# Best Agent frameworks — Open source

> Source: https://theagentsindex.com/best/frameworks/open-source (curated, quality-gated, re-verified)

10 of 14 agent frameworks have a confirmed yes for “Open source”. 3 are qualified or tier-scoped, 1 are confirmed no, and 0 remain not established. Only confirmed yes answers appear in this set; an unknown is never treated as a negative.

| # | Name | What it is | Pricing | API |
| --- | --- | --- | --- | --- |
| 1 | [Agent Development Kit (ADK)](https://theagentsindex.com/google-adk.md) | Google's open-source framework for building, evaluating and deploying multi-agent systems. | freemium | Yes |
| 2 | [AgentsKit.js](https://theagentsindex.com/agentskit.md) | Composable TypeScript foundation for agent runtime, tools, memory, RAG and chat UI bindings. | — | Yes |
| 3 | [AutoGen](https://theagentsindex.com/microsoft-autogen.md) | Open-source Python and .NET framework for building single- and multi-agent AI applications; now in maintenance mode. | free | No |
| 4 | [CrewAI](https://theagentsindex.com/crewai.md) | Open-source Python framework for multi-agent crews, with a hosted platform for deploying and governing them. | freemium | Yes |
| 5 | [LangGraph](https://theagentsindex.com/langgraph.md) | MIT-licensed orchestration framework and runtime for stateful, long-running LLM agents. | paid | Yes |
| 6 | [Letta](https://theagentsindex.com/letta.md) | Stateful agent harness with git-backed memory, run from a CLI, desktop app, web, or SDK | freemium | Yes |
| 7 | [Mastra](https://theagentsindex.com/mastra.md) | Open-source TypeScript framework for AI agents, with a hosted platform for observability and deployment | freemium | Yes |
| 8 | [Microsoft Agent Framework](https://theagentsindex.com/microsoft-agent-framework.md) | Open-source Microsoft SDK for building AI agents and multi-agent workflows in .NET, Python and Go. | freemium | Yes |
| 9 | [OpenAI Agents SDK (Python)](https://theagentsindex.com/openai-agents-sdk.md) | MIT-licensed Python framework for multi-agent workflows, tools, guardrails and tracing | free | Yes |
| 10 | [Pydantic AI](https://theagentsindex.com/pydantic-ai.md) | Typed Python agent framework: one agent definition, any model, terminal, web, editor or voice surface. | free | No |

## Methodology

Projected from the live listing datasheets using the tenant’s “Is the source available?” axis. Membership requires a sourced yes; partial and unknown answers are reported above but excluded from the ranking. Entries retain the directory’s paid-tier ordering, visibly labelled on every promoted card.