tikz foreach loop variables in pgfplots legend in math mode

You need to expand the content of the entry, for example with

\ExpandArgs {e} \addlegendentry {$a=\a$, $b=\b$} 

\ExpandArgs {⟨spec⟩} {⟨cmd⟩} {⟨arg1⟩} ... is part of the LaTeX kernel and expands the ⟨arguments⟩ of the command ⟨cmd⟩ according the the specification ⟨spec⟩, as documented in the LaTeX usrguide. ⟨spec⟩ consists of one letter per argument, controlling its treatment according to the LaTeX3 programming interface, for example : V for the value of a variable, o for a one-level expansion or e for a full expansion.

Example:

\documentclass[tikz]{standalone} \usepackage{pgfplots} \pgfplotsset{compat=1.18} \begin{document} \begin{tikzpicture} \begin{axis} \foreach \a in {1,2}{ \foreach \b in {1,2}{ \addplot {\a*\b*x}; \ExpandArgs {e} \addlegendentry {$a=\a$, $b=\b$} } } \end{axis} \end{tikzpicture} \end{document}