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This paper introduces a novel nine-dimensional framework to diagnose algebraic reasoning failures in LLMs, varying factors like expression nesting depth, operator hardness, and dependency chain length independently. They automatically generate and verify algebraic problems of increasing complexity to track model performance across these dimensions. Evaluating seven instruction-tuned models (8B-235B), the study reveals working memory limitations as a dominant, scale-invariant bottleneck, with all models collapsing between 20-30 parallel branches.
LLMs hit a hard wall in algebraic reasoning, choking on problems with just 20-30 parallel branches regardless of model size, suggesting an architectural bottleneck, not just a capacity issue.
Algebraic reasoning remains one of the most informative stress tests for large language models, yet current benchmarks provide no mechanism for attributing failure to a specific cause. When a model fails an algebraic problem, a single accuracy score cannot reveal whether the expression was too deeply nested, the operator too uncommon, the intermediate state count too high, or the dependency chain too long. Prior work has studied individual failure modes in isolation, but no framework has varied each complexity factor independently under strict experimental control. No prior system has offered automatic generation and verification of problems of increasing complexity to track model progress over time. We introduce a nine-dimension algebraic complexity framework in which each factor is varied independently while all others are held fixed, with problem generation and verification handled by a parametric pipeline requiring no human annotation. Each dimension is grounded in a documented LLM failure mode and captures a structurally distinct aspect of algebraic difficulty, including expression nesting depth, simultaneous intermediate result count, sub-expression complexity, operator hardness, and dependent reasoning chain length. We evaluated seven instruction-tuned models spanning 8B to 235B parameters across all nine dimensions and find that working memory is the dominant scale-invariant bottleneck. Every model collapses between 20 and 30 parallel branches regardless of parameter count, pointing to a hard architectural constraint rather than a solvable capacity limitation. Our analysis further identifies a minimal yet diagnostically sufficient subset of five dimensions that together span the full space of documented algebraic failure modes, providing a complete complexity profile of a model's algebraic reasoning capacity.
