A harness is any wrapper around llm calls that manipulates llm interactions to achieve the process for which it is designed. Claude code et al are just one type of harness, focusing on writing code. The kind of UI used to interact with the harness and underlying models does not matter.

TUI?

Aren't VS Code, Claude Code, Hermes Agent, Goose or Letta harnesses, but with UI, too?

I thought the harness was mainly a TAI (tangible AGENT interface). Its a harness for the agent, not a user interface. That is bolted on top of the harness.