‘X9cV7Q’ turned out to be an invalid metric ID for my test account, but even accounting for that this updated API call:

https://a.klaviyo.com/api/events?include=metric&filter=equals(profile_id,'PROFILE_ID_HERE'),any(metric_id,['RFe5Su', 'SeUc96'])

still gives an invalid filter provided error

https://a.klaviyo.com/api/events?include=metric&filter=equals(profile_id,'PROFILE_ID_HERE'),equals(metric_id,'RFe5Su') works, though.

OK, issue found:

{"errors":[{"id":"f87eba95-26f4-4238-9af2-3f9fd0f06ee3","status":400,"code":"invalid","title":"Invalid input.","detail":"'any' is not an allowed filter operator for metric_id.  Allowed operators are equals","source":{"parameter":"filter"},"links":{},"meta":{}}]}

I was only able to get this helpful error message when I tried a single-item array in the any() filter: https://a.klaviyo.com/api/events?include=metric&filter=and(equals(profile_id,'PROFILE_ID_HERE'),any(metric_id,['RFe5Su']))

Then it gives a specific error message.

So, the answer:

1. you must use a valid metric_id, check your klaviyo dashboard to get this, it’s part of the URL when viewing a metric
2. You cannot have an any() filter for metric_id, you can only use equals()
3. therefore, you’ll have to iterate over any metric IDs you want to test for in your own code, issue a single API call per metric ID, and aggregate the results yourself
4. the working filter format is this: https://a.klaviyo.com/api/events?include=metric&filter=equals(profile_id,'PROFILE_ID_HERE'),equals(metric_id,'RFe5Su')

Glad you were able to solve this, @dartacus! 

The docs for each endpoint list out which filter operators are supported on each field. As you noted, only the equals operator is supported for the metric_id and profile_id fields for this endpoint. 

Also, metric_ids used must be valid for the account. A given metric, e.g. “Opened Email”, will have a unique metric_id for each Klaviyo account. Metric IDs can be retrieved via the Get Metrics API or are visible in the Klaviyo UI.