Claude API 2026-10-21: What now returns a 400 error

What changed

A routine dependency bump could start failing your Claude calls. Each SDK sends the API version that was current when its major version came out, so moving to a new major version also moves you to the new API version.

What changed

Earlier SDK and CLI versions sent 2023-06-01 with every request. The new major versions of the Python, TypeScript, C#, Go, Java, PHP and Ruby SDKs and the ant CLI send 2026-10-21, including their Amazon Bedrock and Google Cloud clients.

If you call the API without an SDK, you switch by setting the anthropic-version header to 2026-10-21 yourself.

What breaks on the new version

• The sampling settings are gone. Sending temperature, top_p or top_k returns a 400 error, even at the default value. Models that already accept only the defaults sample as before, but on models that accept other values, a lowered temperature loses its effect and nothing replaces it.
• A document block placed directly in an assistant message returns a 400 error. Send documents in user messages instead.
• A text block with a non-empty citations array in a user message returns a 400 error.
• Some Admin API and Claude Enterprise Analytics API fields drop their old names. API keys, for example, replace workspace_id with a scope object that holds different values: a key bound to a workspace, the Default Workspace included, has that workspace’s real ID in scope.workspace_id, and an all-workspaces key has no workspace ID. Code that treated a null workspace_id as the Default Workspace needs a change.

What to do

1. Remove temperature, top_p and top_k from your requests, or keep a request that relies on a non-default value on 2023-06-01.
2. Upgrade the SDK and make the code changes its new major version needs. The migration guide lists them per language, including new Go import paths.
3. Test against 2026-10-21 before you release. On Amazon Bedrock, older models don’t support the new version, so the guide says to stay on your current SDK for those calls.
4. If you track dependency licenses, update the entry for the switch to Apache 2.0.