  Effective field theories (EFT) of dark energy (DE) -- built to parameterise
the properties of DE in an agnostic manner -- are severely constrained by
measurements of the propagation speed of gravitational waves (GW). However, GW
frequencies probed by ground-based interferometers lie around the typical
strong coupling scale of the EFT, and it is likely that the effective
description breaks down before even reaching that scale. We discuss how this
leaves the possibility that an appropriate ultraviolet completion of DE
scenarios, valid at scales beyond an EFT description, can avoid present
constraints on the GW speed. Instead, additional constraints in the lower
frequency LISA band would be harder to escape, since the energies involved are
orders of magnitude lower. By implementing a method based on GW multiband
detections, we show indeed that a single joint observation of a GW150914-like
event by LISA and a terrestrial interferometer would allow one to constrain the
speed of light and gravitons to match to within $10^{-15}$. Multiband GW
observations can therefore firmly constrain scenarios based on the EFT of DE,
in a robust and unambiguous way.
