1. A method comprising: at a multi-tenant work distribution system: responsive to a first work item API request provided by a first external application server system of a first account of a plurality of accounts of the multi-tenant work distribution system, determining a first worker identifier of a plurality of worker identifiers configured for the first account to be assigned to a first work item corresponding to the first work item API request; providing first assignment information to the first external application server system via a first callback configured for the first account, wherein the first assignment information specifies at least the first worker identifier and work item attributes of the first work item, wherein the work distribution system receives the first work item API request from the first external application server system via an application protocol communication, wherein the work distribution system provides the first assignment information to the first external application server system via an application protocol communication.