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Claude Code v2.1.284 ·

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

The sonnet alias now means Sonnet 5.5 (claude-sonnet-5-5) on the Anthropic API, Sonnet 5 stays pickable, and refusal handling now covers Sonnet

Group of 7 Use it now Notable New Features
JSON All of v2.1.284
Use it nowTier: how much it should matter to you
5Useful: my rating, 1 to 5
4Signal: worth watching, 1 to 5
ModelsArea: what it touches
New FeaturesKind: in v2.1.284,
What probably matters to youSection of the release

Unclear Model settings sent from the server may override these defaults.

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.

Read from
Names in the bundlesonnet-5-5claude-sonnet-5-5sonnet
Since it was published

The entry above is what we published on the day. These lines were added later, as Anthropic's own pages caught up, and they sit beside the original rather than replacing it.

Added since A small documentation edit on Agent SDK reference - Python touched a line naming claude-sonnet-5-5 after this was published. On the Claude API, the `context-1m-2025-08-07` beta is retired for Claude Sonnet 4.5 and Claude Sonnet 4. If you still pass it with either model, requests that exceed the standard 200K-token context window return an error, so remove it fro… agent-sdk/python see the edit
Added since A small documentation edit on Explore the context window touched a line naming sonnet-5-5 after this was published. Sonnet 5.5 and Sonnet 5 run with the 1M context window and have no `[1m]` variant to select. See [Sonnet 5.5 and Sonnet 5 context window](/docs/en/model-config#sonnet-5-5-and-sonnet-5-context-window) for their auto-compaction thresholds, a… context-window see the edit
Confirmed since Anthropic's documentation has since written up claude-sonnet-5-5, on How Claude Code uses prompt caching. `DISABLE_PROMPT_CACHING_SONNET` and `DISABLE_PROMPT_CACHING_OPUS` each apply to the model the `sonnet` or `opus` alias resolves to. If you set any other Sonnet or Opus model ID as your main model, that model keeps caching. For example, a s… prompt-caching see the edit
Confirmed since Anthropic's documentation has since written up sonnet-5-5, on How Claude Code uses prompt caching. `DISABLE_PROMPT_CACHING_SONNET` and `DISABLE_PROMPT_CACHING_OPUS` each apply to the model the `sonnet` or `opus` alias resolves to. If you set any other Sonnet or Opus model ID as your main model, that model keeps caching. For example, a s… prompt-caching see the edit
Confirmed since Anthropic's documentation has since written up sonnet, on Error reference. Model "Sonnet5" is not a recognized model id. Did you mean 'claude-sonnet-5'? errors see the edit
Added since A small documentation edit on Model configuration touched a line naming sonnet after this was published. Administrators can use `availableModels` in [managed or policy settings](/docs/en/managed-settings) to restrict which models users can select. Entries match a model family such as `sonnet`, a version prefix such as `claude-sonnet-4-5`, or … model-config see the edit
How sure we are
Something disagreesSomething we can check disagrees with this entry, or the writer said they could not settle it.
The writer flagged doubtModel settings sent from the server may override these defaults.
Anthropic's documentation agreesA small documentation edit on Model configuration touched a line naming sonnet after this was published.
Anthropic's release notes agreeAdded Claude Sonnet 5.5 (claude-sonnet-5-5), now the default Sonnet model on the Anthropic API — 1M context, $2/$10 per Mtok with $0.20/Mtok…

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