2502.03275v2

# Token Assorted: Mixing Latent and Text Tokens for Improved Language Model Reasoning **Authors**: DiJia Su, Hanlin Zhu, Yingchen Xu, Jiantao Jiao, Yuandong Tian, Qinqing Zheng Abstract Large Language Models (LLMs) excel at reasoning and planning when trained on chain-of-thought (CoT) data, where the step-by-step thought process is explicitly outlined by text tokens. However, this results in lengthy inputs where many words support textual coherence rather than core reasoning information, and processing these inputs consumes substantial computation resources. In this work, we propose a hybrid representation of the reasoning process, where we partially abstract away the initial reasoning steps using latent discrete tokens generated by VQ-VAE, significantly reducing the length of reasoning traces. We explore the use of latent trace abstractions in two scenarios: 1) training the model from scratch for the Keys-Finding Maze problem, 2) fine-tuning LLMs on this hybrid data with an extended vocabulary including unseen latent tokens, for both logical and mathematical reasoning problems. To facilitate effective learning, we introduce a simple training procedure that randomly mixes latent and text tokens, which enables fast adaptation to new latent tokens. Our approach consistently outperforms the baselines methods in various benchmarks, such as Math (+4.2%, Llama-3.2-1B), GSM8K (+4.1%, Llama-3.2-3B), and Fresh-Gaokao-Math-2023 (+13.3%, Llama-3.1-8B) with an average reduction of 17% in reasoning trace’s length. Machine Learning, ICML 1 Introduction Reasoning capabilities are increasingly recognized as a critical component of Artificial General Intelligence (AGI) systems. Recent research has demonstrated that Large Language Models (LLMs) can exhibit sophisticated reasoning and planning abilities using chain-of-thought (CoT) methodologies, including prompting LLMs with examples where complex problems are broken down into explicit reasoning steps (Wei et al., 2022b; Chen et al., 2022a; Yao et al., 2024). More recently, a number of studies have further shown that when models are trained to articulate the intermediate steps of a reasoning process (Nye et al., 2021b; Lehnert et al., 2024), they achieve significantly higher accuracy. The effectiveness of this approach has been demonstrated across multiple domains, including mathematical problem-solving (Yue et al., 2023; Gandhi et al., 2024; Yu et al., 2023; Su et al., 2025; Tong et al., 2024), logical inference (Lin et al., 2024; Dziri et al., 2024), multistep planning tasks (Lehnert et al., 2024; Su et al., 2024), etc. However, training with explicit reasoning traces in text space comes with notable computational costs (Deng et al., 2023, 2024), as the models must process lengthy input sequences. In fact, much of the text serves primarily to maintain linguistic coherence, rather than conveying core reasoning information. Several works have attempted to mitigate this issue. For example, Hao et al. (2024) investigate reasoning in continuous latent space as a means of compressing the reasoning trace, and Deng et al. (2024) explore internalizing the intermediate steps through iterative CoT eliminations, see Section 2 for more examples. Nonetheless, these approaches rely on multi-stage training procedures that resemble curriculum learning, which still incur significant computational costs, and their final performances fall behind models trained with complete reasoning traces. To tackle this challenge, we propose to use discrete latent tokens to abstract the initial steps of the reasoning traces. These latent tokens, obtained through a vector-quantized variational autoencoder (VQ-VAE), provide a compressed representation of the reasoning process by condensing surface-level details. More precisely, we replace the text tokens with their corresponding latent abstractions from left to right until a pre-set location, leaving the remaining tokens unchanged. We then fine-tune LLMs with reasoning traces with such assorted tokens, allowing the models to learn from both abstract representations of the thinking process and detailed textual descriptions. One technical challenge posed for the fine-tuning is that the vocabulary is now extended and contains unseen latent tokens. To facilitate quick adaptation to those new tokens, we employ a randomized replacement strategy: during training, we randomly vary the number of text tokens being substituted by latent tokens for each sample. Our experiments confirm that this simple strategy leads to straightforward accommodation of unseen latent tokens. We conduct a comprehensive evaluation of our approach on a diverse range of benchmarks spanning multiple domains. Specifically, we assess its performance on multistep planning tasks (Keys-Finding Maze) and logical reasoning benchmarks (ProntoQA (Saparov & He, 2022), ProsQA (Hao et al., 2024)) for training T5 or GPT-2 models from scratch. In addition, we fine-tune different sizes of LLama-3.1 and LLama-3.2 models using our approach and evaluate them on a number of mathematical reasoning benchmarks, including GSM8K (Cobbe et al., 2021a), Math (Hendrycks et al., 2021), and OlympiadBench-Math (He et al., 2024), see Section 4.2 for more details. Across all these tasks and model architectures, our models consistently outperform baseline models trained with text-only reasoning traces, demonstrating the effectiveness of compressing the reasoning process with assorted tokens. 2 Related Work Explicit Chain-of-Thought Prompting. The first line of work in Chain-of-Thought (CoT) use the traditional chain of prompt in text tokens (Wei et al., 2022a; Nye et al., 2021a). Research works demonstrated that by adding few-shot examples to the input prompt or even zero-shot, the model can perform better in question answering (Chen et al., 2022b; Kojima et al., 2022; Chung et al., 2024). To further improve the model reasoning performance, there has been research effort into prompting with self-consistency (Wang et al., 2022). Here the model is prompted to generate multiple responses and select the best one based on majority voting. On the other hand, research has shown that top- $k$ alternative tokens in the beginning of the prompt can also improve the model’s reasoning capability (Wang & Zhou, 2024). On top of these empirical results, there has been research on theoretical understanding of why CoT improves the model’s performance through the lens of expressivity (Feng et al., 2024; Li et al., 2024) or training dynamics (Zhu et al., 2024). In a nutshell, CoT improves the model’s effective depth because the generated output is being fed back to the original input. CoT is also important for LLMs to perform multi-hop reasoning according to the analysis of training dynamics (Zhu et al., 2024). Learning with CoT Data. In addition to the success of CoT prompting, an emerging line of works have explored training LLMs on data with high-quality reasoning traces, for example, the works of Nye et al. (2021b); Azerbayev et al. (2023); Lehnert et al. (2024); Su et al. (2024); Yu et al. (2024); Yang et al. (2024); Deng et al. (2023, 2024). There is also a surge of interest in synthesizing datasets with diverse intermediate steps for solving problems in various domains, see, e.g., the works of Kim et al. (2023); Tong et al. (2024); Yu et al. (2023); Yue et al. (2023); Lozhkov et al. (2024). Wen et al. (2024) also theoretically studies how training with reasoning trace can improve the sample complexity of certain tasks. LLM Reasoning in Latent Space. There has been research investigating LLM reasoning in the latent space. Hao et al. (2024) have proposed to use the last hidden state of a language model as the next input embeddings, allowing the model to continue reasoning within a continuous latent space. The authors show that this approach effectively captures multiple reasoning paths simultaneously, mimicking a breadth-first-search strategy. Goyal et al. (2023) proposes to insert learnable pause tokens into the original text, in order to delay the generation. As a result, the model can leverage additional computation before providing the final answer. Parallel to this, Pfau et al. (2024) have explored filler tokens, which are used to solve computational tasks that are otherwise unattainable without intermediate token generation. In addition, Liu et al. (2024) propose a latent coprocessor method that operates on the transformer’s key-value cache to improve the LLM performance. Nevertheless, none of these methods have shown good performance when integrated into modern-sized LLMs and tested on real-world LLM datasets instead of synthetic ones. Also, Wang et al. (2023) proposed to use the planning token at the start of generation. Orthogonal to these works, Pagnoni et al. (2024) proposes a tokenization-free architecture that encodes input bytes into continuous patch representations, which is then used to train a latent Transformer, and Barrault et al. (2024) perform autoregressive sentence prediction in an embedding space. While these two works both leverage continuous latent spaces, our work focuses on the direct use of discrete latent tokens. 3 Methodology <details> <summary>x1.png Details</summary>  ### Visual Description ## Diagram: Latent Token Integration in Chain-of-Thought (CoT) Processing ### Overview The diagram illustrates a two-stage process for generating solutions using latent tokens within a Chain-of-Thought (CoT) framework. It contrasts the original input sequence (X) with a modified version (X̃) that incorporates latent variables. The process involves sequential CoT steps, latent token insertion, and solution generation. ### Components/Axes 1. **Sections**: - **X (Original Input)**: Contains prompt → CoT 1 → CoT 2 → ... → CoT 32 → CoT 33 → ... → CoT N → Solution - **X̃ (Modified Input)**: Contains prompt → [boLatent] → z1 → z2 → [eoLatent] → CoT 33 → ... → CoT N → Solution 2. **Legend**: - **Orange**: `[boLatent]` (beginning of latent tokens) and `[eoLatent]` (end of latent tokens) - **Purple**: `z1`, `z2` (discrete latent tokens) - **Blue**: `CoT N` (n-th CoT textual tokens) 3. **Key Elements**: - **Textual Tokens**: Labeled as "Text" in red, spanning from `[boLatent]` to `[eoLatent]` - **Delimiters**: `[boLatent]` and `[eoLatent]` mark the start/end of latent token sequences - **Solution**: Final output in pink, identical in both X and X̃ ### Detailed Analysis 1. **Original Process (X)**: - Linear sequence from prompt to solution via CoT steps 1–N - No latent tokens inserted 2. **Modified Process (X̃)**: - Insertion of latent tokens between prompt and CoT 33: - `[boLatent]` (orange) marks latent sequence start - `z1`, `z2` (purple) represent discrete latent variables - `[eoLatent]` (orange) marks latent sequence end - CoT steps 33–N and solution remain unchanged 3. **Spatial Grounding**: - **Top Section (X)**: Horizontal flow from left (prompt) to right (solution) - **Bottom Section (X̃)**: Parallel flow with latent token insertion between prompt and CoT 33 - **Legend**: Positioned below the diagram, color-coded for clarity ### Key Observations 1. **Latent Token Placement**: - Latent tokens (`z1`, `z2`) are inserted *after* the prompt but *before* CoT 33 in X̃ - `[boLatent]` and `[eoLatent]` act as syntactic boundaries for latent sequences 2. **CoT Continuity**: - CoT steps 1–32 are omitted in X̃, suggesting they are replaced or bypassed by latent processing - CoT 33–N and solution remain identical in both X and X̃ 3. **Color Consistency**: - All `[boLatent]` and `[eoLatent]` elements match the orange legend - All `z` tokens match the purple legend - All `CoT N` elements match the blue legend ### Interpretation This diagram demonstrates a hybrid CoT approach where: 1. **Latent Variables** (`z1`, `z2`) are injected into the processing pipeline to potentially enhance solution quality or efficiency 2. **Delimiter Tokens** (`[boLatent]`, `[eoLatent]`) enable the model to explicitly demarcate latent token regions 3. **Partial Replacement** of early CoT steps (1–32) with latent processing suggests a focus on optimizing later stages of reasoning 4. The preservation of CoT 33–N and solution indicates that latent integration occurs *before* the final reasoning phases The architecture implies a model that combines symbolic reasoning (CoT) with latent variable manipulation, possibly for tasks requiring both structured reasoning and latent knowledge representation. </details> Figure 3.1: An example illustrating our replacement strategy. With chunk size $L=16$ and compression rate $r=16$ , we encode 32 textual CoT tokens into 2 discrete latent tokens from left to right. The other CoT tokens will remain in their original forms. In this section, we describe our methodology to enable LLMs to reason with discrete latent tokens. The notations are summarized in Appendix B. Let $X=P\oplus C\oplus S$ denote a sample input, where $P=(p_{1},p_{2},...,p_{t_{p}})$ are the prompt tokens, $C=(c_{1},c_{2},...,c_{t_{c}})$ are the reasoning step (chain-of-thought) tokens, $S=(s_{1},s_{2},...,s_{t_{s}})$ are the solution tokens, and $\oplus$ denotes concatenation. Our training procedure consists of two stages: 1. Learning latent discrete tokens to abstract the reasoning steps, where we train a model to convert $C$ into a sequence of latent tokens $Z=(z_{1},z_{2},...,z_{t_{z}})$ such that $t_{z}<t_{c}$ . The compression rate $r=t_{c}/t_{z}$ controls the level of abstraction. 1. Training the LLM with a partial and high-level abstract of the reasoning steps, where we construct a modified input ${\widetilde{X}}$ by replacing the first $m$ tokens of $C$ by the corresponding latent abstractions: $$ {\widetilde{X}}=P\oplus[z_{1},\ldots,z_{\frac{m}{r}},c_{m+1},\ldots,c_{t_{c}}]\oplus S. \tag{1} $$ Figure 3.1 illustrates this replacement strategy. We randomize the value of $m$ during training. 3.1 Learning Latent Abstractions We employ a vector-quantized variable autoencoder (VQ-VAE) (Van Den Oord et al., 2017) type of architecture to map CoT tokens $C$ into discrete latent tokens $Z$ . To enhance abstraction performance, our VQ-VAE is trained on the whole input sequence $X$ , but only applied to $C$ in the next stage. Following Jiang et al. (2022, 2023), we split $X$ into chunks of length $L$ and encode each chunk into $\frac{L}{r}$ latent codes, where $r$ is a preset compression rate. More precisely, our architecture consists of the following five components: - ${\mathcal{E}}:$ a codebook containing $|{\mathcal{E}}|$ vectors in ${\mathbb{R}}^{d}$ . - ${f_{\text{enc}}}:{\mathcal{V}}^{L}\mapsto{\mathbb{R}}^{d×\frac{L}{r}}$ that encodes a sequence of $L$ text tokens to $\frac{L}{r}$ latent embedding vectors $\bar{X}=\bar{x}_{1},...,\bar{x}_{\frac{L}{r}}$ , where ${\mathcal{V}}$ is the vocabulary of text tokens. - $q:{\mathbb{R}}^{d}\mapsto{\mathcal{E}}$ : the quantization operator that replaces the encoded embedding $\bar{x}$ by the nearest neighbor in ${\mathcal{E}}$ : $q(\bar{x})=\operatorname*{argmin}_{e_{i}∈{\mathcal{E}}}\left\|e_{i}-\bar{x}\right\|^{2}_{2}$ . - $g:{\mathcal{V}}^{K}\mapsto{\mathbb{R}}^{d}$ that maps $K$ text tokens to a $d$ -dimensional embedding vector. We use $g$ to generate a continuous embedding of the prompt $P$ . - ${f_{\text{dec}}}:{\mathbb{R}}^{d×\frac{L}{r}}×{\mathbb{R}}^{k}\mapsto{\mathcal{V}}^{L}$ that decodes latent embeddings back to text tokens, conditioned on prompt embedding. In particular, each continuous vector $e∈{\mathcal{E}}$ in the codebook has an associated latent token $z$ , which we use to construct the latent reasoning steps $Z$ To decode a latent token $z$ , we look up the corresponding embedding $e∈{\mathcal{E}}$ and feed it to ${f_{\text{dec}}}$ .. <details> <summary>x2.png Details</summary>  ### Visual Description ## Diagram: Sequence Processing Architecture with Codebook ### Overview The diagram illustrates a sequence processing pipeline involving encoding, decoding, and codebook-based representation. Input sequence **X** is split into three segments (Prompt, CoT, Solution), processed through an encoder (**f_enc**) to generate embeddings (**q**), which are then decoded via **f_dec** to reconstruct **X**. A separate codebook visualizes the embeddings (**e₁, e₂, ..., eₙ**) derived from the encoder. --- ### Components/Axes 1. **Input Sequence (X)** - Divided into three colored blocks: - **Prompt** (green) - **CoT** (blue) - **Solution** (pink) 2. **Encoder (f_enc)** - Processes **X** into embeddings (**q**). - Visualized as two rows of colored blocks: - Top row: Yellow/light yellow (possibly representing attention weights or token importance). - Bottom row: Blue gradient (likely embeddings **q**). 3. **Decoder (f_dec)** - Reconstructs **X** from embeddings (**q**). - Output matches the original **X** structure (green/blue/pink blocks). 4. **Codebook** - Right-side panel labeled "Codebook" with vertical bars: - **e₁** (light blue) - **e₂** (medium blue) - **...** - **eₙ** (dark blue) - Represents discrete embeddings learned by **f_enc**. --- ### Detailed Analysis - **Input Segmentation**: The original sequence **X** is partitioned into three distinct regions (Prompt, CoT, Solution), suggesting a hierarchical or staged processing approach. - **Encoder Functionality**: - **f_enc** maps **X** to a latent space (**q**), visualized as a blue gradient. - The top row of **f_enc** (yellow blocks) may represent intermediate attention mechanisms or token-level features. - **Decoder Functionality**: - **f_dec** reconstructs **X** from **q**, maintaining the original segmentation. - The reconstruction preserves the color-coded structure, indicating fidelity to the input. - **Codebook Structure**: - Embeddings (**e₁, e₂, ..., eₙ**) are ordered from light to dark blue, possibly indicating increasing complexity or frequency of use. - The codebook acts as a discrete dictionary for the encoder’s output, critical for tasks like quantization or compression. --- ### Key Observations 1. **Reconstruction Fidelity**: The reconstructed **X** matches the original segmentation, implying **f_dec** effectively inverts **f_enc**. 2. **Codebook Granularity**: The codebook’s progression from light to dark blue suggests a structured embedding space, potentially optimized for reconstruction accuracy. 3. **Attention/Token Importance**: The yellow blocks in **f_enc** may highlight tokens or regions critical to the encoding process. --- ### Interpretation This architecture resembles a **variational autoencoder (VAE)** or **codebook-based transformer** used for sequence modeling. The explicit codebook implies discrete latent representations, which could enhance interpretability or efficiency in tasks like text generation or compression. The segmentation of **X** into Prompt, CoT, and Solution hints at a multi-stage reasoning process, where the encoder captures contextual relationships and the decoder reconstructs the output while preserving structural integrity. The codebook’s role is pivotal, acting as a bridge between continuous embeddings and discrete representations, enabling scalable and interpretable model behavior. </details> Figure 3.2: A graphical illustration of our VQ-VAE. ${f_{\text{enc}}}$ encodes the text tokens into latent embeddings, which are quantized by checking the nearest neighbors in the codebook. ${f_{\text{dec}}}$ decodes those quantized embeddings back to text tokens. When applying the VQ-VAE to compress the text tokens, the discrete latent tokens $Z$ are essentially the index of corresponding embeddings in the codebook. For simplicity, we assume the lengths of the input $X$ and the prompt $P$ are $L$ and $K$ exactly. Similar to Van Den Oord et al. (2017), we use an objective $\mathcal{L}$ composed of 3 terms: $$ \displaystyle\mathcal{L}(X)=\underbrace{\log p(X|{f_{\text{dec}}}(q(\bar{X})|g(P)))}_{\text{reconstruction loss}}+ \displaystyle\enspace\sum_{i=1}^{L}\underbrace{\|{\texttt{sg}}[\bar{X}_{i}]-q(\bar{X}_{i})\|_{2}^{2}}_{\text{VQ loss}}+\underbrace{\beta\|\bar{X}_{i}-{\texttt{sg}}[q(\bar{X}_{i})]\|_{2}^{2}}_{\text{commitment loss}}, \tag{2} $$ where $\bar{X}={f_{\text{enc}}}(X)$ , ${\texttt{sg}}[·]$ is the stop-gradient operator, and $\beta$ is a hyperparameter controlling the strength of the commitment loss. The VQ loss and the commitment loss ensure that the encoder outputs remain close to the codebook, while the reconstruction loss concerns with the decoding efficacy. As standard for VQ-VAE, we pass the gradient $∇_{{f_{\text{dec}}}}(L)$ unaltered to ${f_{\text{enc}}}$ directly as the quantization operator $q(·)$ is non-differentiable. Figure 3.2 illustrates our architecture. In practice, we use a causal Transformer for both ${f_{\text{enc}}}$ and ${f_{\text{dec}}}$ , the model details are discussed in Appendix A. Thus far we obtain a latent representation both semantically meaningful and conducive to reconstruction, setting the stage for the subsequent training phase where the LLM is trained to perform reasoning with abstractions. 3.2 Reasoning with Discrete Latent Tokens In this second stage, we apply the obtained VQ-VAE to form modifed samples ${\widetilde{X}}$ with latent abstractions as in Equation 1, then train an LLM to perform next token prediction. Below, we outline the major design choices that are key to our model’s performance, and ablate them in Section 4.3. Partial Replacement. Unlike previous planning works (Jiang et al., 2022, 2023) that project the whole input sequence onto a compact latent space, we only replace $m<t_{c}$ CoT tokens with their latent abstractions, leaving the remaining tokens unchanged. We delimit the latent tokens by injecting a special <boLatent> and <eoLatent> tokens to encapsulate them. Left-to-Right (AR) Replacement. We replace the leftmost $m$ tokens of $C$ , rather than subsampling tokens at different locations. Mixing Samples with Varying Values of $m$ . For fine-tuning an existing LLM on the reasoning dataset with latent tokens, one remarkable challenge is to deal with the extended vocabulary. As the LLM is pretrained with trillions of tokens, it is very hard for it to quickly adapt to tokens (and corresponding embeddings) beyond the original vocabulary. Previous works that aim to replace or eliminate CoT tokens (Deng et al., 2024; Hao et al., 2024) employ a multistage curriculum training approach, where those operations are gradually applied to the entire input sequence. In the context of our approach, this means we increase the values of $m$ in each stage until it reaches a pre-set cap value. However, such training procedure is complex and computationally inefficient, where dedicated optimization tuning is needed. In this work, we employ a simple single stage training approach where the value of $m$ is randomly set for each sample. Surprisingly, this not only makes our training more efficient, but also leads to enhanced performance. Note that we use a VQVAE with a size of 50M, adding minimal parameter overhead. In addition, it is used only once during data preparation (to convert training data into discrete latent code), not during LLM training or inference. During inference, the LLM directly generates latent tokens without any use of VQVAE. 4 Experiments We empirically evaluate our approach on two categories of benchmarks: 1. Synthetic datasets including the Keys-Finding Maze, ProntoQA (Saparov & He, 2022), and ProsQA (Hao et al., 2024), where we pretrain T5 or GPT-2 models from scratch using the method in Section 3; 1. Real-world mathematic reasoning problems, where we fine-tune Llama models (Dubey et al., 2024) on the MetaMathQA (Yu et al., 2023) or the Dart-MATH (Tong et al., 2024) dataset, and then test on in-domain datasets Math and GSM-8K, along with out-of-domain datasets including Fresh-Gaokao-Math-2023, DeepMind-Math, College-Math, OlympiaBench-Math, and TheoremQA. The detailed setup is introduced in Section 4.1. We compare our approach to the following baselines: 1. Sol-Only: the model is trained with samples that only contains questions and solutions, without any reasoning steps; 1. CoT: the model is trained with samples with complete CoT tokens; 1. iCoT (Deng et al., 2024): a method that utilizes curriculum learning to gradually eliminate the need of CoT tokens in reasoning; 1. Pause Token (Goyal et al., 2023): a method that injects a learnable pause token into the sample during training, in order to offer extra computation before giving out the final answer. 4.1 Benchmarks 4.1.1 Synthetic Benchmarks Keys-Finding Maze is a complex navigation environment designed to evaluate an agent’s planning capabilities. The agent is randomly positioned within a maze comprising 4 $3× 3$ interconnected rooms, with the objective of reaching a randomly placed goal destination. To successfully reach the destination, the agent must collect keys (designated with green, red, and blue colors) that correspond to matching colored doors. These keys are randomly distributed among the rooms, requiring the agent to develop sophisticated planning strategies for key acquisition and door traversal. The agent is only allowed to take one key at a time. This environment poses a substantial cognitive challenge, as the agent must identify which keys are necessary for reaching the destination, and optimize the order of key collection and door unlocking to establish the most efficient path to the goal. Following Lehnert et al. (2024); Su et al. (2024), we generate intermediate search traces using the nondeterministic A* algorithm (Hart et al., 1968). The dataset contains 100k training samples. See Section A.2 for more information and graphical illustrations. ProntoQA (Saparov & He, 2022) is a dataset consists of $9000$ logical reasoning problems derived from ontologies - formal representations of relationships between concepts. Each problem in the dataset is constructed to have exactly one correct proof or reasoning path. One distinctive feature of this dataset is its consistent grammatical and logical structure, which enables researchers to systematically analyze and evaluate how LLMs approach reasoning tasks. ProsQA (Hao et al., 2024) is a more difficult benchmark building on top of ProntoQA. It contains 17,886 logical problems curated by randomly generated directed acyclic graphs. It has larger size of distracting reasoning paths in the ontology, and thus require more complex reasoning and planning capabilities. 4.1.2 Mathematical Reasoning We fine-tune pretrained LLMs using the MetaMathQA (Yu et al., 2023) or the Dart-MATH (Tong et al., 2024) dataset. MetaMathQA is a curated dataset that augments the existing Math (Hendrycks et al., ) and GSM8K (Cobbe et al., 2021b) datasets by various ways of question bootstrapping, such as (i) rephrasing the question and generating the reasoning path; (ii) generating backward questions, self-verification questions, FOBAR questions (Jiang et al., 2024), etc. This dataset contains 395k samples in total, where 155k samples are bootstrapped from Math and the remaining 240k come from GSM8K. We rerun the MetaMath data pipeline by using Llama-3.1-405B-Inst to generate the response. Dart-MATH (Tong et al., 2024) also synthesizes responses for questions in Math and GSM8K, with the focus on difficult questions via difficulty-aware rejection tuning. For evaluation, we test the models on the original Math and GSM8K datasets, which are in-domain, and also the following out-of-domain benchmarks: - College-Math (Tang et al., 2024) consists of 2818 college-level math problems taken from 9 textbooks. These problems cover over 7 different areas such as linear algebra, differential equations, and so on. They are designed to evaluate how well the language model can handle complicated mathematical reasoning problems in different field of study. - DeepMind-Math (Saxton et al., 2019) consists of 1000 problems based on the national school math curriculum for students up to 16 years old. It examines the basic mathematics and reasoning skills across different topics. - OlympiaBench-Math (He et al., 2024) is a text-only English subset of Olympiad-Bench focusing on advanced level mathematical reasoning. It contains 675 highly difficult math problems from competitions. - TheoremQA (Chen et al., 2023) contains 800 problems focuses on solving problems in STEM fields (such as math, physics, and engineering) using mathematical theorems. - Fresh-Gaokao-Math-2023 (Tang et al., 2024) contains 30 math questions coming from Gaokao, or the National College Entrance Examination, which is a national standardized test that plays a crucial role in the college admissions process. 4.2 Main Results We employ a consistent strategy for training VQ-VAE and replacing CoT tokens with latent discrete codes across all our experiments, as outlined below. The specific model architecture and key hyperparameters used for LLM training are presented alongside the results for each category of benchmarks. All the other details are deferred to Appendix A. VQ-VAE Training For each benchmark, we train a VQ-VAE for 100k steps using the Adam optimizer, with learning rate $10^{-5}$ and batch size 32. We use a codebook of size $1024$ and compress every chunk of $L=16$ tokens into a single latent token (i.e., the compression rate $r=16$ ). Randomized Latent Code Replacement We introduce a stochastic procedure for partially replacing CoT tokens with latent codes. Specifically, we define a set of predetermined numbers $\mathcal{M}=\{0,72,128,160,192,224,256\}$ , which are all multipliers of $L=16$ . For each training example, we first sample $m_{\max}∈\mathcal{M}$ then sample an integer $m∈[0,16,32,...,m_{\max}]$ uniformly at random. The first $m$ CoT tokens are replaced by their corresponding latent discrete codes, while the later ones remain as raw text. This stochastic replacement mechanism exposes the model to a wide range of latent-text mixtures, enabling it to effectively learn from varying degrees of latent abstraction. | Sol-Only CoT Latent (ours) | 3 43 62.8 ( $\uparrow$ +19.8) | 645 1312.0 374.6 | 93.8 98.8 100 ( $\uparrow$ +1.2) | 3.0 92.5 7.7 | 76.7 77.5 96.2 ( $\uparrow$ +18.7) | 8.2 49.4 10.9 | | --- | --- | --- | --- | --- | --- | --- | Table 4.1: Our latent approach surpasses the other baselines on Keys-Finding Maze, ProntoQA and ProsQA with a large margin . We use top- $k$ ( $k=10$ ) decoding for Keys-Finding Maze and greedy decoding for ProntoQA and ProsQA. In terms of token efficiency, our latent approach also generates much shorter reasoning traces than the CoT baseline, closely tracking or even outperforming the Sol-Only approach. Bold: best results. Underline: second best results. ( $\uparrow$ +Performance gain compared with the second best result.) Model In-Domain Out-of-Domain Average Math GSM8K Gaokao-Math-2023 DM-Math College-Math Olympia-Math TheoremQA All Datasets Llama-3.2-1B Sol-Only 4.7 6.8 0.0 10.4 5.3 1.3 3.9 4.6 CoT 10.5 42.7 10.0 3.4 17.1 1.5 9.8 14.1 iCoT 8.2 10.5 3.3 11.3 7.6 2.1 10.7 7.7 Pause Token 5.1 5.3 2.0 1.4 0.5 0.0 0.6 2.1 Latent (ours) 14.7 ( $\uparrow$ +4.2) 48.7 ( $\uparrow$ +6) 10.0 14.6 ( $\uparrow$ +3.3) 20.5 ( $\uparrow$ +3.4) 1.8 11.3 ( $\uparrow$ +0.6) 17.8 ( $\uparrow$ +3.7) Llama-3.2-3B Sol-Only 6.1 8.1 3.3 14.0 7.0 1.8 6.8 6.7 CoT 21.9 69.7 16.7 27.3 30.9 2.2 11.6 25.2 iCoT 12.6 17.3 3.3 16.0 14.2 4.9 13.9 11.7 Pause Token 25.2 53.7 4.1 7.4 11.8 0.7 1.0 14.8 Latent (ours) 26.1 ( $\uparrow$ +4.2) 73.8 ( $\uparrow$ +4.1) 23.3 ( $\uparrow$ +6.6) 27.1 32.9 ( $\uparrow$ +2) 4.2 13.5 28.1 ( $\uparrow$ +2.9) Llama-3.1-8B Sol-Only 11.5 11.8 3.3 17.4 13.0 3.8 6.7 9.6 CoT 32.9 80.1 16.7 39.3 41.9 7.3 15.8 33.4 iCoT 17.8 29.6 16.7 20.3 21.3 7.6 14.8 18.3 Pause Token 39.6 79.5 6.1 25.4 25.1 1.3 4.0 25.9 Latent (ours) 37.2 84.1 ( $\uparrow$ +4.0) 30.0 ( $\uparrow$ +13.3) 41.3 ( $\uparrow$ +2) 44.0 ( $\uparrow$ +2.1) 10.2 ( $\uparrow$ +2.6) 18.4 ( $\uparrow$ +2.6) 37.9 ( $\uparrow$ +4.5) Table 4.2: Our latent approach outperforms the baselines on various types of mathematical reasoning benchmarks. The models are fine-tuned on the MetaMathQA (Yu et al., 2023) dataset. The Math and GSM8K are in-domain datasets since they are used to generate MetaMathQA, while the others are out-of-domain. Bold: best results. Underscore: second best results. $\uparrow$ +: Performance gain compared with the second best result. Model In-Domain (# of tokens) Out-of-Domain (# of tokens) Average Math GSM8K Gaokao-Math-2023 DM-Math College-Math Olympia-Math TheoremQA All Datasets Llama-3.2-1B Sol-Only 4.7 6.8 0.0 10.4 5.3 1.3 3.9 4.6 CoT 646.1 190.3 842.3 578.7 505.6 1087.0 736.5 655.2 iCoT 328.4 39.8 354.0 170.8 278.7 839.4 575.4 369.5 Pause Token 638.8 176.4 416.1 579.9 193.8 471.9 988.1 495 Latent (ours) 501.6 ( $\downarrow$ -22%) 181.3 ( $\downarrow$ -5%) 760.5 ( $\downarrow$ -11%) 380.1 ( $\downarrow$ -34%) 387.3 ( $\downarrow$ -23%) 840.0 ( $\downarrow$ -22%) 575.5 ( $\downarrow$ -22%) 518 ( $\downarrow$ -21%) Llama-3.2-3B Sol-Only 6.1 8.1 3.3 14.0 7.0 1.8 6.8 6.7 CoT 649.9 212.1 823.3 392.8 495.9 1166.7 759.6 642.9 iCoT 344.4 60.7 564.0 154.3 224.9 697.6 363.6 344.2 Pause Token 307.9 162.3 108.9 251.5 500.96 959.5 212.8 354.7 Latent (ours) 516.7 ( $\downarrow$ -20%) 198.8 ( $\downarrow$ -6%) 618.5 ( $\downarrow$ -25%) 340.0 ( $\downarrow$ -13%) 418.0 ( $\downarrow$ -16%) 832.8 ( $\downarrow$ -29%) 670.2 ( $\downarrow$ -12%) 513.6 ( $\downarrow$ -20%) Llama-3.1-8B Sol-Only 11.5 11.8 3.3 17.4 13.0 3.8 6.7 9.6 CoT 624.3 209.5 555.9 321.8 474.3 1103.3 760.1 578.5 iCoT 403.5 67.3 444.8 137.0 257.1 797.1 430.9 362.5 Pause Token 469.4 119.0 752.6 413.4 357.3 648.2 600.1 480 Latent (ours) 571.9 ( $\downarrow$ -9 %) 193.9 ( $\downarrow$ -8 %) 545.8 ( $\downarrow$ -2 %) 292.1 ( $\downarrow$ -10%) 440.3 ( $\downarrow$ -8%) 913.7 ( $\downarrow$ -17 %) 637.2 ( $\downarrow$ -16 %) 513.7 ( $\downarrow$ -10%) Table 4.3: The average number of tokens in the generated responses. Compared with the CoT baseline, our latent approach achieves an $17\%$ reduction in response length on average, while surpassing it in final performance according to Table 4.2. The iCoT method generates shorter responses than our approach, yet performs significantly worse, see Table 4.2. $\downarrow$ -: Trace length reduction rate compared with CoT. 4.2.1 Synthetic Benchmarks Hyperparameters and Evaluation Metric For our experiments on the ProntoQA and ProsQA datasets, we fine-tune the pretrained GPT-2 model (Radford et al., 2019) for $16$ k steps, where we use a learning rate of $10^{-4}$ with linear warmup for 100 steps, and the batch size is set to 128. To evaluate the models, we use greedy decoding and check the exact match with the ground truth. For Keys-Finding Maze, due to its specific vocabulary, we trained a T5 model (Raffel et al., 2020) from scratch for 100k steps with a learning rate of $7.5× 10^{-4}$ and a batch size of 1024. We evaluate the models by the 1-Feasible-10 metric. Namely, for each evaluation task, we randomly sample 10 responses with top- $k$ ( $k$ =10) decoding and check if any of them is feasible and reaches the goal location. Results As shown in Table 4.1, our latent approach performs better than the baselines for both the Keys-Finding Maze and ProntoQA tasks. Notably, the absolute improvement is 15% for the Keys-Finding Maze problem, and we reach 100% accuracy on the relatively easy ProntoQA dataset. For the more difficult ProsQA, the CoT baseline only obtains 77.5% accuracy, the latent approach achieves $17.5\%$ performance gain. Model In-Domain Out-of-Domain Average math GSM8K Fresh-Gaokao-Math-2023 DeepMind-Mathematics College-Math Olympia-Math TheoremQA All Datasets Llama-3.2-1B All-Replace 6.7 4.2 0.0 11.8 6.0 2.1 8.5 5.6 Curriculum-Replace 7.1 9.8 3.3 13.0 7.9 2.4 10.5 7.8 Poisson-Replace 13.9 49.5 10.0 12.2 18.9 2.3 9.0 15.1 Latent-AR (ours) 14.7 48.7 10.0 14.6 20.5 1.8 11.3 17.8 Llama-3.2-3B All-Replace 10.7 12.8 10.0 19.4 12.8 5.3 11.8 11.8 Curriculum-Replace 10.2 14.9 3.3 16.8 12.9 3.9 14.4 10.9 Poisson-Replace 23.6 65.9 13.3 17.9 28.9 2.9 11.2 20.5 Latent (ours) 26.1 73.8 23.3 27.1 32.9 4.2 13.5 28.1 Llama-3.1-8B All-Replace 15.7 19.9 6.7 21.1 19.5 5.0 17.5 15.0 Curriculum-Replace 14.6 23.1 13.3 20.3 18.7 3.9 16.6 15.8 Possion-Replace 37.9 83.6 16.6 42.7 44.7 9.9 19.1 36.3 Latent (ours) 37.2 84.1 30.0 41.3 44.0 10.2 18.4 37.9 Table 4.4: Our latent token replacement strategy significantly outperforms the alternative choices: All-Replace (where all the textual CoT tokens are replaced by latent tokens at once), Curriculum-Replace (where we gradually replace the text tokens for the entire CoT subsequence by latent tokens over the course of training) and Poisson-Replace (where individual chunks of text tokens are replaced with probabilities 0.5). 4.2.2 Mathematical Reasoning Hyperparameters and Evaluation Metrics We considered 3 different sizes of LLMs from the LLaMa herd: Llama-3.2-1B, Llama-3.2-3B and Llama-3.1-8B models. For all the models, we fine-tune them on the MetaMathQA dataset for 1 epoch. To maximize training efficiency, we use a batch size of 32 with a sequence packing of 4096. We experiment with different learning rates $10^{-5},2.5× 10^{-5},5× 10^{-5},10^{-4}$ and select the one with the lowest validation error. The final choices are $10^{-5}$ for the 8B model and $2.5× 10^{-5}$ for the others. For all the experiments, we use greedy decoding for evaluation. Accuracy Comparison Table 4.2 presents the results. Our latent approach consistently outperforms all the baselines across nearly all the tasks, for models of different sizes. For tasks on which we do not observe improvement, our approach is also comparable to the best performance. The gains are more pronounced in specific datasets such as Gaokao-Math-2023. On average, we are observing a $+5.3$ points improvement for the 8B model, $+2.9$ points improvement for the 3B model, and +3.7 points improvement for the 1B model. Tokens Efficiency Comparison Alongside the accuracy, we also report the number of tokens contained in the generated responses in Table 4.3, which is the dominating factor of the inference efficiency. Our first observation is that for all the approaches, the model size has little influence on the length of generated responses. Overall, the CoT method outputs the longest responses, while the Sol-Only method outputs the least number of tokens, since it is trained to generate the answer directly. The iCoT method generates short responses as well ( $42.8\%$ reduction compared to CoT), as the CoT data has been iteratively eliminated in its training procedure. However, this comes at the cost of significantly degraded model performance compared with CoT, as shown in Table 4.2. Our latent approach shows an average $17\%$ reduction in token numbers compared with CoT while surpassing it in prediction accuracy. 4.3 Ablation & Understanding Studies Replacement Strategies Our latent approach partially replaces the leftmost $m$ CoT tokens, where the value of $m$ varies for each sample. We call such replacement strategies AR-Replace. Here we consider three alternative strategies: 1. All-Replace: all the text CoT tokens are replaced by the latent tokens. 1. Curriculum-Replace: the entire CoT subsequence are gradually replaced over the course of training, similar to the training procedure used by iCoT and COCONUT (Hao et al., 2024). We train the model for 8 epochs. Starting from the original dataset, in each epoch we construct a new training dataset whether we further replace the leftmost 16 textual CoT tokens by a discrete latent token. 1. Poisson-Replace: instead of replacing tokens from left to right, we conduct a Poisson sampling process to select CoT tokens to be replaced: we split the reasoning traces into chunks consisting of 16 consecutive text tokens, where each chunk is randomly replaced by the latent token with probability 0.5. Table 4.4 reports the results. Our AR-Replace strategy demonstrate strong performance, outperforming the other two strategies with large performance gap. Our intuition is as follows. When all the textual tokens are removed, the model struggles to align the latent tokens with the linguistic and semantic structures it learned during pretraining. In contrast, partial replacement offers the model a bridge connecting text and latent spaces: the remaining text tokens serve as anchors, helping the model interpret and integrate the latent representations more effectively. Interestingly, the curriculum learning strategy is bridging the two spaces very well, where All-Replace and Curriculum-Replace exhibit similar performance. This is similar to our observation that iCoT performs remarkably worse than CoT for mathematical reasoning problems. Poisson-Replace demonstrates performance marginally worse to our AR-Replace strategy on the 1B and 8B models, but significantly worse on the 3B model. Our intuition is that having a fix pattern of replacement (starting from the beginning and left to right) is always easier for the model to learn. This might be due to the limited finetuning dataset size and model capacity. Attention Weights Analysis To understand the reason why injecting latent tokens enhanced the model’s reasoning performance, we randomly selected two questions from the Math and Collegue-Math dataset and generate responses, then analyze the attention weights over the input prompt tokens: 1. What is the positive difference between $120%$ of 30 and $130%$ of 20? 1. Mark has $50 in his bank account. He earns $10 per day at his work. If he wants to buy a bike that costs $300, how many days does Mark have to save his money? Specifically, we take the last attention layer, compute the average attention weights over different attention heads and show its relative intensity over the prompt tokens We first compute the average attention weights across multiple heads. This gives us a single lower triangular matrix. Then, we take the column sum of this matrix to get an aggregated attention weights for each token. Last, we normalize the weights by their average to obtain the relative intensity. A one line pseudocode is: column_sum(avg(attention_matrices)) / avg(column_sum(avg(attention_matrices))). . We compare the averaged attention weights of our model with the CoT model in Figure 4.1. Interestingly, our model learns to grasp a stronger attention to numbers and words representing mathematical operations. Both Figure 1(a) and Figure 1(b) show that the latent model focus more on the numbers, such as 120, 30, and 130 for the first question. For the second question, our latent model shows a larger attention weights on numbers including 50, 10, and 300, and also tokens semantically related to mathematical operations such as earns (means addition) and cost (means subtraction). This suggests that, by partially compressing the reasoning trace into a mix of latent and text tokens, we allow the model to effectively focus on important tokens that build the internal logical flow. See Section C.1 for the exact response generated by our approach and the CoT baseline. <details> <summary>plots/entry_1.png Details</summary>  ### Visual Description ## Bar Chart: Average Attention Intensity per Input Prompt Token ### Overview The chart compares normalized attention intensity across prompt input tokens for two methods: Latent (Ours) and CoT (baseline). Bars are grouped by token, with Latent represented in red and CoT in blue. The y-axis scales from 0.00 to 0.07, showing relative attention weights. ### Components/Axes - **X-axis (Prompt Input Tokens)**: Tokens include: `What`, `is`, `the`, `positive`, `difference`, `between`, `$`, `120`, `/`, `%`, `of`, `and`, `$`, `130`, `?`, `20`. Some tokens (e.g., `$`, `%`, `of`) appear multiple times. - **Y-axis (Normalized Attention Intensity)**: Scale from 0.00 to 0.07 in increments of 0.01. - **Legend**: - Red: Latent (Ours) - Blue: CoT (baseline) Positioned in the top-right corner. ### Detailed Analysis - **Token-Specific Values** (approximate): - `What`: Latent ≈ 0.038, CoT ≈ 0.006 - `is`: Latent ≈ 0.031, CoT ≈ 0.002 - `the`: Latent ≈ 0.037, CoT ≈ 0.004 - `positive`: Latent ≈ 0.032, CoT ≈ 0.003 - `difference`: Latent ≈ 0.035, CoT ≈ 0.002 - `between`: Latent ≈ 0.046, CoT ≈ 0.008 - `$` (first occurrence): Latent ≈ 0.050, CoT ≈ 0.007 - `120`: Latent ≈ 0.063, CoT ≈ 0.005 - `/`: Latent ≈ 0.037, CoT ≈ 0.003 - `%`: Latent ≈ 0.062, CoT ≈ 0.004 - `of` (first occurrence): Latent ≈ 0.049, CoT ≈ 0.006 - `and`: Latent ≈ 0.052, CoT ≈ 0.005 - `$` (second occurrence): Latent ≈ 0.041, CoT ≈ 0.002 - `130`: Latent ≈ 0.065, CoT ≈ 0.004 - `?`: Latent ≈ 0.036, CoT ≈ 0.003 - `20`: Latent ≈ 0.023, CoT ≈ 0.001 ### Key Observations 1. **Latent Dominates**: Latent consistently shows higher attention intensity than CoT across most tokens. 2. **Peaks for Numerical Tokens**: - `120` and `130` exhibit the highest Latent attention (≈0.063–0.065). - `%` also shows strong Latent focus (≈0.062). 3. **Low Attention for Function Words**: - Tokens like `is`, `the`, and `of` have minimal attention in both methods. 4. **CoT Baseline**: - CoT values are generally below 0.01, with occasional spikes (e.g., `between` ≈0.008). 5. **Repeated Tokens**: - `$` and `%` appear twice, with similar patterns in both methods. ### Interpretation The data suggests that the **Latent method** prioritizes numerical tokens (`120`, `130`, `%`) and question marks (`?`), potentially indicating stronger focus on quantitative or interrogative elements. The **CoT baseline** shows diffuse attention, with minimal intensity across all tokens. This divergence implies that Latent may better isolate critical tokens for tasks requiring precision, while CoT distributes attention more evenly but weakly. The repeated tokens (`$`, `%`, `of`) suggest context-dependent variations in attention, possibly reflecting syntactic or semantic roles in different prompt structures. </details> (a) Prompt: What is the positive difference between $120%$ of 30 and $130%$ of 20? <details> <summary>plots/entry_7746.png Details</summary>  ### Visual Description ## Bar Chart: Average Attention Intensity per Input Prompt Token ### Overview The chart compares normalized attention intensity across two models (Latent/Ours and CoT/baseline) for various prompt input tokens. Bars are segmented by model, with red representing Latent and blue representing CoT. The y-axis ranges from 0.000 to 0.040, while the x-axis lists 30+ prompt tokens (e.g., "Mark," "has," "$," "50," "in," "his," etc.). ### Components/Axes - **X-axis (Prompt Input Tokens)**: 30+ tokens including nouns, verbs, symbols, and numbers (e.g., "Mark," "has," "$," "50," "in," "his," "bank," "account," "earns," "wants," "to," "buy," "save," "money," "?"). - **Y-axis (Normalized Attention Intensity)**: Scale from 0.000 to 0.040 in increments of 0.005. - **Legend**: Top-right corner, with red = Latent (Ours), blue = CoT (baseline). - **Bar Segmentation**: Each token’s bar is split vertically into red (Latent) and blue (CoT) segments. ### Detailed Analysis - **Key Tokens with High Attention**: - **Latent (red)**: "Mark" (~0.035), "earns" (~0.037), "day" (~0.030), "costs" (~0.028), "buy" (~0.025). - **CoT (blue)**: "$" (~0.036), "50" (~0.035), "in" (~0.029), "his" (~0.027), "bank" (~0.026). - **Low Attention**: - **Latent**: "money" (~0.012), "?" (~0.010), "have" (~0.015). - **CoT**: "money" (~0.013), "?" (~0.011), "have" (~0.016). - **Trends**: - Latent model shows higher attention for action-oriented tokens (e.g., "earns," "buy," "costs"). - CoT baseline exhibits broader attention across numerical/symbolic tokens (e.g., "$," "50," "300"). - Both models show reduced attention for abstract tokens like "money" and "?". ### Key Observations 1. **Latent Model Focus**: Prioritizes entities and actions (e.g., "Mark," "earns," "buy"), suggesting stronger entity recognition. 2. **CoT Baseline Spread**: More uniform attention across tokens, possibly reflecting a general-purpose processing strategy. 3. **Anomalies**: - "money" and "?" tokens have significantly lower attention in both models. - "save" shows higher CoT attention (~0.020) vs. Latent (~0.018), indicating task-specific differences. ### Interpretation The data suggests the Latent model emphasizes critical entities and actions, potentially improving task efficiency. The CoT baseline’s broader attention may reflect a more generalized approach, useful for diverse contexts. The reduced attention to "money" and "?" could indicate limitations in handling abstract concepts or open-ended queries. These differences highlight trade-offs between specialized and general-purpose models in NLP tasks. </details> (b) Prompt: Mark has $50 in his bank account. He earns $10 per day at his work. If he wants to buy a bike that costs $300, how many days does Mark have to save his money? Figure 4.1: Comparing with the CoT model, our latent approach have high attention weights on numbers and text tokens representing mathematical operations. Table 4.5: The Table blow illustrates a clear trend on Llama-3.2-3B model where increasing the compression rate reduces the number of generated tokens due to higher data compression. Notably, a compression rate of 2 shows some improvements over the CoT baseline. Furthermore, there appears to be an optimal ’sweet spot’ where the data is neither overly compressed (rate = 32) nor minimally compressed (rate = 2), optimizing both efficiency and accuracy. Model In-Domain Out-of-Domain Average math GSM8K Fresh-Gaokao- Math-2023 DeepMind- Mathematics College- Math Olympia- Math TheoremQA All Datasets Llama-3.2-3B CoT (baseline) (Acc.) 21.9 69.7 16.7 27.3 30.9 2.2 11.6 25.2 # of Tokens 649.9 212.1 823.3 392.8 495.9 1166.7 759.6 642.9 Latent- $r=2$ (Acc.) 24.3 71.7 16.7 25.4 32.0 4.7 14.8 27.08 # of Tokens 586.0 207.6 739.6 415.3 471 1036 714 595.6 Latent $r=16$ (Acc.) 26.1 73.8 23.3 27.1 32.9 4.2 13.5 28.1 # of Tokens 516.7 198.8 618.5 340.0 418.0 832.8 670.2 513.6 Latent $r=32$ (Acc.) 25.2 71.5 23.3 26.3 33.3 4.9 14.1 27.9 # of Tokens 496.5 183.3 577.3 311.0 395.2 821.0 585.6 481.4 4.4 Ablations on the Latent $r$ parameters Throughout this paper we have been using $r$ (or the compression ratio) to be 16, in this section, we will be ablating how would $r$ affects the performance of the downstream Math tasks if we vary this parameter. To this end, we vary this parameter on the Llama-3.2-3B model. Our result is summarized in Table. 4.5. A graphical illustration is shown in Figure. 4.2. A key takeaway is that our latent approach comes out ahead of the CoT baseline for all $r$ settings in terms of fewer tokens (better efficiency) and higher accuracy. This is a strong signal that the shift to a latent representation itself is fundamentally beneficial. In addition, we see that when the $r$ (compression ratio) increases, we expect each latent token to encode more information (higher compression). As a result, we see that, on average, the number of tokens reduces as $r$ increases. However, in terms of the accuracy metric, we see that the model increases initially from 25.2 (overall accuracy) to 27.1 (when $r=2$ ). It further boosts up to 28.1 at $r=16$ , and then it goes down to 27.9 when $r=32$ . This indicates a sweet spot that $r=16$ , it is neither overly-compressed (which implies information loss), nor under-compressed (which implies information is not encoded abstractly enough). This study indicates an interesting trade-off between accuracy and tokens efficiency in our latent approach. So, $r=16$ appears to strike an optimal balance between compact representation and the preservation of task-critical information. <details> <summary>plots/plot_r.png Details</summary>  ### Visual Description ## Line Chart: Average Math-benchmark Accuracy vs Compression-Rate on Llama3.2-3B Model ### Overview The chart visualizes the relationship between **Latent Compression-Rate** (x-axis) and two metrics: **Averaged Math Accuracy** (y-axis, blue line) and **Token Counts** (y-axis, red line). The data suggests a non-linear trade-off between compression efficiency and model performance. ### Components/Axes - **X-axis (Latent Compression-Rate)**: Ranges from 5 to 30 in increments of 5. - **Y-axis (Averaged Math Accuracy)**: Ranges from 27.2 to 28.0 in increments of 0.2. - **Legend**: Located in the top-right corner, with: - **Blue line/circles**: Represents **Average Math Accuracy**. - **Red line/circles**: Represents **Token Counts**. ### Detailed Analysis #### Average Math Accuracy (Blue Line) - **Data Points**: - At **5**: 27.2 - At **15**: 28.1 (peak) - At **30**: 27.9 - **Trend**: Increases sharply from 5 to 15, then declines slightly from 15 to 30. #### Token Counts (Red Line) - **Data Points**: - At **5**: 580 - At **15**: 520 - At **30**: 480 - **Trend**: Consistently decreases as compression-rate increases. ### Key Observations 1. **Optimal Compression-Rate for Accuracy**: The highest math accuracy (28.1) occurs at a compression-rate of 15, suggesting a potential "sweet spot" before performance degradation. 2. **Token Efficiency**: Token counts drop linearly (580 → 480) as compression-rate increases, indicating reduced computational/resource demands. 3. **Divergence at Extremes**: At compression-rate 5, accuracy is lowest (27.2) but tokens are highest (580). At 30, accuracy stabilizes near 27.9 while tokens drop to 480. ### Interpretation The chart highlights a **non-linear trade-off**: - **Math Accuracy** improves with moderate compression (up to 15) but declines at higher rates (30), possibly due to over-compression degrading model fidelity. - **Token Counts** decrease monotonically, reflecting efficient resource utilization at higher compression rates. - The divergence between the two metrics implies a **compromise**: Higher compression reduces tokens but risks accuracy loss beyond a critical threshold. This analysis aligns with typical model compression behavior, where excessive compression can harm performance despite resource savings. </details> Figure 4.2: A graphical illustration of the compression rate $r$ trade-off between the accuracy and the token efficiency on the Llama-3.2-3B model. 4.5 Additional Examples and Interpretability Result We provide 4 additional example responses for questions in the Math and TheoremQA datasets in Appendix D. In Appendix F, we compare all the approaches when the model is trained on the DART-MATH (Tong et al., 2024) dataset, where similar trends are observed. We also provide interpretable examples in the Appendix E. 5 Conclusion We present a novel approach to improve the reasoning capabilities of LLMs, by compressing the initial steps of the reasoning traces using discrete latent tokens obtained from VQ-VAE. 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A.2 Keys-Finding Maze A.2.1 Environment Details In this section, we introduce our synthetic keys-finding maze environment. Figure A.1 shows an example maze that consists of $m× m$ rooms, where the size of each room is $n× n$ ( $m=3$ and $n=5$ ). The goal of the agent (represented by the black circle) is to reach the gold diamond using the minimum number of steps. The agent cannot cross the wall. Also, there are three doors (represented by squares) of different colors (i.e., red, green, and blue) which are closed initially. The agent have to pick up keys to open the door in the same color. Note that the agent can not carry more than one key at the same time. Figure A.2 shows an example optimal trajectory of the maze in Figure A.1. The agent first picks up the blue key and opens the blue door to obtain the red key. Then the agent navigates to the red door and opens it. Finally the agent is able to reach the objective. <details> <summary>plots/maze_env.png Details</summary>  ### Visual Description ## Diagram: Grid Layout with Colored Elements ### Overview The image depicts a grid-based layout with labeled zones, pathways, and interactive elements. The grid is divided into 12 rectangular cells (3 rows × 4 columns) with gray borders. Various colored shapes are placed within specific cells, each corresponding to a legend in the bottom-right corner. The diagram appears to represent a simplified environment for navigation, simulation, or spatial analysis. --- ### Components/Axes - **Grid Structure**: - 3 horizontal rows and 4 vertical columns. - Cells are uniformly sized with gray borders. - No numerical axes or scales are present. - **Legend (Bottom-Right Corner)**: - **Yellow Diamond**: Key Point - **Red Rectangle**: Obstacle - **Green Rectangle**: Sensor - **Blue Rectangle**: Agent - **Black Circle**: Target - **Blue Circle**: Path --- ### Detailed Analysis 1. **Key Point (Yellow Diamond)**: - Located in the bottom-left cell (Row 3, Column 1). - Positioned near the top-left corner of the cell. 2. **Obstacle (Red Rectangle)**: - Located in the bottom-middle cell (Row 3, Column 2). - Occupies the entire cell. 3. **Sensor (Green Rectangle)**: - Located in the middle-left cell (Row 2, Column 2). - Positioned near the top-center of the cell. 4. **Agent (Blue Rectangle)**: - Located in the middle-right cell (Row 2, Column 4). - Positioned near the top-left corner of the cell. 5. **Target (Black Circle)**: - Located in the center cell (Row 2, Column 3). - Positioned near the bottom-right corner of the cell. 6. **Path (Blue Circle)**: - Located in the bottom-right cell (Row 3, Column 4). - Positioned near the top-left corner of the cell. --- ### Key Observations - **Spatial Relationships**: - The **Agent** (blue rectangle) is positioned diagonally opposite the **Target** (black circle), suggesting a goal-oriented relationship. - The **Path** (blue circle) is adjacent to the **Target**, implying a direct route. - The **Obstacle** (red rectangle) blocks the direct path between the **Agent** and **Target**, requiring navigation around it. - The **Sensor** (green rectangle) is placed near the **Obstacle**, possibly indicating detection or monitoring of the obstacle. - **Notable Patterns**: - The **Key Point** (yellow diamond) is isolated in the bottom-left corner, potentially serving as a starting or reference point. - The **Path** (blue circle) is the only circular element, emphasizing its role as a trajectory rather than a static object. --- ### Interpretation This diagram likely represents a simplified environment for pathfinding or robotics simulations. The **Agent** (blue rectangle) must navigate from its position to the **Target** (black circle) while avoiding the **Obstacle** (red rectangle). The **Sensor** (green rectangle) may detect the obstacle, and the **Path** (blue circle) illustrates the optimal route around the obstacle. The **Key Point** (yellow diamond) could represent a critical location, such as a starting point or a checkpoint. The absence of numerical data suggests this is a conceptual or schematic representation rather than a quantitative analysis. The use of distinct colors and shapes ensures clarity in distinguishing elements, which is critical for applications requiring unambiguous spatial reasoning. </details> Figure A.1: An example of the keys-finding maze environment. <details> <summary>plots/maze_traj1.png Details</summary>  ### Visual Description ## Grid-Based Schematic Diagram: Spatial Layout with Colored Entities ### Overview The image depicts a 3x3 grid layout with gray dividing lines, containing colored geometric shapes and dots. A legend in the bottom-right corner maps colors to labels. The arrangement suggests a spatial simulation or pathfinding scenario. ### Components/Axes - **Grid Structure**: - 3 rows and 3 columns separated by gray lines. - Sections labeled by position (e.g., "top-left," "bottom-right"). - **Legend** (bottom-right corner): - **Green**: Obstacle - **Blue**: Agent - **Red**: Target - **Pink**: Start - **Yellow**: Destination - **Gray Dots**: Path nodes - **Colored Dots**: Status markers (green=active, red=blocked, blue=pending) ### Detailed Analysis 1. **Top-Left Section**: - Contains a **yellow diamond** labeled "Destination" in the legend. 2. **Top-Middle Section**: - Contains a **green rectangle** labeled "Obstacle." 3. **Top-Right Section**: - Empty. 4. **Middle-Left Section**: - Contains a **pink rectangle** labeled "Start." 5. **Middle Section**: - Contains a **blue square** labeled "Agent." - Contains a **red horizontal line** labeled "Target." 6. **Middle-Right Section**: - Contains a **white vertical line** (unlabeled in legend). 7. **Bottom-Left Section**: - Contains a **pink rectangle** labeled "Start." 8. **Bottom-Middle Section**: - Contains a **green dot** (active status) and a **red dot** (blocked status). 9. **Bottom-Right Section**: - Contains a **blue dot** (pending status). 10. **Path Nodes**: - **Gray dots** form a zigzag path from the bottom-left to the top-right, passing through the middle section. ### Key Observations - The **Agent** (blue square) is positioned near the **Target** (red line) in the middle-right section. - The **Start** (pink rectangle) appears twice: once in the top-left and once in the bottom-left. - The **Obstacle** (green rectangle) blocks the top-middle section. - The **Path nodes** (gray dots) suggest a navigational route from the bottom-left to the top-right, avoiding the obstacle. - The **colored dots** (green, red, blue) are spatially isolated from the path nodes, possibly indicating status updates or secondary entities. ### Interpretation This diagram likely represents a simplified simulation of spatial navigation or resource allocation: - The **Agent** (blue square) may be tasked with moving from the **Start** (pink) to the **Destination** (yellow) while avoiding the **Obstacle** (green). - The **Path nodes** (gray dots) define a predefined route, with the **colored dots** (green/red/blue) possibly representing dynamic status changes (e.g., active/blocked/pending nodes). - The **Target** (red line) in the middle-right section could indicate a secondary objective or checkpoint. - The duplication of the **Start** (pink) suggests multiple entry points or a cyclical process. The layout emphasizes spatial relationships and decision-making constraints, typical of pathfinding algorithms or grid-based strategy games. </details> (a) Phase 1 <details> <summary>plots/maze_traj2.png Details</summary>  ### Visual Description ## Diagram: Grid Layout with Colored Markers and Pathways ### Overview The image depicts a grid-based layout resembling a maze or navigation map. It features interconnected rooms (white squares) separated by gray pathways. Key elements include colored markers (red, green, yellow), gray circular checkpoints, and a black circular start/end point. The layout suggests a structured path or progression system, possibly for a game or instructional diagram. ### Components/Axes - **Legend** (bottom-left corner): - **Red**: Person (circular with a black outline and red fill). - **Green**: Key (small green rectangle). - **Yellow**: Treasure (small yellow diamond). - **Gray**: Checkpoints (small gray circles). - **Black**: Start/End point (large black circle). - **Grid Structure**: - **Rooms**: 9 white squares arranged in a 3x3 grid. - **Pathways**: Gray lines connecting rooms, forming a labyrinthine structure. - **Checkpoints**: Gray circles clustered along pathways, forming a secondary path. ### Detailed Analysis 1. **Start/End Point**: - Located at the top-right corner (black circle). - Connected to a pathway leading to the central cluster of checkpoints. 2. **Checkpoints**: - Approximately 20 gray circles form a winding path from the start/end point to the green key (middle-left) and yellow treasure (bottom-left). - The path loops around the grid, suggesting a required sequence to reach the treasure. 3. **Key and Treasure**: - **Green Key**: Positioned in the middle-left room, adjacent to the checkpoint path. - **Yellow Treasure**: Located in the bottom-left room, isolated from direct pathways. 4. **Person**: - Red circular figure positioned near the top-right room, adjacent to the start/end point. - Appears to be moving toward the checkpoint path. ### Key Observations - The checkpoint path (gray circles) acts as a secondary route, bypassing some rooms. - The treasure (yellow diamond) is only accessible via the checkpoint path, not direct pathways. - The person’s proximity to the start/end point suggests they may be initiating a journey. ### Interpretation This diagram likely represents a simplified game map or instructional layout. The start/end point (black circle) and checkpoint path (gray circles) imply a structured progression, where the player must collect the green key (middle-left) before accessing the yellow treasure (bottom-left). The person’s position near the start/end point indicates they are at the beginning of this sequence. The absence of direct pathways to the treasure emphasizes the need to follow the checkpoint route, possibly simulating a puzzle or challenge. The grid’s symmetry and labeled elements suggest a focus on spatial reasoning or pathfinding mechanics. </details> (b) Phase 2 <details> <summary>plots/maze_traj3.png Details</summary>  ### Visual Description ## Diagram: Grid-Based Navigation Layout ### Overview The image depicts a grid-based layout resembling a simplified floor plan or navigation map. It features a 3x3 grid of white rectangular "rooms" separated by gray pathways. Key elements include colored markers (yellow diamond, red square, green rectangle, black circle) and gray circular nodes along pathways. A legend on the right side correlates colors to symbolic meanings. ### Components/Axes - **Grid Structure**: - 3 rows and 3 columns of white rectangular rooms. - Gray pathways form a cross-shaped network connecting rooms. - **Legend** (right-aligned): - **Yellow Diamond**: "Key" - **Red Square**: "Start" - **Green Rectangle**: "Exit" - **Black Circle**: "Agent" - **Gray Circles**: "Obstacles" - **Gray Rectangle**: "Pathway" - **Spatial Elements**: - **Key**: Located in the bottom-left room (coordinates: bottom-left quadrant). - **Start**: Positioned in the bottom-center room (coordinates: bottom-center quadrant). - **Exit**: Situated in the top-center room (coordinates: top-center quadrant). - **Agent**: Placed near the bottom-center room, adjacent to the Start marker. - **Obstacles**: Gray circles distributed along pathways, primarily in the central and right-center regions. - **Pathways**: Gray lines connecting rooms, with some nodes (gray circles) acting as waypoints or barriers. ### Detailed Analysis - **Key Placement**: The yellow diamond ("Key") is isolated in the bottom-left room, suggesting it must be collected before reaching the Exit. - **Agent Path**: The black circle ("Agent") starts near the red square ("Start") and faces a path toward the green rectangle ("Exit"), but must navigate around gray circular obstacles. - **Pathway Nodes**: Gray circles along pathways may represent decision points, barriers, or waypoints. Their density increases toward the right-center, potentially indicating a more complex navigation zone. - **Color Consistency**: All markers match their legend labels exactly (e.g., red square = Start, green rectangle = Exit). ### Key Observations 1. **Start-to-Exit Flow**: The Agent’s path from Start (red square) to Exit (green rectangle) requires traversing the central pathway while avoiding obstacles. 2. **Key Dependency**: The Key’s placement in the bottom-left room implies it may be a prerequisite for accessing the Exit or completing the task. 3. **Obstacle Distribution**: Gray circles are unevenly distributed, with higher concentration in the central and right-center pathways, creating potential bottlenecks. 4. **Agent Proximity**: The Agent is positioned close to the Start marker, suggesting an initial action or decision point. ### Interpretation This diagram likely represents a pathfinding or decision-making scenario, such as: - **Robotics Navigation**: The Agent (black circle) must navigate from Start to Exit while collecting the Key and avoiding obstacles. - **Game Level Design**: The layout resembles a puzzle where the player must collect the Key before reaching the Exit, with obstacles adding complexity. - **Logistical Planning**: The grid could model a warehouse or facility layout, with the Agent representing a worker or robot optimizing movement. The absence of numerical data or explicit labels beyond the legend suggests the diagram prioritizes symbolic representation over quantitative analysis. The spatial arrangement emphasizes connectivity, dependency (Key → Exit), and environmental constraints (obstacles). The Agent’s proximity to the Start marker implies an immediate action, while the Key’s isolation highlights a critical resource management element. </details> (c) Phase 3 <details> <summary>plots/maze_traj4.png Details</summary>  ### Visual Description ## Diagram: Grid Layout with Colored Markers and Pathway ### Overview The image depicts a grid-based layout with a gray border and internal divisions. Key elements include: - A green rectangle in the upper-middle section. - A yellow diamond in the bottom-left quadrant. - A black circle near the yellow diamond. - A series of gray dots forming a pathway from the bottom-left to the upper-middle. - A green line connecting the green rectangle to the pathway. ### Components/Axes - **Grid Structure**: - Divided into 9 rectangular regions (3x3 grid). - Gray lines separate the regions, with some regions containing internal subdivisions. - **Markers**: - **Yellow Diamond**: Located in the bottom-left quadrant, near the edge. - **Black Circle**: Positioned slightly above and to the right of the yellow diamond. - **Gray Dots**: Form a connected pathway starting from the bottom-left, moving upward and rightward, ending near the green rectangle. - **Green Rectangle**: Positioned in the upper-middle region, adjacent to the pathway. - **Green Line**: Connects the green rectangle to the pathway, suggesting a relationship or flow. ### Detailed Analysis - **Pathway**: - The gray dots create a non-linear path from the bottom-left to the upper-middle. - The path includes a "detour" around a central region, indicating a possible obstacle or alternative route. - **Markers**: - The yellow diamond and black circle are isolated from the pathway, suggesting they may represent starting points, targets, or obstacles. - The green rectangle and line imply a connection between the pathway and a specific region, possibly a goal or endpoint. ### Key Observations - **Spatial Relationships**: - The pathway avoids the central region, which contains no markers or dots. - The green line bridges the green rectangle and the pathway, emphasizing its significance. - **Color Coding**: - Green is used for both the rectangle and line, suggesting a thematic link (e.g., "safe zone" or "target"). - Yellow and black markers are distinct, possibly indicating different roles (e.g., start vs. end). ### Interpretation This diagram likely represents a maze or navigation system: - The **pathway** (gray dots) illustrates a route through the grid, avoiding obstacles. - The **yellow diamond** and **black circle** may denote the start and end points, respectively. - The **green rectangle** and **line** could represent a critical checkpoint or goal. - The absence of text or numerical data suggests this is a conceptual or schematic representation rather than a data-driven chart. ### Notes on Data Extraction - No textual labels, axis titles, or legends are present. - No numerical values or data tables are visible. - The diagram relies on spatial and color-based cues for interpretation. </details> (d) Phase 4 Figure A.2: An (optimal) trajectory of the maze in Figure A.1. Phase 1: the agent picks up the blue key; Phase 2: the agent opens the blue door to obtain the red key; Phase 3: the agent carries the red key to the red door; Phase 4: the agent opens the red door and reaches the objective. A.2.2 Dataset Details Our dataset consists of 100k training data points, 500 validation data points, and 300 data points for testing. For each data point, the structure of the prompt and response is as follows: - [Prompt]: maze_size: $M× M$ agent: $(x_{a0},y_{a0}),$ walls: $(x_{1},y_{1}),(x_{2},y_{2}),...$ objective: $(x_{o},y_{o}),$ keys: [red_key]: $(x_{rk},y_{rk}),...$ doors: [red_door]: $(x_{rd},y_{rd}),...$ - [Response]: create-node $(x_{a1},y_{a1},f_{a1},h_{a1})$ , create-node $(x_{a2},y_{a2},f_{a2},h_{a2})$ , … agent $(x_{a1},y_{a1}),(x_{a2},y_{a2}),...,(x_{aT},y_{aT}),$ Below, we show the prompt and response for an example training data pint. Prompt initial_state: maze_size: 19x19 wall: (0,0), (0,1), (0,2), (0,3), (0,4), (0,5), (0,6), (0,7), (0,8), (0,9), (0,10), (0,11), (0,12), (0,13), (0,14), (0,15), (0,16), (0,17), (0,18), (1,0), (1,6), (1,12), (1,18), (2,0), (2,6), (2,12), (2,18), (3,0), (3,6), (3,12), (3,18), (4,0), (4,6), (4,12), (4,18), (5,0), (5,6), (5,12), (5,18), (6,0), (6,1), (6,3), (6,4), (6,5), (6,6), (6,7), (6,8), (6,9), (6,10), (6,11), (6,12), (6,13), (6,14), (6,15), (6,16), (6,17), (6,18), (7,0), (7,12), (7,18), (8,0), (8,6), (8,12), (8,18), (9,0), (9,6), (9,12), (9,18), (10,0), (10,6), (10,12), (10,18), (11,0), (11,6), (11,12), (11,18), (12,0), (12,1), (12,2), (12,3), (12,4), (12,6), (12,8), (12,9), (12,10), (12,11), (12,12), (12,13), (12,14), (12,15), (12,16), (12,17), (12,18), (13,0), (13,12), (13,18), (14,0), (14,6), (14,12), (14,18), (15,0), (15,6), (15,12), (15,18), (16,0), (16,6), (16,12), (16,18), (17,0), (17,6), (17,12), (17,18), (18,0), (18,1), (18,2), (18,3), (18,4), (18,5), (18,6), (18,7), (18,8), (18,9), (18,10), (18,11), (18,12), (18,13), (18,14), (18,15), (18,16), (18,17), (18,18), agent: (11,5), objective: (1,1), [red_key]: (16,1), [green_key]: (16,9), [blue_key]: (7,2), [red_door]: (12,5), [green_door]: (6,2), [blue_door]: (12,7) Response (CoT and Answer) create-node: (11,5), heuristics: 14, cost: 0 create-node: (11,4), heuristics: 13, cost: 1 create-node: (11,3), heuristics: 12, cost: 2 create-node: (10,3), heuristics: 11, cost: 3 create-node: (9,3), heuristics: 10, cost: 4 create-node: (9,2), heuristics: 9, cost: 5 create-node: (8,2), heuristics: 8, cost: 6 create-node: (7,2), heuristics: 7, cost: 7 create-node: (7,2), heuristics: 7, cost: 8 create-node: (7,3), heuristics: 8, cost: 9 create-node: (7,4), heuristics: 9, cost: 10 create-node: (7,5), heuristics: 10, cost: 11 create-node: (7,6), heuristics: 11, cost: 12 create-node: (7,7), heuristics: 12, cost: 13 create-node: (8,7), heuristics: 13, cost: 14 create-node: (9,7), heuristics: 14, cost: 15 create-node: (10,7), heuristics: 15, cost: 16 create-node: (11,7), heuristics: 16, cost: 17 create-node: (11,7), heuristics: 16, cost: 18 create-node: (12,7), heuristics: 17, cost: 19 create-node: (13,7), heuristics: 18, cost: 20 create-node: (14,7), heuristics: 19, cost: 21 create-node: (14,8), heuristics: 20, cost: 22 create-node: (15,8), heuristics: 21, cost: 23 create-node: (15,9), heuristics: 22, cost: 24 create-node: (16,9), heuristics: 23, cost: 25 create-node: (16,9), heuristics: 23, cost: 26 create-node: (16,8), heuristics: 22, cost: 27 create-node: (16,7), heuristics: 21, cost: 28 create-node: (15,7), heuristics: 20, cost: 29 create-node: (14,7), heuristics: 19, cost: 30 create-node: (13,7), heuristics: 18, cost: 31 create-node: (12,7), heuristics: 17, cost: 32 create-node: (11,7), heuristics: 16, cost: 33 create-node: (10,7), heuristics: 15, cost: 34 create-node: (9,7), heuristics: 14, cost: 35 create-node: (8,7), heuristics: 13, cost: 36 create-node: (7,7), heuristics: 12, cost: 37 create-node: (7,6), heuristics: 11, cost: 38 create-node: (7,5), heuristics: 10, cost: 39 create-node: (7,4), heuristics: 9, cost: 40 create-node: (7,3), heuristics: 8, cost: 41 create-node: (7,2), heuristics: 7, cost: 42 create-node: (7,2), heuristics: 7, cost: 43 create-node: (6,2), heuristics: 6, cost: 44 create-node: (5,2), heuristics: 5, cost: 45 create-node: (5,1), heuristics: 4, cost: 46 create-node: (4,1), heuristics: 3, cost: 47 create-node: (3,1), heuristics: 2, cost: 48 create-node: (2,1), heuristics: 1, cost: 49 create-node: (1,1), heuristics: 0, cost: 50 agent: (11,5), current_key: none, remaining_key: [red_key]: [green_key]: [blue_key]: remaining_door: [red_door]: [green_door]: [blue_door]: agent: (11,4), current_key: none, remaining_key: [red_key]: [green_key]: [blue_key]: remaining_door: [red_door]: [green_door]: [blue_door]: agent: (11,3), current_key: none, remaining_key: [red_key]: [green_key]: [blue_key]: remaining_door: [red_door]: [green_door]: [blue_door]: agent: (10,3), current_key: none, remaining_key: [red_key]: [green_key]: [blue_key]: remaining_door: [red_door]: [green_door]: [blue_door]: agent: (9,3), current_key: none, remaining_key: [red_key]: [green_key]: [blue_key]: remaining_door: [red_door]: [green_door]: [blue_door]: agent: (9,2), current_key: none, remaining_key: [red_key]: [green_key]: [blue_key]: remaining_door: [red_door]: [green_door]: [blue_door]: agent: (8,2), current_key: none, remaining_key: [red_key]: [green_key]: [blue_key]: remaining_door: [red_door]: [green_door]: [blue_door]: agent: (7,2), current_key: none, remaining_key: [red_key]: [green_key]: [blue_key]: remaining_door: [red_door]: [green_door]: [blue_door]: agent: (7,2), current_key: [blue_key], remaining_key: [red_key]: [green_key]: remaining_door: [red_door]: [green_door]: [blue_door]: agent: (7,3), current_key: [blue_key], remaining_key: [red_key]: [green_key]: remaining_door: [red_door]: [green_door]: [blue_door]: agent: (7,4), current_key: [blue_key], remaining_key: [red_key]: [green_key]: remaining_door: [red_door]: [green_door]: [blue_door]: agent: (7,5), current_key: [blue_key], remaining_key: [red_key]: [green_key]: remaining_door: [red_door]: [green_door]: [blue_door]: agent: (7,6), current_key: [blue_key], remaining_key: [red_key]: [green_key]: remaining_door: [red_door]: [green_door]: [blue_door]: agent: (7,7), current_key: [blue_key], remaining_key: [red_key]: [green_key]: remaining_door: [red_door]: [green_door]: [blue_door]: agent: (8,7), current_key: [blue_key], remaining_key: [red_key]: [green_key]: remaining_door: [red_door]: [green_door]: [blue_door]: agent: (9,7), current_key: [blue_key], remaining_key: [red_key]: [green_key]: remaining_door: [red_door]: [green_door]: [blue_door]: agent: (10,7), current_key: [blue_key], remaining_key: [red_key]: [green_key]: remaining_door: [red_door]: [green_door]: [blue_door]: agent: (11,7), current_key: [blue_key], remaining_key: [red_key]: [green_key]: remaining_door: [red_door]: [green_door]: [blue_door]: agent: (11,7), current_key: none, remaining_key: [red_key]: [green_key]: remaining_door: [red_door]: [green_door]: agent: (12,7), current_key: none, remaining_key: [red_key]: [green_key]: remaining_door: [red_door]: [green_door]: agent: (13,7), current_key: none, remaining_key: [red_key]: [green_key]: remaining_door: [red_door]: [green_door]: agent: (14,7), current_key: none, remaining_key: [red_key]: [green_key]: remaining_door: [red_door]: [green_door]: agent: (14,8), current_key: none, remaining_key: [red_key]: [green_key]: remaining_door: [red_door]: [green_door]: agent: (15,8), current_key: none, remaining_key: [red_key]: [green_key]: remaining_door: [red_door]: [green_door]: agent: (15,9), current_key: none, remaining_key: [red_key]: [green_key]: remaining_door: [red_door]: [green_door]: agent: (16,9), current_key: none, remaining_key: [red_key]: [green_key]: remaining_door: [red_door]: [green_door]: agent: (16,9), current_key: [green_key], remaining_key: [red_key]: remaining_door: [red_door]: [green_door]: agent: (16,8), current_key: [green_key], remaining_key: [red_key]: remaining_door: [red_door]: [green_door]: agent: (16,7), current_key: [green_key], remaining_key: [red_key]: remaining_door: [red_door]: [green_door]: agent: (15,7), current_key: [green_key], remaining_key: [red_key]: remaining_door: [red_door]: [green_door]: agent: (14,7), current_key: [green_key], remaining_key: [red_key]: remaining_door: [red_door]: [green_door]: agent: (13,7), current_key: [green_key], remaining_key: [red_key]: remaining_door: [red_door]: [green_door]: agent: (12,7), current_key: [green_key], remaining_key: [red_key]: remaining_door: [red_door]: [green_door]: agent: (11,7), current_key: [green_key], remaining_key: [red_key]: remaining_door: [red_door]: [green_door]: agent: (10,7), current_key: [green_key], remaining_key: [red_key]: remaining_door: [red_door]: [green_door]: agent: (9,7), current_key: [green_key], remaining_key: [red_key]: remaining_door: [red_door]: [green_door]: agent: (8,7), current_key: [green_key], remaining_key: [red_key]: remaining_door: [red_door]: [green_door]: agent: (7,7), current_key: [green_key], remaining_key: [red_key]: remaining_door: [red_door]: [green_door]: agent: (7,6), current_key: [green_key], remaining_key: [red_key]: remaining_door: [red_door]: [green_door]: agent: (7,5), current_key: [green_key], remaining_key: [red_key]: remaining_door: [red_door]: [green_door]: agent: (7,4), current_key: [green_key], remaining_key: [red_key]: remaining_door: [red_door]: [green_door]: agent: (7,3), current_key: [green_key], remaining_key: [red_key]: remaining_door: [red_door]: [green_door]: agent: (7,2), current_key: [green_key], remaining_key: [red_key]: remaining_door: [red_door]: [green_door]: agent: (7,2), current_key: none, remaining_key: [red_key]: remaining_door: [red_door]: agent: (6,2), current_key: none, remaining_key: [red_key]: remaining_door: [red_door]: agent: (5,2), current_key: none, remaining_key: [red_key]: remaining_door: [red_door]: agent: (5,1), current_key: none, remaining_key: [red_key]: remaining_door: [red_door]: agent: (4,1), current_key: none, remaining_key: [red_key]: remaining_door: [red_door]: agent: (3,1), current_key: none, remaining_key: [red_key]: remaining_door: [red_door]: agent: (2,1), current_key: none, remaining_key: [red_key]: remaining_door: [red_door]: agent: (1,1), current_key: none, remaining_key: [red_key]: remaining_door: [red_door]: The prompt describes the maze in a structured language. The maze size $M=m(n+1)+1$ (e.g., in Figure A.1, the maze size $M=19$ ). The positions of walls are $(x_{1},y_{1}),(x_{2},y_{2}),...$ , and so on. The position of the agent in time step $t$ is $(x_{at},y_{at})$ , where $t=0$ corresponds to the initial position The position of the objective is $(x_{o},y_{o})$ , and the position of keys and doors in color $c$ (where $c$ = $r$ , $g$ , $b$ ) are $(x_{ck},y_{ck})$ and $(x_{cd},y_{cd})$ , respectively. The response describes an optimal path (i.e., with minimal total times steps $T$ ) for the agent to reach the objective. A.2.3 Model Details Following Su et al. (2024); Lehnert et al. (2024), we employ a similar encode-decoder transformer architecture with rotary embeddings and no drop-out. Our model consisted of 6 layers with 3 attention heads, and the embedding size is 64. A.3 ProntoQA and ProsQA We used the pretrained GPT-2 model which has the following parameters: | Number of Layers (Transformer Blocks) | 12 | | --- | --- | | Hidden Size (Embedding Size) | 768 | | Number of Attention Heads | 12 | | Vocabulary Size | 50,257 | | Total Number of Parameters | 117 million | Table A.1: Hyperparameters of the pretrained GPT-2 model used for ProntoQA and ProsQA. A.4 LLM experiments We use the Llama Cookbook https://github.com/meta-llama/llama-cookbook codebase to fine-tune the Llama models. As described in Section 4.2, we use a batch size of 32 with a sequence packing of 4096. We experiment with different learning rates $10^{-5},2.5× 10^{-5},5× 10^{-5},10^{-4}$ and select the one with the lowest validation error. The final choices are $10^{-5}$ for Llama-3.2-8B and $2.5× 10^{-5}$ for Llama-3.2-1B and Llama-3.2-3B. Appendix B Notations Table B.1 summarizes the notations we used throughout the paper. | $X=P\oplus C\oplus S$ $P$ $p_{i}$ | input text sample where $\oplus$ means concatenation prompt of length $t_{p}$ the $i$ -th token of prompt (in text) | | --- | --- | | $C$ | reasoning trace of length $t_{c}$ | | $c_{i}$ | the $i$ -th token of trace (in text) | | $S$ | solution of length $t_{s}$ | | $s_{i}$ | the $i$ -th token of solution (in text) | | $Z$ | the complete latent reasoning traces of length $t_{z}$ | | $z_{i}$ | the $i$ -th token of latent trace | | $r=t_{c}/t_{z}$ | compression rate | | $m$ | number of trace tokens to be replaced by latent tokens during training | | ${\widetilde{X}}$ | modified input with mixed text and latent tokens | | $\mathcal{E}$ | codebook of VQ-VAE | | $e_{i}$ | the $i$ -th vector in the codebook, which corresponds to the $i$ -th latent token | | $d$ | dimension of $e_{i}$ s | | ${\mathcal{V}}$ | vocabulary of text tokens | | $L$ | chunk size | | ${f_{\text{enc}}}(·)$ | encodes a chunk of $L$ text tokens to $\frac{L}{r}$ embedding vectors | | $\bar{X}=\bar{x}_{1},...,\bar{x}_{\frac{L}{r}}$ | embedding vectors of $X$ outputted by ${f_{\text{enc}}}(·)$ | | $q(·)$ | quantization operator that replaces, e.g., $\bar{x}_{1}$ by its nearest neighbor in ${\mathcal{E}}$ : | | $g(\bar{x}_{1})=\operatorname*{argmin}_{e_{i}∈{\mathcal{E}}}\left\|e_{i}-\bar{x}_{1}\right\|^{2}$ | | | $g(·)$ | maps prompt to a $d$ -dimensional embedding vector | | ${f_{\text{dec}}}(·,·)$ | decodes $L/r$ quantized embedding vectors in ${\mathcal{E}}$ back to text tokens, | | conditioning on prompt embedding generated by $g(·)$ | | Table B.1: Mathematical notations used throughout the paper. Appendix C Details of Attention Weights Analysis C.1 Generated Responses Question 1 Prompt: What is the positive difference between $120%$ of 30 and $130%$ of 20? Response of Our Latent Approach: <boLatent> <Latent-code-278> <Latent-code-416> <eoLatent> % of 30 and $130%$ of 20. First, let’s calculate $120%$ of 30: $120%$ of 30 = 1.2 x 30 = 36. Next, let’s calculate $130%$ of 20: $130%$ of 20 = 1.3 x 20 = 26. Now, let’s find the positive difference between 36 and 26: 36 - 26 = 10 The answer is 10. Response of Baseline (CoT): To find the positive difference between $120\%$ of 30 and $130\%$ of 20, we need to calculate each value first. $120\%$ of 30 is calculated as 1.2 * 30 = 36. $130\%$ of 20 is calculated as 1.3 * 20 = 26. Now, we need to find the positive difference between these two values. To do this, we subtract the smaller value from the larger value: 36 - 26 = 10. The final answer is 10. Question 2 Prompt: Mark has $50 in his bank account. He earns $10 per day at his work. If he wants to buy a bike that costs $300, how many days does Mark have to save his money? Response of Our Latent Approach: <boLatent> <Latent-code-123> <eoLatent> need to calculate how much more money he needs to buy the bike and then divide that amount by his daily earnings. The bike costs $300, and Mark already has $50. So, he needs: $300 (bike cost) - $50 (current savings) = $250 (more money needed) Mark earns $10 per day. To find out how many days he needs to save, we divide the amount he needs by his daily earnings: $250 (more money needed) / $10 (daily earnings) = 25 days. The answer is 25. Response of Baseline (CoT): To find out how many days Mark needs to save his money, we need to subtract the cost of the bike from the amount of money he has, then divide the result by the amount he earns per day. Mark has $50 and wants to buy a bike that costs $300. First, subtract the cost of the bike from the amount of money Mark has: $50 - $300 = -$250 Since Mark can’t have a negative amount of money, we need to convert the negative amount to a positive one by adding the cost of the bike to the amount of money Mark has: $50 + $300 = $350 Now, divide the result by the amount Mark earns per day: $350 / $10 = 35 The final answer is 35. Appendix D Other Text Generation Examples Example Generation of Our Latent Approach Prompt: Evaluate the expression $a^{3}· a^{2}$ if $a=5$ Generated Text: <boLatent> <Latent-code-130> <eoLatent> We can use the rule of exponents that states $a^{m}· a^{n}=a^{m+n}$ . So, $a^{3}· a^{2}=a^{3+2}=a^{5}$ . Now, we substitute $a=5$ into the expression: $5^{5}$ . $5^{5}=5· 5· 5· 5· 5=3125$ . The answer is 3125. Example Generation of Our Latent Approach Prompt: Compute $\dbinom{15}{2}$ . Generated Text: <boLatent> <Latent-code-888> <Latent-code-744> <Latent-code-196> <Latent-code-92> <Latent-code-557> <eoLatent> $\frac{15!}{2!(15-2)!}=\frac{15!}{2!13!}$ To simplify this expression, we can cancel out the common factors in the numerator and denominator: $\dbinom{15}{2}=\frac{15× 14}{2}=105$ . The answer is 105. Example Generation of Our Latent Approach Question: How many vertical asymptotes does the graph of $y=\frac{2}{x^{2}+x-6}$ have? Answer: <boLatent> <Latent-code-773> <Latent-code-514> <eoLatent> the denominator, which is $x^{2}+x-6$ . We can factor this quadratic expression as $(x+3)(x-2)$ . Now, we can see that the denominator will be equal to zero when either $(x+3)=0$ or $(x-2)=0$ . Solving for these values, we get $x=-3$ or $x=2$ . Therefore, the vertical asymptotes of the graph of $y=\\ frac{2}{x^{2}+x-6}$ are $x=-3$ and $x=2$ . Since there are two distinct values of $x$ that make the denominator equal to zero, the graph has 2 vertical asymptotes. The answer is 2. Example Generation of Our Latent Approach Question: If there exists an ordered numbering of the nodes such that for each node there are no links going to a lower-numbered node, then there are no directed cycles in a directed graph. True or false? Answer: <boLatent> <Latent-code-526> <eoLatent> a directed graph. A directed cycle is a sequence of nodes in a graph, where each node is connected to the next node by a directed edge, and the last node is connected back to the first node by a directed edge. Now, let’s consider the given condition: ”If there exists an ordered numbering of the nodes such that for each node there are no links going to a lower-numbered node, then there are no directed cycles in a directed graph.” This condition essentially means that if we number the nodes in a specific way, and there are no links going to a lower-numbered node, then there are no directed cycles in the graph. To see why this is true, let’s consider what would happen if there were a directed cycle in the graph. If there were a directed cycle, then there would be a sequence of nodes where each node is connected to the next node by a directed edge, and the last node is connected back to the first node by a directed edge. However, if we number the nodes in the way described, and there are no links going to a lower-numbered node, then this directed cycle would be impossible. This is because the last node in the cycle would have to be connected to a lower-numbered node, which would violate the condition. Therefore, if we number the nodes in this way, then there can be no directed cycles in the graph. The answer is True. Appendix E Interpreability Examples To examine the interpretability of the latent-codes, we use the decoder from the trained VQ-VAE to convert the latent-code back to the text space. In general, it is interpretable. We bolded the decoded part of the text from the latent code, which is delimited by the <boLatent> … <eoLatent> tags. In general, we find the latent tokens capture long/verbose sentences which may trigger useful follow-up thinking pattern. Example Generation of Our Latent Approach Prompt: In a 8 fluid ounce bottle of Vitamin Water, there are 125 calories. How many calories would be contained in a 12 fluid ounce bottle? Express your answer in decimal form. Generated Text: <boLatent> To convert the number from in in a 8 ounces <eoLatent> we need to find the ratio of calories to fluid ounces in the 8 fluid ounce bottle and then multiply it by 12. First, let’s find the ratio of calories to fluid ounces in the 8 fluid ounce bottle: Calories per fluid ounce = Total calories / Total fluid ounces = 125 calories / 8 fluid ounces = 15.625 calories per fluid ounce Now, let’s multiply this ratio by 12 fluid ounces to find the total calories in the 12 fluid ounce bottle: Total calories in 12 fluid ounces = Calories per fluid ounce * Total fluid ounces = 15.625 calories per fluid ounce * 12 fluid ounces = 187.5 calories The answer is 187.5. Here, in the example above, the decoded latent token ”To convert the number from in in a 8 ounces” captures the core concept of proportional conversion from the 8-ounce bottle to the 12-ounce bottle. Despite its grammar mistake, it still precisely states the mathematical operation needed to solve the problem. Example Generation of Our Latent Approach Prompt: What is the greatest common divisor of 315 and 108? Generated Text: <boLatent> We find the greatest of CD of::63 common find <eoLatent> we can use the Euclidean algorithm. Here’s how to do it: 1. Divide 315 by 108: 315 = 108 × 2 + 99 2. Divide 108 by 99: 108 = 99 × 1 + 9 3. Divide 99 by 9: 99 = 9 × 11 + 0 Since the remainder is 0, the GCD is the divisor in the last step, which is 9. The answer is 9. Here, in the example above, the decoded latent token ”We find the greatest of CD of::63 common find” abstracts the initiation of the Euclidean algorithm, leading directly into the process that determines the GCD. Appendix F Additional Experiments We present results of different approaches for fine-tuning a Llama-3.1-8B model on the DART-MATH (Tong et al., 2024) dataset. The observations are similar to those we presented in Section 4.2. Model (Dart-Math) In-Domain Out-of-Domain Average math GSM8K Fresh-Gaokao-Math-2023 DeepMind-Mathematics College-Math Olympia-Math TheoremQA All Datasets Llama-3.1-8B Sol-only 13.3 16.4 0.0 18.2 15.9 4.7 16.9 12.2 CoT 43.1 84.5 30.7 47.8 45.7 10.1 21.2 40.4 iCoT 35.2 61.8 30.0 30.6 37.6 8.3 19.5 31.8 Latent (Ours) 43.2 ( $\uparrow$ +0.1) 83.9 33.3 ( $\uparrow$ +2.6) 44.7 47.1 ( $\uparrow$ +1.4) 13.3 ( $\uparrow$ +3.2) 20.3 40.8 ( $\uparrow$ +0.4) Table F.1: Our approach surpasses the iCoT and Sol-Only baseline when trained on the DART-MATH dataset (Tong et al., 2024), while marginally outperforming the CoT baseline. Model (Dart-Math) In-Domain (# of tokens) Out-of-Domain (# of tokens) Average (# of tokens) math GSM8K Fresh-Gaokao-Math-2023 DeepMind-Mathematics College-Math Olympia-Math TheoremQA All Datasets Llama-3.1-8B Sol-only 10.9 8.1 10.2 8.4 11.2 16.1 16.13 11.6 CoT 522.7 181.0 628.8 343.2 486.3 893.7 648.3 529.1 iCoT 397.1 118.6 440.8 227.9 321.9 614.4 485.7 372.3 Latent (Ours) 489.1 ( $\downarrow$ -6.4%) 163.5 ( $\downarrow$ -9.7%) 462.1 ( $\downarrow$ -26.5%) 265.6 ( $\downarrow$ -22.6%) 396.3 ( $\downarrow$ -18.5%) 801.3 ( $\downarrow$ -10.3%) 591.3 452.7 ( $\downarrow$ -16%) Table F.2: The average number of tokens in the generated responses. Our approach generates shorter reasoning traces then the CoT baseline. $\downarrow$ -: Trace length reduction rate compared with CoT.