When a configured primary/fallback model isn't found in the registry,
check if the provider has any known models and construct a model
on-the-fly using that provider as a template (mirroring the pi CLI's
buildFallbackModel logic). This lets any valid provider model ID work
(e.g. any OpenRouter model string) without requiring it to be in the
built-in or custom model list. Only throw if the provider itself is
completely unknown.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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