Founding Reinforcement Learning (RL) Engineer

Fruition Executive Search • San Francisco, CA

2d • $125,000 - $190,000 • Onsite

About The Position

The Founding Reinforcement Learning Engineer will build the company’s core reinforcement learning systems from the ground up. Working directly with the founders, this person will own the full process from environment design through model training and evaluation, and help shape the company’s technical direction. The role also contributes to establishing the engineering culture and hiring future engineers.

The position is full-time and on-site in San Francisco, California.

Requirements

• At least 2 years of hands-on experience in reinforcement learning or machine learning engineering.
• Strong Python skills and deep experience with PyTorch or JAX.
• Hands-on experience training models with reinforcement learning, such as policy gradient methods, reward modeling, or RLHF.
• Comfort with distributed training and GPU infrastructure.
• A builder mindset, ability to ship quickly, and comfort working in a small, early-stage team.
• A degree in computer science, mathematics, physics, or a related field; an MS or PhD is a plus.

Nice To Haves

• Experience with LLM post-training or agent training.
• Publications or open-source work in reinforcement learning.
• Experience at an AI lab or early-stage AI startup.
• Experience with simulation or environment frameworks such as Gymnasium, Ray RLlib, Isaac, or MuJoCo.
• Experience with training libraries such as TRL, verl, or OpenRLHF.

Responsibilities

• Design and build reinforcement learning environments, reward functions, and training pipelines.
• Train and fine-tune models using reinforcement learning methods, including PPO, GRPO, DPO, RLHF/RLAIF, and similar methods.
• Build evaluation frameworks to measure model and agent performance.
• Run experiments, review results, and determine next steps.
• Scale training on GPU clusters and maintain reliable pipelines.
• Turn research ideas into production systems.
• Help establish engineering culture and hire future engineers.

What This Job Offers

• Job Type: Full-time
• Career Level: Entry Level