Anthropic will retire Claude Sonnet 4.5 on November 30, 2026

On September 30, 2026, Anthropic notified developers that Claude Sonnet 4.5 (claude-sonnet-4-5-20250929) is deprecated. It will be retired on the Claude API on November 30, 2026. After that date, requests to the model fail. Anthropic recommends moving to Claude Sonnet 5.5 (claude-sonnet-5-5).

Where the date applies

The date covers the platforms Anthropic runs itself: the Claude API, Claude Platform on AWS and Microsoft Foundry. Amazon Bedrock and Google Cloud set their own retirement schedules, so the model may stay up longer there. Check your cloud provider’s model table if you call Claude through one of them.

What the replacement costs

Per 1M tokens	Sonnet 4.5	Sonnet 5.5
Input	$3.00	$2.00
Output	$15.00	$10.00
Cache read	$0.30	$0.10

The per-token price is lower, but Sonnet 5.5 uses Anthropic’s newer tokenizer, which turns the same text into about 30% more tokens than Sonnet 4.5 does. Real savings depend on your prompts.

What breaks

Sonnet 5.5 is not a drop-in swap. Its model page and Anthropic’s deprecation notes list changes that affect code written for older models:

• Non-default temperature, top_p or top_k values return a 400 error.
• Forced tool use returns an error.
• Adaptive thinking is on by default, and text between tool calls comes back in thinking blocks, so an app that streams that text goes quiet until it changes a setting.

The rest of the schedule

This follows two retirements earlier in the summer: Claude Opus 4 and Sonnet 4 on June 15, 2026, and Claude Opus 4.1 on August 5, 2026. Claude Sonnet 4.6 stays active, with no retirement before February 17, 2027.

What to do

Anthropic suggests exporting a usage report from the Claude Console (Usage, then Export) to find any API key still calling Sonnet 4.5. Test your prompts on Sonnet 5.5 or Sonnet 4.6 well before November 30.