Which sippy cups are actually leak-proof in a diaper bag?

True leak-proof claims are rare. Across high-trust r/toddlers threads, the Zak Designs Kelso tumblers earned the most consistent 'still going strong after 2 years' verified-use endorsements, with the caveat that the silicone gasket must be seated correctly. Owala's kids tumbler is described as leak-resistant rather than fully leak-proof due to a vent on the mouthpiece.