[simonw]

Some people use it to demystify, but a whole lot of people seem to be using it to dismiss the technology entirely.

Personally I like to remind people that these things are next-token predictors, but then emphasize how truly astonishing the results we can get out of sufficiently advanced next-token predictors are.

[wren6991]

The "next-token predictor" framing is also a bit shaky. It's an accurate description of pre-training, where next-token prediction is a useful learning objective to force the model to learn higher-level representations. It's wildly misleading for a model put through an RL post-training campaign. The tokens it "predicts" aren't sampled from any naturally occurring distribution; the model's output is the result of an optimisation process that rewarded behaviour that was useful, and that's fundamentally different.

[thesz]

  > The tokens it "predicts" aren't sampled from any naturally occurring distribution; the model's output is the result of an optimisation process that rewarded behaviour that was useful, and that's fundamentally different.

https://arxiv.org/abs/2504.13837

"Surprisingly, we find that the current training setup does not elicit fundamentally new reasoning patterns. While RLVR-trained models outperform their base models at small k (e.g., k = 1), the base models achieve a higher pass@k score when k is large. Coverage and perplexity analyses show that the observed reasoning abilities originate from and are bounded by the base model. "

[adw]

This paper is less dramatic than you think it is and really just re-explains what RLVR does.

Let's stipulate that what pretraining does is train next token prediction over a gigantic corpus. You can then sample from this distribution repeatedly (cf the Large Language Monkeys paper) and count how often it passes some deterministic verifier.

What GRPO-style RLVR does is precisely this, but then reward the trajectories which passed the verifier. These distributions are _by construction_ within the accessible output space of the pretrained model; you're reweighting the distribution so that pass@k goes up, because that's (for applications like programming) very useful. RLVR is about making sampling more efficient; the only new information being added to the system is the presence of the verifier, and note that you only get a reward when the verifier passes, so there's essentially no mechanism for "teaching new facts" here.

[wren6991]

Ooh. This looks like an interesting paper and there were a couple of things in the intro that I found counter-intuitive. It'll take me a while to digest the whole thing.

> Coverage and perplexity analyses show that the observed reasoning abilities originate from and are bounded by the base model

On the face of it this seems unsurprising given the policy gradient term directly minimises this difference.

I don't have a good feel for how the output of an RLVR-trained model concretely differs from the base model. My guess would be there are a fairly small number of "forks" where the training creates a token flip that sends the model down a more useful path.

The fact that the straight paths between the forks resemble the base model would again be unsurprising since (a) those are exactly the right context to continue to elicit more output that's relevant to solving the problem (so not penalised by RLVR), and (b) preservation drops naturally out of the policy gradient term you add to limit catastrophic forgetting in the base model.

Low perplexity could be explained by the relative sparsity of the forks in the output stream, and/or by forks already having high entropy in the base model. That also aligns with the pass-at-high-k: yes it's doing more exploration without training but it's a bit of a monkeys-on-typewriters situation.

Lack of novelty is readily explained by the fact that you need some nonzero pass rate in the base model to actually get some useful training signal from RLVR. That's a limitation of contemporary RLVR techniques, not a limitation on post-training in general.

I think there's room in that forks-and-straights characterisation for the RLVR'd model to be doing something that looks a lot like computation, while having low perplexity vs the base model. I don't see anything in my admittedly incredibly shallow skim of the paper that refutes that.

[CamperBob2]

VibeThinker 3B is highly problematic for the thesis expressed in that paper. There is no possible way it could derive its reasoning capabilities from a bag of 3B parameters alone.

[simonw]

Right, but it's still useful to think of these models in terms of next-tokens because it helps explain that they look at every token that came before and use that to put out the next one.

You can get into RL as part of explaining why it's so unnervingly good at picking a next token.

[wren6991]

That's true. The fact that an LLM is a pure function of (all previous tokens) -> (next token), with internal state like KV cache only existing for optimisation purposes, is pretty mind-blowing.

I guess it was more the "predictor" part I had issue with. There's a tendency to reach for statistical or probabilistic terminology to describe things that aren't usefully understood in those terms. For example in the "Speed Always Wins" LLM technical survey (https://arxiv.org/pdf/2508.09834):

> The gate is a crucial component to bring sparsity in MoE models. For a batch of input token representations X ∈ RT×D, the gate function G determines the probabilities of dispatching token xi to each expert e

...which is nonsense: the gate simply, directly, selects the experts. There's nothing probabilistic about it.

[didibus]

> which is nonsense: the gate simply, directly, selects the experts. There's nothing probabilistic about it.

Isn't G a learned probability?

[wren6991]

I don't think so. Probability of what?

If you put something through a softmax the output is (trivially) a valid PMF. Does that matter? You're not sampling from it.

[didibus]

> the model's output is the result of an optimisation process that rewarded behaviour that was useful, and that's fundamentally different

I'm not understanding, can you explain this more? How does it become more than a next token predictor? Isn't the post-training simply altering the sampled distribution? And isn't that distribution naturally occurring? It's the distribution of "useful" next token?

[IanCal]

IMO that doesn’t sound so much like prediction any more.

It’d be prediction if it’s “predict what would come next in this text sampled from distribution X”.

But what’s it predicting if we’re looking for new useful outputs? It’s finding a distribution that’s useful, and generating tokens, but it’s not predicting what comes next in a known sequence.

[ruszki]

Just because you change loss function or dataset, it doesn’t become not a prediction. Just a prediction of something else.

[ameliaquining]

I'm curious, what are you hoping to convey by reminding people that LLMs are next-token predictors? They are, of course, but most people without an AI background won't fully understand what that means, so I assume you're using it at least partly as a proxy for something else.

[simonw]

I think understanding how this stuff works is really important. For technical people it gives them a useful starting point for understanding it all. For less technical people it's crucial to help them understand that it's not some weird new magical science-fiction AI - it's still computer programs that turn text into numbers and do stuff with the numbers and turn those back into text.

It's harder to believe something is conscious or threatening to achieve word domination once you understand that it's a machine that statistically figures out which word should come next.

[ameliaquining]

I don't think the next-token-predictor thing should increase anyone's confidence that LLMs aren't conscious or can't escape the control of their operators. A very closely analogous argument would "prove" that humans aren't conscious or can't do [insert task here] either. (No, I'm not saying that any of this is true of today's LLMs, I'm saying this particular argument doesn't work.)

I recommend this explanation: https://www.astralcodexten.com/p/next-token-predictor-is-an-...

[dgellow]

You can say that for any argument regarding consciousness, because we don’t have an actual, all encompassing definition of what consciousness is. In general I don’t think comparison with humans makes much sense, we should be able to discuss LLMs without always falling back to “but what about humans” (sorry for the caricature)

[ameliaquining]

Shouldn't that imply that agnosticism is the proper view, rather than asserting that something is impossible on a next-token-predictor architecture?

(Note: I don't actually think the consciousness question is the most important one in the near term. Where I think this line of reasoning gets really dangerous is when people use it to assert that LLMs can't or won't engage in certain behaviors no matter much they advance; this doesn't have anything to do with consciousness.)

[inigyou]

If you believe that matrix multiplication with random sampling is conscious, then you probably believe everything is conscious, like rocks.

Most people would expect that matrix multiplication is not conscious, and autocomplete is not conscious either.

We can't prove matrix multiplication isn't conscious, but it doesn't seem likely unless everything is conscious.

[slopinthebag]

I think you can reliably assert that X != Y without having a complete definition of Y, as long as you can identify at least one property or condition that Y possesses which X violates.

So for consciousness and LLMs it could be Qualia, lack of semantic understanding, lack of continuity in time, lack of a high degree of integrated causal feedback, etc.

Or perhaps those are just features of human consciousness but not integral to consciousness as a whole. To me this then implies panpsychism to some degree, which I'm alright with too.

[cyanydeez]

I think it's important to understand the humans _can_ do what LLMs do: predict next tokens from prior ones.

But LLMs are only operating on text and humans are only operating on <waves hands>

[dgellow]

While you’re active in this thread, I just want to say thank you for all your writing, you’re such a reliable source of sanity in that crazy new world :)

[CamperBob2]

It's harder to believe something is conscious or threatening to achieve word domination once you understand that it's a machine that statistically figures out which word should come next.

The problem is, these models challenge our definition of "consciousness." Or at least they point out how hopelessly-inadequate our thinking on the subject is. Some people really, really don't like having their personal definition of consciousness challenged.

The correct response to "So what, it's just a next-token predictor" isn't a long dissertation on RLHF, training architectures, scaling laws and whatever, but rather to turn around and respond, "Sure, and how is that different from what we do?"

[amenhotep]

LLMs self-evidently have no experience when not inferencing, seems like the most consequential difference to me. If we are next token predictors then we are next token predictors that are inferencing at every waking moment and arguably much of our sleeping moments too; when we train an LLM that can inference as much as we do and remain coherent then I'll be more worried about whether it might have experience.

[Forgeties79]

> It's harder to believe something is conscious or threatening to achieve word domination once you understand that it's a machine that statistically figures out which word should come next.

At the risk of sounding overly flippant, all world domination has been achieved by some person(s) figuring out which word should come next. Words quite literally = action when it comes to LLM’s with tools access

[KPGv2]

> At the risk of seeming X, statement that overwhelmingly demonstrates X-ness.

It's exhausting to even consider where to begin addressing the assertion that good leadership is just predicting the next word to say. Especially considering the corpus available to most great leaders in history was extremely small. To think Hannibal's military campaigns were just because he'd read like ten books in his life and could accurately forecast effective rhetoric is...indescribably divorced from reality.

At the risk of seeming like a jerk.

[notathrowaway51]

"Competent strategy" is just as good a target for a word predictor to optimize as "effective rhetoric" is.

[KPGv2]

I suggest you re-read what I wrote. In particular the part where I said there was no large corpus to perform this prediction back then.

After you edit, I'll address your point about how military strategy is nothing but skill with words.

[slopinthebag]

I think this is smart, it seems to me that the best way to use these models its to approach them as a next-token predictor instead of an intelligent entity. That's how I've gotten the best results from them and allows me to avoid some of the pitfalls people fall into by anthropromising them.