AI Agent

An AI agent is software that uses an LLM to decide what to do, takes actions, observes results, and iterates toward a goal. Unlike a chatbot that just generates text, an agent has tools, memory, and a loop.

Anatomy: a planning step (LLM generates next action), a tool layer (executes that action — read file, run shell command, search web), an observation step (feeds result back into LLM context), and termination logic (when does the loop stop?). The simplest agent is "ReAct" — Reasoning + Acting in alternation.

Examples: Coding agents like Claude Code, Cursor's agent mode, Aider, Cline, Codex. Browser agents that operate web pages. Workflow agents like Devin or Replit Agent 3 that run for hours on tasks. For local-only agents: Open Interpreter, Aider with a local Ollama backend, Continue.dev with local models. The agent quality scales hard with model quality — a 7B local agent is meaningfully worse than Claude Sonnet on multi-step tasks.