  We perform a global effective-field-theory analysis to assess the precision
on the determination of the Higgs trilinear self-coupling at future lepton
colliders. Two main scenarios are considered, depending on whether the
center-of-mass energy of the colliders is sufficient or not to access Higgs
pair production processes. Low-energy machines allow for ~40% precision on the
extraction of the Higgs trilinear coupling through the exploitation of
next-to-leading-order effects in single Higgs measurements, provided that runs
at both 240/250 GeV and 350 GeV are available with luminosities in the few
attobarns range. A global fit, including possible deviations in other SM
couplings, is essential in this case to obtain a robust determination of the
Higgs self-coupling. High-energy machines can easily achieve a ~20% precision
through Higgs pair production processes. In this case, the impact of additional
coupling modifications is milder, although not completely negligible.
