The Demi-Baguette Garnie is the working-lunch unit of the Parisian boulangerie, a half-length of baguette split horizontally and filled with whatever the shop has prepared that morning. The baguette is cut from the same loaf that goes out whole to the dinner table, then halved before it's slit, which gives the sandwich the proportions of a lunchroom portion rather than a picnic. The crust is the same crackling crust; the crumb is the same open, slightly chewy interior; the only thing that has changed is the math. A full baguette will sleep at most one person if it's filled generously; a demi-baguette feeds a salaried Parisian who has thirty minutes before they have to be back at their desk.
The filling logic is the same as the standard Baguette Garnie, with the constraint that the smaller bread has less room to forgive a heavy hand. Jambon de Paris and butter is the most-ordered version, and the one that the boulangerie has at the counter from 11 a.m. forward. Pâté de campagne with cornichons fills the demi-baguette without much butter to bridge it. A few sliced tomatoes with mozzarella and basil works for the summer menu, though the bread doesn't really do justice to the ingredients the way an Italian ciabatta would. The point of the format is portability, restraint, and the fact that you can eat one standing up at a counter without losing the contents to the floor. A full baguette demands a table; a demi-baguette demands only a hand and a paper sleeve.
The sandwich's sibling formats trade size for shape. The ficelle, a thinner baguette, gives you a longer profile and a higher crust-to-crumb ratio, which works for fillings that want bite. The flûte is between the two. The cocktail-sized petit pain, three bites long, lives at the apéro tray rather than the lunch counter. See Pain Garni for the broader category of non-baguette French sandwich breads, and for the way the choice of loaf shape determines what the sandwich can be asked to do.