Saves a new draft head for the page from a full item list. Live content is never changed and no cache is invalidated. Publishing is a separate call that needs content:publish.

Workflow. Drafts and publishing work together like this:

1. GET /projects/{projectId}/page-drafts/{pageId} reads the editable content and its revision tokens.
2. PUT on the same path saves a new draft head (versionId). Live content is not touched.
3. Publish the draft with POST /publish-versions/{versionId}, or schedule it with POST /versions/{versionId}/schedule.
4. Manage history and the schedule with GET /projects/{projectId}/versions, /versions/{versionId} (read, reschedule, replace content, cancel), publish-now and restore.

Body

• items: the complete item graph, 1 to 200 items. Each item needs id and type; children are { "id": ... } references and data is an object. Every child must reference an item in the list, ids must be unique, and every item must be reachable from the root. A malformed body is 400; a broken graph (duplicate ids, dangling child, orphaned items) is 404 PAGE_NOT_FOUND. Nothing is saved in either case.
• The root is the item whose id matches the page's current rootId; otherwise the first item.
• name (optional): renames the draft. Defaults to the live page name.

Revision fence. All three tokens are required and must be copied exactly from the GET response:

• expectedHeadVersionId: GET's headVersionId (null means "no draft yet").
• expectedPageUpdatedAt: GET's pageUpdatedAt, the exact ISO string or null.
• expectedProjectUpdatedAt: GET's projectUpdatedAt, the exact ISO string or null.

null is a real baseline for a legacy row and only matches a row that is still null. If the draft head, the live page or the project moved since your GET, the save is refused with 409 STALE_PAGE and nothing is written. Re-read with GET, reapply your edit and save again.

Retries. Retrying a save that already committed returns the same head instead of a conflict.

Response. versionId is the new draft head. Pass it to POST /publish-versions/{versionId} or POST /versions/{versionId}/schedule, and send it as expectedHeadVersionId on your next save or discard.

Required scope: content:write