Claude API adds 2026-10-21 version with three breaking changes
11/06/2026 — 11/06, 14:18·1 sources (all)·1 reports (all)
Key takeaways
- Anthropic added a 2026-10-21 version to the Claude API, the first update to anthropic-version since 2023-06-01. The Messages API carries thr…
- the Admin API and enterprise features see separate adjustments.
Background & analysis
On November 6, 2026, DEV Community reported that the Claude API has introduced version 2026-10-21, the first new anthropic-version value since 2023-06-01. The Messages API carries three breaking changes: the temperature, top_p, and top_k parameters now return a 400 error, document blocks in assistant messages return a 400, and citations in text blocks of user messages also return a 400. In addition, several fields across the Admin API and the Enterprise Analytics API have been renamed. To enable the new version, developers can send anthropic-version: 2026-10-21 or upgrade to the latest version of the API. This is the only public report available so far, and there is no information yet on what happened after developers started adopting it.
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Reports on this story headlines open the original
Anthropic has introduced a new Claude API version, 2026-10-21, the first change to anthropic-version since 2023-06-01. The Messages API now returns a 400 error for the temperature, top_p and top_k parameters, for document blocks in assistant messages, and for citations in text blocks of user messages; several Admin API and Enterprise Analytics API fields are also renamed. Callers opt in by sending anthropic-version: 2026-10-21.
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