Each assistant message in the session JSON now carries:
backend, backend_label, host (platform.node())
These fields are shown as model tags in the UI — on live responses and
when loading session history. Session log entries (sessions/YYYY-MM-DD.md)
include the backend label and host in the turn header.
The local (OpenAI-compat) backend strips non-standard fields before
sending messages to the API so extra fields don't leak upstream.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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