ML Research Engineer

Moonfire; Greater London; Hybrid; GBP 90,000 - 130,000; Full time

Benefits offered by this job

Hybrid office in London/Paris

Relocation support

Health insurance

Lunch and dinner at the office

Learning and development

Job summary

White Circle is seeking several ML Engineers to train, post-train, and evaluate the LLMs powering our platform. This hands-on role involves building petabyte-scale data pipelines, SFT/RLHF/DPO-style alignment, reward models, and distributed multi-GPU training.

You’ll translate insights into product decisions and create self-serve analytics for the team. The team focuses on safety, reliability, and optimization of AI systems, with opportunities to influence how safety policies are implemented at

Qualifications

• Strong Python + SQL with an engineering mindset.
• Hands-on NLP/ML with real-world text.
• Experience with scale and large data pipelines.
• Ability to evaluate fuzzy problems and monitor drift.

Responsibilities

• Turn petabytes of unstructured text into a structured, explorable view.
• Build scalable representation pipelines, embeddings at scale, indexing, retrieval.
• Use LLMs for labeling, supervision, data enrichment, and diagnostics.
• Deliver insights that translate into product and operational actions.
• Ship self-serve analytics with datasets, models, and lightweight dashboards.
• Partner with engineering to align pipelines with production constraints.

Skills

Python

SQL

NLP/ML

Distributed processing

Embeddings

Clustering

Topic modeling

Semantic search

Classification

Data engineering

Tools

HuggingFace

GitHub

Job description

TL;DR: 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 recently raised our Series A funding round, taking our total funding to $70M. Our investors include top funds, founders, and senior leaders at OpenAI, Anthropic, HuggingFace, Mistral, DeepMind, 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.

What you’ll do

• 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

• Have strong Python + SQL with an engineering mindset: you can build reliable pipelines, not just notebooks.
• Have 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.
• Are comfortable at scale: distributed processing, large-scale storage‑querying, and performance-cost tradeoffs.
• 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 HuggingFace/GitHub, 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

Compensation & benefits

• Competitive compensation, including equity
• Flexible time off
• Office in central London/Paris with flexible hybrid setup
• Relocation support if you're moving to Paris, available after your probationary period
• Premium private health insurance
• Mental health support, including coverage for therapy when you need it
• Lunch and dinner covered when you work from the office
• Learning and development support for courses, conferences, and opportunities to grow your skills
• All the hardware, subscriptions, tools, and services you need
• Team off‑sites twice a year: we've recently been to the Alps, Saint‑Tropez, and Marbella

Process

1. Intro call with Talent Team
2. Test assignment
3. Technical interview with Head of Applied Research
4. Final conversation with CEO