The fallback is its own retrieval

chooseLaws takes two lists: the pool it reranks and the fallback it returns when it cannot. The fallback is not the pool’s first eight. It is a separate call to retrieve with the limit at eight, and the pipeline in Chapter 10 makes both calls.

The deployed pipeline’s comment says why, and the number in it is the kind of number worth having:

fallback is its own retrieval rather than the pool’s first eight, because the two differ: retrieve fuses BM25 over twice its limit with the meaning index, so a 24-law pool reaches BM25 ranks an 8-law retrieval never did, and its first eight are other laws (175 of 175 topic questions, with the index).

The fallback is what the box gave the model before the reranker existed, and the rule from Chapter 5 holds: with the stage absent, the pipeline is the earlier pipeline, not an approximation of it. A fallback that was the pool’s first eight would be a third pipeline that nobody had measured.