Theoretical Barriers of Modern Tokenizer on Symbolic and Arithmetic Computation in Language Models

ACL ARR 2025 May Submission4824 Authors

Abstract: Tokenization is the first—and often underappreciated—layer of computation in language models. While Chain-of-Thought (CoT) prompting enables transformer models to approximate recurrent computation by externalizing intermediate steps, we show that the success of such reasoning is fundamentally bounded by the structure of tokenized inputs. This work presents a theoretical and empirical investigation into how tokenization schemes, particularly subword-based methods like byte-pair encoding (BPE), impede symbolic computation by merging or obscuring atomic reasoning units. We introduce the notion of *Token Awareness* to formalize how poor token granularity disrupts logical alignment and prevents models from generalizing symbolic procedures. Through systematic evaluation on arithmetic and symbolic tasks, we demonstrate that token structure dramatically affect reasoning performance, causing failure even with CoT, while atomically-aligned formats unlock strong generalization, allowing small models (e.g., GPT-4o-mini) to outperform larger systems (e.g., o1) in structured reasoning. Our findings reveal that symbolic reasoning ability in LLMs is not purely architectural, but deeply conditioned on token-level representations.

Paper Type: Long

Research Area: Interpretability and Analysis of Models for NLP

Research Area Keywords: Reasoning

Contribution Types: Model analysis & interpretability, Theory

Languages Studied: English

Submission Number: 4824