I believe the quickest "fix" to this issue is to correct "help" in sqlite3 shell as well online documentation as @jake suggested: require to wrap arbitrary literal strings with parentheses with single quotes in the same way as it going with **`VALUES`** for example to match syntax:
--------------------------------
```
.parameter set @str1 ('string')
.parameter set @str2 ('+1+2+3')
.parameter set @num (2+3+4)
```
--------------------------------
in these cases, **@str1** & **@str2** will be strings and parameter **@num** will be 9.
IMHO it will be more intuitively understandable and may be some one would find calculation on assignment as a feature.
And as always, - thank you for the sqlite !