Added AnthropicOptions.fallbacks + wire types + response parsing gated on the opt-in — server-side fallback stays fully inert on every request that does not set the option.
Coding-agent surfaces the feature via providers.anthropic.serverSideFallback (default off). When enabled, Fable/Mythos requests inject fallbacks: [{ model: claude-opus-4-8 }]; caller-supplied fallbacks always win.
transformMessages centrally strips persisted fallback blocks on cross-provider hops and non-official Anthropic replays so a stored fallback turn never wedges downstream converters. Retry resets restore output.model to the requested id.
Fixes#4177