Research Engineer, Post-Training (All Industry Levels)

Character.AI; California; On-site; USD 225,000 - 400,000; Full time

Benefits offered by this job

Health coverage

401(k) contribution

Paid parental leave

PTO 4 weeks

Doordash stipend

Wellness stipend

Job summary

Character.AI is seeking a Post-Training Researcher on the Post-Training team to advance fine-tuning, alignment, and efficiency of our large language models. You will collaborate across teams to enhance model performance, data pipelines, and training methods, shaping the conversational experiences of millions of users daily.

The role emphasizes production-grade coding, research excellence, and working with GPU workloads to push the boundaries of AI alignment and applicability.

Qualifications

• PhD or equivalent required.
• Ability to write clean production-grade code.
• Experience with GPUs for training, serving, debugging.
• Experience building data pipelines and data infrastructure.
• Strong knowledge of modern ML techniques (reinforcement learning, transformers).
• Proven track record of exceptional research or applied ML projects.

Responsibilities

• Develop alignment algorithms and loss functions to improve data sample efficiency.
• Write data pipelines to process diverse web data into ingestible format for models.
• Identify quality signals to understand model performance in the real world.
• Design sampling algorithms to improve serving efficiency of large generative models.

Skills

PhD or equivalent

Production code

GPUs

Data pipelines

ML techniques (RL/transformers)

Research track record

Education

PhD or equivalent

Job description

About the role and team

Joining us as a Research Engineer on the Post-Training team, you’ll be diving into the exciting world of fine-tuning AI models, optimizing their performance, and ensuring they meet the highest standards of quality and efficiency. Your work will directly contribute to our groundbreaking advancements in AI, helping shape an era where technology is not just a tool, but a companion in our daily lives. At Character.AI, your talent, creativity, and expertise will not just be valued—they will be the catalyst for change in an AI-driven future.

The Post-Training team is responsible for developing our powerful pretrained language models into intelligent, engaging, and aligned products.

As a Post-Training Researcher, you will work across teams and our technical stack to improve our model performance and training methods, including data, compute and algorithms. You will get to shape the conversational experience of millions of users per day.

What you’ll do

• Develop alignment algorithms and loss functions to improve data sample efficiency.

• Write data pipelines to process diverse web data into a format models can ingest.

• Identify quality signals to understand our model’s performance in the real world.

• Design sampling algorithms to improve serving efficiency of large generative models.

Who you are

• “All Industry Levels”: have at least PhD (or equivalent)

• Write clear and clean production-facing and training code

• Experience working with GPUs (training, serving, debugging)

• Experience with data pipelines and data infrastructure

• Strong understanding of modern machine learning techniques (reinforcement learning, transformers, etc)

• Track-record of exceptional research or creative applied ML projects

Nice to Have

• Experience with product experimentation and A/B testing

• Experience training large models in a distributed setting

• Familiarity with ML deployment and orchestration (Kubernetes, Docker, cloud)

• Publications in relevant academic journals or conferences in the field of machine learning

About Character.AI

Character.AI empowers people to connect, learn and tell stories through interactive entertainment. Over 20 million people visit 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.

Compensation

$225K – $400K Offers Equity

• Top-notch health coverage for you & your family, with majority of the premium covered

• We invest in your future with a generous 401(K) contribution

• New parents, we’ve got you covered with incredible paid leave -up to 20 weeks

• 4 weeks of PTO to explore, unwind & come back recharged

• Daily in-office catering plus a monthly Doordash stipend to help keep you fueled no matter where you are

• Monthly wellness stipend to support you in your health journey