Two halves of the Bitter Lesson

In 2019, Richard Sutton published The Bitter Lesson, arguing that the most important advances in AI come from general methods that scale with computation, rather than methods that encode human knowledge.

He identified two such methods: learning and search.

For the seven years since, the vast majority of frontier AI compute has been devoted to learning. The internet gave us an enormous source of training data, and scaling laws provided an extremely predictable return per FLOP spent. These favorable conditions gave rise to the pre-training era.

Reasoning models and AI agents ushered in the post-training era, where search and reinforcement learning become important parts of shaping and aligning model behavior, in many cases rivaling the compute spent on pre-training. But the conditions that made pre-training successful are changing, and early evidence suggests that we are approaching the limits of what we can extract from today’s post-training methods.

We believe overcoming these limits requires making search a first-class citizen of intelligent systems. We call the process that does so cognition, and we predict that the next five years will be defined by a new cognition era. Just as scaling learning defined the last era of AI, we believe scaling cognition will define the next, pulling model capabilities beyond their current limits.

Figure 1. Evolution of the compute split: pre-training → RL post-training → cognition. Pre-training and post-training shares fall; the share devoted to searching for solutions rather than learning from known answers rises. Prediction, not observation.

This is the other half of the Bitter Lesson that we believe the field has yet to fully embrace.

The pre-training era and the success of learning

The pre-training era was one of the most successful periods of empirical science in AI. The recipe was remarkably simple: gather more data, train larger networks, and spend more compute optimizing next-token prediction.

And it worked.

GPT-3 demonstrated that scaling a single autoregressive model could produce capabilities across a remarkable range of tasks. Kaplan et al. showed that language-model loss followed predictable power-law relationships with model size, dataset size, and compute. Chinchilla refined the recipe by showing how to allocate compute more efficiently between parameters and training tokens.

Language modeling test loss improves smoothly as model size, dataset size, and training compute increase, following power laws.
Figure 2. The classic pre-training scaling-law curve. Predictable improvements in loss as compute increases. The message is clear: investment → intelligence, practically guaranteed.

This success rested on three unusually favorable properties:

First, data was abundant. The internet had accumulated an enormous corpus of human-generated text, code, and other knowledge. We could train on it without first needing our models to discover new information.

Second, supervision was dense. Every training token came with an observed target. The optimizer did not need to solve the underlying problem to obtain a learning signal. Thanks to gradient descent, an incorrect prediction supplied a direction for reducing the loss.

Third, progress was predictable. Given a model architecture, dataset, and compute budget, researchers could often estimate how much the next training run would improve cross-entropy loss.

As a result, entire areas of research and engineering matured around this recipe: distributed training, data curation, numerical precision, optimizers, kernels, parallelism, and scaling laws.

The next phase of AI, if we ever hope that it will exceed human capabilities, does not enjoy these luxuries.

Reasoning models and the birth of post-training

Several developments have nudged us beyond the pre-training era. In September 2024, OpenAI introduced o1, showing that performance could improve along an additional axis: reasoning compute during inference. There is a clear lesson: letting models iteratively work towards or refine an answer through a long reasoning chain is a key lever to improve their intelligence.

OpenAI o1 test-time compute versus performance.
Figure 3. o1’s test-time compute vs. performance plot.
Several benchmarks showing o1 advantages over GPT-4o.
Figure 4. Benchmark results showing advantages of o1 over GPT-4o.

Since then, post-training and RL have become central components of frontier-model development. Although figures from leading closed and open model providers vary wildly and can be hard to estimate, in some cases the amount of compute devoted to this process is beginning to rival pre-training itself.

Ai2’s OLMo 3 series is estimated to have spent 15% of all H100 GPU-hours on RL across the training pipeline (pre-training, mid-training, supervised fine-tuning, and reinforcement learning), with RL occupying a disproportionate share of wall-clock accelerator time relative to its arithmetic FLOPs. Ai2’s public recipe breaks out these stages in detail.

Pre-training40,704H100 GPU-days
Post-training~7,550H100 GPU-days
Pre-training 80% Mid-training 3% Long-context 2% Post-training 15%
Figure 5. OLMo 3 share of GPU-hours spent on RL, in approximate allocated H100 GPU-days. Calculated from reported GPU counts and durations, with an illustrative DPO sweep allocation. Includes the additional 21 days of Olmo 3.1 RL. Allocated GPU time, not utilization-adjusted FLOPs.

On the extreme end, Reflection’s Beam was pre-trained on 23.8 trillion tokens using 6,144 NVIDIA GB300 GPUs in under four weeks. Its subsequent high-compute RL campaign ran on 10,500 GB300 GPUs for four weeks and generated more than 100 million rollouts across coding, reasoning, and agentic environments.

Pre-training≤172KGB300 GPU-days
RL~294KGB300 GPU-days
≤37% ≥63%
Figure 6. Share of RL GPU-hours vs. pre-training GPU-hours for Reflection’s Beam model. Calculated from Reflection’s reported GPU counts and training durations: pre-training on 6,144 GPUs for under four weeks, RL on 10,500 GPUs for four weeks. Excludes mid-training and other post-training stages. GPU allocation is not equivalent to executed model FLOPs.

This means that the RL campaign consumed at least 1.7 times as many allocated GPU-hours as pre-training, based on the published hardware counts and durations.

Similarly, xAI’s Grok 4 announcement claims RL training at pre-training scale, with a launch chart illustrating the growing contribution of RL compute relative to earlier model generations.

Grok 2Next-token prediction
Grok 3Pre-training compute
10×
Grok 3 ReasoningPre-training + RL
Grok 4 ReasoningRL compute
10×
Compute
Pre-training RL
Figure 7. xAI’s “Ludicrous rate of progress” slide from the Grok 4 launch livestream, redrawn: xAI claims RL post-training at pre-training scale. The slide gives no scale; bar lengths follow it as drawn. The 10× labels are xAI’s: Grok 3’s pre-training against Grok 2’s, and Grok 4’s RL against Grok 3’s.

These examples do not establish a universal percentage split for all frontier model organizations, but the direction is difficult to miss: Increasing amounts of frontier AI infrastructure are being devoted not simply to updating model parameters, but to generating, evaluating, and learning from candidate solutions. The post-training era is born.

But a trillion FLOPs spent on pre-training and a trillion FLOPs spent generating RL trajectories do not produce the same amount of learning signal.

Compute has shifted from learning to search

In pre-training, a model predicts a token, it is compared against the token in the training corpus, and the model is nudged to predict the target if its prediction is incorrect. The learning signal is immediate and plentiful. Within certain known bounds, scaling laws virtually guarantee that a FLOP spent on this process will lead to a better model.

In contrast, RL involves the model producing thousands of intermediate reasoning tokens, or invoking dozens of tools. At the end, the verifier marks the attempt as correct or incorrect.

Animation. Dense signal vs. sparse signal, on the same search tree. In pre-training the correct path is known in advance. At each token, a narrow search over that node’s alternatives is reconciled with the known next token before the next token is searched, so all of the searching, lit again at the end, stays in a narrow band along the path. In post-training nothing is known until a finished attempt is scored, so the search starts from the root and covers the whole tree: every attempt runs to the end, unchecked on the way, until one finds the answer.

Here we must realize the fundamental difference between the two regimes: Pre-training starts with the target and learns how to reproduce it. Search starts without the target and must first spend compute discovering it.

Three factors make RL much more inefficient than it seems:

The right answer must be searched for. Before any model updates are possible, compute must first be spent to search for the answer. For hard problems, this search process is prohibitively expensive, and it is difficult to predict if FLOPs spent will result in finding the right answer.

An additional, commonly overlooked consequence is the cost induced by autoregressive generation: since LLMs generate answers token-by-token, not knowing the right reasoning chain ahead of time forces RL pipelines to use GPU hours in this much more expensive regime that pre-training does not suffer from.

Incorrect predictions provide no learning signal. In pre-training, an incorrect prediction supplies the exact feedback needed to align the model to the correct behavior. In RL, the model receives no information about what it should have done instead, or where in the process the mistake was made. Consequently, unlike pre-training, future forward passes (search attempts) do not get progressively better.

We should not mistake these observations for proof that RL cannot scale. Instead, they tell us that the assumptions underlying pre-training’s success no longer apply automatically.

Limits of scaling post-training

These challenges have already manifested in concrete consequences and limits on what post-training can and cannot achieve, even when we invest in it in a scale that rivals pre-training.

Bottlenecks in post-training scaling laws. Although post-training scaling laws are not as mature as pre-training laws, in The Art of Scaling Reinforcement Learning Compute, Khatri et al. study more than 400,000 GPU-hours of RL experiments and fit sigmoidal rather than pre-training-style power-law performance curves. In stark contrast to the power laws observed in pre-training, there is a ceiling to what current RL pipelines can extract from base models.

Pre-training scaling laws: performance improves as a power law with compute.
Figure 8. Pre-training scaling laws: test loss improves as a power law with compute (Kaplan et al.).
Post-training scaling laws: sigmoidal curves with a performance ceiling.
Figure 9. Post-training RL scaling laws: sigmoidal curves with a performance ceiling (Khatri et al.).

RL training increases one-shot accuracy at the expense of solution coverage. In Does Reinforcement Learning Really Incentivize Reasoning Capacity Beyond the Base Model?, Yue et al. compare the pass@k of base models and their RL-trained counterparts. At small k, RL-trained models often outperform their base models, but at large k, the advantage often reverses: the base model can solve more distinct problems when allowed enough attempts.

Pass at k of base models versus RL-trained models across tasks from Yue et al.
Figure 10. Pass@k of base models vs. RL-trained models across tasks (Yue et al.).

New capabilities require an external source. More recently, knowledge distillation transfers capabilities from a teacher into a student. It can introduce capabilities that the student previously lacked, but those capabilities must first exist in the teacher or its generated data. On-policy distillation has become a popular framework used in many frontier open-model releases, including recent Kimi K-series models.

These pieces of evidence hint at a shared limitation: an optimizer can only use the information available in its training experience.

Does this mean that reasoning, post-training, and RL were the wrong direction for LLMs? No. We instead take these as signs that the other half of the Bitter Lesson has not been fully embraced.

Cognition: Breaking through the limits of post-training and reasoning

Reasoning and post-training proved to be necessary steps for more intelligent LLMs. Together with the scaling of pre-training, models, and training datasets, they form a two-axis backbone of general-purpose AI.

Then how can we break through the limits of the pre- and post-training eras? We think we just need to embrace the other half of the Bitter Lesson: we must bring back search as a first-class citizen of intelligent systems.

Test-Time Cognition (TTC) is our first instantiation of this principle in LLMs, providing a third, independent scaling axis. Unlike the reasoning axis, which always trades off higher performance for additional cost, we show that this new cognition axis can outperform the model's native reasoning while reducing cost:

Scaling reasoning effort Scaling test-time cognition
Figure 11. Test-time cognition is a new, independent scaling axis. On Terminal-Bench 2.0, our best TTC system beats GPT-5.2's native reasoning performance at a lower cost. TTC coordinates compute between three models: GPT-5.2, MiniMax-M2.5, and DeepSeek-v3.2.

This allowed TTC to beat the GPT-5.4 performance-cost Pareto frontier on Terminal-Bench 2.0, using models released purely before it.

Cognition: Coordinating the components of search

How is this possible? This result comes from taking the Bitter Lesson seriously: we must bring back search as a first-class citizen of intelligent systems. AlphaGo's Move 37, described as "surely creative" by grandmaster Lee Sedol, was discovered through a deliberative search process that the expert policy found unremarkable.

Search depends on considering all the important or interesting possibilities, even those that the strongest models consider unlikely, and recognizing promising partial solutions and rejecting bad ones. TTC approximates that process in LLMs.

Learning and reasoning are still very useful tools, and we do not claim that we should abandon them. Instead, we view search and learning as two halves of the same coin. We call the process that coordinates between them, and utilizes their complementary strengths to the fullest extent, cognition.

Figure 12. Cognition is the coordination of search over time, as on our homepage. At each step of an agent’s trajectory, cognition calls on the model’s learned weights, episodic memory and explicit search, and saves what each step finds back to memory.

This is not a proposal to abandon neural networks or gradient descent. It is a proposal to build a more capable process around them, one that leverages their strengths to achieve things the models could not previously do. Neural networks can serve as candidate generators, policies, value functions, and world models. Gradient descent can improve each component, while search determines which experiences to acquire and which possibilities deserve further computation.

Cognition is how the two halves of the Bitter Lesson can reinforce one another.

The Cognition Era: The next frontier in model capability

As we hit the limits of the pre-training and post-training eras, we believe the cognition era is the next stage: making search itself more intelligent, so that every unit of compute produces more useful information.

The key is to increase the effective density of rewards. By enumerating better candidates, estimating the value of partial solutions, and extracting information from failures, cognition can turn sparse, delayed feedback into richer signals that guide both search and learning. Instead of relying on a rare successful trajectory to learn something new, an intelligent system should improve from much more of the experience it generates. This is how we believe search can become a reliable engine for discovering capabilities beyond those of the underlying models.

Our prediction is that by 2031, most compute devoted to advancing frontier AI intelligence will be spent on cognition. Not simply on training larger models, nor on generating ever more independent reasoning attempts, but on systems that deliberately explore possibilities, evaluate progress, and accumulate knowledge from experience. The next frontier of intelligence will be driven by how effectively these systems convert compute into discovery.

Further reading: Intelligence Is Search: Cognition Coordinates It