The models service starts a conversation with start({agent, cwd, ownership?}, context). The agent field names a built-in definition or a project overlay. The service stores that name in models.profile. It applies the definition during creation and before each new run. Definitions without an explicit model follow their family, then the default family.

Each resolution reads /src/sausage/agents/models.json and the definition files through host.fs. A file edit applies at the next run boundary without a restart. The project definitions live under <repo>/.sausage/agents/. The existing T37 parser controls validation, overlays, model suffixes, and tool denials.

The optional models.pin service supplies pin(tx, conversationId) for an external model override. Without that provider, the definition or default family selects the model. No login fallback runs.

Existing registered models retain their transport. New logical model references reuse the host's OpenAI-compatible proxy provider and its auth resolver. The extension never reads a credential or registers a direct provider SDK.

host/tests/core_ext.rs tests these services through the snapshot loader.