Dishwasher-safe is a label, not a material

You can put some lunch boxes in the dishwasher, but it depends on what the maker says, not just what it’s made of. “Plastic” doesn’t automatically mean safe, and “stainless steel” doesn’t automatically mean unsafe.

Plastic is the tricky one. If it’s marked dishwasher-safe, sure, but keep it on the top rack and away from the heating element. Thin plastic lunch containers will still warp no matter what the label says. And over time, even “safe” plastic gets cloudy and starts holding onto food smells.

Stainless steel is usually fine for plain metal containers, but insulated ones with a vacuum seal can lose their heat-keeping ability if you run them through a hot cycle. Also, the dishwasher won’t hurt the metal much, but it will make it look dull. Hand washing keeps it like new.

Glass is the safest bet in the dishwasher, but the lid is the weak point. Most glass lunch boxes have plastic lids with rubber gaskets, and those will crack or warp. Take the lid off and wash that by hand. The glass body can go on the bottom rack.

Final rule: if the label doesn’t say dishwasher-safe, don’t risk it. A ruined lunch box is not worth saving ten minutes.