GUWonder

What is allowed depends upon a combination of the following: (a) the country/countries on the trip where a foreign citizen will present herself/himself for entry using a foreign passport; and (b) the conditions and circumstances under which the foreign citizen's entry is attempted/occurring.

Reciprocal agreements between countries have something to do with what is generally applicable. However, even in the absence of such agreements between a person's country of citizenship and the relevant foreign country, it may be possible to enter even with less than 6 months before passport expiration date, depending upon the conditions and circumstances under which a foreign citizen's entry is attempted/occurring.

I've entered at least a dozen countries using US passports with an expiration date within 6 months of my date of arrival in such countries and not all of them have reciprocal agreements with the US specifying that travel is allowed within 6 months of passport expiration date. But, as I said, it depends on the country/countries on the trip where you will present yourself for entry using a foreign passport and the conditions and circumstances under which entry is attempted/occurring by the foreigner.

If you are unsure about what is applicable for your situation, consider asking the embassy/consulate of the country/countries where you will present yourself for entry. If not willing to do that, consider renewing the passport at some point that is more than 6 months before it's expiration date and would get you your renewal passport in time for the trip.