LangChain

LangChain is an open-source framework and platform for building LLM applications and agents, with a free tier for solo developers. It includes the langchain framework for quick starts, langgraph for low-level control, and deepagents for long-running tasks. The LangSmith platform offers tracing, evaluation, deployment, sandboxed code execution, and no-code Fleet agents. Pricing: Developer plan is free with up to 5,000 base traces per month, then pay-as-you-go; Plus is $39 per seat per month with 10,000 base traces, and Enterprise is custom. Best for developers and teams building production agent systems, from solo projects to enterprise deployments, enabling iterative improvement through observability and evaluation.