Anthropic retires Claude Sonnet 4.5 (claude-sonnet-4-5-20250929) on 30 November 2026. After that, requests to that model ID on Anthropic's API fail. Anthropic recommends Claude Sonnet 5.5 as the replacement (Anthropic's notice).

The ID rarely sits in one place. Check:

  • application code and SDK calls
  • .env.example and other environment defaults
  • Terraform variables and Helm values
  • notebooks and eval scripts
  • fallback paths that only run when something else fails

The free way 

rg -n "claude-sonnet-4-5" --hidden -g '!node_modules'

It misses IDs built from parts (f"claude-sonnet-{ver}") and aliases, so read the hits around your config loaders too.

On Bedrock or Google Cloud?

Those platforms set their own retirement dates, which can differ from Anthropic's. Check your platform's page.

The automatic way

We built SunsetLint, a GitHub App that runs this check against 59 retirement rules (Anthropic, OpenAI, Google Gemini), each taken from the provider's own notice. It opens one issue per repo with file, line, date and replacement, and it's read-only. Here's what it posts on our demo repo: issue #1.

Free for public repos and 3 private repos.