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Defining Agents

Agents represent a bundle of Tools, Hooks, Skills, Instructions and Configuration for interacting with models.

fast-agent supports two complementary ways to define agents:

  • Python API - define agents and workflows in Python with decorators such as @fast.agent or @fast.parallel.
  • Agent Cards - portable markdown files that can be loaded by fast-agent go, the TUI, and card tooling.

Use the Python API when you are want to build a self contained Python application or script. Use Agent Cards when you want portable, reusable agent definitions that can be easily loaded, edited or shared.

Agents can be used in a number of ways:

  • From the Harness API: a high level session based automation framework.
  • Directly from a Python program using fast_agent.run()
  • Interactively, from the fast-agent TUI or ACP
  • Deployed as an MCP Server