Summary fields are refused headlessly with capability dwgprops before any write; custom properties are $CUSTOMPROPERTYTAG/$CUSTOMPROPERTY pairs and survive save + reopen. Validation is shared, by name, and never str()-coerces. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01FKFStjEFzpGyLq5bMpWxTd