Best Open-Source Agent Frameworks and Platforms

AutoGen is a broad open-source agent framework family focused on multi-agent interaction and extensibility. PydanticAI is a typed Python framework built around validation and structured outputs. Mastra sits in between framework and platform, offering an open-source TypeScript framework plus a broader paid platform layer.

That means the right choice is often more about team shape and operating model than about raw feature count.

AutoGen and PydanticAI are straightforward open-source framework choices with no public framework subscription fee. Mastra is more hybrid: the framework is free, but the platform is explicitly commercial. That makes Mastra easier to compare with product platforms, while AutoGen and PydanticAI are cleaner comparisons with other OSS-first libraries.

For budgeting, framework price is usually the least important line item. Model, infra, and operations still dominate.

If the team wants broad multi-agent patterns and extension surfaces, AutoGen is the stronger fit. If the team wants typed, validated Python agents with familiar schema ergonomics, PydanticAI is often the cleaner answer. If the team wants a TypeScript-first framework with an obvious managed upgrade path, Mastra is usually the better fit.

The highest-quality choice is the one that matches how the team already builds software.