Claude Sonnet 4.5 retires Nov 30: migrate to Sonnet 5.5 now
Claude Sonnet 4.5 retires from the Claude API on November 30, 2026, and requests to it will fail. Sonnet 5.5 is cheaper, but test your tool calls first.
If any script, agent or integration you run still calls claude-sonnet-4-5-20250929, you have four weeks. Anthropic's Claude Sonnet 4.5 retirement is set for November 30, 2026 on the Claude API. After that date, requests to it do not get rerouted. They fail. The replacement, Sonnet 5.5, costs less per token, so the move pays for itself. The risk is the code you forgot about.
What actually happened
Anthropic told Sonnet 4.5 developers on September 30, 2026 that the model would retire. Its model deprecations page lists Sonnet 4.5 as deprecated, with November 30, 2026 in the "tentative retirement date" column, and names claude-sonnet-5-5 as the recommended replacement. The same page is blunt about what retired means: "Requests to retired models fail."
The price moves in your favor. On Anthropic's pricing page, Sonnet 4.5 is $3 per million input tokens and $15 per million output. Sonnet 5.5 is $2 and $10. Cache hits drop from $0.30 to $0.10 per million.
One more Nov 30 deadline sits in the release notes. Fast mode for Claude Opus 4.8 is deprecated, and after November 30, requests to claude-opus-4-8 with speed: "fast" return an error. Anthropic points those users to Opus 5.5.
Why it matters for your business
The hard part is finding the calls. Model IDs hide in places nobody reads: a Zapier step, an env var on a forgotten Lambda, a vendor plugin, a cron job someone wrote in 2025. Grep your repos and your automation platforms for sonnet-4-5 and opus-4-8 today. Check your Anthropic console usage by model too. It shows calls your code search will miss.
A new model is a new model. Anthropic publishes a Sonnet 4.5 to 5.5 migration guide, and it lists behavior changes. Some touch tool use. If your agent forces a tool call with tool_choice, read that section before you switch. Run your real prompts against Sonnet 5.5 and compare outputs before you change the production string.
Cloud resellers run their own clocks. If you call Claude through Amazon Bedrock or Google Cloud Vertex AI, check that provider's model page for its date. Do not assume it matches Anthropic's.
Put the model ID in config, not code. Every model on that deprecations page has a date or will get one. One setting per environment turns the next one into a five-minute change.
Key takeaways
- Claude Sonnet 4.5 retires from the Claude API on November 30, 2026, and requests to it will fail
- Anthropic recommends Claude Sonnet 5.5, priced at $2/$10 per million tokens against $3/$15 for Sonnet 4.5
- Opus 4.8 fast mode is removed the same day, and
speed: "fast"requests will error - Read the migration guide's tool-use changes and test real prompts before you switch
- Bedrock and Vertex AI set their own retirement dates, so check yours
Not sure where your Claude calls live? We audit your stack for hardcoded model IDs, move them to config, and run your prompts against the replacement before cutover. See how we build model-agnostic systems, or send us your stack.
Sources: Anthropic: Model deprecations, Anthropic: Pricing, Anthropic: Release notes.
- #anthropic
- #claude
- #sonnet-4-5
- #model-deprecation
- #api-migration
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