The deployment default used when an entry point creates an Agent that has no session-local model selection. `AgentDefaultModelService` provides `ctx.agentDefaultModel`; direct entry points such as `dsh --profile headless` and Host-backed entry points such as ApiProxy read the same service instead of owning parallel provider/model defaults.
The plugin config requires `{ provider, model }`. That composition entry is the base of the `agent-default-model` Settings section; a mounted settings provider layers the user's choice over it and changes are visible on the next `currentSelection()` read. `reasoningEffort` belongs to the Settings section but deliberately not to plugin config: a complete saved selection can clear an effort when the next selected model has none, while a composition value would be inherited again.
-`ctx.agentDefaultModel.currentSelection()` returns a detached `{ provider, model, reasoningEffort? }` selection for a newly created Agent.
-`ctx.agentDefaultModel.saveSelection(selection)` saves the complete user selection. Without a settings provider it is a no-op and the composition entry remains current.
The service does not validate catalog membership. A provider route may serve an unadvertised model, and the consumer that actually opens a model request owns availability diagnostics.
Changing the default affects only Agents that subsequently resolve from it. An existing session whose request log already names a selection keeps that selection, so this service does not invalidate its established prefix.
