S        This expression returns the received text message.

This text will be blank when no Text/Any Message event is running, although text messages can also be blank.


Relay Client:

Relay does not do any tests at all for the content; if there are embedded nulls, the string will simply cut off at the first embedded null.

The text sent will be ANSI only in Lacewing Relay, so non-ANSI characters may be corrupted. More details on that here.

In Relay, if you read this expression before a message is received, the app will crash.


Blue Client:

Using this expression when it is the wrong type of message will result in an error, and blank will be returned.


The Unicode text is not tested for validity on client-side. However, the server checks all text messages, using the usual Unicode validation and potentially using a Unicode allowlist.

If you need a more complete test or limitation of content, you should do the checks on the server side, but there is a Convert string to UTF-8 and check against Unicode allowlist expression on the Client.