  We prove convergence of the 2- and 4-point fermionic observables of the
FK-Ising model on simply connected domains discretised by a planar isoradial
lattice in massive (near-critical) scaling limit. The former is alternatively
known as a (fermionic) martingale observable (MO) for the massive interface,
and in particular encapsulates boundary visit probabilties of the interface.
The latter encodes connection probabilities in the 4-point alternating
(generalised Dobrushin) boundary condition, whose exact convergence is then
further analysed to yield crossing estimates for general boundary conditions.
Notably, we obtain a massive version of the so-called Russo- Seymour-Welsh
(RSW) type estimates on isoradial lattice. These observables satisfy a massive
version of s-holomorphicity [Smi10], and we develop robust techniques to
exploit this condition which do not require any regularity assumption of the
domain or a particular direction of perturbation. Since many other
near-critical observables satisfy the same relation (cf. [BeDu12, CIM21,
Par19]), these strategies are of direct use in the analysis of massive models
in broader setting.
