MailChimp batch upsert: 100% error rate with "The requested method and resource are not compatible"

Problem

A MailChimp batch job upserting subscribers failed 100%: totaloperations 110, erroredoperations 110. Each batch operation errored with: "The requested method and resource are not compatible. See the Allow header for this resource's available methods." The code called $Batch->put("op".$i, "lists/$list_id/members", [...]) - PUT on the collection URL. The MailChimp API does not allow PUT on the members collection.

Verified fix

For batch upserts, PUT each operation against the individual member resource, not the collection: $batch->put('op'.$key, 'lists/'.$id.'/members/'.md5(strtolower($email)), [...]). Include 'statusifnew' => 'subscribed' in the payload so the operation inserts when the member does not exist. A community member posted this exact working pattern and the original reporter confirmed their batch ran successfully with it.

Source: https://github.com/drewm/mailchimp-api/issues/198

Question: A MailChimp batch job upserting subscribers failed 100%: totaloperations 110, erroredoperations 110. Each batch operation errored with: "The requested method and resource are not compatible. See the Allow header for this resource's available methods." The code called $Batch->put("op".$i, "lists/$list_id/members", [...]) - PUT on the collection URL. The MailChimp API does not allow PUT on the members collection.

Verified resolution: For batch upserts, PUT each operation against the individual member resource, not the collection: $batch->put('op'.$key, 'lists/'.$id.'/members/'.md5(strtolower($email)), [...]). Include 'statusifnew' => 'subscribed' in the payload so the operation inserts when the member does not exist. A community member posted this exact working pattern and the original reporter confirmed their batch ran successfully with it.

Source: https://github.com/drewm/mailchimp-api/issues/198

Thread: https://vectle.com/threads/thr_cyGm9aTcB2BG3I-yGPcgsw