Senior Software Engineer

About Mainstay

Are you ready to be a part of something groundbreaking? Mainstay is the leading market intelligence platform purpose-built for buying, managing, and selling homes in the single-family rental industry. We empower informed decision-making and unlock opportunities for owners, operators, and residents by aggregating and standardizing data from over 50 unique sources. Originally founded within Opendoor, Mainstay launched as an independent company in 2024, driven by our mission to simplify the complex and bring clarity and trust to the single-family rental ecosystem. Join us as we reshape the future of the industry!

About the role

What you should expect as a full-stack senior software engineer - maximum agency, tight feedback loops with customers, and no shortage of product challenges that directly translate to business value! You’ll have the opportunity to design, build, and ship complex AI-powered workflow management systems and tools that enable our operations team to execute at scale.

We’re looking for someone who can both fine-tune and optimize models and ship reliable, production ready AI features end to end - from retrieval pipelines and guardrails to backend systems and user facing product experiences.

Our products are built with Python, Typescript, and React on AWS.

What you'll do

In this role, you’ll be responsible for both shaping new AI-powered product experiences and ensuring they’re reliable at scale. That means fine-tuning and optimizing models for our domain, designing retrieval pipelines and guardrails, and building the backend systems that make these features production-ready. You’ll work closely with product managers, designers, and operators to bring AI features from idea to launch, balancing innovation with stability.

In this role, you will:

• Contribute to all phases of product development, from ideation and prototyping to implementation and iteration

• Build and evolve both frontend product experiences and backend services that power them

• Fine-tuning existing models and optimize their performance for Mainstay use cases

• Design reliable, scalable backend services to support AI-powered features in production

• Leverage AI to modernize and automate legacy workflows in the real estate industry

• Evaluate and benchmark LLM performance across latency, cost, accuracy, and hallucination rates

• Tune prompts, chain-of-thought logic, and implement retrieval strategies such as RAG and vector stores

• Monitor performance, safety, and cost of LLM-driven features in production environments

• Define and build useful LLM-powered experiences in partnership with cross-functional teams

• Mentor and support the growth of other engineers on the team

• Collaborate closely with product managers, designers, and operators to identify and solve customer pain points

What we're looking for

• 5+ years of experience building and delivering production-grade software, ideally in fast-paced, high-growth startup environment

• At least 3 years of hands-on experience developing and deploying production-ready AI features (LLM applications, fine-tuning, RAG systems, guardrails, evals, etc.)

• Strong full-stack engineering skills with a focus on backend scalability and system reliability

• Experience fine-tuning and deploying LLMs

• Deep understanding of LLM evaluation methods, including prompt testing, retrieval precision, hallucination detection, and cost/performance tradeoffs

• Experience building custom eval pipelines using techniques like model-as-a-judge or embedding similarity

• Hands-on work with agentic frameworks (e.g. LangGraph, AutoGen, CrewAI) or custom agent systems

• Strong full-stack engineering skills with a focus on scalable systems and adaptability

• Clear, concise communicator - essential for our remote, highly collaborative team

• Proactive and ownership-driven - comfortable navigating ambiguity and shifting priorities

• Customer-focused mindset with a drive to build solutions that solve real problems

• Experience with AI/ML tools or APIs; bonus if you’ve worked with Python-based RAG, voice agents, or AI task automation

Bonus points if

• Experience working with retrieval-augmented generation (RAG) pipelines

• Exposure to building or integrating voice agents

• Familiarity with task automation powered by AI

• Strong Python background with applied AI or ML workflows

#LI-AW

Mainstay Built on Trust

At Mainstay, trust is at the core of everything we do. We build trust with everyone -- from candidates to employees to partners. We believe in fairness and openness throughout our hiring process, evaluating candidates based on qualifications without regard to race, color, religion, national origin, age, gender, marital status, disability, veteran status, sexual orientation, gender identity, or any other protected status. Our privacy practices describe how we handle applicant information, you can find the policy details for California residents here.

We are committed to providing reasonable accommodations throughout our recruitment processes for candidates with disabilities, pregnancy, religious beliefs, or other reasons protected by applicable laws. If you require assistance or a reasonable accommodation, please contact us at [email protected]