Or HaChaim on Deuteronomy 13:4/אור החיים על דברים י״ג:ד׳

1 כי מנסה ה׳ אלוקיכם אתכם לדעת, for the Lord your G'd is testing you, wanting to know, etc." You may well ask why such a prophet should be executed if G'd sent him to test the loyalty of the Jewish people! Clearly then the words כי מנסה do not mean that G'd instructed this prophet to say what he said, but it explains only why the miracles this prophet performed in order to legitimise himself were allowed to happen.

2Alternatively, the fact is that G'd never allowed any miracles to be performed by a prophet except to demonstrate His dominion over the forces of nature, i.e. that He created these forces. The words כי מנסה השם, "for G'd conducts a test, etc." are only an answer to the question why G'd allows some people to perform such miracles instead of preventing them so as not to mislead the Israelites. The Torah answers that G'd allows the world to function in its normal manner. If it were not for the fact that G'd on occasion wants to test His people, He would not allow such charlatans to perform miracles so as not to reinforce the power of the wicked.