I made a small JOSM plugin called Fillet Tools (fillettools.jar).

It adds an interactive mode for rounding corners of ways, similar to a fillet tool in CAD software. The motivation is simple: in the real-world, many objects are not designed with sharp corners. Roads, sidewalks, residential landuse areas, sports fields, running tracks, and other planned infrastructure often have smooth, consistent curves. Mapping these curves by hand usually means placing several nodes one by one, which is slow and often inconsistent. Fillet Tools is an attempt to make this specific editing task easier in JOSM.

You can find the project in the GitLab repository, and download the first test jar from the v0.1.0 release.

How it works:

• enable the Fillet Tools mode from the toolbar or by pressing v;
• move the mouse over a way or inside a closed polygon;
• small handles appear near available corners;
• drag a handle to preview the radius;
• release the mouse button to apply the rounded corner.

There are also two useful modifier keys:

• Shift snaps the radius to a fixed step, so it is easier to make several corners with the same radius;
• Alt / Option allows a larger “oversize” drag for bigger arcs. The plugin still checks the final geometry before applying the change.

I tried to make the plugin careful with data. It uses the normal JOSM command stack, so undo works as expected. It also tries not to replace important existing nodes. In the usual case, the original corner node is moved onto the new arc instead of being deleted, so its local/OSM id history is preserved as much as possible.

Important note: I am an absolute beginner in programming, and I used an AI coding agent to help write this plugin. So there may be strange, inefficient, or simply wrong things in the code. Please keep this in mind if you look at the repository or test the plugin.

For now the plugin is not available from the JOSM plugin list. To test it, you need to download fillettools.jar from the release page and copy it manually into your JOSM plugins directory.

After copying the file, open Preferences → Plugins in JOSM, search for Fillet Tools, and enable the checkbox next to it. Then restart JOSM. After restart, the plugin should be ready for testing.

On Windows this is usually:

%APPDATA%/JOSM/plugins/

On macOS this is usually:

~/Library/JOSM/plugins/

I would be happy if someone could test it on real mapping tasks.

Feedback, bug reports, strange geometry cases, and suggestions are very welcome. You can write them here in the comments, or on GitLab.