Agnosticism, based on various definitions that I have seen, is defined as one who is beliefs that G-d or supernatural powers cannot be proved or disproved.
Other definitions I have seen explain it as either doubt or non committal to the idea of a G-d.
Is one who is doubtful ie. Does not make a definitive statement about the existence of G-d or the Torah’s divine source classified halachically as a min (a heretic)?
The presumption of this question is that we find in Rashi (beginning of the 10 perek in Sanhedrin 90a) about who denies the belief of תחיית המתים מן התורה, that there is no proof from within the torah about the rising of the dead in the Messianic times is considered a Kofer. Rashi explains, even if one were to belief in the concept but deny that it has a source in the torah is also considered a heretic. The suggestion being that the torah requires a clear and definitive proof that stems from the torah, and not just a logically based belief.
This Gemara and rashi’s explanation connects to agnosticism in the sense that it, per the definitions above, is inherently an doubtful stance. But we see clearly that one cannot even logically deduce such a belief, all the more so if one is not even sure.
Thus, is agnosticism included in the category of heresy?