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[GH-ISSUE #3957] Why does AgentMiddleware allow untracked side-effect mutations of state in hooks instead of enforcing reducer-only updates? #2771

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opened 2026-06-05 17:26:42 -04:00 by yindo · 0 comments
yindo commented 2026-06-05 17:26:42 -04:00
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Originally created by @goodpostidea-tech on GitHub (May 12, 2026).
Original GitHub issue: https://github.com/langchain-ai/docs/issues/3957

Type of issue

question

Language

Python

Description

I noticed that before_model can mutate state directly (e.g., modifying HumanMessage.content via reference) and these mutations persist even when None is returned, bypassing the reducer mechanism entirely. This creates a 'shadow update' path that isn't tracked by the framework. What was the rationale behind not deep-copying state before passing it to hooks, or using frozen/immutable structures? Was there a performance concern, or a deliberate choice to trust developers with mutable access?

Originally created by @goodpostidea-tech on GitHub (May 12, 2026). Original GitHub issue: https://github.com/langchain-ai/docs/issues/3957 ### Type of issue question ### Language Python ### Description I noticed that before_model can mutate state directly (e.g., modifying HumanMessage.content via reference) and these mutations persist even when None is returned, bypassing the reducer mechanism entirely. This creates a 'shadow update' path that isn't tracked by the framework. What was the rationale behind not deep-copying state before passing it to hooks, or using frozen/immutable structures? Was there a performance concern, or a deliberate choice to trust developers with mutable access?
yindo added the langchainexternal labels 2026-06-05 17:26:42 -04:00
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Reference: langchain-ai/docs#2771
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