Both come down to the same part: the cliff sensors underneath the robot. They fire infrared straight down to spot a drop-off. Very dark, black or high-gloss surfaces absorb that beam, so the robot reads it as stairs and refuses to cross.

Why the black rug keeps getting skipped

It isn’t a bug and it isn’t your robot being stupid. The sensor sends out an infrared beam and waits for it to bounce back. A dark surface absorbs the beam, nothing comes back, and the robot concludes there’s nothing there to drive onto. It’s the exact same signal it gets at the top of a staircase, so it does the safe thing and turns around.

High-gloss and mirror-finish floors cause the same problem for the opposite reason — the beam bounces off at an angle and never returns to the sensor. Direct sunlight hitting the floor can wash the sensors out too, which is why some robots behave differently in the afternoon than at night.

What you can actually do about it

Clean the sensors first. They’re small windows on the underside and they collect dust and hair, which makes false readings much worse. A dry cloth once every couple of weeks fixes more of these complaints than anything else.

Some apps let you reduce cliff sensitivity for a specific zone. That’s a real fix, but do it carefully and never in a zone that touches a staircase.

The flip side is the reassuring part: those same sensors are why robots don’t fall down stairs. Across the robots I’ve tested, a working machine with clean sensors stops at the edge every time. The failure cases I’ve seen have all involved sensors caked with dust or a robot that was already damaged.

Dad’s Take

Wipe the little windows on the bottom. Half the “my robot is broken” emails I get are solved by that alone.