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

What you'll do

The work is the same as the mid-level tier: build real applications and tools on top of a frontier lab's pre-release models, integrate the APIs and scaffolding (tool interfaces, evaluation harnesses, telemetry), and turn every model or integration failure you hit into a clear write-up for the research team. What changes at this level is ownership. The ad expects you to have owned systems end to end, and to design the API and data model rather than slot into someone else's.

Responsibilities, per the ad:

• Building and shipping full-stack apps, services and internal tools in Python, Java, Rust, C#, C++ or TypeScript
• Integrating pre-release model APIs and their scaffolding
• Diagnosing and documenting model and integration failure modes
• Prototyping quickly against shifting requirements
• Keeping code quality, architecture and documentation consistent across the team

Who qualifies

• 6+ years building and shipping production software at a recognised, top-tier organisation, including end-to-end system ownership
• Deep production experience in one of Python, Java, Rust, C# or C++, plus demonstrated delivery in a second language from a different ecosystem
• Full-stack ownership: backend service and API design, React or equivalent, data modelling, cloud deployment and operations
• Fast ramp-up on unfamiliar codebases
• Demonstrable career progression
• 40 hours a week on weekdays
• Must be located in the United States

The second-language requirement is stricter here than at the mid-level tier: "delivery across a different ecosystem", not just working proficiency. A Java and Kotlin engineer, for instance, may not count; Java plus Python or Rust would.

What it pays

$90–110 per hour, W-2 through Cincinnatus LLC with payroll and benefits. No payment schedule is published. That is $40–45 an hour above the mid-level US listing ($50–65) for three more years of experience. There is also an India tier at $25–30.

Worth knowing

Good:

• W-2 with benefits, at a solid senior rate
• Hands-on work with unreleased frontier models
• Direct line to the lab's engineering manager and researchers

Less good:

• Full-time and US-only
• Strict two-ecosystem requirement
• No duration published; the lab is not named
• "Top-tier organisation" is undefined

About this listing

Posted on Mercor's platform, read on 24 September 2026. Employment is through Cincinnatus LLC, placed at a leading AI lab as extended workforce. Pay, location and qualifications are from the ad. See Mercor.