Evaluating Faithful Formalization of Natural Language to Lean Statements

Ke Zhang, Patricio Gallardo Candela, Sudhir Murthy, Yi Xie, Zhi Wang, Maziar Raissi· July 1, 2026 View original

Summary

This research introduces a new protocol for evaluating the faithfulness of natural-language-to-Lean statement formalization, going beyond mere compilation success. It reveals a significant gap between compilable and semantically faithful outputs, highlighting challenges in accurately translating mathematical statements.

A new study investigates the fidelity of converting natural language mathematical statements into formal Lean declarations, moving beyond simple compilation checks. The researchers developed a rigorous evaluation protocol that combines Lean compilation, cross-model semantic judging, and human expert calibration to assess "faithful statement formalization." Using a 400-entry benchmark covering advanced mathematics, the study uncovered a substantial discrepancy: while a tool-augmented agent achieved an 89.5% compilation rate, only 60.5% of these outputs were deemed semantically faithful. This 29.0-point gap indicates that a statement can be syntactically correct in Lean but still misrepresent the original natural language meaning. The research also analyzed the impact of various interventions, finding that Lean elaboration feedback is crucial for validity but can expose more semantic failures. Context search improves grounding, and expert drafting can be substituted when feedback and grounding are strong. The findings emphasize the need to separately report formal validity, proof-oriented competence, and faithful statement generation.

Why it matters

For professionals working with formal verification, automated theorem proving, or AI-assisted code generation, understanding the faithfulness gap is critical for ensuring that AI-generated formalizations accurately reflect human intent.

How to implement this in your domain

  1. 1Adopt multi-faceted evaluation protocols that go beyond syntax checks to assess semantic faithfulness in AI-generated code or formalizations.
  2. 2Implement human expert calibration and cross-model judging in AI development workflows for critical applications.
  3. 3Prioritize robust feedback mechanisms, like Lean elaboration, in formalization pipelines to catch errors early.
  4. 4Differentiate between formal validity, proof-oriented competence, and faithful generation when reporting AI system performance.

Who benefits

Software EngineeringAI Research & DevelopmentAcademia (Mathematics/Logic)LegalTechCybersecurity

Key takeaways

  • Evaluating natural-language-to-Lean formalization requires assessing semantic faithfulness, not just compilation.
  • A significant gap exists between compilable and semantically faithful AI-generated formal statements.
  • Lean elaboration feedback is a key intervention but can expose more semantic failures.
  • Faithful statement generation, formal validity, and proof competence should be evaluated distinctly.

Original post by Ke Zhang, Patricio Gallardo Candela, Sudhir Murthy, Yi Xie, Zhi Wang, Maziar Raissi

"arXiv:2606.31002v1 Announce Type: new Abstract: Theorem-proving benchmarks evaluate proof search against fixed formal statements, but natural-language-to-Lean formalization must generate the formal statement itself. In this setting, compilation is only a validity check: a Lean de…"

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Originally posted by Ke Zhang, Patricio Gallardo Candela, Sudhir Murthy, Yi Xie, Zhi Wang, Maziar Raissi on X · view source

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