Default models for sonnet, opus and haiku can come from a server document

What

Claude Code picks a default model for each model family alias (sonnet, opus and haiku). These defaults used to be fixed in each build. Now:

• The default for a family can come from a document. Such a default is marked fromDocument, and it can carry a documentTarget naming a model ID this build does not know.
• The ANTHROPIC_DEFAULT_*_MODEL environment variables, such as ANTHROPIC_DEFAULT_SONNET_MODEL, keep priority over these defaults.
• In two places that check modelOverrides (the setting that maps model IDs to your provider's IDs), entries that have a documentTarget are no longer suppressed by a modelOverrides entry. One place returns the tier, environment variable and value for model tiers set by environment variables; the other checks the environment variables of each model entry.
• A new check lists candidate models that fail an availability check.
• On Bedrock and Vertex, startup now also runs checkBedrockDocumentDefaults or checkVertexDocumentDefaults in the time left. When a document-configured default is not available on the account, a startup notice says "Default model: ... not available on this account - not used as this session's default".
• New helpers decide whether a model name configured for a third-party provider (Bedrock, Vertex or Mantle) is usable. They check per-provider and everywhere default pins, and fall back to an older model in the same family when modelOverrides covers the newer one.

Why

In practice, which model an alias such as sonnet points to is no longer decided only by the installed build. On Bedrock and Vertex you are told when such a default is not available to your account instead of it being used anyway.