Anthropic deprecates Claude Sonnet 4.5 with 61 days' notice, one day above its floor

Anthropic deprecated Claude Sonnet 4.5 on September 30 and gave the model a tentative retirement date of November 30, 2026, which is 61 days later. Its published policy commits it to "at least 60 days' notice before model retirement for publicly released models", so the timetable clears Anthropic's own floor by a single day.

Anthropic's illustration of two hands cupping a branching, plant-like network on a terracotta background. Image: Anthropic

The model ID going away is claude-sonnet-4-5-20250929, and the recommended replacement is claude-sonnet-5-5, according to Anthropic's deprecations documentation. Sonnet 4.5 launched on September 29, 2025, so it will have been available for 14 months when it goes.

Requests to a retired model fail outright

Deprecated and retired are not the same state. Deprecated means "the model is still functional but no longer recommended", with a replacement named and a retirement date assigned. Retired means "the model is no longer available for use", and "requests to retired models will fail".

Between now and then, Anthropic's own warning is that "deprecated models are likely to be less reliable than active models". Anything still pointing at the dated Sonnet 4.5 ID keeps working until November 30 and then stops, which is the part that catches side projects and plugins nobody has touched in a year.

Bedrock and Google Cloud run their own clocks

The dates apply to Anthropic-operated platforms only: the Claude API, Claude Platform on AWS and Microsoft Foundry. "Partner-operated platforms (Amazon Bedrock and Google Cloud) set their own retirement schedules, so a model's lifecycle status and dates can differ," Anthropic says.

If you run Sonnet 4.5 through Bedrock or Google Cloud, November 30 is not your date, and Anthropic points at each partner's own model tables rather than giving one. Sonnet 5.5 shipped on both of those platforms as well as the Claude API on the day Anthropic launched it, September 28.

Two months is close to Anthropic's usual

The 61 days is not unusual for the company. Claude Opus 4.1 got 61 days between its June 5 deprecation and its August 5 retirement, Claude Sonnet 4 and Claude Opus 4 got 62, and Claude Haiku 3 got exactly 60. The outlier runs the other way: Claude Sonnet 3.7 was given 114 days in October 2025.

Anthropic is candid about the cost of the policy. It says it retires models "to ensure capacity for new model releases", and names the downsides itself: users who value specific models must migrate, researchers lose access for comparative studies, and "model retirement introduces safety- and model welfare-related risks". It says it has committed to long-term preservation of model weights.

Find out which keys still call it

Anthropic's suggested audit is short. Open the Usage page in the Claude Console, click Export, and read the downloaded CSV, which breaks usage down by API key and model.

Migrating is not a drop-in swap. Code written for Sonnet 5 breaks on Sonnet 5.5 in five ways, including forced tool use returning a 400 error, so code written for 4.5 is worth testing rather than redeploying. On price and scores, Anthropic puts Sonnet 5.5 at 70.6% on Terminal-Bench.

One caveat on the date itself: Anthropic calls November 30 a "tentative retirement date", the wording it uses for every model on the clock. That is the plan rather than a promise, and the 61-day clock is running from September 30.