## Problem
A MailChimp batch job upserting subscribers failed 100%: total_operations 110, errored_operations 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 'status_if_new' => '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