train our own LLMs so they run faster and cheaper than any open or proprietary model

We’re a small, highly focused team.

If you want to work deeply on hard problems, see your work ship to production quickly, and influence how AI safety is actually built – you’re the one we need.

You will

• Turn petabytes of unstructured text into a structured, explorable view (topics, clusters, segments, trends, anomalies): iterate from “unknown unknowns” to stable definitions we can track.
• Build scalable representation pipelines: sampling strategies, preprocessing/normalization, embeddings at scale, indexing, and retrieval to make the corpus searchable and analyzable.
• Use LLMs pragmatically: labeling/classification, weak supervision, data enrichment, summarization, and automated diagnostics of inbound volumes (with cost/quality controls).
• Deliver insights that change decisions: translate findings into product and operational actions (what data we have, what’s missing, where quality breaks, what to prioritize next).
• Ship self-serve analytics: datasets, data models, and lightweight tools/dashboards so the team can explore and answer questions without ad-hoc requests.
• Partner closely with engineering/research: align pipelines with production constraints (latency/cost/privacy), and integrate outputs into workflows.

You'll fit right in if you

• Strong Python + SQL with an engineering mindset: you can build reliable pipelines, not just notebooks.
• Solid applied NLP/ML experience on real-world text: embeddings, clustering, topic modeling, semantic search, classification; you understand failure modes and how to debug them.
• Comfortable at scale: distributed processing, large-scale storage-querying, and performance-cost tradeoffs.
• You know how to evaluate fuzzy problems: offline/online metrics, human-in-the-loop labelling, inter-annotator agreement, drift monitoring, and reproducibility.
• Have prior work with safety/moderation datasets, policy/rule systems, or high-volume logging/observability.

A big plus

• A public builder footprint: open-source models, datasets, or training frameworks on Hugging Face/Git Hub, benchmarks, papers (workshop or main conference), or technical posts with real usage.
• Experience training models at a frontier or near-frontier lab, or leading open-source model releases with documented adoption.
• Experience with RL methods for LLMs beyond standard RLHF: online RL, GRPO-style methods, or novel alignment approaches.
• Experience with moderation, safety, or classification models at scale.
• Multilingual model training experience.
• Why White Circle
• Paid time off in line with your local regulations, no matter where you work from.
• Work from Paris (hybrid) with a relocation package available, or work from London (note: we are currently unable to provide relocation support and medical insurance for London-based roles).
• Comprehensive medical insurance for our France-based team.
• All the hardware, tools, and services you need.
• Covered subscriptions for AI agents and IDEs.
• Team off-sites twice a year: we've recently been to the Alps and to Saint-Tropez.
• How we hire
• Introductory call with HR (25 min)
• Take-home test task
• Technical interview with Head of Applied Research (60 min)
• Final conversation with our CEO (45 min)

Please submit your application in English.

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