Strict mode turns warnings into errors, as JavaScript's strict mode refuses lax code. Write use strict at the top of the program, or run it with warp --strict:

use strict
"a" + warning("careful")   // Error("careful")

Warnings from level normal up fail; --strict=<level> picks another level (--strict=severe, --strict=weak). Hints never fail. Levels and what they cover: warning.

An undefined name is an error with or without strict mode: f(y) := y*y+u fails with "undefined variable: u". Annotations are strict on their own too: a declared type is a promise the compiler checks with or without strict mode, see typing.