# The 9 best AI agent builders for developers (2026) — The Agents Index

> Source: https://theagentsindex.com/category/agent-builders (curated, re-verified)

Open-source and hosted builders where you assemble agents from models, tools and your data, then deploy them.

9 agents in this category, ranked by what we can confirm about each (a public price, an API and docs, independent sources, stated limits; payment never changes the order). Each name links to its answer-first markdown twin.

| Name | What it is | Pricing | API |
| --- | --- | --- | --- |
| [Letta](https://theagentsindex.com/letta.md) | Stateful agent harness, cloud and SDK for agents with persistent, git-versioned memory. | Free, then $20/month | Yes |
| [Sim](https://theagentsindex.com/sim.md) | Open-source AI workspace for building, deploying and running agents across 1,000+ integrations. | Free, then $25/seat/month | Yes |
| [MindStudio](https://theagentsindex.com/mindstudio.md) | No-code platform for building, running and deploying AI agents as drag-and-drop workflows. | Free, then $20/month | Yes |
| [Dify](https://theagentsindex.com/dify.md) | Open-source platform for building agentic workflows, AI agents and RAG knowledge pipelines. | Free, then $59/month | Yes |
| [Relevance AI](https://theagentsindex.com/relevance-ai.md) | Platform for building and running specialist business agents and multi-agent workforces. | On quote | Yes |
| [Agno](https://theagentsindex.com/agno.md) | Python SDK, self-hosted AgentOS runtime and hosted control plane for running agents as a service. | Free, then $150/month | Yes |
| [Gumloop](https://theagentsindex.com/gumloop.md) | Multiplayer AI agent builder: teams build agents over their apps while IT keeps governance and spend control. | From $37/month | Yes |
| [Pickaxe](https://theagentsindex.com/pickaxe.md) | No-code platform for building AI agents and selling them through branded portals with billing built in. | Free, then $37/month | Yes |
| [StackAI](https://theagentsindex.com/stack-ai.md) | No-code enterprise platform for building, deploying and governing AI agents across business systems. | Free | Yes |