The straight answer: no, you should not put a stainless steel container in the microwave. Metal reflects microwaves instead of letting them through, which stops your food heating and can cause sparking. This is the one honest limitation of steel, and we’d rather state it plainly than bury it.
Why not. A microwave heats by penetrating food with waves. Metal blocks them. At best your food stays cold; at worst, thin or damaged metal can arc against the oven wall.
What to do instead — it’s simple. Tip your food onto a plate or into a bowl and heat that. Ten seconds of habit. This is how most people reheat a steel-stored lunch and never think about it again.
Other ways to warm food without a microwave. A stovetop pan, an oven, or letting a portion come to room temperature all work. Our container bodies are fine in an oven with the lid off — the lid is not, because it carries a food-grade silicone seal. If you specifically want a container you can heat food in, glass is the material for that job, and we say so.
What steel is better at. Most things that repeat: it won’t warp, won’t stain, won’t hold smells, and it lasts — we guarantee the steel for ten years.
Two things it is not better at, while we’re being straight: it won’t keep your food cold (ours is single-wall, no vacuum — there’s no insulation in it), and it isn’t inert. We’ve never claimed steel doesn’t leach, because that isn’t a claim anyone can honestly make.
So the real question isn’t “is steel microwave-safe” — it’s “is plating my food to reheat a fair price for a container that doesn’t warp, doesn’t stain, and doesn’t need replacing?” For most people, easily yes.
Food should be stored without doubt.