Fill to the neck, not the brim.

Hot soup leaks out of a thermos for one of two reasons: you overfilled it or the gasket didn’t seat.

The secret is heat plus headroom. Preheat the jar by filling it with boiling water and letting it sit for a few minutes, then dump that and fill it with soup that’s already bubbling hot. Leave a solid inch of empty space below the threads where the lid screws on. If you fill to the absolute top, the steam pressure and liquid expansion force soup past the seal the moment you tilt the thing.

Before you screw the lid on, check that the silicone gasket sits flat in the lid and is bone dry — one escaped grain of rice will break the seal. Wipe the rim of the jar clean too. Screw on the lid fully, but don’t torque it like you’re changing a tire; that warps the gasket. Then do the thirty-second test: turn it upside down and shake it over the sink. If it dribbles, open it, dry rim and gasket, re-seal.

That’s the whole trick. Future you doesn’t want chili in the laptop bag.