langchainjs

This is the framework that connects your code to LLMs. It handles the plumbing: talking to OpenAI/Anthropic/local models, managing conversation memory, chaining prompts together, and calling tools.

What's free: Everything. MIT license, no paid tier in the library itself. LangSmith (their observability platform) has a free tier with limits.

LangChain JS has become the default starting point for JS/TS AI applications. Active development, huge community. The abstractions for chains, agents, and retrieval are battle-tested.

The catch: LangChain is famously over-abstracted. Simple things that take 5 lines with the OpenAI SDK directly take 20 lines through LangChain. The abstraction layers add latency and debugging complexity. If you're just calling an API and formatting the response, you don't need this. It earns its keep when you're building complex agent workflows with tool calling, retrieval-augmented generation (feeding your own documents to AI), or multi-step reasoning chains.