Which size chart is your AI shopping assistant reading?
Ordering two sizes and returning one can eat the lunch break you hoped online shopping would save. Before asking an AI assistant which size to buy, check what the numbers in the chart actually measure. UNIQLO's existing US help page makes a distinction that's easy to miss: product measurements describe the actual garment; the size on the tag refers to body dimensions. It also warns that fit can vary between garments with the same size label, and that its US charts may differ from other countries. I'd give the assistant the chart for the exact item and ask it to identify which kind of measurement it is using before suggesting a size. If we're comparing finished garments, start with a similar shirt that already feels comfortable and follow the retailer's measuring instructions. Don't silently compare a width measured across a shirt laid flat with a measurement taken around your body. Then say what comfortable means to you: room to reach forward, sleeves that stay clear of your hands, or space for a layer underneath. A size letter alone cannot tell the assistant that. This is a check I'd suggest, not a fit test I've run. I'd rather get a useful comparison with an honest uncertainty than a confident ‘buy medium’ and another parcel to send back.
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I would also resist a recommendation that wins by a tiny measurement difference. UNIQLO's page warns of fabric-dependent sizing differences of 0.5 to 1.15 inches. That is not a tolerance I'd apply to every retailer, but it is a reason not to treat this chart as an exact measurement of the shirt that will arrive. If two options are close, I'd ask the assistant to explain the uncertainty rather than turn an extra decimal place into confidence. Someone still has to wear the result, not just match the spreadsheet.
Before getting the tape measure out, I'd check whether a shirt you already like is in your purchase history. UNIQLO's same guide says you can log in and compare previously purchased products with the item you're considering. Some products lack the data, so it won't always work. I'd start with one familiar shirt and ask the assistant to point out where the new one differs, rather than pick a size from scratch. ‘Longer sleeves than the shirt I keep rolling up’ would help me decide more than another decimal place. That comparison is a suggestion, not a fit guarantee or a tool I've tested.