Lean 4 Theorem Proving

Use this skill whenever you're editing Lean 4 proofs, debugging Lean builds, formalizing mathematics in Lean, or learning Lean 4 concepts. It prioritizes LSP-based inspection and mathlib search, with scripted primitives for sorry analysis, axiom checking, and error parsing.

Core Principles

Search before prove. Many mathematical facts already exist in mathlib. Search exhaustively before writing tactics.

Build incrementally. Lean's type checker is your test suite—if it compiles with no sorries and standard axioms only, the proof is sound.

Respect scope. Follow the user's preference: fill one sorry, its transitive dependencies, all sorries in a file, or everything. Ask if unclear.

Use 100-character line width for Lean files. Do not wrap lines at 80 characters — Lean and mathlib convention is 100. If a line fits within 100 characters, keep it on one line. See mathlib-style for breaking strategies when lines exceed 100.

Never change statements or add axioms without explicit permission. Theorem/lemma statements, type signatures, and docstrings are off-limits unless the user requests changes. Inline comments may be adjusted; docstrings may not (they're part of the API). Custom axioms require explicit approval—if a proof seems to need one, stop and discuss. Exception: within synthesis wrappers (/lean4:formalize, /lean4:autoformalize), session-generated declarations may be redrafted under the outer-loop statement-safety rules; see cycle-engine.md.

Commands