Fixing "theoretical" nondeterminism for a totally closed individual input-output pair doesn't solve the two "practical" nondeterminism problems, where the exact same input gives different results given different preceding context, and where a slightly transformed input doesn't give a correctly transformed result.
Until those are addressed, closed-system nondeterminism doesn't really help except in cases where a lookup table would do just as well. You can't use "correct" unit tests or evaluation sets to prove anything about inputs you haven't tested.
There is no such thing as "exactly the same input, but with different preceding context". The preceding context is input!
If you were to obtain exactly the same output for a given input prompt, regardless of context, then that would mean that the context is being ignored, which is indistinguishable from the session not maintaining any context such that each prompt is in a brand new empty context.
Now what some people want is requirements like:
- The different wording of a prompt with exactly the same meaning should not change anything in the output; e.g. whether you say "What is the capital of France" or "What is France's capital" the answer should be verbatim identical.
- Prior context should not change responses in ways that don't have any interaction with the context. For instance, a prompt is given "what is 2 + 2", then the answer should always be the same, except if the context instructs the LLM that 2 + 2 is to be five.
These kinds of requirements betray a misunderstanding of what these LLMs are.
While I get that this is how LLMs work, I think you should think backwards from the user / from what AI as a field is aiming for and recognize that the „naive“ way of the parent to ask for reliable responses no matter what the „context“ is, is exactly what a good AI system should offer.
„The context is the input“ betrays a misunderstanding of what (artificial) intelligence systems are aiming for.
I agree mostly. They are all that you say, but if you think about the conditional distribution that you are learning, there is nothing preventing us in principle from mapping different contexts to the same responses.
It is rather a practical limitation that we don’t have sufficient tools of shaping these distributions very soundly. All we can do is throw data at them and hope that they generalize to similar contexts.
We have observed situations where agentic LLM traces on verifiable problems with deterministic (greedy) decoding lead to either completely correct or completely wrong solutions depending on the minutes on the clock which are printed as coincidental output of some tool that the LLM used.
I think there may be some mild fixes to current models available , for example it is worrying that the attention mechanism can never fully disregard any token in the input, because the softmax will always assign a > 0 weight everywhere (and the NN has no way of setting a logit to -infinity).
This directly causes that it is extremely difficult for the LLM to fully ignore any part of the context reliably.
However Yann LeCun actually offers some persuasive arguments that autoregressive decoding has some limitations and we may need something better.
> They are simple statistical predictors, now universal anwsering machines.
I see this a lot. I kinda' doubt the "simple" part, but even beyond that, is there any evidence that statistical predictor can't be a universal answering machine? I think there's plenty of evidence that our thinking is at least partially a statistical predictor (e.g. when you see a black sheep you don't think "at least one side of this sheep is black", you fully expect it to be black on both sides)
I'm not saying that LLMs _are_ universal answering machines. I'm wondering why people question that they are/they can become one, based on the argument that "fundamentally they are statistical predictors". So they are. So what?
Sure. But the context always starts with the first input, right?
And how can you guarantee—or why should you guarantee—that the reply to the first input will always be the same?
And if that’s not the case, how can we ensure the preceding context remains consistent?
If an input along with the context generated some random seed or hash this would certainly be possible. Just paste your seed over to your coworker, they supply it to the model and it contains all contextual information.
It's going to backfire. In real scenarios (not regression testing) users don't want to see the exact same thing twice out of the LLM in the same session in spite of trying to refine the result with more context.
There are going to be false positives: text that is subtly different from a previous response is misidentified as a duplicate such that the previous response is substituted for it, frustrating the user.
Not an expert, but I've been told RAG in combination with a database of facts is one way to get more consistency here. Using one of the previous examples, you might have a knowledge store (usually a vector database of some kind) that contains a mapping of countries to capitols and the LLM would query it whenever it had to come up with an answer rather than relying on whatever was baked into the base model.
> where the exact same input gives different results given different preceding context
Why and how is this a problem?
If 'preceding context' doesn't cause different results, it means you can simply discard the context. Why do I want that? It's not how I expect a tool to work (I expect vim responds differently to my input after I switch to the insert mode). It's absolutely not how I expect intelligence to work either. It sounds like the most extreme form of confirmation bias.
When the context is auto-generated and may include irrelevant data.
This is a common AI benchmark and has been for years before GPT-2 even existed. LLMs need to not get distracted by irrelevant facts and there are tests that measure this. It's the motivation for attention mechanisms, which are the breakthrough that enabled LLMs to scale up.
An example is translation. I MTLed some text recently where the name of a (fictional) city was translated about a dozen different ways. Sometimes you'd get a calque, sometimes you'd get a transliteration (including several wrong ones). Ironically "dumb" MTLs are often much more consistent about this than LLMs.
Why do you care about determinism in a probabilistic system? What difference does it make to the end user if the input "How do I X?" always produces the same deterministic output when semantically equivalent inputs "how do i x?", "how do I x", and "how do I X??" are bound to produce different answers that often won't even be semantically equivalent.
What LLMs need is the ability to guarantee semantically-equivalent outputs for all semantically-equivalent inputs, but that's very different from "determinism" as we understand it from other algorithms.
Not all LLM based applications are a user facing free form chat.
If you take an LLM that makes 10 tool calls in a row for an evaluation, any reduction in unpredictable drift is welcome. Same applies to running your prompt through DSPy Optimizer. [0] Countless other examples. Basically any situation where you are in control of the prompt, the token level input to the LLM, so there's no fuzziness.
In this case, if you would've eliminated token level fuzziness and can yourself guarantee that you're not introducing it from your own end, you can basically map out a much more reliable tree or graph structure of your system's behavior.
> If you take an LLM that makes 10 tool calls in a row for an evaluation, any reduction in unpredictable drift is welcome
why use an ambiguous natural language for a specific technical task? i get that its a cool trick but surely they can come up with another input method by now?
You aren't wrong, but that doesn't mean this level of determinism isn't useful. If you don't even have the level of determinism that the exact same input tokens produce the exact same output tokens, then it's very hard to share reproducible results with peers, which can be useful if you are say, red teaming an LLM to produce a very rare / unreliable output.
I'm actually working on something similar to this where you can encode information into the outputs of LLM's via steganography: https://github.com/sutt/innocuous
Since I'm really looking to sample the only the top ~10 tokens, and I mostly test on CPU-based inference of 8B models, there's probably not a lot of worries getting a different order of the top tokens based on hardware implementation, but I'm still going to take a look at it eventually, and build in guard conditions against any choice that would be changed by an epsilon of precision loss.
It would be very useful for AI platform customers. You could run prompts with 0 temperature and check if the results are the same making sure that AI provider is not switching the PRO model in the background for a cheap one and ripping you off.
For "bug" reproduction purposes. It is easier to debug a model if the same string always produces the same incorrect or strange LLM output, not every 100th time you run it.
If there is a bug (a behavior defined by whatever criteria), it is just a single path in a very complex systems with high connectivity.
This nonlinear and chaotic behavior regardless of implementation details of the black box makes LLM seem to be nondeterministic. But LLM is just a pseudo random number generator with a probability distribution.
(As I am writing this on my iPhone with text completion, I can see this nondeterministic behavior)
Was my thinking exactly - but also semantically equivalent is also only relevant when it needs to be factual, not necessarily for ALL outputs (if we're aiming for LLM's to present as "human" - or for interactions with LLMs to be natural conversational...). This excludes the world where LLMs act as agents - where you would of course always like the LLM to be factual and thus deterministic.
When you do MCP-style applications, an LLM is more like RegEx on steroids, and since you expect your regex to return the same matches on the same input, it is a very desirable attribute for LLMs as well. I would say it is more than desirable, it is necessary.
If i want to covert "how do I x" to `api.howTo("x")` it is very important that i get the exact same result every time.
Deterministic output is needed when LLMs are used for validations. This can be anything from input validation at runtime to a CI check leveraging LLMs. It can be argued this is not an acceptable use of AI, but it will become increasingly common and it will need to be tweaked/tested. You cannot tweak/test a response you don't know you're going to get.
yeah indeed, regression testing for chatbots that use RAGs would involve making sure the correct response comes from the RAG.
Today we have a extremely hacky workaround by ensuring that at least the desired chunk from the RAG is selected, but it's far from ideal and our code is not well written (a temporary POC written by AI that has been there for quite some months now ...)
I agree that we need stochasticity in a probabilistic system, but I also think it would be good to control it. For example, we need the stochasticity introduced at high temperatures since it is inherent to the model, but we don’t need stochasticity in matrix computations, as it is not required for modeling.
I don't think the claim is that this is particularly helpful for consumer-facing applications. But from a research perspective, this is invaluable for allowing reproducibility.
Sometimes, the reason for non-determinism is implementation-specific. For instance, in GPT-2's source code (I haven't checked other model versions), setting the temperature in the GUI does not lead to a value of 0 but "epsilon" (a very small value larger than 0), to avoid a division by zero error in the code, which makes sense.
For many applications, non-determinism implies "useless".
This has been a long standing issue with LDA topic models. In particular in the legal, financial and regulatory domains, if a method is not deterministic, it may be illegal to use it or it may lead to follow-on requirements that one does not want (e.g. all screens shown to humans must be preserved to be able to go back and reconstruct what exactly happened to a particular user in a particular second).
"in collaboration with others at Thinking Machines"
If you're old enough, you might remember Danny Hillis' Thinking Machines from the late 80s. I wish they had chosen a different name (I say this for nostalgic reasons, having been in front of one of those cubes glowing with red LEDs back in the late 80s at MIT's AI Lab" (renamed to CSAIL at some point). Feynman did some amazing work on that, too: https://longnow.org/ideas/richard-feynman-and-the-connection...
In the U.S., the “THINKING MACHINES” trademarks were owned by Thinking Machines Corporation (the company Hillis co-founded), not Hillis personally, and those registrations were cancelled in 1998–1999.
USPTO Report
+1
The company itself went bankrupt in 1994 and its assets were dispersed (e.g., to Sun Microsystems, later Oracle).
There’s a new, pending USPTO application for “THINKING MACHINES” filed in 2025 by Thinking Machines Lab Inc., the company founded by Amira Murati.
I had no idea. But I believe the same deal with Einstein being a dick to his wife and never acknowledging his friend who taught him the math he used in his work (I read about that recently from a respectable source.) I guess that makes sense; no one is void of some deep flaw, it's just selectively hidden.
I love high quality blog post style research discussion - Anthropic has been leading the charge with this recently and it's great to see it spreading. OpenAI was also doing this during all the RL research days.
Natural language is ambiguous. It needs to be. I think the approach here of trying to figure out how to make circles into squares, and argue why circles should be squares, is misguided.
Discussions of this type are going to eventually morph into better understanding of how to accept ambiguity and randomness in language, and further shape it with other larger sub-patterns beyond the little proto-grammars that the QKV projection matrices extract.
“Language ambiguity with determinism”? Sure I can juxtapose the terms but if it’s semantically inconsistent, then what we mean by that is not a deterministic, definitive thing. You’re chasing your tail on this ‘goal’.
Ambiguity: The request/prompt leaves a lot of room for interpretation. Many qualitatively different answers may be correct, relative to the prompt. Different or non-deterministic models will return highly variance results.
Determinism: If a model is given the exact same request/prompt twice, its two responses will also be identical. Whether or not the consistent response qualifies as correct.
The two concepts are very different.
(Ambiguous vs. precise prompt) x (Deterministic vs. Non-deterministic model) = 4 different scenarios.
A model itself can be non-deterministic without being ambiguous. If you know exactly how it functions, why it is non-deterministic (batch sensitive for instance), that is not an ambiguous model. Its operation is completely characterized. But it is non-deterministic.
An ambiguous model would simply be model whose operation was not characterized. A black box model for instance. A black box model can be deterministic and yet ambiguous.
Maybe I got this wrong but I thought ambiguity refered to the input. So in a deterministic system I would assume that a input of "Give an example of a dice roll" Will always output the exact same example (unless the model also gets the context of the message history).
Ambiguity is what happens when you change the prompt slightly, e.g. by adding a word: "Give an example of a single dice roll". Now as a human our expectation would be that this is the same question and should thus (in a deterministic system) receive the same answer. But to an LLM it may not be.
> Ambiguity: [...] Different or non-deterministic models will return highly variance results.
Yes, and thanks. That was my intended point - but you point out a better example. Slightly different prompts may also produce highly varied responses.
(My subsequent comments on ambiguous models was in case I was misinterpreting the comment I was replying to. I also generally think of ambiguity as a property of input. Either way, ambiguity is not the same as non-deterministic.)
So is ‘2’ or ‘3’ or ‘19’ or ‘99’ or ‘a jam sponge cake with gaming dice for frosting’… The point is in natural language there are many perfectly acceptable answers. Usually any particular answer is arbitrary, and it would probably be undesirable to have the same answer everytime. For a majority of use cases.
Are you talking about the “Thinking Machines” company that shut down in 1994? Took me some digging to figure it out, doesn’t seem well-known enough to be the reason - it’s just a nice (and relatively obvious) name.
Yes. Danny Hillis’ Thinking Machines Corporation, an AI company which created its own massive parallel processing supercomputer hardware.
“We are building a machine that will be proud of us” was their corporate motto. And that was in 1983.
One of those Machines is on view at the Computer History Museum in Mountain View.
Back then, they could be ordered in “Darth Vader Black”, no kidding here. You can also see a couple of them (the CM-5) as the stereotypical supercomputer in the original Jurassic Park.
It may not be a household name like Apple or Microsoft but its flagship product the Connection Machine is somewhat iconic in (super)computing history. The physical design of the machine is cool and unforgettable looking, plus recurring HN favorite Richard Feynman contributed to the original architecture.
> But why aren’t LLM inference engines deterministic? One common hypothesis is that some combination of floating-point non-associativity and concurrent execution leads to nondeterminism based on which concurrent core finishes first. We will call this the “concurrency + floating point” hypothesis for LLM inference nondeterminism. For example, a recent arXiv preprint writes
I'm honored to see that Mira and co. appreciated my feedback on the very topic I made 7 months ago here :D
> You don't need RNG since the whole transformer is an extremely large floating-point arithmetic unit. A wild guess - how about the source of non-determinism is coming from the fact that, on the HW level, tensor execution order is not guaranteed and therefore (T0 * T1) * T2 can produce slightly different results than T0 * (T1 * T2) due to rounding errors?
I really hope we will get deterministic LLMs in the future. Even if it causes slightly slower response times.
Nondeterminism is what currently keeps me from working with other developers.
As I wrote in "Prompt Coding" [1], these days I am not looking for good code. I am looking for prompts that create good code. But how do you share prompts among developers when they produce different code every time? You cannot simply state "Here, I found a prompt that makes gpt-5-2025-08-07 output a solution with all the desired attributes".
Similar with images. At the moment, for most image models, you cannot outsource the task of writing prompts that create the desired images. Because most image models will not create the same image when given the same prompt and parameters.
Surely if you end up relying on a given prompt to produce the exact same code every time you should instead just check that code into source control the first time you generate it?
A deterministic LLM isn't going to behave appreciably differently from a non deterministic one if your input or context varies by even a tiny bit (pun intended) each time.
If nothing has changed, caching the result would certainly be cheaper. But if you're doing that as part of a test, it's not really running the test and it might defeat the purpose of the test.
i tried to create a makefile driven workflow based on this idea and ended up with https://github.com/khimaros/enc -- it suffers from the issues you raised
i'm hoping that it becomes more useful as models improve and become more reliable at producing working code (though determinism would be great for improving prompts).
Deterministic reproducibility is very different from replicability, and imo the latter is more important; even if the details of the reproducibility are interesting I think they're irrelevant.
There's a similar situation in other scientific disciplines. People want source code and data so they can reproduce results - that basically tells you someone didn't cheat and they documented everything. But it does not tell you whether a real phenomenon was observed.
It's much more interesting to know if roughly the same cause and effect relationships exist so we can predict behavior.
Concretely, there are studies that show e.g. randomly capitalizing letters can lead to completely different responses from and LLM. That speaks to a fragility that doesn't have anything to do with deterministic reproduction.
For fun over the last few days, I've built a compressor / decompressor that uses the logits from an LLM, for each token in the input, then takes the ranks and exponential goolomb encodes them. Then you work in reverse to regenerate the original
It took me ages to get the prediction for the second token after "hello" to match the same as the prediction for the second token when running the model on the string "hello world", despite the fact that I was using a causal model. I tried all kinds of things before discovering that `quantized: false` was the important setting.
What's the Weissman score? Or more seriously :) did it perform well. Sounds like it should. If more and more text is AI slop it should do well.
I dont fully understand what you said but I guess higher probability logits are encoded with fewer bits. If your text is the LLM output then you may need a bit or two per token?
I used exponential golomb coding, so the rank 0 logit is encoded with a single bit, ranks 1 and 2 are encoded with three bits, ranks 3-6 are encoded with 5 bits, etc.
In terms of performance, I've not done any serious testing, but e.g. the wikipedia article on volcanos compresses to about 20% using GPT2. I've seen other strings compress even further.
The big issue is that while encoding is not unreasonable, decoding any significant amount of data is incredibly slow, since I'm doing a model run for every token in the output. It's bad enough that the scheme is probably unworkable as it is. I'm thinking about changing my code so that it streams out the tokens as it decodes them, so you're not just left there waiting for ages.
I don't know about golomb coding, but with Arithmetic coding you can do stream decoding(AC), if I remember correctly.
I supervised a student's project whose goal was exactly that : implement compression with LLMs using AC.
Since AC is optimal, if your LLM has an average cross entropy x on some dataset, you can expect that the compression will compress data using x nats per token on average!
Arithmetic coding looks like an extremely interesting approach, given that you can use the model at each step to give you the probabilities of each token.
Very impressive! I guess this still wouldn't affect their original example
> For example, you might observe that asking ChatGPT the same question multiple times provides different results.
even with 0.0 temperature due to MOE models routing at a batch level, and you're very unlikely to get a deterministic batch.
> Not because we’re somehow leaking information across batches — instead, it’s because our forward pass lacks “batch invariance”, causing our request’s output to depend on the batch size of our forward pass.
The router also leaks batch-level information across sequences.
> even with 0.0 temperature due to MOE models routing at a batch level, and you're very unlikely to get a deterministic batch.
I don’t think this is correct - MoE routing happens at per token basis. It can be non deterministic and batch related if you try to balance out your experts load in a batch but that’s performance optimization (just like all of the blogpost) and not the way models are trained to work.
As others have pointed out, these phenomena are well known to many folks across companies in the AI infra space. It doesn't really break new ground. This article is a good exposition of the basic strategies though.
What I would have loved is a discussion around collectives/multi-node setups. And showing how to get determinism at low performance penalty for multi-node reduction collectives.
As the bottom of LLM inference, it is sampling for the next token based on the probability distribution conditioned on the tokens currently in the context window. If the distribution exhibits degeneracy in probability for more than token, outcome of the sampling will naturally, as it should, be nondeterministic. It should be left alone.
By setting the temperature to 0 you get greedy decoding, which does a lot more than just making it predictable, and can degrade outputs. Random sampling exists for a reason! Gemini 2.5 Pro in particular doesn't like temp 0, for example.
His solution still relies on greedy (temperature 0) sampling, which is probably not optimal for model performance on various tasks. For example, Gemini 2.5 uses temperature 1 by default. But deterministic inference with temperature >0 can still be achieved by using pseudorandom sampling with a fixed seed.
Conceptually setting temperature to be >0 doesn't actually introduce any non-determinism. If your sampler is seeded then it will always choose the same next token. Higher temperature only flattens the logit distribution.
The point of the blog is that even at "supposed" deterministic generative sampling, non-determinism creeps in. This in turn has disastrous effects in very real experiments.
I think this means that the results might also be non-deterministic across hardware revisions b/c I don't think they verified that the kernels will work the same on different GPU & TPU versions b/c how do they know that the compiler will not re-order the operations behind their back?
Yes, there’s usually no guarantee on how different hardware does operations (for example, even if the hardware is correctly rounding intermediate results, different hardware may use different tile sizes). The reproducibility here is for runs on the same machine.
Compilers can also reorder operations but in practice this is rarely an issue because kernels typically synchronize frequently and this limits the ability for compilers to reorder things. This isn’t to say it doesn’t happen, but even if it does happen it’s likely because the compiler changed because the code they generate is generally run-to-run identical.
You can prevent reordering with sufficient amounts of compiler abuse.
With revisions, you're trying to ensure a consistent floating point environment where the operations used are deterministic, and used in the same order with the same inputs. The best way to do that is to use operations that adhere to a mostly deterministic standard like IEEE-754.
Ensuring the same floating-point algorithm workload behaves exactly the same on two distinct workstations is a heck of a lot of work that almost no one is willing to pay for.
Not only that but heterogeneous clusters (inevitable at a large enough scale) will also have non-deterministic outputs. So it's great that they wrote kernels to make the forward pass deterministic but getting rid of it entirely at data center scale would mean that they'd also have to do this type of work across cluster nodes as well to maintain "cluster" invariance & not just batch invariance.
A = torch.randn(2048, 2048, device='cuda', dtype=torch.bfloat16)
B = torch.randn(2048, 2048, device='cuda', dtype=torch.bfloat16)
ref = torch.mm(A, B)
for _ in range(1000):
assert (torch.mm(A, B) - ref).abs().max().item() == 0
I’m sort of surprised that Torch doesn’t have some kind of lazy evaluation thing to avoid computing anything here. I thought that was one of the nice things about all these fancy frameworks (if I wanted the computer to actually do silly things when I asked it to, I would use BLAS directly, right?).
Maybe I'm missing something, but in this case, wouldn't being lazy would be pure overhead? I don't see anything can be lazy here. The reference computed once, nanoseconds before it's needed, and test cases computed at the time of comparison, then tossed away.
What would hope to be achieved by making this case lazy? If you wanted these to run in parallel, with a multi-gpu system, you would use the appropriate parallel interface.
I don't understand. Since it's not using the parallel interface, only one operation can happen at a time. This would be, literally, sequential execution with extra overhead, in this case. Again, in this case, what would hope to be achieved from doing things lazily, since the lazy operations would immediately be followed by their evaluation?
The parallel interface, which is async, is probably what you're lookin for.
If evaluation is lazy, then the subtraction operator gets fed two unevaluated matrix multiplies.
If it's a dumb subtraction operator, this gives us no benefit. Eventually it evaluates both and then subtracts. And it has some extra overhead like you said.
But if it's a smart subtraction operator, it can realize that both parameters are the same equation, and then it can return all 0s without evaluating anything.
And even better than just skipping the matrix math, "all 0s" can be a stub object that takes O(1) time to set up. And then .abs().max() will be instant too.
The Python commands are encountered sequentially. One could image a library where the Python commands build the computation under the hood. Then, the library would be able to take advantage of situations like this one (or, more practically, reorder multiplications and/or avoid unnecessary temporaries).
A bit off topic from the technical discussion but does anyone recognize what blog layout or engine this is? I really like the layout with sidenotes and navigation.
This is eternal struggle. - Hardware developers will constantly scale horizontally and make less (time) deterministic hardware, because wall of memory, and scientists could constantly develop new ways to make calculations deterministic.
So, even if will be achieved progress just now, I think in predictable future this will be constant dead-end.
THANK YOU! Great work and writeup. Hope it finally silences the "concurrency + floating point" crowd and the "LLMs can never be deterministic" zealots.
Where this gets really complicated is when you are chaining many LLM calls together (basically any agent). A slight deviation in the call stack can throw off everything else.
"I had no problem getting deterministic LLM outputs when I experimented with this 6 months ago" looks like you're using llama-cpp in that repo. This is about vllm serving many requests at once, at long sequence lengths.
> As it turns out, our request’s output does depend on the parallel user requests. Not because we’re somehow leaking information across batches — instead, it’s because our forward pass lacks “batch invariance”, causing our request’s output to depend on the batch size of our forward pass.
Seed your PRNGs and call torch.use_deterministic_algorithms(True) to get the deterministic kernels. They may be slightly slower, but in practice, you probably will not notice.
Note that results will still differ between different drivers and GPUs. It would be great if NVIDIA tried harder in that regard.
The blog post is about LLM non-determinism in the context of serving at scale (variable batch size). The page you link is only about run-to-run determinism implicitly assuming a fixed batch size.
Seems a buried lede is that on-policy RL is unlocked by bitwise identical results between training and sampling. I'm not an expert here but my understanding is that this would allow for stronger guarantees about deployment/training alignment for the RL training that the labs already do.
I don't fully understand the BigMath example though. They show that off-policy RLVR requires off-policy correction, which avoids divergence, but is suboptimal because it results in noisy rewards. Then they say "we fixed the sampler and trainer numerical mismatch, which allows for on-policy RL, look how much better it is." It's not clear to me whether this is an artificial example that deliberately uses different trainer/sampler setups, or if it's actually impossible to have the same numerics between trainer/sampler without their fixes (even if we use same batch size, no atomics, etc.).
I think this is an excellent article which addresses the issue that I personally have been thinking about a long time. And no its not just some slop they put but actual an engineering blog(with open source code and reproducible results!)
I think the company is off to a good start
cool project but if this is what you are producing with $2 billion funding, i doubt you will survive. This is the type of article a grad student would write over a weekend.
The team is good, and I enjoyed the read. But this is just an engineering blog post. They're promoting this like it's ground breaking research and it's on their front-page. Ultimately this paper is not very meaningful and just a fun debugging session.
I've seen this play out dozens of times. So many startups that have come and go in the bay area were composed of extremely talented individuals, but almost all of them failed.
Who needs a working product when you can spend all day designing the most WEWORK looking website and slap some pseud slop on it. It's like crypto "startups" but it's not even fun.
I am baffled that I still run against these statement years after LLM's have been around. LLM's are deterministic and always have been. The reason people are having issues with them is because they are basing their assumptions on api based experiments. Like my man, how can you be making these statements when you haven't done the due diligence of running the LLM on your own hardware with all of the variables locked down and accounted for? If you do just that it would become obviously clear that they are deterministic and most of the time the reason you see the non deterministic behavior is because you have not controlled for a variable. Usually prompt caching, batch processing or some other obvious variable. Now this is related to within same system deterministic behavior. You might get different answers when running on a different gpu, but at least for same systems the behavior is 100% identical if you account for all server startup flags and properly account for things like prompt cashing, slot contamination etc...
I suggest you look up the name of the main author of TFA before assuming they don’t know what they are talking about.
This is literally one of the most knowledgeable person on the topic. I think you are the one that hasn’t peeled enough layers to connect with what they are saying.
1. they aren't, they are just popular online.
2. the author has nothing to do with the original comment. Why do you think academic reviews are double blind?
One of the top 5 most active contributors to pytorch over the last few years, and specifically working on some of it's most hardcore components is "just popular online"?
If you say so.
> the author has nothing to do with the original comment
Except for the part of the comment that was assuming the author had no idea how this all works, has only used LLMs through API and has never run a local model, you mean?
Hold on a second. A transformer produces deterministically a probability distribution over the token alphabet from the context. Then one samples from this distribution. This is random and meant to be random.
The sampling process isn't random. If you sample with identical sampling parameters and identical values for said parameters, you will always get same results. You only start getting "non deterministic" behavior when you start using more complex systems outside the scope of your control like multi gpu systems and batch processing. One llm sampled with cash prompting off and and batch processing off will always generate same results if all values are same.
It's possible to deterministically sample from a probability distribution. For example, just seed your RNG with a constant, or with the SHA256 hash of the context.
Well yes, you can "hack" the pseudorandom number generator, but... that's not really the point when talking about determinism in LLMs is it? I mean the mathematical idea of the standard LLM is certainly truly random.
> I mean the mathematical idea of the standard LLM is certainly truly random.
Not really, LLMs give you a distribution over possible next tokens.
You are free to then sample from this distribution how you want.
There is no need to hack RNG or whatever, for example you can simply just take a greedy approach and always output the most likely token, in which case the LLM becomes deterministic (mathematically). This is equivalent to setting the temperature to 0.
I suppose I have issues with the way "determinism" is used in the title of this article. It can mean different things to different people and in my mind stating that "Defeating Nondeterminism in LLM Inference" frames it as an actual issue with LLM inference. But its not, its an issue with LLM inference when you start using large scale inference with more complex parts such as systems which use multi gpu inference systems or batching processes and other mechanisms. It is not an issue when using an LLM without those more complex parts. Stating it this way muddies the signal and gives a false sense that this is a fundamental issue with architecture, where its an issue of the systems at scale...
Fixing "theoretical" nondeterminism for a totally closed individual input-output pair doesn't solve the two "practical" nondeterminism problems, where the exact same input gives different results given different preceding context, and where a slightly transformed input doesn't give a correctly transformed result.
Until those are addressed, closed-system nondeterminism doesn't really help except in cases where a lookup table would do just as well. You can't use "correct" unit tests or evaluation sets to prove anything about inputs you haven't tested.
There is no such thing as "exactly the same input, but with different preceding context". The preceding context is input!
If you were to obtain exactly the same output for a given input prompt, regardless of context, then that would mean that the context is being ignored, which is indistinguishable from the session not maintaining any context such that each prompt is in a brand new empty context.
Now what some people want is requirements like:
- The different wording of a prompt with exactly the same meaning should not change anything in the output; e.g. whether you say "What is the capital of France" or "What is France's capital" the answer should be verbatim identical.
- Prior context should not change responses in ways that don't have any interaction with the context. For instance, a prompt is given "what is 2 + 2", then the answer should always be the same, except if the context instructs the LLM that 2 + 2 is to be five.
These kinds of requirements betray a misunderstanding of what these LLMs are.
While I get that this is how LLMs work, I think you should think backwards from the user / from what AI as a field is aiming for and recognize that the „naive“ way of the parent to ask for reliable responses no matter what the „context“ is, is exactly what a good AI system should offer.
„The context is the input“ betrays a misunderstanding of what (artificial) intelligence systems are aiming for.
Then we need something else. This is not how LLMs work. They are simple statistical predictors, now universal anwsering machines.
I agree mostly. They are all that you say, but if you think about the conditional distribution that you are learning, there is nothing preventing us in principle from mapping different contexts to the same responses. It is rather a practical limitation that we don’t have sufficient tools of shaping these distributions very soundly. All we can do is throw data at them and hope that they generalize to similar contexts.
We have observed situations where agentic LLM traces on verifiable problems with deterministic (greedy) decoding lead to either completely correct or completely wrong solutions depending on the minutes on the clock which are printed as coincidental output of some tool that the LLM used.
I think there may be some mild fixes to current models available , for example it is worrying that the attention mechanism can never fully disregard any token in the input, because the softmax will always assign a > 0 weight everywhere (and the NN has no way of setting a logit to -infinity). This directly causes that it is extremely difficult for the LLM to fully ignore any part of the context reliably.
However Yann LeCun actually offers some persuasive arguments that autoregressive decoding has some limitations and we may need something better.
> They are simple statistical predictors, now universal anwsering machines.
I see this a lot. I kinda' doubt the "simple" part, but even beyond that, is there any evidence that statistical predictor can't be a universal answering machine? I think there's plenty of evidence that our thinking is at least partially a statistical predictor (e.g. when you see a black sheep you don't think "at least one side of this sheep is black", you fully expect it to be black on both sides)
I'm not saying that LLMs _are_ universal answering machines. I'm wondering why people question that they are/they can become one, based on the argument that "fundamentally they are statistical predictors". So they are. So what?
Does your definition of "universal answering machine" include the answers being correct?
If it does, statistical predictors can't help you because they're not always correct or even meaningful (correlation does not imply causation).
If it doesn't then, by all means, enjoy your infinite monkeys
> These kinds of requirements betray a misunderstanding of what these LLMs are.
They do not. Refusing to bend your requirements to a system that can't satisfy them is not evidence of misunderstanding the system.
And if you tack on "with X 9s of reliability" then it is something LLMs can do. And in the real world every system has a reliability factor like that.
Sure. But the context always starts with the first input, right? And how can you guarantee—or why should you guarantee—that the reply to the first input will always be the same? And if that’s not the case, how can we ensure the preceding context remains consistent?
If an input along with the context generated some random seed or hash this would certainly be possible. Just paste your seed over to your coworker, they supply it to the model and it contains all contextual information.
I wonder if there's a way to use an LLM to rewrite the prompt, standardizing the wording when two prompts mean the same thing?
It's going to backfire. In real scenarios (not regression testing) users don't want to see the exact same thing twice out of the LLM in the same session in spite of trying to refine the result with more context.
There are going to be false positives: text that is subtly different from a previous response is misidentified as a duplicate such that the previous response is substituted for it, frustrating the user.
Google search rewrites misspelled search queries and also lets you override it if that's not what you want. Maybe something similar would work?
Not an expert, but I've been told RAG in combination with a database of facts is one way to get more consistency here. Using one of the previous examples, you might have a knowledge store (usually a vector database of some kind) that contains a mapping of countries to capitols and the LLM would query it whenever it had to come up with an answer rather than relying on whatever was baked into the base model.
Deterministically, you mean? ;)
oh so you want it to be thinking???? now we talking
> where the exact same input gives different results given different preceding context
Why and how is this a problem?
If 'preceding context' doesn't cause different results, it means you can simply discard the context. Why do I want that? It's not how I expect a tool to work (I expect vim responds differently to my input after I switch to the insert mode). It's absolutely not how I expect intelligence to work either. It sounds like the most extreme form of confirmation bias.
When the context is auto-generated and may include irrelevant data.
This is a common AI benchmark and has been for years before GPT-2 even existed. LLMs need to not get distracted by irrelevant facts and there are tests that measure this. It's the motivation for attention mechanisms, which are the breakthrough that enabled LLMs to scale up.
An example is translation. I MTLed some text recently where the name of a (fictional) city was translated about a dozen different ways. Sometimes you'd get a calque, sometimes you'd get a transliteration (including several wrong ones). Ironically "dumb" MTLs are often much more consistent about this than LLMs.
This is really useful in reproducing bugs.
I was with you until you said it “doesn’t really help”. Did you mean “doesn’t completely solve the problem “?
I thought this was pretty well known (at least in the JAX/XLA world). I've hit this many times and got batch variance explained to me before: https://github.com/google-deepmind/penzai/issues/82 and https://github.com/jax-ml/jax/issues/20047#issuecomment-1975...
should be the top comment.
Why do you care about determinism in a probabilistic system? What difference does it make to the end user if the input "How do I X?" always produces the same deterministic output when semantically equivalent inputs "how do i x?", "how do I x", and "how do I X??" are bound to produce different answers that often won't even be semantically equivalent.
What LLMs need is the ability to guarantee semantically-equivalent outputs for all semantically-equivalent inputs, but that's very different from "determinism" as we understand it from other algorithms.
Not all LLM based applications are a user facing free form chat.
If you take an LLM that makes 10 tool calls in a row for an evaluation, any reduction in unpredictable drift is welcome. Same applies to running your prompt through DSPy Optimizer. [0] Countless other examples. Basically any situation where you are in control of the prompt, the token level input to the LLM, so there's no fuzziness.
In this case, if you would've eliminated token level fuzziness and can yourself guarantee that you're not introducing it from your own end, you can basically map out a much more reliable tree or graph structure of your system's behavior.
[0]: https://dspy.ai/#2-optimizers-tune-the-prompts-and-weights-o...
> If you take an LLM that makes 10 tool calls in a row for an evaluation, any reduction in unpredictable drift is welcome
why use an ambiguous natural language for a specific technical task? i get that its a cool trick but surely they can come up with another input method by now?
You aren't wrong, but that doesn't mean this level of determinism isn't useful. If you don't even have the level of determinism that the exact same input tokens produce the exact same output tokens, then it's very hard to share reproducible results with peers, which can be useful if you are say, red teaming an LLM to produce a very rare / unreliable output.
I'm actually working on something similar to this where you can encode information into the outputs of LLM's via steganography: https://github.com/sutt/innocuous
Since I'm really looking to sample the only the top ~10 tokens, and I mostly test on CPU-based inference of 8B models, there's probably not a lot of worries getting a different order of the top tokens based on hardware implementation, but I'm still going to take a look at it eventually, and build in guard conditions against any choice that would be changed by an epsilon of precision loss.
It would be very useful for AI platform customers. You could run prompts with 0 temperature and check if the results are the same making sure that AI provider is not switching the PRO model in the background for a cheap one and ripping you off.
For "bug" reproduction purposes. It is easier to debug a model if the same string always produces the same incorrect or strange LLM output, not every 100th time you run it.
If there is a bug (a behavior defined by whatever criteria), it is just a single path in a very complex systems with high connectivity.
This nonlinear and chaotic behavior regardless of implementation details of the black box makes LLM seem to be nondeterministic. But LLM is just a pseudo random number generator with a probability distribution.
(As I am writing this on my iPhone with text completion, I can see this nondeterministic behavior)
Was my thinking exactly - but also semantically equivalent is also only relevant when it needs to be factual, not necessarily for ALL outputs (if we're aiming for LLM's to present as "human" - or for interactions with LLMs to be natural conversational...). This excludes the world where LLMs act as agents - where you would of course always like the LLM to be factual and thus deterministic.
When you do MCP-style applications, an LLM is more like RegEx on steroids, and since you expect your regex to return the same matches on the same input, it is a very desirable attribute for LLMs as well. I would say it is more than desirable, it is necessary.
If i want to covert "how do I x" to `api.howTo("x")` it is very important that i get the exact same result every time.
Deterministic output is needed when LLMs are used for validations. This can be anything from input validation at runtime to a CI check leveraging LLMs. It can be argued this is not an acceptable use of AI, but it will become increasingly common and it will need to be tweaked/tested. You cannot tweak/test a response you don't know you're going to get.
yeah indeed, regression testing for chatbots that use RAGs would involve making sure the correct response comes from the RAG.
Today we have a extremely hacky workaround by ensuring that at least the desired chunk from the RAG is selected, but it's far from ideal and our code is not well written (a temporary POC written by AI that has been there for quite some months now ...)
I agree that we need stochasticity in a probabilistic system, but I also think it would be good to control it. For example, we need the stochasticity introduced at high temperatures since it is inherent to the model, but we don’t need stochasticity in matrix computations, as it is not required for modeling.
I don't think the claim is that this is particularly helpful for consumer-facing applications. But from a research perspective, this is invaluable for allowing reproducibility.
Easier to debug deterministic inference
Sometimes, the reason for non-determinism is implementation-specific. For instance, in GPT-2's source code (I haven't checked other model versions), setting the temperature in the GUI does not lead to a value of 0 but "epsilon" (a very small value larger than 0), to avoid a division by zero error in the code, which makes sense.
For many applications, non-determinism implies "useless". This has been a long standing issue with LDA topic models. In particular in the legal, financial and regulatory domains, if a method is not deterministic, it may be illegal to use it or it may lead to follow-on requirements that one does not want (e.g. all screens shown to humans must be preserved to be able to go back and reconstruct what exactly happened to a particular user in a particular second).
"in collaboration with others at Thinking Machines"
If you're old enough, you might remember Danny Hillis' Thinking Machines from the late 80s. I wish they had chosen a different name (I say this for nostalgic reasons, having been in front of one of those cubes glowing with red LEDs back in the late 80s at MIT's AI Lab" (renamed to CSAIL at some point). Feynman did some amazing work on that, too: https://longnow.org/ideas/richard-feynman-and-the-connection...
In the U.S., the “THINKING MACHINES” trademarks were owned by Thinking Machines Corporation (the company Hillis co-founded), not Hillis personally, and those registrations were cancelled in 1998–1999. USPTO Report +1
The company itself went bankrupt in 1994 and its assets were dispersed (e.g., to Sun Microsystems, later Oracle).
There’s a new, pending USPTO application for “THINKING MACHINES” filed in 2025 by Thinking Machines Lab Inc., the company founded by Amira Murati.
I make this mistake every time I see their name.
[flagged]
I had no idea. But I believe the same deal with Einstein being a dick to his wife and never acknowledging his friend who taught him the math he used in his work (I read about that recently from a respectable source.) I guess that makes sense; no one is void of some deep flaw, it's just selectively hidden.
I love high quality blog post style research discussion - Anthropic has been leading the charge with this recently and it's great to see it spreading. OpenAI was also doing this during all the RL research days.
Natural language is ambiguous. It needs to be. I think the approach here of trying to figure out how to make circles into squares, and argue why circles should be squares, is misguided.
Discussions of this type are going to eventually morph into better understanding of how to accept ambiguity and randomness in language, and further shape it with other larger sub-patterns beyond the little proto-grammars that the QKV projection matrices extract.
Yes, but determinism != ambiguity, because determinism means: for this exact input the same exact output needs to follow.
If I ask the same model the same question I should be able to deterministically get the same answer.
Now if we phrase the same question slightly differently we would expect to get a slightly different answer.
> Now if we phrase the same question slightly differently we would expect to get a slightly different answer.
You wouldn't get this from an LLM though, a tiny change in starting point gets a massive change in output, its a chaotic system.
Maybe predictability is what is meant?
Me: What’s an example of a dice roll?
LLM: 1
“Language ambiguity with determinism”? Sure I can juxtapose the terms but if it’s semantically inconsistent, then what we mean by that is not a deterministic, definitive thing. You’re chasing your tail on this ‘goal’.
Ambiguity: The request/prompt leaves a lot of room for interpretation. Many qualitatively different answers may be correct, relative to the prompt. Different or non-deterministic models will return highly variance results.
Determinism: If a model is given the exact same request/prompt twice, its two responses will also be identical. Whether or not the consistent response qualifies as correct.
The two concepts are very different.
(Ambiguous vs. precise prompt) x (Deterministic vs. Non-deterministic model) = 4 different scenarios.
A model itself can be non-deterministic without being ambiguous. If you know exactly how it functions, why it is non-deterministic (batch sensitive for instance), that is not an ambiguous model. Its operation is completely characterized. But it is non-deterministic.
An ambiguous model would simply be model whose operation was not characterized. A black box model for instance. A black box model can be deterministic and yet ambiguous.
Maybe I got this wrong but I thought ambiguity refered to the input. So in a deterministic system I would assume that a input of "Give an example of a dice roll" Will always output the exact same example (unless the model also gets the context of the message history).
Ambiguity is what happens when you change the prompt slightly, e.g. by adding a word: "Give an example of a single dice roll". Now as a human our expectation would be that this is the same question and should thus (in a deterministic system) receive the same answer. But to an LLM it may not be.
> Ambiguity: [...] Different or non-deterministic models will return highly variance results.
Yes, and thanks. That was my intended point - but you point out a better example. Slightly different prompts may also produce highly varied responses.
(My subsequent comments on ambiguous models was in case I was misinterpreting the comment I was replying to. I also generally think of ambiguity as a property of input. Either way, ambiguity is not the same as non-deterministic.)
If you really want that to work while being reproducible, maybe give it a random number tool and set the seed?
> LLM: 1
A perfectly acceptable answer.
If it answers 1 every time it's still a perfectly acceptable answer.
So is ‘2’ or ‘3’ or ‘19’ or ‘99’ or ‘a jam sponge cake with gaming dice for frosting’… The point is in natural language there are many perfectly acceptable answers. Usually any particular answer is arbitrary, and it would probably be undesirable to have the same answer everytime. For a majority of use cases.
I am still irritated by the name of the company.
What is the reasoning behind these schemes? The hope that bits of the properties of legendary companies will rub off onto the new venture?
As if naming the next best venture PARC will inevitably create a breakthrough in networking just by the arrangement of four letters.
Are you talking about the “Thinking Machines” company that shut down in 1994? Took me some digging to figure it out, doesn’t seem well-known enough to be the reason - it’s just a nice (and relatively obvious) name.
Yes. Danny Hillis’ Thinking Machines Corporation, an AI company which created its own massive parallel processing supercomputer hardware.
“We are building a machine that will be proud of us” was their corporate motto. And that was in 1983.
One of those Machines is on view at the Computer History Museum in Mountain View. Back then, they could be ordered in “Darth Vader Black”, no kidding here. You can also see a couple of them (the CM-5) as the stereotypical supercomputer in the original Jurassic Park.
More here: https://en.m.wikipedia.org/wiki/Thinking_Machines_Corporatio...
And in the original Jurassic Park! https://www.google.com/search?q=jurassic+park+cm-5
[addendum: posted this too quickly & didn't see it in the comment above. duh.]
It may not be a household name like Apple or Microsoft but its flagship product the Connection Machine is somewhat iconic in (super)computing history. The physical design of the machine is cool and unforgettable looking, plus recurring HN favorite Richard Feynman contributed to the original architecture.
The thinking is free marketing and the same reason trademarks were invented
Super interesting. For those unaware, this is the company Mira Murati (OpenAI previous CTO) started
> But why aren’t LLM inference engines deterministic? One common hypothesis is that some combination of floating-point non-associativity and concurrent execution leads to nondeterminism based on which concurrent core finishes first. We will call this the “concurrency + floating point” hypothesis for LLM inference nondeterminism. For example, a recent arXiv preprint writes
I'm honored to see that Mira and co. appreciated my feedback on the very topic I made 7 months ago here :D
> You don't need RNG since the whole transformer is an extremely large floating-point arithmetic unit. A wild guess - how about the source of non-determinism is coming from the fact that, on the HW level, tensor execution order is not guaranteed and therefore (T0 * T1) * T2 can produce slightly different results than T0 * (T1 * T2) due to rounding errors?
https://news.ycombinator.com/item?id=42952605#42960047
I really hope we will get deterministic LLMs in the future. Even if it causes slightly slower response times.
Nondeterminism is what currently keeps me from working with other developers.
As I wrote in "Prompt Coding" [1], these days I am not looking for good code. I am looking for prompts that create good code. But how do you share prompts among developers when they produce different code every time? You cannot simply state "Here, I found a prompt that makes gpt-5-2025-08-07 output a solution with all the desired attributes".
Similar with images. At the moment, for most image models, you cannot outsource the task of writing prompts that create the desired images. Because most image models will not create the same image when given the same prompt and parameters.
[1]: https://www.gibney.org/prompt_coding
Surely if you end up relying on a given prompt to produce the exact same code every time you should instead just check that code into source control the first time you generate it?
A deterministic LLM isn't going to behave appreciably differently from a non deterministic one if your input or context varies by even a tiny bit (pun intended) each time.
If nothing has changed, caching the result would certainly be cheaper. But if you're doing that as part of a test, it's not really running the test and it might defeat the purpose of the test.
i tried to create a makefile driven workflow based on this idea and ended up with https://github.com/khimaros/enc -- it suffers from the issues you raised
i'm hoping that it becomes more useful as models improve and become more reliable at producing working code (though determinism would be great for improving prompts).
> most image models will not create the same image when given the same prompt and parameters.
Really? If you include the seed as one of the parameters most produce pixel identical output.
E.g. "Generate deterministic images" https://cloud.google.com/vertex-ai/generative-ai/docs/image/...
Deterministic reproducibility is very different from replicability, and imo the latter is more important; even if the details of the reproducibility are interesting I think they're irrelevant.
There's a similar situation in other scientific disciplines. People want source code and data so they can reproduce results - that basically tells you someone didn't cheat and they documented everything. But it does not tell you whether a real phenomenon was observed.
It's much more interesting to know if roughly the same cause and effect relationships exist so we can predict behavior.
Concretely, there are studies that show e.g. randomly capitalizing letters can lead to completely different responses from and LLM. That speaks to a fragility that doesn't have anything to do with deterministic reproduction.
For fun over the last few days, I've built a compressor / decompressor that uses the logits from an LLM, for each token in the input, then takes the ranks and exponential goolomb encodes them. Then you work in reverse to regenerate the original
It took me ages to get the prediction for the second token after "hello" to match the same as the prediction for the second token when running the model on the string "hello world", despite the fact that I was using a causal model. I tried all kinds of things before discovering that `quantized: false` was the important setting.
What's the Weissman score? Or more seriously :) did it perform well. Sounds like it should. If more and more text is AI slop it should do well.
I dont fully understand what you said but I guess higher probability logits are encoded with fewer bits. If your text is the LLM output then you may need a bit or two per token?
I used exponential golomb coding, so the rank 0 logit is encoded with a single bit, ranks 1 and 2 are encoded with three bits, ranks 3-6 are encoded with 5 bits, etc.
In terms of performance, I've not done any serious testing, but e.g. the wikipedia article on volcanos compresses to about 20% using GPT2. I've seen other strings compress even further.
The big issue is that while encoding is not unreasonable, decoding any significant amount of data is incredibly slow, since I'm doing a model run for every token in the output. It's bad enough that the scheme is probably unworkable as it is. I'm thinking about changing my code so that it streams out the tokens as it decodes them, so you're not just left there waiting for ages.
I don't know about golomb coding, but with Arithmetic coding you can do stream decoding(AC), if I remember correctly.
I supervised a student's project whose goal was exactly that : implement compression with LLMs using AC.
Since AC is optimal, if your LLM has an average cross entropy x on some dataset, you can expect that the compression will compress data using x nats per token on average!
Arithmetic coding looks like an extremely interesting approach, given that you can use the model at each step to give you the probabilities of each token.
Very impressive! I guess this still wouldn't affect their original example
> For example, you might observe that asking ChatGPT the same question multiple times provides different results.
even with 0.0 temperature due to MOE models routing at a batch level, and you're very unlikely to get a deterministic batch.
> Not because we’re somehow leaking information across batches — instead, it’s because our forward pass lacks “batch invariance”, causing our request’s output to depend on the batch size of our forward pass.
The router also leaks batch-level information across sequences.
> even with 0.0 temperature due to MOE models routing at a batch level, and you're very unlikely to get a deterministic batch.
I don’t think this is correct - MoE routing happens at per token basis. It can be non deterministic and batch related if you try to balance out your experts load in a batch but that’s performance optimization (just like all of the blogpost) and not the way models are trained to work.
Ah interesting, good point. So I guess expert-choice routing leaks across the batch. Now I'm not sure.
As others have pointed out, these phenomena are well known to many folks across companies in the AI infra space. It doesn't really break new ground. This article is a good exposition of the basic strategies though.
What I would have loved is a discussion around collectives/multi-node setups. And showing how to get determinism at low performance penalty for multi-node reduction collectives.
As the bottom of LLM inference, it is sampling for the next token based on the probability distribution conditioned on the tokens currently in the context window. If the distribution exhibits degeneracy in probability for more than token, outcome of the sampling will naturally, as it should, be nondeterministic. It should be left alone.
By setting the temperature to 0 you get greedy decoding, which does a lot more than just making it predictable, and can degrade outputs. Random sampling exists for a reason! Gemini 2.5 Pro in particular doesn't like temp 0, for example.
Focus on correctness, not determinism.
Determinism does not require temperature=0. You can have a deterministic behavior even with >0 temperature as long as you fix your random seeds.
His solution still relies on greedy (temperature 0) sampling, which is probably not optimal for model performance on various tasks. For example, Gemini 2.5 uses temperature 1 by default. But deterministic inference with temperature >0 can still be achieved by using pseudorandom sampling with a fixed seed.
Conceptually setting temperature to be >0 doesn't actually introduce any non-determinism. If your sampler is seeded then it will always choose the same next token. Higher temperature only flattens the logit distribution.
The point of the blog is that even at "supposed" deterministic generative sampling, non-determinism creeps in. This in turn has disastrous effects in very real experiments.
My point is that greedy sampling is not just not sufficient but also not necessary for deterministic inference.
I think this means that the results might also be non-deterministic across hardware revisions b/c I don't think they verified that the kernels will work the same on different GPU & TPU versions b/c how do they know that the compiler will not re-order the operations behind their back?
Yes, there’s usually no guarantee on how different hardware does operations (for example, even if the hardware is correctly rounding intermediate results, different hardware may use different tile sizes). The reproducibility here is for runs on the same machine.
Compilers can also reorder operations but in practice this is rarely an issue because kernels typically synchronize frequently and this limits the ability for compilers to reorder things. This isn’t to say it doesn’t happen, but even if it does happen it’s likely because the compiler changed because the code they generate is generally run-to-run identical.
You can prevent reordering with sufficient amounts of compiler abuse.
With revisions, you're trying to ensure a consistent floating point environment where the operations used are deterministic, and used in the same order with the same inputs. The best way to do that is to use operations that adhere to a mostly deterministic standard like IEEE-754.
Ensuring the same floating-point algorithm workload behaves exactly the same on two distinct workstations is a heck of a lot of work that almost no one is willing to pay for.
Not only that but heterogeneous clusters (inevitable at a large enough scale) will also have non-deterministic outputs. So it's great that they wrote kernels to make the forward pass deterministic but getting rid of it entirely at data center scale would mean that they'd also have to do this type of work across cluster nodes as well to maintain "cluster" invariance & not just batch invariance.
> will not re-order the operations behind their back?
Valid point. Floating point summation is not always commutative.
From their code:
I’m sort of surprised that Torch doesn’t have some kind of lazy evaluation thing to avoid computing anything here. I thought that was one of the nice things about all these fancy frameworks (if I wanted the computer to actually do silly things when I asked it to, I would use BLAS directly, right?).Maybe I'm missing something, but in this case, wouldn't being lazy would be pure overhead? I don't see anything can be lazy here. The reference computed once, nanoseconds before it's needed, and test cases computed at the time of comparison, then tossed away.
What would hope to be achieved by making this case lazy? If you wanted these to run in parallel, with a multi-gpu system, you would use the appropriate parallel interface.
I mean if you wait long enough, it is asking for
of something that can be identified as definitionally zero.I don't understand. Since it's not using the parallel interface, only one operation can happen at a time. This would be, literally, sequential execution with extra overhead, in this case. Again, in this case, what would hope to be achieved from doing things lazily, since the lazy operations would immediately be followed by their evaluation?
The parallel interface, which is async, is probably what you're lookin for.
Let's look at the subtraction in this case.
If evaluation is lazy, then the subtraction operator gets fed two unevaluated matrix multiplies.
If it's a dumb subtraction operator, this gives us no benefit. Eventually it evaluates both and then subtracts. And it has some extra overhead like you said.
But if it's a smart subtraction operator, it can realize that both parameters are the same equation, and then it can return all 0s without evaluating anything.
And even better than just skipping the matrix math, "all 0s" can be a stub object that takes O(1) time to set up. And then .abs().max() will be instant too.
I see now, thank you. I was stuck on the "lazy evaluation" part, rather than the optimization part they were actually suggesting.
The Python commands are encountered sequentially. One could image a library where the Python commands build the computation under the hood. Then, the library would be able to take advantage of situations like this one (or, more practically, reorder multiplications and/or avoid unnecessary temporaries).
A bit off topic from the technical discussion but does anyone recognize what blog layout or engine this is? I really like the layout with sidenotes and navigation.
Seems like a Thufte inspired style, something like this: https://clayh53.github.io/tufte-jekyll/articles/20/tufte-sty...
This is eternal struggle. - Hardware developers will constantly scale horizontally and make less (time) deterministic hardware, because wall of memory, and scientists could constantly develop new ways to make calculations deterministic.
So, even if will be achieved progress just now, I think in predictable future this will be constant dead-end.
It reminded me of this wonderful talk by the late Joe Armstrong (Erlang's creator): https://www.youtube.com/watch?v=lKXe3HUG2l4
Great post.
THANK YOU! Great work and writeup. Hope it finally silences the "concurrency + floating point" crowd and the "LLMs can never be deterministic" zealots.
Where this gets really complicated is when you are chaining many LLM calls together (basically any agent). A slight deviation in the call stack can throw off everything else.
This work is extremely consequential. When building agentic systems determinism will significantly improve the reliability.
I hope all the model providers adopt this.
Are the results of the matmuls really that far apart in size that you have to lose significant bits when adding them up at FP32?
Job one is have every bit of software involved also be deterministic, which stagex takes care of.
I had no problem getting deterministic LLM outputs when I experimented with this 6 months ago.
Run two of these with the same prompts and same seed and you get the same results.
Obviously in GPU clusters with different hardware things get more complicated.
https://git.distrust.co/public/llmshell
That's not what this is about.
"I had no problem getting deterministic LLM outputs when I experimented with this 6 months ago" looks like you're using llama-cpp in that repo. This is about vllm serving many requests at once, at long sequence lengths.
> As it turns out, our request’s output does depend on the parallel user requests. Not because we’re somehow leaking information across batches — instead, it’s because our forward pass lacks “batch invariance”, causing our request’s output to depend on the batch size of our forward pass.
Your situation isn't really comparable.
What’s stagex?
supply chain security focused linux distro that does not trust its own maintainers by design.
It should also be noted that PyTorch has a page about reproducibility: https://docs.pytorch.org/docs/stable/notes/randomness.html
TL;DR
Seed your PRNGs and call torch.use_deterministic_algorithms(True) to get the deterministic kernels. They may be slightly slower, but in practice, you probably will not notice.
Note that results will still differ between different drivers and GPUs. It would be great if NVIDIA tried harder in that regard.
The blog post is about LLM non-determinism in the context of serving at scale (variable batch size). The page you link is only about run-to-run determinism implicitly assuming a fixed batch size.
Some great discussion on twitter: https://x.com/thinkymachines/status/1965826369721623001
Seems a buried lede is that on-policy RL is unlocked by bitwise identical results between training and sampling. I'm not an expert here but my understanding is that this would allow for stronger guarantees about deployment/training alignment for the RL training that the labs already do.
I don't fully understand the BigMath example though. They show that off-policy RLVR requires off-policy correction, which avoids divergence, but is suboptimal because it results in noisy rewards. Then they say "we fixed the sampler and trainer numerical mismatch, which allows for on-policy RL, look how much better it is." It's not clear to me whether this is an artificial example that deliberately uses different trainer/sampler setups, or if it's actually impossible to have the same numerics between trainer/sampler without their fixes (even if we use same batch size, no atomics, etc.).
We know what thinking machines does yet?
I think this is an excellent article which addresses the issue that I personally have been thinking about a long time. And no its not just some slop they put but actual an engineering blog(with open source code and reproducible results!) I think the company is off to a good start
cool project but if this is what you are producing with $2 billion funding, i doubt you will survive. This is the type of article a grad student would write over a weekend.
on the contrary this makes me bullish about their team, it shows that people here care about the craft
The team is good, and I enjoyed the read. But this is just an engineering blog post. They're promoting this like it's ground breaking research and it's on their front-page. Ultimately this paper is not very meaningful and just a fun debugging session.
I've seen this play out dozens of times. So many startups that have come and go in the bay area were composed of extremely talented individuals, but almost all of them failed.
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Who needs a working product when you can spend all day designing the most WEWORK looking website and slap some pseud slop on it. It's like crypto "startups" but it's not even fun.
I am baffled that I still run against these statement years after LLM's have been around. LLM's are deterministic and always have been. The reason people are having issues with them is because they are basing their assumptions on api based experiments. Like my man, how can you be making these statements when you haven't done the due diligence of running the LLM on your own hardware with all of the variables locked down and accounted for? If you do just that it would become obviously clear that they are deterministic and most of the time the reason you see the non deterministic behavior is because you have not controlled for a variable. Usually prompt caching, batch processing or some other obvious variable. Now this is related to within same system deterministic behavior. You might get different answers when running on a different gpu, but at least for same systems the behavior is 100% identical if you account for all server startup flags and properly account for things like prompt cashing, slot contamination etc...
I suggest you look up the name of the main author of TFA before assuming they don’t know what they are talking about.
This is literally one of the most knowledgeable person on the topic. I think you are the one that hasn’t peeled enough layers to connect with what they are saying.
1. they aren't, they are just popular online. 2. the author has nothing to do with the original comment. Why do you think academic reviews are double blind?
One of the top 5 most active contributors to pytorch over the last few years, and specifically working on some of it's most hardcore components is "just popular online"?
If you say so.
> the author has nothing to do with the original comment
Except for the part of the comment that was assuming the author had no idea how this all works, has only used LLMs through API and has never run a local model, you mean?
Hold on a second. A transformer produces deterministically a probability distribution over the token alphabet from the context. Then one samples from this distribution. This is random and meant to be random.
The sampling process isn't random. If you sample with identical sampling parameters and identical values for said parameters, you will always get same results. You only start getting "non deterministic" behavior when you start using more complex systems outside the scope of your control like multi gpu systems and batch processing. One llm sampled with cash prompting off and and batch processing off will always generate same results if all values are same.
It's possible to deterministically sample from a probability distribution. For example, just seed your RNG with a constant, or with the SHA256 hash of the context.
Well yes, you can "hack" the pseudorandom number generator, but... that's not really the point when talking about determinism in LLMs is it? I mean the mathematical idea of the standard LLM is certainly truly random.
> I mean the mathematical idea of the standard LLM is certainly truly random.
Not really, LLMs give you a distribution over possible next tokens. You are free to then sample from this distribution how you want. There is no need to hack RNG or whatever, for example you can simply just take a greedy approach and always output the most likely token, in which case the LLM becomes deterministic (mathematically). This is equivalent to setting the temperature to 0.
The article literally justifies This in the second paragraph.
I suppose I have issues with the way "determinism" is used in the title of this article. It can mean different things to different people and in my mind stating that "Defeating Nondeterminism in LLM Inference" frames it as an actual issue with LLM inference. But its not, its an issue with LLM inference when you start using large scale inference with more complex parts such as systems which use multi gpu inference systems or batching processes and other mechanisms. It is not an issue when using an LLM without those more complex parts. Stating it this way muddies the signal and gives a false sense that this is a fundamental issue with architecture, where its an issue of the systems at scale...