# 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 — 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](https://theagentsindex.com/openai-agents-sdk.md) | OpenAI's lightweight open-source framework for building agents in Python and TypeScript, with handoffs, guardrails, tracing and a sandboxed workspace mode for long-running work. | Free | Yes |
| 2 | [Google Agent Development Kit](https://theagentsindex.com/google-adk.md) | Google's open-source, code-first agent SDK with five language runtimes (Python, TypeScript, Go, Java, Kotlin), native A2A and MCP support, and an optional managed runtime on Google Cloud. | Free | Yes |
| 3 | [Microsoft Agent Framework](https://theagentsindex.com/microsoft-agent-framework.md) | Microsoft's unified, MIT-licensed agent SDK for .NET and Python: the supported successor to AutoGen and Semantic Kernel. | Free | Yes |
| 4 | [Claude Agent SDK](https://theagentsindex.com/claude-agent-sdk.md) | Anthropic's own agent-building library: the same agent loop, tools and context management that power Claude Code, for Python and TypeScript. Claude-only, with no multi-provider escape hatch. | Free | Yes |
| 5 | [LangChain](https://theagentsindex.com/langchain.md) | The most widely-used framework for building LLM apps and agents, with LangGraph for stateful multi-agent control: open-source and free to self-host. | Free | Yes |
| 6 | [CrewAI](https://theagentsindex.com/crewai.md) | Open-source Python framework for role-based multi-agent crews, with a hosted build and runtime that adds enterprise governance, native SDKs for the major LLMs, and Ollama via LiteLLM. | Free (month (50 workflow executions)) | Yes |
| 7 | [LlamaIndex](https://theagentsindex.com/llamaindex.md) | Open-source data framework for LLM apps, with LlamaParse as the paid agentic OCR for messy documents and LlamaAgents for document workflows. | $0 / month | Yes |
| 8 | [Pydantic AI](https://theagentsindex.com/pydantic-ai.md) | The Python AI SDK for type-safe agents, realtime voice, image generation and embeddings, with every model a string swap away. | Free | — |
| 9 | [Mastra](https://theagentsindex.com/mastra.md) | The TypeScript-native agent framework. Agents, durable workflows, memory and observability in one Apache 2.0 package, with an optional managed cloud platform. | Free | Yes |
| 10 | [Letta](https://theagentsindex.com/letta.md) | Letta is the open-source framework behind Letta Code, a stateful coding agent whose memory, skills, and identity persist across sessions rather than rebuilding context on every call. | $0 / month | Yes |
| 11 | [Agno](https://theagentsindex.com/agno.md) | An Apache 2.0 Python agent SDK and AgentOS runtime, with a Control Plane you run in your own cloud. | Free | Yes |
| 12 | [Microsoft AutoGen](https://theagentsindex.com/microsoft-autogen.md) | Microsoft's open-source framework for orchestrating conversations between multiple AI agents. Now in maintenance mode. | Free | — |
| 13 | [AgentsKit](https://theagentsindex.com/agentskit.md) | Open-source, MIT-licensed JS/TS toolkit for building AI agents with a 10KB zero-dependency core and 50 built-in integrations. | — | Yes |
| 14 | [LangGraph](https://theagentsindex.com/langgraph.md) | Open-source, MIT-licensed framework for building stateful, multi-agent AI workflows on cyclical graphs. | $0/seat/month | Yes |

## Methodology

Every entry below is a Published listing that already cleared this site's per-listing quality gate — 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.