The first version is running at https://stretch.parametricfab.com/ .

The idea started as a PrusaSlicer add-on. I wanted to use two cut planes to choose a section of an STL, then stretch or shrink that section by a set amount.

I went with a standalone webpage so it could be shared and used with different slicers. The STL is read and edited in the browser, then downloaded as a new STL.

The current version has draggable planes, exact number inputs, custom angles, a choice of which side stays fixed, and undo/redo for applied edits.

The example bracket in the Files section shows a 40 mm section stretched to 60 mm. The middle hole stretches with it. The ends and their holes keep their shape and size.

There are 12 automated geometry tests so far, plus browser checks for editing, export, and reopening the exported STL. The tests and most of the implementation were generated with Codex from the requirements I supplied. Independent review and more real STL testing are welcome.

The source, a ready-to-host webpage, and the before/after STLs are available here under the MIT License.

Next I want to see how it handles a wider range of models and get more checks with actual prints. If you try it and find a problem, a shareable STL and the settings you used would help.