Roblox's UICorner now has four radii of its own (TopLeftRadius, TopRightRadius, BottomRightRadius, BottomLeftRadius), available as a Studio beta under File > Beta Features > New UI Capabilities. The UI Builder had to learn them without breaking every existing project.

CornerRadius becomes a shorthand

CornerRadius is now a shorthand over the four. Setting it sets all four, and reading it gives the top left. A radius is worked out the way the docs describe: X and Y the same, the Scale taken of the layer's smaller side, and never more than half of it, so a large radius gives a pill.

In the Inspector

The Corner row keeps its one slider, which sets all four, and gains a button that opens a field for each corner. While the corners differ, the row says Mixed. A layer with no UICorner shows 0. Dragging the slider creates one with Roblox's default of 8px, then moves it to the value. Giving a single corner a radius creates the UICorner with the other three at 0, in the same undo step.

Keeping old files working

Corners that are equal are saved exactly as before, as CornerRadius. Old projects, older Studio and the MCP tools see no change. Only when a UICorner's corners differ does it write the four and no CornerRadius.

The script exports (Luau, React-Luau, Fusion) and Studio Sync follow that per instance, and put a note in the script's header when the beta is needed. The model files (.rbxm, .rbxmx) are different, because a model file gives all instances of a class the same properties. So they write the four on every UICorner in the file as soon as one needs them. The importer reads all four instead of reducing them to the top left.

A ScrollingFrame can't have one

A UICorner can't be applied to a ScrollingFrame. It can't be added there, and the message says why. The Corner row explains instead of offering a slider, and one that arrives from a file has no effect. The "Can't add..." message now gives the reason in each case (a UICorner on a ScrollingFrame, a second one on a layer, a layout already arranging the children).

The canvas, the share preview PNG, the smart scale, Draw's export and animation tweens all follow the four radii. The rules live in corner-radius.js.