Sonnet 5.5 becomes the default Sonnet model, with Sonnet 5 kept in the picker

What

A new model, Sonnet 5.5 (claude-sonnet-5-5, display name "Sonnet 5.5", 1M context window), is added and becomes the default for the sonnet family when you use Anthropic's own API. A context window is how much text the model can hold at once.

• The sonnet alias, and /model sonnet, now resolve to claude-sonnet-5-5 instead of claude-sonnet-5. It is also listed as the latest Sonnet.
• A short name sonnet-5-5 maps to claude-sonnet-5-5, checked ahead of sonnet-5 so the new ID is no longer treated as Sonnet 5. It is added to the known-models list and gets its own Vertex region environment variable.
• Other providers keep their own Sonnet defaults: Bedrock, Vertex, Foundry and Mantle stay on claude-sonnet-4-5, and anthropic_aws and gateway stay on claude-sonnet-4-6.
• The /model picker gains a row labelled "Sonnet 5 · Previous Sonnet version" when Sonnet 5 is available.
• Internal model names move on: the main Opus and Sonnet IDs are now claude-opus-5-5 and claude-sonnet-5-5 ("Claude Sonnet 5.5"), the previous ones are claude-opus-5 and claude-sonnet-5, and the separate "next" entries are gone.
• Refusal fallback: when a Sonnet 5.5 session (claude-sonnet-5-5 or claude-sonnet-5-5[1m]) has a request refused in the cyber or frontier_llm category, Claude Code falls back to claude-sonnet-5. There is no fallback for bio. Before, Sonnet 5.5 would have used the default list, which sends bio and frontier_llm to claude-opus-5 and cyber to claude-opus-4-8.
• Safeguard notices: the "You may be seeing this for the first time…" explanation, shown when safeguards flag a message, used to appear only for claude-opus-5-5. It now also appears for any Sonnet model in the cyber category, reading "on a Sonnet model". Sonnet also gets the frontier_llm recovery text ("is answering instead" / "Edit and retry"). The notices now come from a per-family table, and the Opus wording is built the same way instead of being fixed text.

Why

If you pick sonnet on the Anthropic API, you move to a new model without changing anything, and you can still choose Sonnet 5 from the picker. Sonnet users who hit a safeguard refusal now get an explanation and a fallback route instead of a bare refusal notice.