Leanstral 1.5: Proof abundance for all

374 98 19 days ago

boulos• 19 days ago

This is nice work, but I found the bug finding example to be weird:

One such bug was in the sign function for zigzag decoding of the datrs/varinteger library. On input Std.U64.MAX, the expression (value + 1) overflowed, causing crashes in debug mode and silent corruption in release mode—an edge case that testing and fuzzing would typically miss.

In what way would this boundary condition case be considered something that "testing [...] would typically miss"? It's certainly something that bad tests would miss or not think about, but I find that (a) careful people and (b) ML coding systems are actually really good at "oh, I should test the extreme values". Especially for things that parse user input.

I'm curious if they found other bugs that were more interesting, but found them too hard to explain quickly.

fjdjshsh• 19 days ago

Maybe it's not something they would "typically miss", but, from proof by existence, it's something they sometimes miss.

It does speak to the benefits of using lean in that you don't need to be clever about the different examples you test.

Exoristos• 19 days ago

Yes, it's basic QA. If tests missed this kind of thing, they would be of much more limited use than we generally expect them to be. It raises questions about the authors' background.

Groxx2• 19 days ago

particularly "and fuzzing", yea. fuzzing generally does intentionally explore boundary values, from what I've seen. for an encoding library like this, I think it's fair to say that fuzzing is a baseline expectation for any decent code, and it almost certainly would've caught this in seconds.

--- edit

concretely, I made a very simple round-trip test with proptest, and got dozens of failures and this in less than a second:

    thread 'signed_round_trip' (50528) panicked at tests/test.rs:72:1:
    Test failed: attempt to multiply with overflow.
    minimal failing input: value = 4611686018427387904
        successes: 2
        local rejects: 0
        global rejects: 0

Quekid52• 19 days ago

Because this is garbage PR. That's it.

Every property-based testing system (invented ca. 1980) will explore boundary values. The semantics (or lack thereof) of C and C++ can make this difficult to actually test for because the compiler is allowed to say "test passed" to any input leading to UB.

vlovich123• 19 days ago

Property based testing is good at generating boundary values for inputs. But for any more complicated piece of code getting boundary value coverage of interior values is an open problem that requires instrumentation feedback to understand branch coverage and value coverage of the code that got tested. It’s not an easy thing at all.

teiferer• 19 days ago

The semantics (or lack thereof) of C and C++ can make this difficult to actually test for because the compiler is allowed to say "test passed" to any input leading to UB.

I get what you are saying but does this actually apply to a test? If the code under test is in one compilation unit and the test harness in another and they are linked together then the UB optimization issue ends at the API boundary and can't possibly make the test pass ..?

satvikpendem• 19 days ago

I also submitted the HuggingFace link itself here: https://news.ycombinator.com/item?id=48779902

nullc• 19 days ago

It would be nice if special purpose models provided a some diverse examples of exactly the input required to get its expected performance on a mix of problem types. Maybe also a document intended for LLMs to read that advises on prompt construction.

I've found that you can get wildly different quality results from these sorts of models due to seemingly insignificant differences in prompt construction. It would be much easier to guess at what it wants if I could just see some RL transcripts -- and so the model author is in a much better position to provide initial advice.

moonset2• 19 days ago

I gave Codex with GPT-5.5 High this prompt:

    Identify bugs in [datrs/varinteger](https://github.com/datrs/varinteger) . Do NOT look at the GitHub issues, just inspect the source

It also found the bug that Leanstral 1.5 found and the authors highlighted. I think this bug wasn't especially tricky; it's just a case of too few eyeballs on this repo.

Congrats on the release regardless! Excited for the direction Lean + automated AI proofs are headed.

Disclosure: I work at OpenAI.

8note• 19 days ago

the mechanism is whats interesting, rather than whether it could do it.

this sounds like a great tool to add to the toolbelt, as part of the "how do we handle all the code output from LLMs" problem

noperator• 19 days ago

Leanstral 1.5 has 6B active parameters. How many parameters does GPT-5.5 have?

lioeters• 19 days ago

We can run Leanstral locally on commodity/consumer hardware. Nuff said.

ilc• 19 days ago

Given that they directly compare to GPT-5.5 in their documentation. This comes off as puppy kicking to me. They state it is not SOTA, even IN its domain!

Honestly: Think twice before dragging your firm into what you say.

Disclaimer: I speak for myself. Not any firm I am associated with.

moonset2• 18 days ago

Sorry, I recognize that these are different classes of model and didn't mean to punch down. I'm genuinely excited for the work Mistral is doing in this space!

adev_• 19 days ago

It also found the bug that Leanstral 1.5 found and the authors highlighted

This is a little bit like someone pointing the moon and you look at the finger.

The formal proof domain goes way beyond just finding bugs.

It has tons of usages in term of functional safety, protocol validation, cryptography, etc...

The fact Mistral tackle this kind of problem is both smart and not so surprising.

Smart because it is niche enough that they do not front face the big competitors (yet).

No so surprising because the French labs have a well known and long time expertise with formal proof tools (Coq and all its Ocaml associated tools). It has been historically mainly pushed by the aerospace and train industries (Airbus, Dassault, Alsthom).

InsideOutSanta9• 19 days ago

GPT-5.5 is what, a trillion+ parameter model? I think the insight here is that you can do this with a tiny model.

henryrobbins00• 19 days ago

Try out Leanstral 1.5 on the latest version of OpenATP! OpenATP is an open-source Python package and CLI for agentic automated theorem provers. It natively supports running provers locally in Docker or remotely in Modal sandboxes.

GitHub: https://github.com/henryrobbins/open-atp

Docs: https://open-atp.henryrobbins.com

strongly-typed• 19 days ago

Lean is such a wonderful language. So hyped by these releases.

Groxx2• 19 days ago

One such bug was in the sign function for zigzag decoding of the datrs/varinteger library. On input Std.U64.MAX, the expression (value + 1) overflowed, causing crashes in debug mode and silent corruption in release mode—an edge case that testing and fuzzing would typically miss.

that library is: https://github.com/datrs/varinteger

it seems probably correct, as there's an identical issue filed on that repo a week before this was published: https://github.com/datrs/varinteger/issues/8 (is this a leanstral employee? they have almost no info and only very sparse activity. or did leanstral perhaps just pick up this issue?)

it's a tiny, surprisingly-poorly tested, long-untouched (8y) library: https://github.com/datrs/varinteger/blob/master/tests/test.r... that has about 1k downloads per day: https://crates.io/crates/varinteger [1] which seems rather low.

I don't think I'd consider that such a smashing success that it's worth bringing up as the sole example tbh. though automated detection is certainly useful. or is this a noteworthy accomplishment for this sub-field? I haven't played with proof-writing LLMs, but given the paucity of training data I wouldn't be surprised if they're a bit rough compared to general coding.

1: https://crates.io/crates/varinteger lists it as https://github.com/mafintosh/varinteger-rs which redirects to https://github.com/datrs/varinteger , so despite looking different at a glance it does appear to be the same library

frostlynx• 19 days ago

The problem with proof is that it’s a bit hard sometimes to convey the value. The point is not to find bugs, but to prove that there are none (of a certain class; under certain assumptions; etc). But it’s a hard story to sell, so often the marketing is around “look at this bug we found”.

Eridrus• 19 days ago

I would be much more interested in "here is a provably memory safe version of openssl with all its memory safety bugs fixed"

Agingcoder• 19 days ago

Yes. Most people don’t actually understand what a program proof is - the answer is usually ‘I have very good tests’.

Now, go write the code for an artificial heart , and sleep at night thanks to strong testing !

andai3• 19 days ago

Discussed the other day:

https://news.ycombinator.com/item?id=48738938

andai3• 19 days ago

Halfway thru the article it shows a comparison with several frontier-ish LLMs. But they're all from half a year ago. "Our new model is better than all these Chinese models from 3 generations ago" is pretty funny to me.

rtaylorgarlock3• 19 days ago

Agreed, though open weights + relatively small is still headline worthy. This thing really cooks.

dannyw• 19 days ago

It’s a 6bn model. Totally different class. I’m more excited about “frontier small language models” tbh.

andai3• 19 days ago

It's a 119B model, 6B active.

That's still 3-10x smaller than the other models in that graph though (400B, 1T, 1.5T).

RossBencina• 19 days ago

Curious that they are pitching Lean 4 for formal verification. I thought that this was more the domain of Isabelle/HOL and TLA+. At least I would have expected a model trained at using all three. Maybe also Isabell/Isar, which seems preferable for forward derivations in linear algebra. Could anyone shed some light on this?

nextos• 19 days ago

It is true that Lean has seen relatively little adoption in software verification compared to e.g. Isabelle and Rocq (previously Coq). Even Agda has had more traction in that domain.

However, Lean is currently gaining significant momentum as an alternative, particularly due to its capabilities as a general-purpose functional programming language.

Personally, I think something based on Hoare or separation logic would be more practical as it'd be easier to align requirements with specifications. I like Dafny and F*.

ChrisArchitect2• 19 days ago

[dupe] https://news.ycombinator.com/item?id=48738938

rtaylorgarlock3• 19 days ago

Have you ever been downvoted for calling 'dupe?' I once was downvoted after calling dupe on a link posted thrice. HN is an interesting place to hang out, that's for sure.

ChrisArchitect2• 19 days ago

oh of course, unfortunately people's self-interest in pushing their own submissions and not being told they missed a story causes adverse reactions, but a bit of sacrifice worth keeping the site fresh and discussions focused.

zuzululu3• 19 days ago

I applaud mistral's efforts but reading this release made me realize that Europe is far far behind and that once the gap is solidified I don't think its recoverable in the same way Canada's brain drain had on its economy

The best and the brightest from Europe have no incentive to build in Europe when they can do it in America and be compensated and treated far better

bjelkeman-again2• 19 days ago

At this point I wouldn’t move to USA if you paid me double the salary. There are more things in life than money.

That said, if (or when) the progress of the LLMs flatten out, then I think even Europe can catch up in a few years. If they don’t, and that seems unlikely to me, if the required compute needs to increase at the rate it does today, then I am not sure any of us can predict where society ends up.

InsideOutSanta9• 19 days ago

I think there is a non-zero chance that Europe stumbled into an optimal scenario where they avoided all the losses incurred by US companies, but still benefit from the research. Having said that, doing so would require moving at the exact right time.

zuzululu3• 19 days ago

Sure some people like to make less money and pay more taxes for ideological reasons. I respect that.

InsideOutSanta9• 19 days ago

Fortunately for Europe, the US is doing its best to make itself both an undesirable and unavailable immigration target.

pbkompasz• 19 days ago

treated far better != earn more money

vrganj• 19 days ago

As somebody who moved to the US and then back to Europe again, you may get more money there, but you'll pay for it with a much worse environment and lifestyle.

Life's not all about the number on your bank account. Once you're past a certain level (which a competent engineer can easily reach in Europe as well), the marginal utility of money diminished quite quickly.

raphinou• 19 days ago

Can this be useful for someone with no prior knowledge of lean? I'd like to verify a software I'm working on, but I have no experience in formal verification. Can I get useful result with the spec, the code and some (limited) learning time on my side?

rubendev• 19 days ago

I think at minimum you would need to understand which theorems you want to prove about your code, and how to express those in Lean. Otherwise you won’t be able to verify the output. It may have proven some statement that is machine checked to be correct, but it’s pointless if you don’t understand what that statement means and if it covers what you want to verify about your code.

camkego• 19 days ago

Read this section of the article “ Bug Discovery: Finding Hidden Flaws”, they appear to have used the model on open source Rust to find issues starting with just the Rust code. You might be also able to have conversations that help you write the Lean to verify your application, but I’m not certain about this.

rzmmm• 19 days ago

You need to understand the bits you are trying to prove, but not the full proof. It's more like reading haskell types than math, even though the vocabulary is heavily inspired by math.

reinitctxoffset3• 19 days ago

I've gone from zero knowledge of lean4 to the point where I'm doing most of my coding with it in ~6 months, and this was dramatically helped by how facile the AI assist is: it's remarkable how consistently fluent models are in lean4. I've found this to be true of the near frontier and smaller local models alike, LLMs just seem to get lean4.

I still have a ways to go before calling myself a lean4 expert, but I don't need assist to get useful programs anymore.

The ability to start with very little knowledge and still be able to trust parts you don't fully understand is a real unlock on learning progress: it's both practical and motivating to get useful programs you can rely on with incomplete knowledge, it sort of drags you in. You're bounded by the subset of the language that describes your axiom and proposition surface, not the subset that describes the intermediate steps. Over time as your ambition goes up, you need to understand more to do more things, but you can operate safely at level N+1 in a sense.

It's also just a delightful programming language irrespective of its theorem proving role, and it's remarkably fast. I've got it bolted to io_uring and in many cases it blows the ass off of C++ with libuv or Rust with Tokio. Now and again you'll see some huge tail at the p99.99 latency or something and you go make a number fixed width or something, but you have to tune C++ and Rust too.

numeri• 19 days ago

Are you writing general use programs in it, then? Have any good examples?

andrewcl• 18 days ago

Any chance you can share any of your process? Super interesting to hear you use lean as a meta language, I'd be curious to see how you've harnessed this (as I'd like to do the same)!

InsideOutSanta9• 19 days ago

There's a lot of criticism of Mistral being unable to compete with large model, and that's fair. But I think it dismisses what Mistral is actually doing, which is making specific capabilities available at high quality in tiny models.

I do a lot of OCR, file analysis, stuff like that. I use Mistral for that. I put 100$ into my account, and it just runs for a year without any worries about the amount of requests I make, because the cost is minuscule. That's valuable, even if it doesn't compete with Opus 4.8.

ekjhgkejhgk• 19 days ago

Stupid Europoors, optimizing for making a good product, instead of optimizing for making as much money as possible /s

nok22kon3• 19 days ago

big AI labs make so much money because they have a good (amazing) product

nicman23• 19 days ago

i would argue it is more valuable

stavros• 19 days ago

But how does it compete on OCR? I find that having good quality at a cheap price is more niche than having the best quality at 10x the cheap price, because for most use cases you want to pay a bit more if it saves you mistakes later.

InsideOutSanta9• 19 days ago

It's really good. I didn't do any type of statistical evaluation or comparison to other models, but it's so good that it doesn't matter to me if there's an option that might be even better.

embedding-shape• 19 days ago

It seems to heavily depend on what exactly you're transcribing, the performance/quality between them is really uneven. Some models work really well for old cursive but then fail reading 8-bit segment LCD digital fonts, vice-versa or any combination out there.

Basically, to find the answer you really need your own benchmark you run with real examples from what you want to do. Basically the same goes for anything ML nowadays as the public benchmarks cannot really be trusted to give you any sort of indication on how we'll it'd work for you.

baridbelmedar• 19 days ago

I'm not sure the "a year of document processing for under 100 USD/y" is such as great thing as you think it is (at least not for European competitiveness)... It means Mistral is essentially setting a revenue ceiling very low. OCR is a commodity at this point, and open source models, AWS, etc already do it out of the box.

Plus, you can't really build loyalty on a 100 USD/Y price tag. Since there are no switching costs holding them back, those buyers will leave the moment somebody offers a lower rate. An easily cloned, low cost tool with zero customer lock in is not a business. It is a feature.

That might sound great for the buyer (you), but it is a terrible strategy if we want a European company to compete long term against global competitors on actual product merit instead of just regulatory arbitrage.

vincnetas2• 19 days ago

well all commodities are like this. replace AI with milk, or plastic. It's easy for me to just move to different milk provider, this does not mean that milk industry is not a business.

And yes, its good that "its good for buyer" after all we do business so that living would be nicer, not the other way around (live to do business)

InsideOutSanta9• 19 days ago

How is that different from any other model provider, though? I used to use Anthropic for 100% of my code. Now I use GLM 5.2 for half of it, and as soon as something better appears, I'll use that.

nejch• 18 days ago

Yeah, I think also efforts like Docling and similar are showing that smaller, specialized models with the right tooling might be more effective (and efficient) at this than throwing everything at Opus.

They don't seem to release as much open-weights at Mistral as they used to though :)

easygenes• 19 days ago

Was fun to see their developers make nods to Le Chaton Fat in the announcements for this on Twitter.

I suspect a true "big new general-purpose" model is around the corner from them, whether or not they were in on Le Chaton Fat for real. They've mentioned it after the media circus. Hopefully more creatively named than just "Large 4".

bjt12345• 18 days ago

I find it a bit ridiculous to be critisizing Mistral, of all companies, as falling behind the Frontier models.

Firstly, who hasn't fallen behind? Grok...Meta....? A lot of big companies are struggling.

Secondly, Mistral are trying to solve a different problem.

Finally, Mistral should be congratulated for staying in the race for so long now.

canada_dry• 18 days ago

Considering the (apparent) heaps of money and brains thrown at GROK I'd argue MISTRAL is relatively futher ahead.

Every time I go back to trying GROK it is an abysmal disappointment.

Leanstral 1.5: Proof abundance for all

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