Claude Sonnet 4.5 retires on November 30: Check the model IDs in use first

Anthropic announced the retirement schedule for the Sonnet 4.5 model in the Claude API. According to the official documentation, claude-sonnet-4-5-20250929 was deprecated on September 30, 2026, and its retirement date is November 30. After retirement, requests calling this model will fail. If this model name is included in code or configuration files, it must be changed before that date.

1. Model ID to change and replacement model

The official table specifies claude-sonnet-5-5 as the recommended replacement for claude-sonnet-4-5-20250929. Search separately for locations using the full ID with the date and those using the short name. Even within the same service, different workers often use different model names.

2. Remaining time and platform differences

Anthropic states in its documentation that it provides notice at least 60 days before retiring a publicly available model. There are 61 days from the September 30 announcement to November 30. However, these dates apply to the Claude API, Claude Platform on AWS, and Microsoft Foundry. Amazon Bedrock and Google Cloud maintain their own model tables and schedules. If you use either platform, check its table separately.

3. Finding actual usage

Searching the code alone can easily miss cron jobs, old batch scripts, and configurations owned by other teams. In the Claude Console's Usage page, click “Export” to download a CSV containing API key and per-model usage. Keys with records showing use of claude-sonnet-4-5-20250929 in this file are replacement targets. Assigning an owner to each key can reduce omissions.

4. Three things to check before replacement

First, compare the outputs of the existing and replacement models using the same set of inputs. The priority is to check whether the response format, JSON structure, or tool-calling method has changed.

Second, measure response length, latency, and cost together. When the model changes, token usage for the same task may also change.

Third, prepare a configuration to revert in case of failure. Extracting the model name into an environment variable allows you to revert without a deployment. This is a practical recommendation, not Anthropic's official procedure.

5. Related schedule and previous article

The same document tentatively lists the retirement date for Claude Opus 4.5 (claude-opus-4-5-20251101) as “after November 24.” Because this is not a confirmed date, check again when an announcement is issued. When changing models, also review the agent's permission scope. Previous article: Why Claude behaved unexpectedly on an actual website and how to manage AI agent permissions ↗

Source: Anthropic, Model deprecations (Claude Platform documentation, September 30, 2026 announcement entry) ↗

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