Claude Sonnet 4.5 (claude-sonnet-4-5-20250929) retires on the Claude API on November 30, 2026. What breaks, what should I migrate to, and how do I find every place that still uses it?

What it is: Anthropic deprecated Claude Sonnet 4.5 on September 30, 2026 and will retire it on the Claude API on November 30, 2026. After that date, requests that name claude-sonnet-4-5-20250929 (or an alias that resolves to it) fail with an error instead of returning a response. Anthropic's recommended replacement is claude-sonnet-5-5. The dates apply to Anthropic-operated platforms (Claude API, Claude Platform on AWS, Anthropic-managed Google Cloud, Microsoft Foundry); Amazon Bedrock and Google-managed Vertex AI set their own schedules, so check their model tables separately. Am I affected: - Any code, config, agent framework setting, prompt-eval harness or environment variable that names claude-sonnet-4-5 (search your repos and deployment configs for 'sonnet-4-5' and 'claude-sonnet-4-5'). - To find usage you forgot about: in the Claude Console go to Usage > Export; the CSV breaks usage down by API key and model. Fix: 1. Switch the model id to claude-sonnet-5-5 (or another active model that fits your cost and latency needs) and test before November 30, not on the day. 2. Follow Anthropic's migration guide: newer models can reject request shapes older ones accepted. In particular, temperature, top_p and top_k are removed in API version 2026-10-21 (400 error) and are rejected on Claude 4.7+ models when set to non-default values even on earlier API versions; remove them and steer behavior with the prompt instead. Newer SDKs default to the 2026-10-21 API version, so upgrading the SDK and the model at the same time can surface both changes at once. 3. Re-run your evals and check token usage: newer models use a different tokenizer, so token counts and costs per request can change; adjust max_tokens and budget alerts. 4. Make the model id a single config value, and alert on model-not-found / 4xx errors so the next retirement doesn't cause an outage. Verify: the usage export shows no traffic to claude-sonnet-4-5-20250929, and your test suite passes against the replacement model. Sources: https://platform.claude.com/docs/en/about-claude/model-deprecations https://platform.claude.com/docs/en/about-claude/models/migration-guide https://platform.claude.com/docs/en/api/migration-2026-10-21 https://codersera.com/blog/claude-sonnet-4-5-retirement-migration-2026/