Anthropic retires claude-sonnet-4-5-20250929 on the Claude API on Nov 30, 2026. After that date, requests to it fail. Anthropic's recommended replacement is claude-sonnet-5-5.

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The facts (from official pages)

Model ID Platform Shutdown Replacement
claude-sonnet-4-5-20250929 Anthropic Claude API Nov 30, 2026 claude-sonnet-5-5 (source)
anthropic.claude-sonnet-4-5-20250929-v1:0 AWS Bedrock Apr 8, 2027 not listed (source)

Anthropic deprecated claude-sonnet-4-5-20250929 on Sep 30, 2026 and retires it on the Claude API on Nov 30, 2026 (Anthropic model deprecations).

AWS Bedrock is different: anthropic.claude-sonnet-4-5-20250929-v1:0 was marked deprecated on Oct 8, 2026 with a shutdown date of Apr 8, 2027. AWS does not list a replacement on that page (AWS Bedrock model lifecycle).

Google Vertex AI publishes its own retirement dates for partner models — check Google's page directly. Do not assume it matches the Claude API date.

Step 1: Find every place you call it 

rg -n --hidden -g '!node_modules' 'claude-sonnet-4-5' . rg -n --hidden -g '*.{yml,yaml,json,toml,tf,tfvars}' -g '.env*' 'sonnet-4-5' .

Check CI workflows, Dockerfiles, notebooks, eval configs, LLM gateway/router configs, feature flags and prompt registries. On Bedrock, the ID looks like anthropic.claude-sonnet-4-5-20250929-v1:0 (often with a region or inference-profile prefix); search for sonnet-4-5 to catch every form.

Step 2: Map each call to its platform and date

The same model retires on different days depending on where you call it. Write down, per call site: platform (Claude API / Bedrock / Vertex / gateway), model ID, shutdown date, owner.

Step 3: Test the replacement

Run claude-sonnet-5-5 against a frozen set of real prompts and compare quality, refusals, latency and cost.

  • Tool use: confirm every tool call still validates against your schemas and that multi-step tool loops end the same way.
  • Structured output: parse 100% of responses with your real JSON parser, not by eye.
  • Parameters: re-check max output tokens, thinking/extended-reasoning settings and any beta headers against Anthropic's docs for the new model.

Step 4: Roll out in stages, well before Nov 30

Put the model ID in config, not code, so rollback is one change. Shadow traffic first, then 1% → 10% → 50% → 100%. Aim to finish at least a week before Nov 30, 2026.

Step 5: Guard against the day-after failure

Alert on model-not-found / 404 errors from the Anthropic SDK. Add a CI check that fails if claude-sonnet-4-5-20250929 appears in the repo after the cutover.

Free checklist + alerts

Dates come from official provider pages. If a date looks wrong, open an issue against the source page first — that page is the source of truth.