I noticed the REST API doesn't support special characters such as the @ symbol in External Ids, either in POST/create or GET/query. Is this by design? Other import mechanisms allow (or have previously allowed?) the @ symbol.

Hi, yes, it is by design currently.

do RESTlets or CSV Import or SOAP or SuiteScript support the @ symbol in external id? If yes, then REST needs to at least be able to query based on that character.