Upcoming breaking change: Marketing Event property updates

Developer Support Developer Announcements
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On July 6th, 2026, we’ll be updating the property definitions for 11 of the standard properties for Marketing Events to be read only. Specifically, this will mean that the readOnlyDefinition field for the properties will be set to true, and any attempts to update the property definitions will result in an error being returned.

Which properties are changing?

The affected properties are listed below. Each property includes the label and API name for the property.

  • Attendees - hs_attendees
  • Cancellations - hs_cancellations
  • End date - hs_end_datetime
  • Event cancelled - hs_event_cancelled
  • Description - hs_event_description
  • Name - hs_event_name
  • Organizer - hs_event_organizer
  • Type - hs_event_type
  • URL - hs_event_url
  • Registrations - hs_registrations
  • Start date - hs_start_datetime

How will it work?

The listed properties will become read-only on July 6th. Changes can still be made to these properties up until that date, at which point they will be set to read-only with the existing definition. See this product update for more information, including instructions being sent to account administrators.

If your integration currently modifies these standard properties, we recommend updating your integration to work within the default property definitions. If your integration needs flexibility or functionality beyond the standard definitions, you will need to migrate to custom properties. If your users reach out requesting specific changes, direct them to the product update which will have further instructions for customers.

Why is this changing?

These default properties are used across apps listed in the HubSpot Marketplace. Changes to the type (i.e. data type) for these properties has led to conflicts and errors for customers using multiple apps for their marketing events. Standardizing these default properties will make it easier for app builders to use the properties that are included in every HubSpot account without needing to worry about custom functionality from other sources, while still allowing customization using custom properties.

Please let us know if you have any questions or comments by replying below.

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Hey @Dadams !
Do these changes affect the following fields

eventName, eventType, startDateTime, endDateTime, eventDescription

in this request:

POST https://api.hubapi.com/marketing/marketing-events/2026-03/events{"endDateTime": "2025-12-17T06:30:00Z","eventCancelled": false,"eventDescription": "test description","eventName": "test 111","eventOrganizer": "test org","eventType": "test type","externalEventId": "111","startDateTime": "2025-12-11T05:30:00Z","externalAccountId": "111"}

Or do they only affect hs_* fields?

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@Dadams could you give me response on my previous question?