Numerical stability is a fundamental problem with interpolating splines defined over Euler spirals because a unique solution is not always guaranteed to exist.

Numerical stability is a fundamental problem with interpolating splines defined over Euler spirals because a unique solution is not always guaranteed to exist. I'm pretty sure there's a theorem in there somewhere if deflections can be bounded, but I haven't found it yet. If you don't need exact Euler spirals and are ok with increasing the tension on high deflection (which imho is valid for design tasks like making fonts), then the approach of the hyperbezier spline might be an appealing alternative. I believe that always has a solution, but the current solver is kinda dumb and can fail when points are very close together. I plan on improving it. The general idea of the hyperbezier solver - use Hermite geometric interpolation, and solve the tangent angles for G2 continuity - is a better approach than what the Spiro code does. That has technically richer capabilities (including G4 continuity if needed), but is less stable. I don't promise anything, but if you want to file an issue against linebender/spline with more detailed requirements of what you need, I will at least take a look at it.