Senior NLP Engineer

Who we are

Joi AI is a platform for AI-lationships — personalized, emotionally intelligent connections between people and AI characters. We build characters that have their own personality, mood, and voice: they can ignore you, argue with you, miss you. If you've seen the film Her — that's roughly where we're headed.

Our platform serves both emotional and intimate needs without judgment, built on the values of rejection-free connection, sex positivity, freedom to be yourself, and ethical integrity. Today we have tens of millions of conversations on the platform. Our goal: 10% of the global population in long-term AI-lationships with our characters.

Joi Lab (joilab.ai) is our open research arm — we build open-source generative models, agent architectures, and training infrastructure. All code, weights, and data are public. We're not chasing the next ChatGPT wrapper; we're working on true AI agency: persistent memory, self-modification, self-authored constraints.

We're looking for a Principal/Senior+ ML Engineer with deep expertise in NLP and large language models. You'll own the NLP direction end-to-end: training and fine-tuning language models, building agent harnesses, and maintaining the conversational core of our AI companions. This is a hands-on role. We move fast and expect you to go from idea to deployed result in days, not months.

WHAT YOU’LL DO

• Train and fine-tune language models powering our AI companions

• Own and improve agent harnesses, agentic loops, and the chat interface algorithm

• Build and maintain the full LLM stack — from model training to production deployment

• Track cutting-edge NLP research and open-source developments; translate them into an NLP roadmap for the ML team

• Collaborate closely with the validation, content, and dataset preparation teams to design experiments and measure model quality

• Optionally: grow into the NLP Tech Lead role as the team scales

WHAT WE’RE LOOKING FOR

• Deep experience training and fine-tuning large language models

• Hands-on experience building agent harnesses and agentic pipelines

• Strong understanding of RLHF, DPO, and other alignment/post-training techniques

• Proficiency with transformers, PyTorch, and related libraries

• Experience at AI-focused startups or companies (Character AI, OpenAI, and similar is a strong plus)

• Backend engineering experience (Python, Go, C#) and knowledge of scalable deployment systems is a significant advantage

Nice to have:

• Experience with multimodal LLMs

• CV background and image/video model experience

• Distributed training on large-scale datasets (millions to billions of samples)

WHY JOI

• You're joining the company that's redefining what relationships with AI look like — before the category gets crowded

• We move fast: ideas become live experiments in days, not quarters

• Joi Lab gives you access to frontier model research — open-source, no corporate gatekeeping

• Small team, high trust, high ownership — you'll see the direct impact of your work

• Work from anywhere with our fully remote, full-time setup.

• Standard 28-day annual leave policy.

• 7 yearly wellness days for life admin or recovery—no sick notes needed.

• Referral rewards up to $5000 for helping us hire top talent.

• We cover 50% of costs for training, conferences, and global meetups.

• Subsidized English classes through our corporate discount.

• Health support: receive up to $1,000 annually for medical fees or private insurance if you aren't on the group plan.

• Optimized workspace: we equip you in-office or provide $1000 every three years to perfect your home office or co-working setup.

• Peer-to-peer recognition: earn gratitude bonuses and swap them for swag, massage vouchers, or team adventures.

NEXT STEPS

• Intro call — conversation with the Recruiter, includes a short live technical quiz (5 questions, camera on — no AI tools)

• Technical interview — live coding session with AI tools allowed, plus ML and ML system design questions with CPO and CTO (90 min)

• Final interview — culture fit and team alignment