Claude’s new API version changes citation inputs and sampling parameters

A Claude SDK upgrade can now change which citation-bearing message histories the API accepts. Anthropic introduced API version 2026-10-21 on October 21, with stricter rules for document placement and citation inputs. Teams that replay saved conversations should inspect the request they actually send before adopting the new default.

The release notes identify a new API contract. This is separate from the model-specific changes covered in the Claude model migration guide; changing a model and changing an API version need separate compatibility checks.

Check the role that carries the evidence

Anthropic’s migration guide documents the relevant input rules. The matrix below summarizes the citation-related cases that can affect a research assistant’s saved history.

Separate sampling changes from citation changes

The new version removes temperature, top_p and top_k request parameters, including explicit default values. Default sampling remains available. An application that depends on non-default sampling can retain the older API version while planning its migration; support also depends on the selected model.

New major SDK releases send the new version by default. An SDK dependency update can therefore expose an old serialization assumption even when the application’s visible prompt has not changed. Inspect dependency versions and outgoing headers alongside the stored conversation.

Replay a small fixture set before rollout

Consider a fictional research assistant that stores a document, the user’s question, and Claude’s cited answer. A later export accidentally moves the document into the assistant turn. The earlier successful response does not prove the new API will accept the exported history.

• Keep one fixture with a document in the user turn and a cited answer in the assistant turn.
• Add separate negative fixtures for a direct assistant document and user text carrying a non-empty citations array.
• Test a returned web-fetch result independently, so its documented exception is not lost in a blanket rewrite.
• Compare the request header, serialized roles, and error response before comparing answer quality.

These are proposed migration checks. SearchEngineAnswer reviewed the documentation and did not execute API requests. A request that passes validation still needs a separate search-result evidence review; acceptance alone cannot establish that a cited passage supports the answer.