Google Open-Sources Long Horizon: 5 Design Patterns for Stable Long-Running AI Agents

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Google has open-sourced Long Horizon, an agent harness built on its Agent Development Kit (ADK) under the Apache 2.0 license, designed to keep AI agents running reliably over days or weeks. The project was released after Google's own team ran the system internally for several weeks, uncovering recurring silent failures where agents continued operating without throwing errors despite underlying problems. Key issues discovered included broken prefix caching due to volatile prompt headers, unnecessary memory retrieval slowing every turn, and sub-agents timing out while parent agents falsely reported task completion. To address these, the team documented five design patterns covering prompt structure optimization, write-behind memory caching, robust sub-agent error handling, stateful session recovery, and tightened security guardrails. Google describes Long Horizon as a reference implementation meant to be read and adapted rather than deployed out of the box.

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