Building Effective AI Agents

This article remains one of the better pieces on this topic, especially since it clearly defines which definition of "AI agents" they are using at the start! They use: "systems where LLMs dynamically direct their own processes and tool usage, maintaining control over how they accomplish tasks".

I also like the way they distinguish between "agents" and "workflows", and describe a bunch of useful workflow patterns.

I published some notes on that article when it first came out: https://simonwillison.net/2024/Dec/20/building-effective-age...

A more recent article from Anthropic is https://www.anthropic.com/engineering/built-multi-agent-rese... - "How we built our multi-agent research system". I found this one fascinating, I wrote up a bunch of notes on it here: https://simonwillison.net/2025/Jun/14/multi-agent-research-s...

Does anyone know which AI agent framework Anthropic uses? It doesn't seem like they ever released one of their own.

Just write the for loop to react to tool calls? It’s not very much code.

They mentioned hand offs, sub agents, concurrent tool calls, etc. You could write that yourself, but you would be inventing your own framework.