Claude API version 2026-10-21

As of: 2026-10-22 · Sources: API versions, migration guide

The SDK release notes only say "add support for new API version 2026-10-21". What changes:

Messages API (any client sending anthropic-version: 2026-10-21)

• temperature, top_p and top_k are removed. A request with any of them returns 400 on every model, even at the default value. Remove them. Requests sample with the defaults (temperature 1.0). Claude 4.7 and later models already accept only the defaults; on older models such as Claude Sonnet 4.6, code that lowered temperature loses that effect, and nothing replaces it. If a request truly needs a non-default value, keep that request on 2023-06-01.
• A document block placed directly in an assistant message returns 400. Send documents in user messages.
• A text block with a non-empty citations array in a user message (also inside search_result) returns 400. Citations in assistant messages, and "citations": {"enabled": true} on documents, still work.

For more consistent output without temperature: a JSON schema / structured output that only allows the valid values, a precise prompt, and caching identical inputs.

SDKs

Behaviour of the new majors:

• temperature, top_p, top_k are gone from the types. Python 2.0 raises TypeError: Messages.create() got an unexpected keyword argument 'temperature' before sending.
• The API key goes out as Authorization: Bearer <key>, not X-Api-Key (the API accepts both). Proxies, gateways and middleware that read x-api-key must read authorization.
• One credential per client. Passing two (api_key + auth_token, apiKey + credentials…) raises ValueError (Python) or TypeError (TypeScript).
• MessageParam is a union of UserMessageParam | AssistantMessageParam | SystemMessageParam. Dict/object literals with a fixed role need no change; calling MessageParam(...) in Python raises TypeError; a TypeScript interface X extends MessageParam must become a type alias.
• Text Completions (client.completions, HUMAN_PROMPT, max_tokens_to_sample) is removed (TS, Go, Java, Ruby, CLI). Use client.messages.create.
• betas is removed from the non-beta Models methods: use client.beta.models.list({ betas }).
• TypeScript tool runner: compactionControl is removed; use server-side compaction (context_management with compact_20260112).

Watch out

• pip install anthropic / npm install @anthropic-ai/sdk without a version now installs the new major. An unpinned anthropic in requirements.txt upgrades on the next build. ^0.x in package.json stays on 0.x.
• On Amazon Bedrock, 2026-10-21 works only for Claude 4.7 and later; stay on the old SDK major (or the old header) for older Bedrock models.
• Raw HTTP clients that send anthropic-version: 2023-06-01 keep working on that version; switch the header and remove the parameters together.

Full write-up with the tests: Claude API 2026-10-21: what breaks when you upgrade.