mailchimp: how to set a merge var value for a specific list member

Question

What API in mailchimp should I use to set a merge var value for a specific list member?

If I'm adding a new list member, I can use lists/subscribe to set a merge var value for that new member. However, I do not see an API to change the value if the list member already exists.

The API lists/merge-var-set sets the value for all list members. And lists/merge-var-update sets attributes for the merge var.

What is the API for setting a specific value?

Answer 1

In Mailchimp API v3 you can set a merge field value this way:

PUT {api_endpoint}/3.0/lists/{list_id}/members/{member_hash}

{
    "merge_fields": {
        "BIRTHDAY": "01/23"
    }
}

Where {member_hash} is md5 hash of member's email address.

You don't have to provide all merge fields. Mailchimp will update only those you've specified.

Answer 2

In APIv2, you still use lists/subscribe, but you pass update_existing as well. This is considerably more intuitive in APIv3, which you should use if you can, since 2.0 is officially deprecated.