# LLMs struggle with planning problems as symbolic heuristics outperform learned models

Digest AI · Research · published 2026-10-21T04:00:00Z

Canonical: https://digestai.news/story/llms-struggle-with-planning-problems-as-symbolic-heuristics-outperform

## Summary

A new arXiv study shows that large language models (LLMs) have difficulty solving classical planning tasks, and that traditional symbolic heuristics can predict problem difficulty better than learned approaches. The zero‑training delete‑relaxation heuristic \(h_{add}\) reaches a mean Spearman correlation of 0.931 on held‑out STRIPS domains, beating ridge regression (0.707), simple object counting (0.657) and the episodic retrieval architecture ARC (0.400). ARC also fails to surpass a linear model that uses the same features.

To isolate the effect of semantic understanding, the researchers progressively renamed predicates while keeping states, transitions and optimal plans unchanged. LLM planning success falls from 67.5% to 40.0% as renaming moves from 0% to 100%, with refusal rates staying at 0%. Supplying an LLM with ARC’s conditioning information improves difficulty prediction (mean \(|\rho|=0.826\)) compared with a trained ARC model. The paper concludes that improving hybrid LLM‑symbolic planners requires better modeling of LLM behavior sensitive to predicate semantics rather than creating new structural difficulty predictors.

## Key points

- Zero‑training delete‑relaxation heuristic h_add achieves 0.931 mean Spearman correlation, outpacing ridge regression (0.707), object count (0.657) and ARC (0.400).
- Renaming all predicates drops LLM planning success from 67.5% to 40.0% while refusal rate remains at 0%.
- Hybrid LLM‑symbolic planning benefits more from calibrating to LLM outcomes than from choosing a structural difficulty predictor.

## Why it matters

The findings reveal that LLMs are more limited by semantic interpretation than structural complexity, guiding future hybrid planning systems and research on integrating symbolic heuristics with language models.

## Sources

1. [When Planning Problems Become Hard for LLMs: Separating Structural Complexity from Semantic Interpretability](https://arxiv.org/abs/2610.26964) (arXiv cs.AI, 2026-10-21, primary source)

## Cite

Digest AI, "LLMs struggle with planning problems as symbolic heuristics outperform learned models", 21 October 2026, https://digestai.news/story/llms-struggle-with-planning-problems-as-symbolic-heuristics-outperform

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