Flue 2 introduces React-style hooks for building dynamic agent harnesses

Flue 2, an open-source agent framework from Fred Schott, the creator of Astro, introduces React-style “Agent Hooks” as its foundation. In Flue, an agent is represented as a JavaScript function that “re-renders on every turn” before each model call, enabling dynamic behavior during conversations or workflows.

Schott, whose company Astro was acquired by Cloudflare in January, said the hooks were inspired by React’s composability. He initially framed Flue as “the Astro for agents” or “the Next.js for agents,” but later concluded the better analogy was “the React for agents.” Hooks are authored in TypeScript and let developers build agents that manage their own state, listen to lifecycle events, and attach resources or capabilities at runtime.

Flue 2 includes 16 built-in hooks such as useSkill(), useTool(), and useSubagent(), with support for custom hooks. This design allows an agent’s configuration to change as the conversation or workflow progresses, which Schott argues is necessary for building “real support bots” and “real triage bots” that cannot be fully configured in advance.

A central concept in Flue is the harness: an environment that gives an agent access to context and capabilities so it can drive itself. Flue is built on Pi, an open-source minimal harness, and Schott describes Pi’s role as analogous to Vite’s under Astro—providing the right abstraction without doing too much. Flue adds opinionated features on top of Pi, including hosted agents built with Vite.

Schott emphasized that Flue’s early bet was that the harness is not just a feature but fundamental to what an agent is. “There is no agent without a harness,” he said. The project began within the Astro repository as an issue-triage system and evolved into a headless, cloud-hostable agent framework inspired by Claude Code.

Flue positions itself among early agent frameworks that treat the harness as foundational, such as Vercel’s eve. Schott called eve the “most directly competitive” and noted that while older frameworks like Vercel’s AI SDK, Cloudflare’s Agents SDK, and Mastra are now adding harnesses, Flue and eve embed them from the start. He also distinguished Flue from emerging “meta-harnesses,” arguing that Flue’s framework and harness are intertwined and that a single API across harnesses would complicate the developer story.

Schott said Flue targets AI coding agents and that the onboarding flow and documentation are designed to be guided by an agent. He also stressed that Flue remains an open-source framework intended to work across hosts, leveraging Cloudflare’s tooling and infrastructure while staying independent.