A harness is the agent program that runs a model and its tools: Claude, Codex, or Cursor. Some models can run on more than one harness. For example, many hosted models that run on AI credits can run on Claude or on Codex. You choose the harness for each model, and you can carry a Claude chat over to Codex, or a Codex chat over to Claude.
Before you start
- To use the Cursor harness, connect Cursor first. See Connect your AI models.
Change the harness from the model picker
- In the composer, open the model picker. See Choose a model and effort level.
- Hover over the model. The harness icon at the right of its row turns into •••. Select it.
- Under Harness, choose the harness that should run the model.
The choice is saved for that model, so new chats that use it start on the harness you picked. The same menu can also show Usage billing, where you choose whether this chat runs on your subscription or on AI Credits.
Set the default harness for each model
- Open Settings and go to Default harness, in the Agents group.
- Each column is a harness: Claude, Codex, and Cursor. Each model sits in the column of the harness that runs it.
- Drag a model to another column. Or hover over it, select •••, and choose Move to Claude, Move to Codex, or Move to Cursor.
To go back to the recommended harness, choose Reset to recommended in the same menu. A model with a lock beside it can run on one harness only, and its tooltip says which one, such as Managed by Claude.
Chats you've already started keep the harness they started with. Your change applies to new chats.
Switch a chat that has already started
Before a chat's first message, you can pick any model. After that, the chat is tied to its harness, so a different harness means a new chat.
Between Claude and Codex. When you pick a model that needs the other harness, the app asks whether to start a new chat with the model you picked. Select Start new chat. The new chat gets a summary of what you've done so far. Your files aren't changed, and the old chat stays open.
Wait for the agent to finish its turn and for any queued messages to send before you switch. If the agent is still working, the picker explains what to finish first.
Cursor and local models. A summary can't be carried to or from Cursor or Ollama. When you pick a model that needs a different harness, the app asks, for example, Start a new chat with the Claude harness? Select Start a new chat to begin fresh. The old chat stays as it is.