Stock @tiptap/extension-image carries no caption attribute, so markdownToProseMirror through docmostExtensions dropped the data-caption the client emits, breaking the lossless claim. Extend the Image node (mirroring editor-ext image.ts and the nearby Highlight extend) to parse/render data-caption. Rebuilt build/. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>