  In $\mathrm{U(1)}$ lattice gauge theory in three spacetime dimensions,
confinement can be analytically shown to persist at all values of the coupling.
Furthermore, the explicit predictions for the dependence of string tension
$\sigma$ and mass gap $m_0$ on the coupling allow one to tune their ratio at
will. These features, and the possibility of obtaining high-precision numerical
results via an exact duality map to a spin model, make this theory an ideal
laboratory to test the effective string description of confining flux tubes. In
this contribution, we discuss our investigation of next-to-leading-order
corrections to the confining potential and of the finite-temperature behavior
of the flux tube width. Our data provide a very stringent test of the
theoretical predictions for these quantities and allow to test their dependence
on the $m_0/\sqrt{\sigma}$ ratio.
