Formal Methods (Lean 4) Expert

About Open Train

Open Train is the #1 platform for building careers in AI training and data labeling. We help AI-training freelancers find specialized projects, consolidate opportunities, and build a unified portfolio so they can grow durable freelance careers in this fast-moving field.

AI training (data labeling, annotation, and human feedback) is the human side of building modern AI systems. People prepare, curate, and evaluate examples that models learn from - everything from formal proofs to transcripts, translations, and image labels.

• 100% remote work you can do from anywhere with an internet connection.
• Flexible, part-time friendly projects that fit around other commitments.
• Accessible entry points, with specialist roles paying more for deep expertise.
• Direct impact on how state-of-the-art AI systems reason and behave.

Open Train AI is hiring a Formal Methods (Lean
4) Expert to design and review challenging formal-verification and theorem-proving problems, then evaluate AI-generated proofs and formalizations for correctness and rigor. This is a contractor, part-time role (20+ hours/week) paid at $95/hour, open worldwide in English.

• Title:
  
  Formal Methods (Lean
  4) Expert (Expert level)
• Work type:
  Contractor, Part-time, 20+ hours/week
• Pay: $95/hour (_HOUR)
• Location:
  
  Fully remote; applicants worldwide (English required)

• Author challenging problems in theorem proving, program verification, and mathematical formalization.
• Review peer-authored problems for clarity, difficulty, and ground-truth correctness.
• Evaluate AI-generated proofs, tactics, and formalizations and assign Accept / Revise / Reject verdicts. Requirements
  • Deep background in formal verification or interactive theorem proving (expert level).
  • Hands-on experience with Lean 4 - ideally with mathlib contributions or equivalent work.
  • Familiarity with other ITP systems such as Coq, Isabelle, or Agda is desirable.
  • Strong understanding of type theory, mathematical logic, and program verification.
  • Clear technical writing and meticulous attention to detail required for written review rationales.
  Helpful background

  Experience comparing model outputs for correctness and rigor will help you judge subtle differences in proofs and formalizations. Prior exposure to creating evaluation rubrics or writing reviewer feedback is a plus.

  Compensation & logistics

  This contract role pays $95 per hour and is expected to 20+ hours per week. Open Train AI is the hiring and contracting organization for this role. Work is remote and available worldwide, with English required for reviews and written rationales. The primary data type is text; tasks include question/answering and evaluation/rating of model outputs.

  • Payment type: _HOUR at $95/hour.
  • Labeling focus: TEXT; label types include  and .
  • Employment:
    Contractor, Part-time.