This manuscript focuses on borderline, gradient, and higher regularity estimates of solutions to fully nonlinear elliptic transmission problems given by \( {\left\{ \begin{array}{ll} \begin{array}{lclcccl} F^+(D^2 u) & = & f^+(x) & \text { in } & \Omega ^+ & = & B_1 \cap \{ x_n > \psi (x') \}, \\ F^-(D^2 u) & = & f^-(x) & \text { in } & \Omega ^- & = & B_1 \cap \{ x_n < \psi (x') \}, \\ u^+_\nu - u^-_\nu & = & g(x) & \text { on } & \Gamma _{\psi } & = & B_1 \cap \{ x_n = \psi (x') \}, \end{array} \end{array}\right. } \) for suitable functions f, continuous data g, and a fixed interface \(\psi \) . Specifically, under the standard uniform ellipticity condition on the governing operators and suitable Hölder continuity assumptions on g and \(\psi \) , we obtain optimal \(C^{1,\alpha }\) -regularity provided the forcing term lies in the Lebesgue space \(L^{p}\) for \(p>n\) . Furthermore, we establish \(C^{0,\text {Log-Lip}}\) regularity in the borderline case \(f \in L^{n}\) . Finally, in the critical borderline case, i.e., when the forcing terms belong to the \(\text {BMO}\) space and under asymptotic concavity conditions for the governing operators with suitable Hölder continuity assumptions on the data, we obtain higher Log-Lipschitz-type estimates, specifically, \(u \in C_{\text {loc}}^{1, \text {Log-Lip}}\) . Our results are remarkable, even within the context of linear transmission problems.
