Model Engineer, Post-Training & RL

MODEL ENGINEER, POST-TRAINING & RL

You will work on the most critical post-training and reinforcement learning challenges at any given time — including reward modeling, preference optimization (RLHF/DPO), and RL for improving reasoning, truthfulness, and real-world capabilities.

RESPONSIBILITIES:

• Design and run rigorous experiments that turn research ideas into measurable gains in model capability
• Improve model recipes through data quality, deduplication, and mixture experiments
• Define what makes RL data and rewards good, including grader design and preventing reward hacking
• Advance training science, from architecture and scaling experiments to post-training recipes for coding and computer-use agents
• Build evals and targeted tasks that expose model weaknesses and guide where the team invests next
• Partner with engineers to take promising results from small experiments to frontier-scale training runs

BASIC QUALIFICATIONS:

• Experience writing production-quality code in Python, PyTorch, JAX, Rust, or C++

PREFERRED SKILLS AND EXPERIENCE:

• You believe truth-seeking AI is the most important and challenging problem
• You are obsessed about building incredibly useful models through post-training and RL techniques
• You are a power user of AI models and eager to push the boundaries of what’s possible with reinforcement learning and alignment methods
• If you previously worked on post-training, RLHF, or trained models used by millions of people it’s a big plus, but relevant experience is not required
• You take pride in your work and thrive in meritocratic environments

COMPENSATION AND BENEFITS:

$160,000 - $250,000 USD

Base salary is just one part of our total rewards package at SpaceXSI, which also includes equity, comprehensive medical, vision, and dental coverage, access to a 401(k) retirement plan, short & long-term disability insurance, life insurance, and various other discounts and perks.