Sr. Model Engineer, RL Training Framework

Palo Alto, CAfull timeOn Site

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senior

$180k–$600k

H1B: no

xAI Sr. Model Engineer role focused on post-training and reinforcement learning challenges including reward modeling and preference optimization. Responsibilities include designing experiments, improving model recipes, and partnering with engineers for large-scale training runs.

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

Qualification

Experience training large models (e

Required

Experience writing production-quality code in Python, PyTorch, JAX, Rust, or C++
Experience training large models (e.g., pre-training, post-training, fine-tuning, or RL training of neural networks with 1B+ parameters)

Preferred

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

Benefits

Dental
Vision
401(k)
Equity/RSU
Life Insurance

Company

xAI

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Source: greenhouse

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