The final countdown for Claude 3.5 Sonnet
I was reading the latest AWS release notes when one line made me stop and reread.
“Claude 3.5 Sonnet has entered legacy status on Amazon Bedrock and will lose public extended access on December 1, 2025, before reaching end of life on March 1, 2026.”
That caught my attention because many of my AI workflows, such as multi-step agents, automated summaries, and background jobs, still run on Claude 3.5 Sonnet through Bedrock. My first instinct was, “No rush, I’ll handle it later.” But the more I thought about it, the more the risk became obvious. This was not a minor version bump, and ignoring it could break everything.
If you run AI workloads on Amazon Bedrock with Claude 3.5 Sonnet, its retirement will impact your workflows as much as mine. So, unless you start planning the migration now, you will face the same risks I did.
This newsletter will discuss the key changes developers need to know, the migration challenges you will likely face, and the steps you can take to move without disrupting your workflows. By the end, you will know how to plan, test, and deploy Claude Sonnet 4 so your AI systems keep running smoothly and take advantage of its new capabilities.
How the retirement impacts your Bedrock workloads#
Even though Claude 3.5 Sonnet is still answering API calls, its “legacy” status on Bedrock already changes a few things behind-the-scenes. It no longer receives performance or reliability updates, and its support window is shrinking. That means if you’re running multi-step agents, batch summarization jobs, chatbots, or any other automated flow on Bedrock, you’re already operating on a model that won’t be fixed if something breaks.
When the endpoint finally shuts off in March 2026, Bedrock will not automatically route your calls to Claude Sonnet 4. Unless you migrate, those agents and jobs will start throwing errors. The sooner you test Claude Sonnet 4 against your current workloads, the easier it will be to smooth out responses, latency, and cost differences before the deadline.
Migrating from Claude 3.5 Sonnet to Claude Sonnet 4 on Bedrock#
Now that you know why retirement matters, the next question is what to do about it. Migrating early gives you control over the process instead of scrambling close to the deadline. Below is a refined playbook that gives you a systematic way to migrate.
