Output-side scanning covers non-streamed responses. Streamed completions skip most of it.
Grounding verification runs only when a request supplies the sources the answer is supposed to rest on. Without sources there is nothing to check the answer against, so it does not run.
When it runs, it checks the claims it can extract: figures, dates, URLs, citations, API and tool references, and statements introduced by phrases such as "according to". When the fine-tuned grounding model is installed and the opt-in fusion mode is off, the other sentences are checked too, one at a time in order until a time budget runs out (5 seconds by default), and a plain prose sentence counts as a claim when that model finds the sources contradict it. Sentences past the budget are not checked and do not fail the response; the result only notes them in its errors, so a long response, or a false sentence placed after enough harmless ones, can pass. Measured end to end, that catches about 57% of false prose sentences, and it flags about 6% of correct sentences, 13% of sentences the sources do not mention, and 16% of responses made of four correct claims; each extra sentence is another chance of a false flag, so the per-response rate grows with length. Without that model, or with the fusion mode on, plain prose is not checked. Verdicts come from a trained classifier over an NLI model that is downloaded from HuggingFace on first use; without it, grounding falls back to keyword overlap, which does not catch a changed figure. Every grounding result states which of the two decided.
Source: AI gateway proxy and Verification Engine code paths.