Fabricated numbers are the highest-signal defect
A figure in the answer that never appeared in the context is almost always invented. Models produce plausible numbers readily, and a plausible number is far more damaging than a vague sentence because people act on it.
The checker extracts every number from the answer and verifies it appears in the source. This is a cheap check with a high hit rate.
What this is not
It measures lexical overlap. A correctly paraphrased claim scores low and a fluent fabrication reusing context vocabulary scores high. It cannot determine truth.
Treat the output as a list of sentences worth a human reading, not as a verdict. Used that way it reliably surfaces the claim that slipped in from the model rather than the source.