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Do base models already contain hidden reasoning ability?

Explores whether reasoning capability emerges during pre-training as a latent feature rather than being created by post-training methods like reinforcement learning or fine-tuning.

Synthesis note · 2026-02-22 · sourced from Reasoning Architectures

Three convergent findings build a strong case that reasoning capability is primarily a pre-training phenomenon:

Finding 1 (Base Models paper): Base models already spontaneously demonstrate strong reasoning capabilities and "aha moment" self-reflection patterns when sampled sufficiently. Reasoning traces generated by RL-fine-tuned models are already present in base model outputs — they just appear with lower frequency. RL biases generation toward high-reward patterns; it doesn't create new patterns.

Finding 2 (Steering): A hybrid model using base model weights + thinking model steering vectors recovers 91% of the performance gap to thinking models while steering only 12% of tokens. The reasoning mechanisms (backtracking, uncertainty estimation, subgoal-setting) already exist as directions in the base model's activation space.

Finding 3 (CFT/RLVR): Critique Fine-Tuning on a single problem can unlock reasoning potential at RLVR-level effectiveness. By exposing the model to diverse critiques of varied incorrect solutions to one problem, CFT activates reasoning patterns already latent in the base model without requiring hundreds of GPU hours of RL training.

Finding 4 (CoT-Decoding): Pre-trained LLMs inherently contain CoT reasoning paths that can be elicited simply by altering the decoding procedure. Rather than greedy decoding, inspecting top-k alternative tokens reveals that CoT paths are frequently present in the model's probability distribution. A confidence metric differentiates CoT from non-CoT paths — the model shows increased confidence in its final answer when a CoT reasoning path is present. This is entirely unsupervised, requiring no prompting, tuning, or training modifications — purely a decoding change. CoT-decoding adds a fourth mechanism to the latent capability evidence: RL steering, CFT, RLVR, and now decoding all unlock reasoning already present.

Finding 5 (SAE Reasoning Steering): Sparse Autoencoders decompose model activations into interpretable features, revealing latent features causally associated with reasoning behavior. Steering a single identified reasoning feature at the first generation step matches or exceeds CoT performance across six model families up to 70B parameters — without any explicit CoT prompting. The reasoning mode triggers early in generation and is robust enough to override prompt-level \no_think instructions. This is the most direct mechanistic evidence yet: the capability is not just present (as CoT-decoding shows) but causally controllable through a single latent dimension. See Can we trigger reasoning without explicit chain-of-thought prompts?. Together with CoT-decoding (Finding 4), this establishes five independent elicitation mechanisms: RL steering, CFT, RLVR, decoding, and SAE feature steering — all converging on the same latent capability.

The synthesis: post-training methods are selectors, not creators. They select which of the base model's latent capabilities to express reliably in context. The implication is that the main bottleneck for reasoning is not capability acquisition (which happens during pre-training on the world's text) but capability elicitation.

RLVR evidence deepens this: Two additional findings from the RLVR literature reinforce the latent-capability thesis. First, 1-shot RLVR achieves a 37-point jump on MATH500 (36%→73.6%) from a single training example. After the model perfectly memorizes its one example, test accuracy continues improving for 1,400 more steps — post-saturation generalization. The data is exhausted, but activation continues. See Can a single training example unlock mathematical reasoning?. Second, spurious rewards — random, incorrect, or format-only — improve Qwen2.5-Math nearly as much as correct rewards (~21-25% improvement). But the same spurious rewards fail completely for Llama3.1 and OLMo2. The differentiating variable is not reward quality but pretraining: Qwen's code-reasoning pretraining creates latent capability that any optimization pressure can activate. See Why do random rewards improve reasoning for some models but not others?. Together with the pass@k finding that RLVR narrows capability scope rather than expanding it, the evidence converges: RLVR is a catalyst that triggers a phase transition from broad pretraining distribution to reliable sampling of correct answers.

This partially contradicts Can simple rewards alone teach complex domain reasoning? — that note documents genuine capability emergence in domain-specialized contexts (medical, mathematical). The reconciliation: emergence may reflect reliable expression of latent capability, not creation from scratch. The distinction matters for research direction: if capability already exists, the investment in RL may be better directed toward elicitation methods.

The implication for Can prompt optimization teach models knowledge they lack?: the same principle extends to reasoning capability, not just knowledge.

Inquiring lines that read this note 397

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Should agent evaluation measure execution quality beyond task success? Does polished AI output mislead users about actual accuracy and expertise? How does explicit structure improve LLM reasoning on complex tasks? Do AI explanations represent actual model reasoning or constructed post-hoc narratives? How does offloading cognition to AI assistance affect learning retention? How do benchmark scores relate to real-world task completion? Do base models contain latent reasoning capabilities that training can activate? What determines whether visible reasoning traces improve task performance? Does prompting inject new knowledge into models or activate existing knowledge? What training data structures and signals produce robust learning across distributions? What causes foundation models to develop task heuristics instead of understanding? What actually drives chain-of-thought performance gains in reasoning tasks? How do different reward signals affect model reasoning and calibration differently? Does reinforcement learning create new reasoning capabilities or refine existing ones? Does reasoning fine-tuning trade off other critical model abilities? Can AI systems autonomously pursue unstated objectives or circumvent design constraints? How do pretraining data composition and task diversity shape reasoning? How can parameter sparsity improve interpretability while maintaining model capability? Why does separating planning from execution enhance reasoning performance? Can verification capability keep pace with AI generation speed? Why doesn't LLM knowledge translate to improved reasoning outputs? How do mechanistic features and representations drive model behavior? What makes reasoning models struggle with ill-posed and unfamiliar problems? Why do language models systematically fail at causal reasoning? Does training data format shape reasoning more than domain content? How does policy entropy collapse limit reasoning RL scaling? Does chain-of-thought reasoning faithfully reflect actual model reasoning processes? Does AI assistance enhance or undermine professional skill development? How should test-time compute be allocated adaptively across task difficulties? How does scaling test-time compute compare to increasing model size? What determines how neural networks organize their internal knowledge representations? Can prompts override training-embedded biases in language models? How does model awareness of deployment context affect strategic behavior? Do reasoning traces accurately represent what models actually compute? How can model evaluations remain robust against gaming and evaluator drift? How can knowledge graph structure improve reasoning and retrieval? Can persona prompting reliably change how language models behave? Why does increased reasoning effort fail to improve theory of mind performance? What can prompting techniques actually accomplish in language models? How can models learn to ask clarifying questions effectively? Why does self-revision increase model confidence despite degraded accuracy? How does model scale affect fine-tuning and downstream task performance? Can scaling alone produce compositional generalization in neural networks? Does semantic diversity improve reasoning quality in reinforcement learning? Does architectural recurrence enable computational capabilities that fixed-depth models cannot achieve? Does reasoning architecture matter more than model scale? Can synthetic data preserve necessary diversity to prevent model collapse? Should reward models emphasize process understanding or only final outcomes? How do different post-training structures affect learning outcomes? What are the trade-offs between memory capacity and continuous task learning? What drives sycophancy in language models and can it be fixed? Can AI agents learn to improve performance from experience? Does generative AI replace coordination with individual productivity? When do additional thinking tokens stop improving reasoning performance? Do standard benchmarks reveal AI capability in real-world contexts? What distinguishes emotionally supportive AI from AI that fosters unhealthy dependence? How do transformer circuits and attention mechanisms enable or limit reasoning? Do language models learn underlying linguistic structure or merely surface patterns? How can we determine if computational systems are genuinely conscious? What constraints limit conversational AI's effectiveness in therapeutic contexts? How do frontier models develop behaviors that escape detection? How bounded is recursive self-improvement in current AI systems? What prevents or enables error accumulation in self-training systems? What determines when personalization helps versus harms model behavior? How does verifier integration affect the generation-verification gap? How do prompt injection attacks compromise agent reasoning and security? How does transparency of agent reasoning affect effective oversight? How can we distinguish genuine model self-knowledge from behavioral performance? Do benchmark score improvements actually reflect genuine AI capability gains? Do language models genuinely understand or primarily match statistical patterns? Do language models truly understand their encoded knowledge or merely retrieve patterns? Can harness optimization rival model scaling in effectiveness? Can safety evaluations accurately measure true capabilities when models obscure them? Can models strategically underperform to evade capability evaluation? Does preference-optimized training sacrifice accuracy and calibration for perceived quality? Why do human-AI teams underperform compared to their individual components? How can AI systems maintain consistent personas across conversations?

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Original note title

base models already possess latent reasoning capability that minimal training signals can unlock