# Principal Research Engineer, Post-Training - Company: [Character]() - Location: Redwood City, CA - Team: Technical Staff - ML - Employment type: Full Time - Salary: $275K – $400K - Posted: June 22, 2026 ## Job description **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](http://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](http://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. **What You'll Do** * **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. **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. **Nice to Have** * 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** [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 adventure*s.* 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 Character, we value diversity and welcome applicants from all backgrounds. As an equal opportunity employer, we firmly uphold a non-discrimination policy based on race, religion, national origin, gender, sexual orientation, age, veteran status, or disability. Your unique perspectives are vital to our success.* ## Apply [Apply on Character]() Canonical job page: