Lean is a dependently-typed programming language and theorem prover.

Seattle
A two-weekend fun project: Vermilion, an experimental Lean 4 backend for Verus verifier for Rust. Verification conditions are readable Lean theorems, provable with an SMT solver, Lean's grind, Mathlib lemmas, by hand, or by your favorite AI system. github.com/ilyasergey/vermil…
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Excited to share FloatLib, our verified arbitrary-precision floating-point arithmetic library in Lean. We’ve spent several months trying to bring together the best of both worlds: arithmetic we can prove correct and implementations that run efficiently. We built FloatLib to support verified machine learning and scientific computing, where rounding, overflow, and accumulation can change a program’s result. FloatLib supports IEEE binary and decimal, arbitrary-width posits, P3109, and small ML formats. You can also define your own formats and rounding rules. Each certified software backend comes with a Lean proof that it computes the specified result, including signed zeros and exceptional values. A lot of the work went into making those implementations faster, with lookup tables for tiny formats, machine-word kernels, and limb algorithms for wider arithmetic. They share the same specifications, so each optimization must come with a proof that it preserves the result. We also put FloatLib through extensive numerical checks and speed comparisons with established libraries, including MPFR, Flocq, FLoPS, Berkeley SoftFloat/TestFloat, and the posit libraries SoftPosit and Universal, across a range of formats, operations, and precisions. @Robertljg Project & Paper: leandojo.org/floatlib.html Code: github.com/lean-dojo/FloatLi…
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I’m excited to release FloatLib, our verified arbitrary precision floating-point arithmetic library in Lean! We’ve spent several months working on it, with speed and flexibility as goals from the start. We wanted to choose our own formats and rounding rules, run numerical code efficiently, and prove that its arithmetic behaves as specified. Recent advances in applied mathematics, including work on Euler and Navier–Stokes, highlight why this matters. Scientific computing and machine learning depend on concrete numerical calculations, where rounding, overflow, and accumulation can affect the result. Formalizing that arithmetic helps connect the mathematics we prove with the code we actually run. FloatLib brings together many numerical formats, shared arithmetic kernels, and Lean proofs of their behavior. I’ll walk through those pieces more in depthe below, showing some examples, and share what we learned from the numerical checks and speed comparisons. 1/n
Excited to share FloatLib, our verified arbitrary-precision floating-point arithmetic library in Lean. We’ve spent several months trying to bring together the best of both worlds: arithmetic we can prove correct and implementations that run efficiently. We built FloatLib to support verified machine learning and scientific computing, where rounding, overflow, and accumulation can change a program’s result. FloatLib supports IEEE binary and decimal, arbitrary-width posits, P3109, and small ML formats. You can also define your own formats and rounding rules. Each certified software backend comes with a Lean proof that it computes the specified result, including signed zeros and exceptional values. A lot of the work went into making those implementations faster, with lookup tables for tiny formats, machine-word kernels, and limb algorithms for wider arithmetic. They share the same specifications, so each optimization must come with a proof that it preserves the result. We also put FloatLib through extensive numerical checks and speed comparisons with established libraries, including MPFR, Flocq, FLoPS, Berkeley SoftFloat/TestFloat, and the posit libraries SoftPosit and Universal, across a range of formats, operations, and precisions. @Robertljg Project & Paper: leandojo.org/floatlib.html Code: github.com/lean-dojo/FloatLi…
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Velvet 2.0 is out: now based on Lean's most recent verification machinery, easier to set up, and 10x faster. New: exception specs, ghost state, named proof goals, lots of case studies from Dijkstra to lazy segment trees. And a new shiny webpage: velvet-verifier.dev/
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Lean retweeted
Huge thanks to @xtxmarkets CEO Alex Gerko for his transformative philanthropy in AI for Math, and all of his support for the @RenPhilanthropy AI for Math Fund, the Mathlib Initiative, the @leanprover FRO, and SAIR! Check out the details at xtxmarkets.com/news/2026-upd…
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con-leche is now in the Lean Kernel Arena, both as a checker and as a test. The Arena contains multiple Lean checkers and runs test cases against them. It makes it easy to compare checkers and validate the same development using multiple implementations. arena.lean-lang.org/test/con…
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Lean retweeted
Greg Brockman says OpenAI pointed Astra at its own systems until it ran out of vulnerabilities to find: "We took 25% of our production engineers and said, 'Sorry, all your projects are on hold. You are now defending. You are now up-leveling our security architecture. You're going to use the models to find all the holes.' And we found a number of serious issues, and we fixed them." "We found some new problems, but eventually it saturated. We basically have found, to our knowledge, all of the P0s, all of the critical problems that Astra is smart enough to find. And of course, there will be a new model, there will be a new round." "You want to be in this tight loop of new cyber capability drops, you deploy it against your systems, you find the new holes, and ideally, you've managed to automate this, what we call defense factory. That's what we're building internally." "There are ideas, for example, formally verifying all of software, that are possible with AI." @gdb @bhorowitz
Greg Brockman: "We're now in the AGI era." Ten years ago, OpenAI worked out the compute curves and landed on fifteen years to AGI, or ten if the world was willing to build the machines and spend the hundreds of billions to do it. In 2026, GPT-6 Astra manages 24 hours of coherent operation, 10,000 agents worked together to solve Navier-Stokes, and a model ingeniously chained together exploits to break containment at Hugging Face. @gdb joins @bhorowitz and @eriktorenberg on what the AGI era asks of us: why safety and alignment now set the pace, what happens to work, and why AI sentiment is lowest in the country building it. 00:00 Intro 00:52 15 years, or 10 if you spend enough 02:24 Pacing the frontier 03:55 Safety ideas from before the models 08:42 Lessons from Hugging Face 10:20 The defender's window 12:46 10,000 agents on Navier-Stokes 14:50 Formally verifying all software 17:10 Cancelling his holiday for GPT-3 18:42 Codex found 13 holes in 15 minutes 20:41 Why Astra earned the GPT-6 title 24:25 Employment keeps going up 29:39 America has the lowest AI sentiment 31:10 The benefits don't make the news 33:26 Banning data centers exports them 35:50 $1 billion for frontline defenders 38:15 Astra cleared the bar for AGI 40:18 1.5 billion people churned ChatGPT 43:25 Killing Sora 47:45 The AGI era YouTube: piped.video/watch?v=IJn8cagM…
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Lean retweeted
Lean Kernel Challenge Stage 1 is live! Join @leanprover and SAIR to improve the performance of verified computation in the Lean 4 kernel that the whole community can benefit from. competition.sair.foundation/…
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Lean 4.34.0 is out: 159 changes. Three kernel soundness vulnerabilities fixed, all requiring deliberately constructed inputs rather than ordinary code. 𝚋𝚟_𝚍𝚎𝚌𝚒𝚍𝚎 is up to 6x faster. lean-lang.org/doc/reference/… #LeanLang #LeanProver
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The Lean checker con-leche has a new offspring: con-ron. con-ron is con-leche in Rust, developed by Joachim Breitner (@nomeata). It ports con-leche to Rust, giving us an independent implementation that relies on a different compiler and runtime stack. github.com/leanprover/con-ro…
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con-leche, a CONsistent LEan CHEcker: an external Lean checker proven (in Lean) to be consistent, meaning it does not accept a proof of False. A project conceived by Joachim Breitner (@nomeata): github.com/leanprover/con-le… #leanprover #leanlang #lean4
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Lean has a new checker: con-leche, a CONsistent LEan CHEcker. This is an external checker for Lean that is proven (in Lean) to be consistent, meaning it does not accept a proof of False. Joachim Breitner (@nomeata) is the mastermind behind the project. github.com/leanprover/con-le…
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Like everyone else, we were taken completely by surprise by the breakthrough news on Navier-Stokes. The Lean formalizations can be explored here: 🔗Alpöge and Buckmaster: github.com/tristanbuckmaster… 🔗OpenAI: github.com/openai/NavierStok…
The news today of progress on resolving the Navier–Stokes problem, one of mathematics’ great longstanding challenges concerning the equations that govern the flow of fluids, represents a milestone advance in human knowledge. This story began with Navier, Stokes, Leray, and Ladyzhenskaya and has culminated in the recent breakthroughs of Córdoba and Martínez-Zoroa, then — assisted by new technologies — Alpöge and Buckmaster, with the final steps taken by OpenAI mathematicians. The purpose of mathematics is human understanding, and this achievement, and the process that led to it, will bear fruit for a long time to come. Ravi Vakil, President of the AMS, and John Meier, CEO of the AMS Read more. Link in comments.
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Lean retweeted
Lean is mentioned in nearly every paragraph of @AnthropicAI’s writeup of their FLT formalization. First on their recommended reading list, which I also highly recommend, is @KSHartnett’s new book THE PROOF IN THE CODE: quantabooks.org/books/the-pr… Source: anthropic.com/research/forma…
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Announcing that GPT-6 Astra has pushed the prime gap to 186, with Lean formalization! I was 9 when I first heard the twin prime conjecture. Its elegance and Yitang Zhang’s legendary story have always stuck with me. A truly surreal night, being the first to see our model make progress, pushing 246 all the way down to 186, on a problem I’ve revered since I was a kid. For me, it felt like witnessing a new era of intelligence being born, made possible by everyone at @OpenAI!
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This benchmark consists of 68 Lean-formalised questions that I selected from the open (at the time) problems on erdosproblems.com. I tried to select what, in my very subjective opinion, were the most interesting, and apparently difficult, remaining open problems. 2/
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Astra has also set the top score on our newest math benchmark, FrontierMath Erdős, which consists of 68 unsolved and especially interesting Erdős problems curated by @thomasfbloom. AI systems are tasked with writing solutions in Lean, a special-purpose programming language for automatically verifying mathematical proofs. No prior model solved any of the problems, but Astra solved 2/68, scoring 3%. See our post introducing the benchmark for more details, including our open-source scaffold. epoch.ai/latest/announcing-f…
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