Cache diagnostics reports previous_message_not_found on the next turn

Symptom

diagnostics shows previous_message_not_found when previous_message_id is setRequest with only the cache-diagnosis-2026-04-07 beta header is accepted but gives no diagnostic history

Cause

Since September 9, 2026, the API stores a request's fingerprint only when the request includes the diagnostics object. Sending just the beta header stores nothing, so a later turn cannot find the earlier message.

Fix

Include the diagnostics object on every turn. Set previous_message_id to null on the first turn and to the prior message id after that. The beta header is no longer required as of September 23, 2026.

ApplyCopy

{"diagnostics": {"previous_message_id": null}} // first turn; use the prior message id on later turns

Affected

Claude API, from 2026-09-09

Sources

• https://platform.claude.com/docs/en/release-notes/overview

Last verified 2026-10-30 · Confirmed by 0 builders