We review the exact thermodynamic description of the classical sine-Gordon field theory, the notorious integrable model. It turns out that existing results in the literature based on soliton gas diagrams did not correctly account for light, but the extension of the soliton led to erroneous results. This problem is regularized by requantization. By taking the semiclassical limit of the quantum model, we derive the correct thermodynamics. Our results are extended to transport settings by generalized fluid dynamics.