App cards (UI extensions) have no REST API; they are built as a developer project with the HubSpot CLI on platform 2026.09.
Source pending
More facts from this area
- Classic CRM cards (the
/crm/v3/extensions/cardsAPI) were deprecated on June 16, 2025.Documented - Classic CRM cards are sunset on November 2, 2026 and must be rebuilt as app cards; the migration UI and migrate API run until December 1, 2026. This replaces the earlier October 31 date.Corrected
- App cards can't use
windoworfetch; outbound calls go throughhubspot.fetchto URLs listed inpermittedUrls.fetch.Source pending - Classic CRM cards stop working on October 31, 2026.Documented
Spriggit uses these facts when it plans changes. See something out of date? Email [email protected]