# The Best AI Agent Frameworks (2026)

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

A framework is not a product you sign up for. It's a toolkit you build an agent with, in your own codebase, calling your own model API key. That makes "which one is best" a different question here than it is for the rest of this site: the real split is between four official SDKs, built and maintained by the lab whose models they run best on, and nine independent frameworks that were never designed around one vendor's model.

OpenAI Agents SDK, Claude Agent SDK, Google Agent Development Kit and Microsoft Agent Framework are each a major AI lab's own answer to "how should you build an agent on our models": free, officially supported, and three of the four reach other vendors' models through a LiteLLM-style integration; Claude Agent SDK is the exception, Claude only, by Anthropic's own design decision. LangChain, CrewAI, LlamaIndex, Pydantic AI, Mastra, Letta, Agno and Microsoft AutoGen are independent and model-agnostic by design, so you can point any of them at whichever provider you choose, at the cost of assembling more of the production plumbing yourself. LangChain remains the default starting point for its integration ecosystem alone; CrewAI is the fastest route to a working multi-agent team; Pydantic AI and Mastra trade breadth for type safety and a native TypeScript stack respectively; and Microsoft AutoGen, the framework that popularized agents talking to agents, is now frozen in favor of its own successor.

| # | Name | What it is | Pricing | API |
| --- | --- | --- | --- | --- |
| 1 | [OpenAI Agents SDK (Python)](https://theagentsindex.com/openai-agents-sdk.md) | Open-source Python framework for building agents with tools, handoffs, guardrails and tracing. | Free | Yes |
| 2 | [Google Agent Development Kit (ADK)](https://theagentsindex.com/google-adk.md) | Open-source framework for building, evaluating and deploying AI agents in Python, TypeScript, Go, Java or Kotlin. | Free | Yes |
| 3 | [Microsoft Agent Framework](https://theagentsindex.com/microsoft-agent-framework.md) | Open-source .NET, Python and Go SDK for building AI agents and graph-based multi-agent workflows. | Free | Yes |
| 4 | [Claude Agent SDK](https://theagentsindex.com/claude-agent-sdk.md) | Python and TypeScript library that runs Claude Code's agent loop and tools inside your own application. | From $1/million input tokens | Yes |
| 5 | [LangChain](https://theagentsindex.com/langchain.md) | Open source agent frameworks plus LangSmith, a hosted platform to trace, evaluate, deploy and govern agents. | Free, then $39/seat/month | Yes |
| 6 | [CrewAI](https://theagentsindex.com/crewai.md) | Open-source multi-agent framework with a managed platform for deploying and governing agent workflows. | Free | Yes |
| 7 | [LlamaIndex](https://theagentsindex.com/llamaindex.md) | Open-source agent framework plus a credit-metered document parsing, extraction and indexing platform. | Free, then $50/month | Yes |
| 8 | [Pydantic AI](https://theagentsindex.com/pydantic-ai.md) | Open-source Python agent framework from the Pydantic team, with a typed agent loop and swappable models. | Free | Yes |
| 9 | [Mastra](https://theagentsindex.com/mastra.md) | Open-source TypeScript framework for building AI agents, with a hosted platform for running and observing them. | Free, then $250/month | Yes |
| 10 | [Letta](https://theagentsindex.com/letta.md) | Stateful agent harness, cloud and SDK for agents with persistent, git-versioned memory. | Free, then $20/month | Yes |
| 11 | [Agno](https://theagentsindex.com/agno.md) | Python SDK, open-source AgentOS runtime and hosted control plane for building and operating agents. | Free, then $150/month | Yes |
| 12 | [AutoGen](https://theagentsindex.com/microsoft-autogen.md) | Open-source Microsoft framework for building single and multi-agent AI applications in Python. | Free | Yes |
| 13 | [AgentsKit](https://theagentsindex.com/agentskit.md) | Composable MIT-licensed TypeScript packages for agent runtime, tools, memory, RAG and chat UI. | Free | Yes |
| 14 | [LangGraph](https://theagentsindex.com/langgraph.md) | Open-source orchestration framework and runtime for stateful, long-running LLM agents, from LangChain. | Free, then $39/month | Yes |
| 15 | [LiveKit Agents](https://theagentsindex.com/livekit-agents.md) | Open-source Python and Node.js framework for voice AI agents, with LiveKit Cloud hosting, telephony and observability. | Free, then $50/month | Yes |
| 16 | [Langflow](https://theagentsindex.com/langflow.md) | Open-source visual builder for AI agents, RAG apps and MCP servers, with an API server included. | Free | Yes |

## Methodology

Every entry below is a Published listing that already cleared this site's per-listing quality gate. That gate wants sourced facts, admitted weaknesses, a "best for" and a "not for," re-verified on a rolling cadence (see /methodology). frameworks is the site's second-largest category by listing count (14, behind only coding-agents' 25) and, unlike every other multi-member category here, had no ranking of its own until it was built. The flagship /best-ai-agents groups these same 14 together in one flat list, which can't give the framework-specific detail a dedicated page can. We split the 14 into the two groups a developer actually chooses between first: an official SDK from the lab whose models you're already committed to, or an independent framework you can point at any provider. Within a group, the order follows rough ecosystem maturity (GitHub stars, funding, production track record), not a score; the live score chip on every card (evidence, honesty, depth, freshness) is computed the same way as that tool's own listing page.