Models
Every task runs on a coding agent and a model. In the app both come from the agent you assign the work to, so there is no per-task model choice. Over the API the choice is per session, so two sessions under the same task can run different models.
Choosing a model
The model belongs to the agent. Open the agent from Agents, go to its Settings, and select Model. The picker groups models under the coding agent that runs them:
- Claude Code (Anthropic)
- Codex (OpenAI)
Pick a coding agent, then a model within it. Leave it on Default to follow the account, which is Opus 4.8 under Claude Code. The choice applies to the tasks that agent starts from then on; a session already running keeps the model it started with. To run the same work on a different model, assign it to an agent set to that model.
Claude Code (Anthropic)
| Model | Notes |
|---|---|
| Opus 4.8 | Default. |
| Opus 5 | Current Opus release. Runs against a one-million-token context window. |
| Fable 5 | Most capable. Costs more per session, so pick it explicitly. |
| Opus 4.7 | Prior Opus release. |
| Sonnet 5 | Faster and lower cost than Opus. Runs against a one-million-token context window. |
| Sonnet 4.6 | |
| Haiku 4.5 | Fastest and lowest cost. |
Context Window
A session on Opus 5 or Sonnet 5 works against the model's full one-million-token window. The other Claude models work against 200,000 tokens, and a session that outgrows that window cannot take another turn.
The difference shows on long-running work: a task that reads a lot of a large repository, or that runs for hours across many turns, keeps going on Opus 5 or Sonnet 5 where it would otherwise stop.
Codex (OpenAI)
| Model | Notes |
|---|---|
| GPT-5.6 Sol | Frontier tier. |
| GPT-5.6 Terra | Balanced intelligence and cost. |
| GPT-5.6 Luna | Efficient, for high-volume work. |
| GPT-5.5 | Prior flagship. |
| GPT-5.4 | The generation before that. |
An unknown or unavailable model falls back to the agent's default rather than failing the task, so the session always runs on a supported model.
Choosing a model over the API
Set the agent and model when you create a task or session with the harness and model fields. See Tasks and Sessions.