Fix natbib: Bibliography Not Compatible With Author-Year

Switch the bibliography style to one that works with natbib, such as \bibliographystyle{plainnat}. Styles like plain write numeric \bibitem entries without the author and year that authoryear mode needs. If your journal supplies its own .bst, use that, or switch natbib to numeric mode with [numbers].

What the message means

With \usepackage[authoryear]{natbib}, commands such as \citet and \citep print names and years. natbib reads those from the bibliography file that BibTeX writes, and that file comes from the style named in \bibliographystyle. If the style is not natbib-aware, the names and years are missing and natbib complains.

The usual fix: a natbib style

Change the style to plainnat (or abbrvnat, unsrtnat, or your journal's .bst). Nothing else in the document needs to change.

What the bad version looks like

This file uses authoryear with the plain style:

\documentclass{article}
\usepackage[authoryear]{natbib}
\begin{document}
As shown by \citet{smith2020}, the method works.
\bibliographystyle{plain}
\bibliography{refs}
\end{document}

In our Tectonic 0.17.0 run it compiled and the PDF showed "(author?) [1]" in the sentence and a numbered reference, instead of "Smith [2020]". The log held the natbib warning "Author undefined for citation". We did not trigger the exact error message from the title on this setup, so your own error may be worded differently.

The fixed version

The .bib file is the same in both cases:

@article{smith2020,
  author  = {Smith, Jane},
  title   = {A Sample Article},
  journal = {Journal of Examples},
  year    = {2020},
  volume  = {1},
  pages   = {1--10}
}

The fixed document with plainnat:

\documentclass{article}
\usepackage[authoryear]{natbib}
\begin{document}
As shown by \citet{smith2020}, the method works \citep{smith2020}.
\bibliographystyle{plainnat}
\bibliography{refs}
\end{document}

We compiled this file as-is with Tectonic 0.17.0 on November 2, 2026, and inspected the rendered pages of both versions. The fixed version shows "Smith [2020]" and "[Smith, 2020]" with an unnumbered reference. We did not compile it with other engines, natbib versions or journal styles.

FAQ

What causes the natbib author-year error?

natbib in authoryear mode needs each \bibitem to carry author and year information. A style such as plain omits it, so natbib cannot build author-year citations. In the compiled example below, this mismatch showed up as (author?) and a numeric [1] in the text.

Which bibliography style should I use with natbib?

Use a natbib-aware style: plainnat, abbrvnat or unsrtnat ship with natbib, or the .bst file your journal provides. The fixed example below uses plainnat.

Can I keep my current style and use numeric citations instead?

Yes. Load natbib with [numbers] and keep a style that produces numeric entries. Author-year commands such as \citet still work with natbib-aware styles, but the output is numeric.

Why do I see (author?) in my PDF?

It is natbib's placeholder when the bibliography entry has no author information. In our compiled bad example the log showed a natbib warning, Author undefined for citation, and the PDF showed (author?). We did not reproduce the exact error message wording on this setup.

Do I need to recompile after changing the style?

Yes. The .bbl file is rebuilt by BibTeX from the style, so run the full LaTeX, BibTeX, LaTeX cycle again. Tectonic in our test ran that cycle automatically.

Further reading

Companion guides in this series cover citing websites with BibTeX, Page X of Y, bold and italic text, and commenting out blocks. More how-tos are on the Bibby AI blog. Source question: Package natbib Error: Bibliography not compatible with author-year citations on TeX Stack Exchange.

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Last updated: November 2, 2026. By the Bibby AI editorial team.