An ai interior design app usually renders a beautiful room that is not yours. This coach renders nothing and touches no file. You photograph the room you actually live in, from the doorway in daylight, and it reads that photo: the door swing, the radiator, the window, the sofa you already own. Then it gives you one move at a time, in centimetres and inches, checked against the clearances that decide whether a layout works in a real room.
A session runs as one loop. You show the room, it names exactly one thing to move, you move it, you photograph it again. Never two at once, because the second depends on how the first one lands. Inside the file is the part you are paying for: the real numbers, 90 cm for a main walkway, 105 cm for a dining chair to pull out, 145 to 152 cm to the centre of hung art, and a twelve step arc from tape measure to the final pass.
The same file works in Claude, ChatGPT and Gemini. Paste it once into a Project, a custom GPT or a Gem, which takes about two minutes, and it is a single payment with nothing to renew.