Is it at all possible to propagate custom event properties through other events, such as email opens?

To follow up on Brian’s note, you can get the ID of the flow, for example, that triggered the email. It is returned in the event properties when you call Get Events:

"Campaign Name": "Abandoned Cart: Email 1",

"$flow": "SbpWYd",

If you know the metric that triggered the flow — Checkout Started in this case — you could find the events for this metric for the profile in question — e.g.

https://a.klaviyo.com/api/events/?filter=equals(metric_id,"QhyxVR"),equals(profile_id,"01H105A9SB52KBSK23XTDZYXQQ")

— and then get the properties of the event that triggered the flow.

There’s also this blog post showing how to use webhooks to save properties of events to profile properties:

https://www.klaviyo.com/blog/klaviyo-event-properties-sms-email

You could then fetch an array of the properties for all the events of the metric for a given profile.

What sort of custom event is this? That might help guide another approach. Is it an event that a profile might have often, or more a “one-off” sort of event?

Two additional possible approaches, again neither of which will likely directly do what you’re looking for.

• in the flow triggered on the custom event, use a webhook to set a profile property that has the property or properties from the event, along with the ID of the email about to be sent (you can get the email ID by editing the URL and fetching it from the URL, e.g. https://www.klaviyo.com/email/flow/VLfHZb/content)
• update a profile property when someone click an email link: https://help.klaviyo.com/hc/en-us/articles/115005255248
  This would let you quickly look at a profile and see if they’ve clicked a specific link.

Hi, we’ll add it as a feature request for our product managers, to be able to associate email events (e.g. Clicked Email) with the event that triggered the flow that sent the email.

For now, you’ll have to use one of the approaches in this thread.