Welcoming Repositories Index Lean Ecosystem
Top Welcoming Open-Source Lean Repositories
Top welcoming Lean repositories. Scored by real 90-day external PR merge rates, maintainer review turnaround speed, and first-time contributor success.
Updated daily • Thu, 15 Oct 2026 UTC
Tracked Repos
39
>100 stars verified
Avg Merge Rate
47%
90-day external PRs
Open GFIs
55
Beginner friendly issues
Welcoming Tier
21
S & A Tier maintainers
Ranked Lean Repositories
Showing top 39 of 39 ranked repositories
| Rank | Repository | Tier | Score | Merge Rate | Good First Issues |
|---|---|---|---|---|---|
| #1 | S•Elite | 70.2 | 78.9% | - | |
| #2 | An AIs-welcome Lean library downstream of Mathlib: AI handle the implementation and review, humans write the roadmaps and review rubrics | A•Welcoming | 68.8 | 10.2% | 4 |
| #3 | A collection of formalized statements of conjectures in Lean. | A•Welcoming | 68.6 | 43.2% | 11 |
| #4 | Lean documentation authoring tool | A•Welcoming | 67.9 | 59.5% | - |
| #5 | A Lean library for machine-checked cryptographic proofs. | A•Welcoming | 66.5 | 68.1% | - |
| #6 | The "batteries included" extended library for the Lean programming language and theorem prover | A•Welcoming | 65.1 | 56.5% | - |
| #7 | Catalog Of Math Problems Formalized In Lean | A•Welcoming | 64.5 | 90.6% | - |
| #8 | The Lean Computer Science Library (CSLib) | A•Welcoming | 61.5 | 36.0% | 1 |
| #9 | leanprover/lean49,465 Lean 4 programming language and theorem prover | A•Welcoming | 61.2 | 7.1% | - |
| #10 | Ongoing Lean formalisation of the proof of Fermat's Last Theorem | A•Welcoming | 61.1 | 65.5% | - |
| #11 | Formally Verified Arguments of Knowledge in Lean | A•Welcoming | 60.8 | 39.8% | - |
| #12 | Definitional implementation of Cedar language and utilities for DRT | A•Welcoming | 60.5 | 64.7% | - |
| #13 | Lean 4 port of Iris, a higher-order concurrent separation logic framework | A•Welcoming | 60.1 | 73.6% | 1 |
| #14 | Formally verified smart contracts gives mathematical certainty across all inputs and execution paths. We bet that agents will make full formal verification practical. | A•Welcoming | 59.8 | 94.2% | - |
| #15 | A formalized proof of Carleson's theorem in Lean | A•Welcoming | 59.3 | 64.3% | - |
| #16 | A project to digitalise results from physics into Lean. | A•Welcoming | 59.3 | 6.5% | 38 |
| #17 | Wasm interpreter in lean, designed for reasoning | A•Welcoming | 59.2 | 47.8% | - |
| #18 | The Lean reference manual | A•Welcoming | 58.5 | 73.1% | - |
| #19 | Formalization of Mathematical Logic | A•Welcoming | 58.5 | 42.2% | - |
| #20 | A•Welcoming | 58.2 | 43.8% | - | |
| #21 | Blueprint for the PNT+ Project | A•Welcoming | 57.4 | 31.3% | - |
| #22 | A blueprint for a formalization of infinity-cosmos theory in Lean. | B•Solid | 56.0 | 71.4% | - |
| #23 | An evaluation benchmark for undergraduate competition math in Lean4, Isabelle, Coq, and natural language. | B•Solid | 52.9 | 87.5% | - |
| #24 | Tactics for discharging Lean goals into SMT solvers. | B•Solid | 50.8 | 100.0% | - |
| #25 | B•Solid | 50.4 | 60.0% | - | |
| #26 | thomasahle/sunfish3,270 Sunfish: a Python Chess Engine in 111 lines of code | B•Solid | 50.4 | 65.4% | - |
| #27 | A verifier for automated and interactive proofs about transition systems. | B•Solid | 49.3 | 37.5% | - |
| #28 | White-box automation for Lean 4 | B•Solid | 48.9 | 83.3% | - |
| #29 | teorth/analysis1,943 A Lean companion to Analysis I | B•Solid | 45.4 | 64.7% | - |
| #30 | B•Solid | 44.4 | 50.0% | - | |
| #31 | Natural Number Game | B•Solid | 44.3 | 28.6% | - |
| #32 | ATLAS Autoformalized Textbook Library At Scale | B•Solid | 41.5 | 26.3% | - |
| #33 | A type-safe, formally verifiable HDL compiler in Lean 4. Inspired by Clash, built for high-assurance hardware synthesis. | B•Solid | 40.2 | 64.0% | - |
| #34 | Source code for the Mathematics in Lean tutorial. | C•Moderate | 34.9 | 12.5% | - |
| #35 | The math library of Lean 4 | D•Risky | 29.9 | 0.0% | - |
| #36 | D•Risky | 17.6 | 0.0% | - | |
| #37 | Helper toolkit for creating your own Lean 4 UserWidgets | D•Risky | 13.0 | 0.0% | - |
| #38 | digama0/mm0412 Metamath Zero specification language | D•Risky | 8.0 | 0.0% | - |
| #39 | Lean certificates accompanying Navier-Stokes and Euler results | D•Risky | 7.5 | 0.0% | - |
How to Get Your Lean PR Merged
1
Check C-Rank™ First: Pick projects with S or A tiers to avoid maintainer ghosts and slow review cycles.
2
Run Local Verification: Ensure test suites and formatting linters pass before opening a pull request.
3
Submit in Maintainer Windows: Check the repository report card to submit when maintainers are most active.
Get this week's top welcoming repos + fresh Good First Issues for Lean
Free weekly email, scoped to Lean. No account needed - confirm once and unsubscribe anytime.
Frequently Asked Questions: Lean Open Source
What is the most welcoming Lean open-source repository?
The highest-ranked Lean repository on GetMerged is currently leanprover/comparator, with a C-Rank score of 70.2/100 and a 78.9% PR merge rate.
How can I find good first issues in Lean?
You can browse verified beginner-friendly issues in Lean by visiting our curated Good First Issues directory at getmerged.abhishekco.de/good-first-issue/lean.
What makes a Lean repository ‘Super Welcoming’ on GetMerged?
A ‘Super Welcoming’ (S-Tier) Lean repository demonstrates an external PR merge rate above 80%, a median first response time within 24 to 48 hours, active review feedback, and clear onboarding documentation.