Fetch the record (v8 Records API) and note its last-modified timestamp.
Send the update with `PUT /crm/v8/{module}/{id}` including an `If-Unmodified-Since` header set to that timestamp (RFC-formatted).
If the server timestamp is newer, the request returns HTTP 412 with code `ALREADY_MODIFIED`.
On 412, re-fetch the current record, reconcile the changes, and retry the update with the refreshed timestamp.
Known gotchas
Zoho CRM v8 uses the `If-Unmodified-Since` header (→ 412 `ALREADY_MODIFIED`) for optimistic concurrency, not `If-Match`/ETag.
The timestamp must match Zoho's expected format and reflect the record's actual `Modified_Time`; a malformed or stale value can either be ignored or cause every update to 412.
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