Claude Sonnet 4.5 Retirement Is Nov 30: What to Change Before Your API Calls Break

The Claude Sonnet 4.5 retirement is scheduled for November 30, 2026, and Anthropic already marked the model deprecated on September 30. If your app still calls claude-sonnet-4-5-20250929, you have about five weeks to move to claude-sonnet-5-5. The bigger surprise may be an API change that starts today.

Here are the facts worth knowing before you open your code editor:

• Sonnet 4.5 was deprecated on September 30, 2026, and its retirement date is November 30, 2026 (marked tentative).
• Anthropic recommends claude-sonnet-5-5 as the replacement.
• Deprecated models still work until retirement, then requests fail.
• Anthropic lists API version 2026-10-21 as removing temperature, top_p and top_k.
• Claude Haiku 4.5 is still active, with no deprecation announced.

That mix of dates is easy to miss, because one deadline is a calendar date and the other is a request that can start failing on its own.

What the Claude Sonnet 4.5 retirement means for your app

Deprecated is not the same as retired. Right now the model still answers requests, but Anthropic no longer recommends it. Once the retirement date passes, calls to that model ID fail, so a chatbot, script or internal tool that hard-codes it simply stops working.

The date is marked tentative on Anthropic’s model deprecations page. Treat it as the deadline anyway. Moving early costs you an afternoon of testing, while moving late costs you an outage.

If you use Claude through a partner such as Amazon Bedrock or Google Cloud, check their pages too. Anthropic notes that partner platforms set their own schedules.

Which Claude models are on the clock

Not every older model is leaving at once. This table shows where each one stands, based on Anthropic’s own deprecations page. Dates marked “not sooner than” are the earliest possible retirement, not a confirmed one.

Why this matters: Opus 4.5 has the next-closest date, so if you run both Sonnet 4.5 and Opus 4.5, plan one migration sweep instead of two. Haiku 4.5 has the earliest “not sooner than” date, but Anthropic has announced no deprecation for it.

We covered the newest small model in our Claude Haiku 5.5 pricing and benchmarks post, which helps if you want a cheaper replacement for lighter jobs.

The October 21 API change that can break requests first

Anthropic’s deprecations page says temperature, top_p and top_k are removed in API version 2026-10-21. Requests that include them return a 400 error, and Anthropic’s advice is to leave them out and steer behavior through prompting.

Newer models already behave this way. Anthropic’s Sonnet 5.5 migration guide lists sampling parameters among settings that return a 400 error, and tells you to remove all three when moving from Sonnet 4.6 or earlier.

That is the catch when you swap model names. Code that worked fine on Sonnet 4.5 can fail on claude-sonnet-5-5 simply because a wrapper library adds temperature=0.7 by default. Search your code for all three names, including inside config files and helper functions.

I could not find a public note saying exactly which accounts get the new API version automatically. Check which anthropic-version value your SDK sends, and test before you assume you are safe.

A simple Sonnet 4.5 migration checklist

You do not need a big project for this. Work through these steps in order, and test each one on a staging key before touching production:

1. Find every place the model ID claude-sonnet-4-5-20250929 appears, including environment variables and saved configs.
2. Remove temperature, top_p and top_k from those requests.
3. Switch the model to claude-sonnet-5-5 and run your usual test prompts.
4. Compare answer quality, speed and cost, because a new model can change all three.
5. Ship the change well before November 30, and keep a rollback plan for a week.

Why this matters: step two is the one most teams skip. It is also the one that turns a quick model swap into a surprise error at 2 a.m.

Anthropic’s Sonnet 5.5 guide also suggests using output_config.effort to control how hard the model works. Start with the medium setting and adjust from there. For related cost changes on the premium side, see our note on Claude Opus 5.5 fast mode pricing.

Frequently Asked Questions

When does Claude Sonnet 4.5 stop working?

Anthropic lists November 30, 2026 as the tentative retirement date. After that, requests to the Sonnet 4.5 model ID are expected to fail.

What should I use instead of Sonnet 4.5?

Anthropic recommends claude-sonnet-5-5. Claude Sonnet 4.6 is also still active, with the earliest retirement listed as February 17, 2027, so it can work as a short stop-gap.

Is Claude Haiku 4.5 being retired too?

Not yet. Anthropic lists Haiku 4.5 as active with no deprecation announced. Its earliest possible retirement is October 15, 2026, but that is a floor, not a scheduled shutdown.

Why does my request return a 400 error after changing models?

The usual cause is a leftover temperature, top_p or top_k value. Remove them and resend the request.

My take: do the swap this week, not in late November. The model change itself is easy, but the removed sampling parameters are what bite, and you want to find that on a quiet Wednesday rather than during a holiday freeze. Pin your versions, strip the old settings, and treat every “tentative” date as final.