Character.AI

Principal Research Engineer, Post-Training

Redwood City, CA · Principal

Sponsorship not specifiedDetected 30 days ago

AlgorithmsDockerKubernetesMachine LearningData EngineeringNLPLLMsA/B TestingResearchLeadershipCommunicationMentoring

About the role

Specifically, your team focuses on post-training of top-tier OSS LLMs (such as Mistral and Qwen) to power the highly immersive role-playing chat features of Character.AI http://Character.AI.
This is a highly cross-functional role that combines deep technical expertise with organizational leadership.
Your work will directly shape the conversational experiences of millions of users every day.

Technical Leadership & Mentorship: Define and drive the technical roadmap for mid- and post-training systems, balancing research innovation with production reliability and scalability.
You will mentor and grow a team of researchers and engineers through technical guidance, design reviews, and career development.
Systems & Infrastructure: Lead the design of efficient training and inference systems for large-scale generative models.
Partner with infrastructure teams to optimize distributed training, GPU utilization, and serving efficiency.
Drive improvements in experimentation platforms, data quality systems, and model observability.
Expertise in designing, building, and maintaining production-quality ML systems and infrastructure.
Excellent communication skills and the ability to influence technical direction across teams. Lead complex, cross-functional initiatives across data, training infrastructure, evaluation, and model serving.
Define and drive the technical roadmap for mid- and post-training systems, balancing research innovation with production reliability and scalability.
You will lead initiatives spanning data, algorithms, infrastructure, and evaluation, helping define how our models learn from feedback and improve over time.
You will partner closely with researchers, engineers, product teams, and infrastructure teams to identify the highest-leverage opportunities for improving model performance and user experience.

Experience scaling and mentoring high-performing research and engineering teams.
Strong track record of delivering impactful research or applied ML systems in production environments.
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 the ability to write clean, maintainable, and scalable code.
Excellent communication skills and the ability to influence technical direction across teams.
Who You Are (Required Qualifications)

Hands-on experience working directly with open-source models like Mistral and Qwen, particularly adapting them via mid- and post-training for specific personas, creative writing, or role-playing applications.
Familiarity with cloud-native ML infrastructure, including Kubernetes, Docker, and modern orchestration platforms.
ABOUT CHARACTER.AI
Character.AI http://Character.AI empowers people to connect, learn and tell stories through interactive entertainment.
Over 20 million people visit Character.AI http://Character.AI every month, using our technology to supercharge their creativity and imagination.
Our platform lets users engage with tens of millions of characters, enjoy unlimited conversations, and embark on infinite adventures.
In just two years, we achieved unicorn status and were honored as Google Play's AI App of the Year-a testament to our innovative technology and visionary approach.
Your unique perspectives are vital to our success.

Identify and execute high-impact research opportunities that improve model behavior, safety, and user engagement.

Drive advances in mid- and post-training methodologies including reinforcement learning, preference optimization, supervised fine-tuning, and emerging alignment approaches.
PhD in Computer Science, Machine Learning, AI, or a related field, or equivalent industry experience.
Significant experience leading technical projects or teams in machine learning, AI research, or large-scale distributed systems.
Deep understanding of modern machine learning techniques, including transformers, reinforcement learning, alignment methods, and large language models.

This listing is sourced directly from Character.AI's careers page and normalized into a canonical job model.