Prism: Persistent Faceted File Memory for Cost-Efficient Coding Agents

Ziyun Zhang, Yihao Liu, Yuzhe Zhang, Huaqian Cai


Abstract
Coding agents working on sequential maintenance tasks re-read the same files across related tasks, wasting more than a third of their input tokens on redundant context. The natural fix, caching file summaries, fails to reliably reduce cost: tool-use traces are consistent with agents over-relying on generic descriptions, skipping re-reads, and offsetting token savings with less effective downstream decisions, leaving net cost roughly unchanged. We introduce Prism, a content-hash-keyed file memory that augments summaries with bounded facets: identifier-grounded facts distilled from the agent’s own prior reasoning. On 12 SWE-Chain dependency-upgrade chains, Prism reduces cost by 24.9% on GPT-5 and 9.9% on GPT-5.4-mini, while maintaining comparable aggregate Build+Fix F1. Against no memory, a summary-only ablation yields only a nominal 2.4% cost decrease that falls within run-to-run variability, while a static Aider-style repo-map baseline costs substantially more at 13% over no memory. Two controlled ablations isolate the causal contributions: wiping the persistent store before every task raises cost by 37% over no memory, and replacing facets with a 3× larger unstructured summary raises it by 41%. Code: https://github.com/WhiteFlamesCN/Prism-Opencode.
Anthology ID:
2026.findings-emnlp.1301
Volume:
Findings of the Association for Computational Linguistics: EMNLP 2026
Month:
October
Year:
2026
Address:
Budapest, Hungary
Editors:
Hung-Yi Lee, Sunipa Dev, Jessy Li, Desmond Elliott
Venue:
Findings
SIG:
SIGDAT
Publisher:
Association for Computational Linguistics
Note:
Pages:
27518–27531
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URL:
https://aclanthology.org/2026.findings-emnlp.1301/
DOI:
Bibkey:
Cite (ACL):
Ziyun Zhang, Yihao Liu, Yuzhe Zhang, and Huaqian Cai. 2026. Prism: Persistent Faceted File Memory for Cost-Efficient Coding Agents. In Findings of the Association for Computational Linguistics: EMNLP 2026, pages 27518–27531, Budapest, Hungary. Association for Computational Linguistics.
Cite (Informal):
Prism: Persistent Faceted File Memory for Cost-Efficient Coding Agents (Zhang et al., Findings 2026)
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PDF:
https://aclanthology.org/2026.findings-emnlp.1301.pdf