Anthropic sets a 30 November retirement for Sonnet 4.5

Anthropic deprecated Claude Sonnet 4.5 on 30 September 2026 US time. Its model deprecations page says claude-sonnet-4-5-20250929 retires on the Claude API on 30 November 2026.

The recommended replacement is claude-sonnet-5-5. The release notes entry for 30 September says the same.

The page says requests to a model past its retirement date "will fail."

Which platforms the date covers

Anthropic says its dates apply to the platforms it operates: the Claude API, Claude Platform on AWS and Microsoft Foundry.

Amazon Bedrock and Google Cloud set their own retirement schedules, the page says, so status and dates there can differ. It points to separate Bedrock and Google Cloud model tables.

Anthropic says it gives at least 60 days' notice before retiring a publicly released model. This notice gives 61.

Where the rest of the 4.5 family stands

The status table, checked at 15:52 SGT on 3 October, lists these dates for the other 4.5 models:

  • claude-haiku-4-5-20251001 is active, with a tentative retirement date of no sooner than 15 October 2026. It is the only Haiku model the table lists as active.
  • claude-opus-4-5-20251101 is active, with a tentative retirement date of no sooner than 24 November 2026.

Neither has a deprecation notice yet. Under the 60-day rule, any retirement of either would come at least 60 days after a notice.

claude-mythos-preview has been deprecated since 9 June 2026, with its retirement date still to be announced.

The replacement changes request behavior

Sonnet 5.5 is not a drop-in rename. Anthropic's 28 September release notes list five ways code written for Sonnet 5 can break on Sonnet 5.5:

  • To turn off up-front thinking, send thinking: {"type": "between_tools"} at high effort or below.
  • Forced tool use, with tool_choice set to any or tool, returns a 400 error.
  • Thinking blocks are tied to the model and the conversation.
  • On the Claude API and Google Cloud, the older computer_20251124 computer use tool is not accepted.
  • The advisor tool rejects Opus 4.8, Opus 4.7 and Sonnet 5 as advisors.

The notes describe these against Sonnet 5. By our reading, teams jumping straight from Sonnet 4.5 should test for all five.

The deprecations page does not list a Sonnet 4.5 date for Bedrock or Google Cloud. Check those tables directly.

Teams pinned to the September 2025 snapshot

Anthropic's page describes a usage audit:

  1. Open the Usage page in Claude Console.
  2. Click Export.
  3. Review the CSV, which breaks usage down by API key and model.

Then plan the switch before 30 November:

  • Search code and config for claude-sonnet-4-5-20250929, including fallbacks and eval harnesses.
  • Rerun regression tests on claude-sonnet-5-5 with tool use and thinking settings enabled.
  • Check prompts that rely on forced tool choice, which now errors.

For Sonnet 5.5's launch details and prices, see Anthropic releases Claude Sonnet 5.5 on 28 September at unchanged Sonnet 5 prices. For the higher tier, see Anthropic ships Claude Opus 5.5 on 22 September with Fable-class claims at lower list prices.