Claude Sonnet 4.5 deprecation: dates, model IDs and migration

Deprecation facts

Read from Anthropic's model-deprecations page on October 11, 2026.

The same page defines the lifecycle terms used here: Active means fully supported; Legacy means no further updates and a possible future deprecation; Deprecated means still functional but no longer recommended, with a named replacement and a retirement date; Retired means requests fail. Sonnet 4.5 is Deprecated as of September 30, 2026.

What the move to Sonnet 5.5 costs

Both columns are Anthropic's published rates per million tokens. Every figure below is read from this site's model registry rather than typed into the copy, so the table cannot drift from the vendor.

Input and output are both 33% cheaper on Sonnet 5.5, and the cache-read rate falls 67%. The context window grows 5× and max output grows 2×, so this is not a trade-down: lower rates and a bigger window. Rates verified against Anthropic's pricing page on 2026-10-11. One caveat — the newer tokenizer counts the same text as more tokens, so a lower sticker price is not automatically a lower bill. Measure with your own traffic.

Migration checklist

Anthropic's guidance is to move workloads before the retirement date, not after.

• Find what still calls Sonnet 4.5

  In the Claude Console open Usage, click Export, and read the CSV — it breaks usage down by API key and model, which is how you locate live calls to claude-sonnet-4-5-20250929.

• Update the model ID string

  Swap claude-sonnet-4-5-20250929 — or the alias claude-sonnet-4-5 — for claude-sonnet-5-5. On Amazon Bedrock, Google Cloud or Microsoft Foundry, use that platform's own model ID.

• Re-test before the deadline

  Run your evaluation set on Sonnet 5.5 well before November 30, 2026. Anthropic recommends testing a replacement on your own tasks rather than assuming the benchmarks carry over.

• Re-check your reasoning settings

  Sonnet 4.5 has no effort parameter; Sonnet 5.5 has one and defaults to high. If you depended on Sonnet 4.5's fixed reasoning behaviour, set effort explicitly instead of inheriting the new default.

• Walk the breaking-change list

  Five documented changes can reject requests that used to succeed, including tool_choice any and tool returning 400 and the computer_20251124 tool being refused on the Claude API.

• Confirm your platform's dates

  November 30, 2026 is the Anthropic-operated date. Amazon Bedrock and Google Cloud are partner-operated and publish their own schedules, so check their tables before you plan a code freeze.

What to check when you move to Sonnet 5.5

Anthropic documents these as breaking changes relative to Sonnet 5. They apply to any client that moves onto Sonnet 5.5, including one arriving from Sonnet 4.5.

• Turning thinking off

  At high effort or below, disabling thinking now sends thinking undefined instead of "disabled". Code that asserts on the old value needs updating.

• tool_choice: any and tool

  Both return 400 on Sonnet 5.5. Use tool_choice auto together with strict tool use instead.

• Thinking blocks are scoped

  Thinking blocks are tied to the model and to the conversation that produced them, so replaying them somewhere else is rejected.

• The older computer-use tool

  computer_20251124 is rejected on the Claude API and on Google Cloud. Those paths need the newer computer_toolset_20260801 toolset.

• Which models the advisor tool accepts

  The advisor tool rejects Opus 4.8, Opus 4.7 and Sonnet 5 when they are passed as advisors.

Source: Anthropic's Sonnet 5.5 release note of September 28, 2026. The wording here is this site's summary; where a change can reject a request, the vendor's migration guide is the authority.

Which platforms follow this date

The retirement date applies to Anthropic-operated platforms. Partner-operated platforms are a separate question.

• Anthropic-operated

  The Claude API, Claude Platform on AWS and Microsoft Foundry follow the dates above: deprecated September 30, 2026, retiring November 30, 2026.

• Partner-operated

  Amazon Bedrock and Google Cloud set their own retirement schedules. A model's lifecycle status and dates can differ there, so read their model tables rather than reusing November 30.

• What retirement actually does

  Retired is not a soft deprecation: requests to a retired model fail. Plan the switch before the date rather than debugging an outage after it.

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