Tǔdòu Sī Juǎn Bǐng (土豆丝卷饼) is the shredded-potato roll, a thin soft pancake wrapped around stir-fried julienned potato dressed with vinegar and chili, eaten as a handheld street snack. The angle is the wrap doing structural work the bread cannot in the mó version. Here there is no sturdy bun to absorb anything, only a flexible flatbread that has to hold a tangle of loose, slick potato together without tearing or going soggy, so the build hinges on a pancake pliable enough to roll tight and a filling drained and crisp enough not to weep through it.
The build is a stir-fry rolled in a thin flatbread. Potatoes are peeled and cut into fine matchsticks, rinsed well to strip surface starch so they stay separate, then stir-fried fast in a hot wok with oil, chili, and a splash of vinegar added late so the acid stays sharp and the potato keeps a crunch. A thin wheat pancake, soft and slightly elastic rather than crisp, is laid flat, often brushed with a savory or chili sauce, the hot potato spooned in a line off-center, sometimes with cilantro, scallion, or a few sheets of lettuce for a fresh edge. The pancake is folded over one end and rolled up snug so it holds on the move. Done well it shows a pancake that stays soft and intact, a roll tight enough to keep its shape without splitting, potato that is still crunchy and clearly tart and chili-warm, and just enough sauce to season without soaking the wrap. Done poorly the failure modes are quick: a filling not drained so it leaks vinegar and oil and the pancake disintegrates, potato cooked limp so the roll has no texture, a wrap rolled loose so the contents push out the open end, or a pancake too dry and brittle that cracks instead of bending.
It shifts mostly by what goes in with the potato and how the pancake is dressed. Some versions stay strictly potato, chili, and vinegar; others add green pepper, carrot, cilantro, or a smear of fermented sauce, and the heat ranges from a mild warmth to a fierce dried-chili and Sichuan-pepper hit. The same potato filling packed into a split baked mó instead of a soft pancake is its own separate preparation with a different texture logic, and the broader family of filled rolled pancakes spans many savory and meat fillings that each get their own treatment. What fixes this one as its own entry is the vinegar-and-chili potato julienne kept crisp and wrapped in a thin soft pancake rather than a bun.