Conversation completion evaluations now use a dedicated internal LLM
feature with GPT 4.1 as the default. The internal route keeps the
completion model separate from the installation wide Captain model
override and from the assistant route, which can use GPT 5.2 for Captain
V2 accounts. Evaluations continue to use the installation API key and do
not consume Captain response credits.
## What changed
Added an internal `conversation_completion` feature to the LLM model
config and excluded internal features from account preferences, the
Captain settings API, and Super Admin model overrides.
Updated `Captain::ConversationCompletionService` to resolve its model
through `Llm::FeatureRouter`.
Added focused service and request coverage for model routing and
settings visibility.
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