Substrate, a Y Combinator-backed startup, is hiring a Harness Engineer to build AI systems for healthcare revenue cycle management. The role offers $140K-$200K salary plus equity at their San Francisco headquarters, where engineers will develop agents that process over 500,000 healthcare claims monthly. The position requires expertise in modern LLM architectures and experience shipping ML/AI products that solve real business problems.

Candidates should be comfortable with both traditional software engineering and AI-assisted development workflows. The ideal applicant has either healthcare domain knowledge or enthusiasm for automating tedious healthcare processes. Previous startup experience or a track record of independent product launches is preferred, along with deep understanding of AI evaluation methodologies and fine-tuning approaches.

The interview process emphasizes practical demonstration over theoretical knowledge. Applicants must showcase a built project, explaining their technical decisions and prompting strategies. The technical interview includes both traditional coding assessments and AI-assisted development evaluations. Substrate's mission focuses on augmenting and replacing manual RCM work through browser-based AI agents, targeting the $14 billion outpatient RCM market.