Principal Research Engineer, Post-Training

Responsibilities

• Who You Are (Required Qualifications)
• 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. Experience scaling and mentoring high-performing research and engineering teams.
• Deep understanding of modern machine learning techniques, including 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 the ability to write clean, maintainable, and scalable code.
• 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.

Requirements

• 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.
• Publications in leading machine learning conferences or demonstrated contributions to the broader AI community.
• About Character.AI
• 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.
• Join us and be a part of establishing this new entertainment paradigm while shaping the future of Consumer AI!
• At Chara

Benefits

Additional Information

About the Role and Team As a Principal Research Engineer on the Post-Training team, you will drive the technical vision, execution, and evolution of the systems that transform foundation models into intelligent, engaging, and aligned products. 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 . You will lead initiatives spanning data, algorithms, infrastructure, and evaluation, helping define how our models learn from feedback and improve over time. This is a highly cross-functional role that combines deep technical expertise with organizational leadership. 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. Your work will directly shape the conversational experiences of millions of users every day. At Character.AI , you will have the opportunity to influence both the direction of our research and the systems that bring it into production, helping build the next generation of AI entertainment.

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