An agent harness is what enables the user to seamlessly interact with both a model and tool calls. Claude Code is an agent harness.

  ┌────────────────────────────┐
  │           User             │
  └──────────────┬─────────────┘
                 │
                 ▼
  ┌────────────────────────────┐
  │       Agent Harness        │
  │   (software interface)     │
  └──────┬──────────────┬──────┘
         │              │
         ▼              ▼
  ┌────────────┐ ┌────────────┐
  │   Models   │ │   Tools    │
  └────────────┘ └────────────┘

Here's an example of a harness with less code: https://github.com/badlogic/pi-mono/blob/fdcd9ab783104285764...