axiom-lean-prover

name: axiom-lean-prover description: Formally verify Lean 4 proofs against Mathlib using the hosted Axiom AXLE API — no local Lean toolchain. Use in the mathematics_lean domain, when a proof needs machine-checked verification but installing Lean/Mathlib locally is undesirable, or when translating informal proofs into formally verified Lean 4 code checked in the cloud.

Axiom Lean Prover (AXLE)

Formal proof verification with Lean 4 + Mathlib, checked by the hosted Axiom AXLE API (https://axle.axiommath.ai). There is no local Lean install, no lake, and no Mathlib cache — .lean files are sent as text to AXLE, which checks them against a preloaded full Mathlib and returns structured errors/warnings with source positions.

This is the cloud-verified counterpart of the lean-prover skill. If you want a local lake build workflow instead, use lean-prover. Do not mix them in one workspace.

When to Use

• Working in the mathematics_lean research domain
• Verifying proofs without provisioning a multi-GB local Lean/Mathlib toolchain
• Translating an informal proof into formally verified Lean 4 code
• Checking whether a mathematical statement is provable / type-checks

Setup

Almost none. You need:

1. AXLE_API_KEY in the environment (already passed through from the project .env). Verify with: echo "${AXLE_API_KEY:0:6}..." (should print pk_...).
2. Python 3.10+ (already present). The verifier uses only the standard library.

Create a directory for your proof files (any name; proofs/ by convention):

mkdir -p proofs

That's it — no elan, no lake, no lake exe cache get, no initial build.

The Verifier: axle_check.py

lake build is replaced by this one script (the ground truth for this domain):

python .claude/skills/axiom-lean-prover/scripts/axle_check.py proofs/MyProof.lean

Exit codes (use them in loops)

A sorry proof returns okay:true from the API — it compiles but is not proven. The script catches this and reports INCOMPLETE_SORRY. Never report a sorry'd result as complete. This is the exact analogue of the old "green build but grep finds sorry".

Error positions

On any non-trivial result the script writes the exact text it sent to AXLE to .axle_last_check.lean in the current directory. Lean positions look like -:7:12-7:18 (line:col); they refer to that normalized file, so open it to see the offending line. (Line numbers differ from your source files because all import lines are collapsed into a single import Mathlib at the top — see Imports below.)

Imports — IMPORTANT (different from the local lean-prover skill)

Always start a proof file with the full import:

import Mathlib

• The AXLE environment has all of Mathlib preloaded; the full import costs nothing (it is checked remotely — there is no local cache to download or build).
• The default AXLE environment's import set is exactly [Mathlib]. Sending import Mathlib.Tactic or specific submodules triggers an "Imports mismatch".
• axle_check.py strips every import line you write and prepends a single import Mathlib (and sets ignore_imports), so submodule imports or local cross-file imports (import Proofs.Definitions) are normalized away and never break the check. You still write import Mathlib at the top of each file for clean, human-readable Lean source.

This is the opposite of the lean-prover skill's "never use bare import Mathlib" rule — that rule exists only to limit local cache size, which is irrelevant here.

Using lemmas that are NOT in Mathlib

Only Mathlib is available remotely — you cannot import an external Lean package (an import SomeProject.Lemmas line is stripped and its names become unknown identifier). When the resource finder surfaces a lemma that lives outside Mathlib (a paper's companion formalization, a GitHub Lean project, or an informal result from the literature), you have two honest options:

Option 1 (preferred) — inline and prove it

Copy the lemma's statement and Lean proof into your proofs/ files and let AXLE verify it against Mathlib like any other declaration. If its proof only depends on Mathlib, this is clean and gives a fully machine-checked result. Re-prove informal (paper) lemmas the same way.

Option 2 (escape hatch) — cite it as a named axiom

If proving a prerequisite would itself require proving a long chain of further non-Mathlib lemmas (too costly for the session), you may assume it rather than re-prove it — but do this with a named axiom, never with sorry:

/-- Assumed without proof; established in [Smith 2020, Theorem 3]. -/
axiom key_prerequisite (G : Type*) [Group G] : SomeProperty G

Then use key_prerequisite freely in downstream proofs. They will verify as COMPLETE (CONDITIONAL on N assumed axiom(s)) — i.e. correct modulo the cited assumption. axle_check.py detects every axiom you declare and lists it, so a conditional result is never mistaken for a fully-closed one.

axiom vs sorry — they mean different things

Never use sorry as a stand-in for a citation: sorry says "not done yet", a named axiom says "assumed on purpose, here's the reference".

Audit what a result depends on

Lean tracks axiom dependence transitively. Confirm exactly which assumptions a theorem rests on with #print axioms:

python .claude/skills/axiom-lean-prover/scripts/axle_check.py --code '#print axioms my_main_theorem' --json
# reads back e.g.  'my_main_theorem' depends on axioms: [key_prerequisite, propext, Classical.choice, …]

propext, Classical.choice, Quot.sound are Mathlib's standard trusted axioms (expected — ignore them). Any of your own axiom names appearing there are cited assumptions that must be listed in REPORT.md's Verification Status, so the result is honestly reported as "verified modulo {…}".

Project Structure (convention)

proofs/
├── Definitions.lean   # types, structures, notation
├── Lemmas.lean        # supporting lemmas
└── MainTheorem.lean   # main results

axle_check.py --dir proofs/ checks them together in dependency order. Because files are concatenated, use one namespace and close it, or keep everything in a single file — repeated namespace Foo ... end Foo blocks across files are fine, but an unclosed namespace will nest into the next file (a harmless linter warning). For multi-file proofs, write each file's local import Proofs.X as usual; the verifier strips them and concatenation supplies the actual definitions.

Verification Workflow (Per Proof)

# Step 1 — write the statement with `sorry`, confirm it type-checks
python .claude/skills/axiom-lean-prover/scripts/axle_check.py proofs/MainTheorem.lean --statement-check
#   exit 0 → statement is well-typed; proceed to prove it
#   exit 1 → fix the statement (read the error) before attempting the proof

# Step 2 — replace `sorry` with tactics, re-check
python .claude/skills/axiom-lean-prover/scripts/axle_check.py proofs/MainTheorem.lean
#   exit 0 → COMPLETE ✓
#   exit 2 → still has sorry
#   exit 1 → tactic/type error; read the message and adjust

# Step 3 — final whole-project check
python .claude/skills/axiom-lean-prover/scripts/axle_check.py --dir proofs/
#   exit 0 with STATUS: COMPLETE → all proofs formally verified

Looking up Mathlib lemmas without a local Lean

#check works through the API — submit it as a snippet:

python .claude/skills/axiom-lean-prover/scripts/axle_check.py --code '#check @Nat.add_comm'
#   COMPLETE → the name exists; the #check output appears in --json `lean_messages.infos`
#   ERROR "unknown identifier" → it does not exist (wrong name)

Add --json to read the #check type from lean_messages.infos. Combine several #check lines in one snippet to probe many names in a single request.

Writing Proofs

Theorem / Lemma syntax

import Mathlib

namespace Proofs

-- Statement only (use sorry to check the type compiles first)
theorem my_theorem (n : ℕ) (h : n > 0) : n * 2 > n := by
  sorry

-- Full proof
theorem my_theorem (n : ℕ) (h : n > 0) : n * 2 > n := by
  omega

end Proofs

Definitions

def MySet := Finset ℕ

structure MyGraph where
  vertices : Finset ℕ
  edges    : Finset (ℕ × ℕ)

inductive Tree (α : Type) where
  | leaf : Tree α
  | node : α → Tree α → Tree α → Tree α

Core Tactic Reference

Note: interactive tactics exact?, apply?, simp? are not useful through the API (they need an editor). Use #check snippets and Mathlib naming conventions instead.

Searching Mathlib

Naming conventions (predict lemma names)

• Nat.add_comm — commutativity of + on ℕ
• Int.mul_neg — negation rule for * on ℤ
• List.map_comp — map composed with comp
• Type-namespaced: Finset., Set., Matrix., Polynomial.

#check to confirm a name exists (via the API, see above)

Web search

leanprover-community.github.io/mathlib4_docs for the exact namespace.

Interpreting Lean Error Messages

(Identical to local Lean — same kernel, same Mathlib. These appear in lean_messages.errors with a -:line:col position.)

Proof Skeleton Template

import Mathlib

namespace Proofs

/-- One-line description of what this proves. -/
theorem theorem_name
    (param1 : Type1) (param2 : Type2)
    (hyp1 : Condition1) (hyp2 : Condition2) :
    Conclusion := by
  sorry  -- replace with the actual proof

/-- Supporting lemma used in the main theorem. -/
lemma lemma_name (n : ℕ) : n + 0 = n := by
  simp

end Proofs

Tips

• State first, prove later. Stub every statement with sorry, run with --statement-check until all type-check, then fill in proofs bottom-up.
• Automation first. Try omega, simp, ring, norm_num, linarith, aesop before hand-writing tactic steps.
• Mind rate limits. AXLE allows 20 concurrent requests per key. The check is fast (50–100 ms), but check meaningful units (a full file) rather than spamming one-line edits.
• Pin the environment with --environment (or $AXLE_ENV) and record it in your report for reproducibility — it determines the Lean and Mathlib versions used.