Pick Your Replacement Model Before Claude Sonnet 4.5 Retires on November 30

What is the retirement schedule for Sonnet 4.5?

According to the entry on the model deprecations page, the schedule is as follows.

It helps to know the terms the documentation uses. Deprecated means the model still works but is no longer recommended, and Retired means it can no longer be used and requests fail. Sonnet 4.5 is in the first state right now. The documentation also warns that a deprecated model is likely to be less stable than the latest models.

On notice periods, it says the retirement of publicly released models is announced at least 60 days ahead. The span from this announcement (September 30) to the retirement date (November 30) meets that standard.

How long can I use the other Claude models?

From the same documentation's status table, here are the earliest retirement times for the models in use today. This column is titled "tentative retirement date," and each value is a lower bound meaning "not earlier than this day." No retirement has been announced yet.

The lower-bound dates for Haiku 4.5 and Opus 4.5 are close. That does not mean those dates are the retirement dates. But once an announcement goes up, you will plan around the 60-day standard above, so it helps to list in advance the places in your code that use these model names.

Which models have already retired?

According to the documentation, claude-opus-4-1-20250805 retired on August 5, 2026, claude-sonnet-4-20250514 and claude-opus-4-20250514 on June 15, 2026, and claude-3-haiku-20240307 on April 20, 2026. If these names are still in your config files or environment variables, those calls may already be failing.

What in your code can break when you move to 5.5?

The September 28 entry in the Claude Platform release notes lists these points where code written for Sonnet 5 can break.

• Code that turned thinking off at the start: use {"type": "between_tools"} instead of thinking: {"type": "disabled"} (when effort is high or lower).
• Code that forces tool use: a 400 error occurs if tool_choice is any or tool.
• The old computer_20251124 tool: the Claude API and Google Cloud do not accept it.

The parameter table on the deprecations page has one more. It says setting temperature, top_p, or top_k to a non-default value causes a 400 error on Claude 4.7 and later models. If your 4.5-era code pins these values, that may be the first thing to break when you migrate. That 5.5 belongs to the 4.7-and-later group is my own reading of the order in which the documentation lists the models. For model-specific behavior, check the migration guide.

In what order should you check before retirement?

1. On the Console's Usage page, download a CSV with Export and look at usage by API key and by model. This shows which keys still use 4.5.
2. Search your code and config files for the string claude-sonnet-4-5-20250929.
3. Sort out the thinking, tool_choice, and sampling parameters above.
4. Send a few representative tasks to 4.5 and 5.5 with the same input and compare the output format and token usage.
5. If there are no problems, change the model name, and before the retirement date download the usage again to confirm no calls remain.

The retirement date can change with a new notice. Just before you start work, reopen the Model deprecations page and the release notes in the Claude Platform Docs. The documentation says affected customers are notified by email and in the documentation.