Auto-Number Field Not Available as External ID for Upsert in Salesforce Data Loader

An Upsert is a Data Loader command that either inserts a new record (if no match is found) or updates an existing record (if a match is found), based on a matching external ID field. Auto-Number fields are system-generated sequential numbers automatically assigned by Salesforce — they cannot be manually set or overwritten.

Why Auto-Number Is Not Available for Upsert in Data Loader

While it is technically possible to designate an Auto-Number field as an external ID, Data Loader does not support using it for the Upsert operation. Here is why

• The Upsert command may need to insert new records if no match is found. Since Auto-Number fields are system-generated and cannot be manually inserted or created, Data Loader cannot allow matching by this field
• Data Loader only displays fields that are either creatable or updatable in the external ID dropdown. Since Auto-Number fields are neither creatable nor updatable, they do not appear as options.

As a work around, consider changing the field type can be switched to Text for the time of the update and then back to Auto number again. It is strongly recommended that you backup the data prior to each field type change in case unexpected results occur during the change.