Aider is an open-source, terminal-based AI pair programmer: you run it inside your project's git repository, describe a change in natural language, and it edits the relevant files directly, then commits the change with a descriptive commit message. There's no IDE plugin or web app — it works entirely from the command line, layered on top of your existing editor and git workflow.

It's popular specifically with developers who want a lightweight, model-agnostic tool rather than being locked into one vendor's IDE or subscription. Aider works with models from Anthropic, OpenAI, Google, DeepSeek, and local models via Ollama, among others — you bring your own API key and pay the model provider directly rather than paying Aider a subscription fee.

Aider itself is completely free and open source. Your actual cost is whatever API usage you generate with your chosen model provider, which can range from a few dollars to $50+/month depending on how heavily you use it and which model you pick.