Senior Software Engineer, Full Stack (Python, Java, Rust, C#, C++)

EMAIL TO APPLY - View email address on us.fitly.work

1. Overview

Join a leading AI lab's cutting-edge GenAI team and help build foundational AI models from the ground up. We're seeking talented Senior Full-Stack Software Engineers with deep, hands-on production expertise across modern application stacks — Python, Java, Rust, C#, C++, and TypeScript — to build, integrate, and stress-test real software on top of frontier models before they ship. Engineers work directly with the AI lab's engineering manager on fast-moving projects and are expected to onboard quickly and contribute working code early. This is a full-time commitment of 40 hours per week.

This is a W-2 employment position with Cincinnatus LLC, with the opportunity to be placed at a leading AI Lab as part of their extended workforce.

2. Key Responsibilities

Build and ship full-stack applications, services, and internal tools — in Python, Java, Rust, C#, C++, or TypeScript — that exercise frontier model capabilities end to end.

Integrate pre-release model APIs into working software, including the surrounding scaffolding: tool interfaces, evaluation harnesses, and telemetry.

Diagnose and document model and integration failure modes surfaced while building, translating engineering observations into clear written feedback for the research team.

Prototype quickly against shifting requirements, delivering working increments with limited up-front specification.

Collaborate with the engineering manager and other engineers to maintain consistency in code quality, architecture, and technical documentation.

3. Core Qualifications

6+ years of dedicated professional experience building and shipping production software at a recognized, top-tier organization, including ownership of systems end to end.

Deep production experience in at least one of Python, Java, Rust, C#, or C++, plus demonstrated delivery in a second language across a different ecosystem — this cohort is intentionally staffed across multiple language ecosystems.

Hands-on ownership across the stack: backend service and API design, a modern front-end framework (React or equivalent), data modelling, and cloud deployment and operations.

Demonstrated ability to onboard onto an unfamiliar codebase and deliver working code quickly with minimal ramp-up.

Demonstrable career progression.

Ability to engage reliably for at least 40 hours/week during weekdays.

Strong written communication skills and the ability to explain complex technical decisions clearly.

About Cincinnatus LLC: Cincinnatus LLC is an enterprise staffing company that partners with leading technology companies to source and employ highly skilled professionals for contingent and contract-based opportunities. Cincinnatus serves as the employer of record for these engagements, providing W-2 employment, payroll, benefits, and compliance, while placing employees directly within client teams to work on high-impact initiatives.