# Change appearance of biblatex undefined references

I am currently using biblatex to handle my bibliography. In comparison to BibTeX, undefined references are visualized as [bold citation key] instead of "[?]". Since my citation keys are rather long, this really messes up my document when I write some text and intend to fill in the citations later.

Is there any way to change the appearance of undefined references in biblatex?

\documentclass{article}
\usepackage[backend=biber]{biblatex}

\begin{document}
\cite{authorlastname2017firstwordoftitle}
\end{document}


This is controlled by the internal command \abx@missing. Its default is

\protected\def\abx@missing#1{%
\mbox{\reset@font\bfseries#1}}


\makeatletter
\protected\def\abx@missing#1{\textbf{?}}
\makeatother


to your preamble to just see a bold question mark if something is missing.