What is a word for when someone speaks or answers you using single words like "yes", "okay", or "probably", but your questions are sensible enough to base a conversation on?
The right word choice depends as much on connotation as definition. You're inviting someone to converse, and your invitation is being declined. I get the idea that you feel you're being rebuffed, and you want a word to describe the mostly monosyllabic responses you get in that light.
"Curt" comes to mind. It strongly connotes discourtesy. "Brusque" is a similar word, with similar connotations. Neither of those words is especially popular, but not to the point that I would shy away from using them in everyday speaking or writing.
"Reticent" has no connotation of discourtesy. It gives the connotation that the speaker's shortness has more to do with the speaker's mood or personality, and nothing to do with you or how you're being treated.
Terms like "tight-lipped" and "closed-mouthed" are synonymous with "reticent," but they are often used in the context of secrets. They imply that information is being withheld. Although neither of them implies discourtesy, they are often used to convey a certain a certain mild disappointment on the part of the person who desires the information.
See the list of synonyms at laconic here:
Using or marked by the use of few words; terse or concise.
"Monosyllabic" is often used for a person who deals in one word answers: technically it refers to words of a single syllable, but if is often extended to include single word, or curt, answers.