Principal Research Engineer, Character.ai

• 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

Desirable

• 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
• 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
• Establish best practices for experimentation, model development, and deployment
• Research & Model Development: Lead the development of alignment algorithms, optimization techniques, and training objectives to improve model capabilities and data efficiency
• Drive advances in mid- and post-training methodologies including reinforcement learning, preference optimization, supervised fine-tuning, and emerging alignment approaches
• Identify and execute high-impact research opportunities that improve model behavior, safety, and user engagement
• Develop robust evaluation frameworks and quality signals to measure real-world model performance
• Systems & Infrastructure: Lead the design of efficient training and inference systems for large-scale generative models
• Architect scalable data pipelines that transform diverse data sources into high-quality training datasets
• Partner with infrastructure teams to optimize distributed training, GPU utilization, and serving efficiency
• Drive improvements in experimentation platforms, data quality systems, and model observability