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Claude Sonnet 4.5 Will Be Retired on 30 November 2026: How to Migrate

Anthropic has deprecated Claude Sonnet 4.5 (claude-sonnet-4-5-20250929) and will retire it on the Claude API on 30 November 2026, recommending Claude Sonnet 5.5. What retirement means for your app, how to find hard-coded model IDs, and the code changes Sonnet 5.5 needs.

On 30 September 2026, Anthropic announced the deprecation of Claude Sonnet 4.5 (claude-sonnet-4-5-20250929). Its retirement on the Claude API is scheduled for 30 November 2026. Anthropic recommends migrating to Claude Sonnet 5.5.

What retirement means

After the retirement date, API requests naming claude-sonnet-4-5-20250929 will fail. Anything that hard-codes that ID — your app, a background job, an n8n workflow, a script — will start returning errors. It may remain available on some cloud platforms for longer; check your provider if you use Bedrock or Google Cloud.

Find every reference

grep -rn "claude-sonnet-4-5" --exclude-dir=node_modules --exclude-dir=.git .

(The grep Command)

Also check environment variables on your servers, configuration in automation tools, and any database tables storing a model name. Better still: put the model ID in one environment variable so the next migration is a one-line change. (Environment Variables Explained)

What changes when you move to Sonnet 5.5

Sonnet 5.5 is cheaper than 4.5 ($2 / $10 vs $3 / $15 per million tokens), but it isn't a drop-in swap:

  • Thinking is adaptive. Responses can include thinking blocks — select text content by type, not by position.
  • Forced tool use returns a 400 error. Remove tool_choice settings that force a specific tool.
  • The newer tokenizer counts the same text as roughly 30% more tokens, so revisit max_tokens and cost estimates.
  • Computer use requires the newer toolset.

Anthropic's Sonnet 5.5 migration guide has a section specifically for moving from Sonnet 4.5. (Claude Sonnet 5.5 Released)

A safe migration

  1. Switch a staging environment to claude-sonnet-5-5.
  2. Run your evaluation set — or at least your 20 most important real prompts — and compare outputs. (Evaluate LLM Outputs)
  3. Fix response parsing and tool-choice code.
  4. Roll out to production with a quick way back (a feature flag or an env var). (Feature Flags)
  5. Do it in October, not the last week of November.

Source: Claude Platform release notes · Model deprecations

EasySpawn gives Claude Code your whole codebase on a persistent server, so "find every hard-coded model ID and migrate it" is a task you can hand over and review as one pull request. See how it works or join the waitlist.

Related: OpenAI API vs Claude API · Claude API Pricing Explained · Update Dependencies Safely · Claude Haiku 5.5 Released

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