Flue is an open-source agent harness framework from the Astro team that lets developers build autonomous agents as programmable TypeScript functions, complete with models, skills, tools, and sandboxes. It has collected more than 8,500 stars on GitHub since launch and is under active development, with updates in October 2026. Its tagline says it plainly: not another SDK, but a harness for real agent work.
Flue starts from the observation that the first generation of agents was built with raw LLM API calls, which works until agents need context, tools, verification, and a safe place to execute. In Flue, the agent is a function: you declare which model it uses, which skills (instruction files) it carries, which tools it may call, and which sandbox it runs in: local, virtual, or a remote container. A triage agent, for example, can be composed to reproduce a bug, diagnose the root cause, check whether behavior is intentional, and attempt a fix, all within that declared harness. Being from the creators of the Astro web framework, Flue emphasizes a developer experience that feels like writing ordinary TypeScript rather than wiring a graph of nodes.
Flue is free and open source under the Apache-2.0 license. There is no paid tier: you install the framework, connect your own model provider accounts, and pay only those providers’ API rates plus your own infrastructure costs for sandboxes.
Flue is for TypeScript developers building autonomous agents that must take real actions safely: automated triage and maintenance bots, internal workflow agents, and product features where an agent needs tools and a sandbox rather than a chat box. Teams already in the Astro and TypeScript ecosystem will feel at home.