Khanmigo is a surprisingly naive product, but this isn't a particularly hard problem:
1. It's not necessary to invent all these problems from scratch. At least for any "normal" math problem the world doesn't need new problems on demand, you just have to fetch one. Khan Academy has a perfectly good bank of problems.
2. If you have a baked problem you should hang onto knowledge about the correct answer. Khanmigo just seems to put it in the chat history where it is easily forgotten.
3. If you want customized problems you might need to invent problems instead of reading problems from a bank. In that case you might want to create a more "formal" problem and an "informal" problem, where the formal problem is expressed clearly and solvable (e.g., by Wolfram or SageMath or something), and therefore can be verified. Translating the formal problem into a word problem is generally much more reliable than inventing a problem and correct solution.
That said, "right answers" aren't IMHO that interesting. Helping the student attack a problem in the right way, or adapt their knowledge to be able to apply it to a problem, is more interesting and helpful.