Harness does a lot of things potentially, but this depends on the harness.

1. Manages what is loaded in the context (I.e. sent to the model), this can be about compaction, summarization, removing things it judges no longer relevant, proving memory 2. Handling retries and how to resend data , lot of model specific behavior here 3. Wires up tools like web search or lsp servers 4. Skills and how to use them 5. Plugins/sdks 6. Management of rules (AGENTS.md)

About everything interesting really, a ton of work goes into this and getting it right is surprisingly hard