A session model picker and a deployment default are two layers of the same preference. If the picker affects only its addressed session, the next blank session can select a different model with no user-facing way to align the default. If the default lives inside a Host gateway, direct Agent entry points cannot share it without depending on Host or duplicating state.
Reasoning effort makes the persistence shape significant: a model selection without an effort must clear a stored effort, or the next Agent may apply an effort that its selected model does not accept.
`AgentDefaultModelService` provides `ctx.agentDefaultModel` and registers `{provider, model, reasoningEffort?}` as the `agent-default-model` Settings section. Its `{provider, model}` composition entry is the base layer and `settings.yaml` supplies the user layer. The service is entry-point-neutral, so direct creation and ApiProxy-backed creation share one default ([headless direct core entry point](../architecture/2026-08-09-headless-direct-core-entry-point.md)).
`reasoningEffort` belongs to the Settings section but not to the plugin config. Settings layers merge by field, so a configured effort would survive a user selection that omits it. `saveSelection()` instead writes the complete user section; absence therefore clears a stored effort. A deployment-wide effort default belongs to the adapter profile, which resolves it per model.
`session.selectModel` applies an accepted `ModelSelection` to its session and calls `saveDefaultModelSelection()` for the shared Agent default. A storage failure is logged without undoing the session selection. A deployment with no settings provider retains the composition entry and keeps the accepted selection only in that session.
`ApiProxyDefaults` carries `defaultModelSelection()` and `saveDefaultModelSelection()` closures, so `createApiProxy` has no dependency on the Settings seam. `ApiProxyService` wires them to `ctx.agentDefaultModel.currentSelection()` and `ctx.agentDefaultModel.saveSelection()`.
`selectionFor(agent)` resolves its tiers on every read: a process-local session selection, otherwise the session's latest logged `request/header`, otherwise the live Agent default. A session with a logged request remains bound to that durable selection. A blank session observes the current default even when it was created before the preference was saved; this matches the New Session surface, which may reuse a blank session.
The stored selection does not require catalog membership. A provider route may serve a model omitted from its advisory catalog. `session.models` therefore reports the stored selection independently of advertised groups and separately reports whether an adapter serves its provider.
`host.describe` reports the live Agent default. A successful model switch stores an `agent-default-model:` section in `settings.yaml`. The gateway does not expose that namespace through its Settings-page allowlist; the model picker is its editor.
`session.prompt` refuses with `model-unavailable` before opening a turn when no adapter serves the session's selected provider. This method is the enforcement boundary; a disabled composer is only a client affordance.
`session.models` reports `routable`. The ui-model plugin projects an unroutable selection through `ctx.conversation.blocks`, and the composer becomes inert while leaving the model seat available. An unknown client-side routability state, including an initial or failed catalog load, does not block input.
Routability is distinct from catalog membership. A live provider route can serve an unadvertised model, so absence from catalog groups does not imply that the session is unusable.
