Principal Research Engineer, Post-Training ML Systems

Character; Redwood City (CA); Hybrid; USD 220,000 - 320,000; Full time

Job summary

Character.AI is seeking a Principal Research Engineer on the Post-Training team to define the technical vision for post-training systems powering our immersive role-playing chat features.

You will lead ML research and systems development, build scalable data pipelines, and collaborate with researchers, engineers, product, and infrastructure to improve model alignment, performance, and user experience.

Qualifications

• PhD in Computer Science, Machine Learning, AI, or equivalent industry experience.
• Experience leading technical projects or teams in machine learning, AI research, or large-scale distributed systems.
• Deep understanding of transformers, reinforcement learning, alignment methods, and large language models.
• Strong track record of delivering impactful research or applied ML systems in production environments.
• Expertise in designing, building, and maintaining production-quality ML systems and infrastructure.
• Experience training, serving, debugging, and optimizing large-scale models on GPU-based systems.
• Experience leading teams working on large language model training, mid-training, or post-training.
• Experience with product experimentation, online evaluation, and A/B testing frameworks.
• Strong software engineering skills with maintainable, scalable code.
• Excellent communication and ability to influence technical direction across teams.

Responsibilities

• Technical Leadership & Mentorship: Define and drive the technical roadmap for mid- and post-training systems and mentor teams.
• Research & Model Development: Lead alignment algorithms and training objectives to improve model capabilities and data efficiency.
• Systems & Infrastructure: Design scalable training/inference systems and data pipelines; optimize distributed training and serving.

Skills

Technical leadership

ML system design

Distributed systems

MLOps

Python

GPU training

LLM training

Experimentation frameworks

Communication

Cross-functional teamwork

Education

PhD in CS/ML

Equivalent industry experience

Tools

Kubernetes

Docker

PyTorch

TensorFlow

Job description

Character.AI is seeking a Principal Research Engineer on the Post-Training team to define the technical vision for post-training systems powering our immersive role-playing chat features.

You will lead ML research and systems development, build scalable data pipelines, and collaborate with researchers, engineers, product, and infrastructure to improve model alignment, performance, and user experience.