Are collapsible water bottles actually leak-proof?

Most listings in this category advertise leak-proof or leak-resistant caps, and verified-purchase reviewers generally agree they hold up for everyday carry. However, none of the collapsible bottles in our pool carry independent lab leak-testing, and ratings cluster in the 4.1–4.4 range, which suggests a meaningful minority of buyers report occasional seepage at the cap or seam. Hand-tighten the lid fully and test over a sink before traveling.