Diagnose LaTeX Compilation

Turn a failed build into a located, classified, minimally-fixed error. Never mass-rewrite the source to "make it compile" — find the one thing that broke.

In Papyrus the app owns compilation (Cmd+S / Ctrl+S) and shows the AUTHOR a clickable diagnostics list. You cannot see that list: read the compiler log the app leaves in the project's top-level directory (<main>.log, named after the main .tex), or ask the author to paste the error. Do not run the compiler yourself in Papyrus (see papyrus-writing). Running the compiler directly applies only outside the app.

Step 1 — Read the FIRST real error

Work from the first error, not the last: LaTeX errors cascade, and the tail of the log is usually damage from the head. The app's list is grouped by kind (file:line errors, then ! errors, then warnings, then boxes), not log order, and is parsed from only the last 20,000 characters of the final pass, so its top row may not be the first error; the .log on disk is complete. In a raw log, the first file:line: message (or ! …) line is the one to fix.

-file-line-error makes a compiler print file:line: message. If citations/refs are wrong (not the compile itself), the full cycle is pdflatex → bibtex/biber → pdflatex → pdflatex; Tectonic runs that cycle itself. In Papyrus, when the app compiles with pdflatex it runs the bibtex cycle for you if the document cites anything, but never biber: a biblatex document on the biber backend will not resolve its citations there — tell the author rather than editing around it.

Step 2 — Classify and fix at the source line

Common cause → fix:

• Unescaped special char (_ % & # $ in text) → escape it (\_, \%, …).
• Unbalanced brace / missing } or \end{env} → match the environment.
• Undefined control sequence → missing \usepackage, or a typo'd/renamed macro.
• File 'x.sty' not found → package not installed in this TeX distribution; prefer a package the document already loads over telling the author to install one.
• File not found (graphic/\input) → wrong path or extension; confirm the file exists.
• Missing $ inserted → math symbol (_, ^, \alpha) used outside math mode; wrap in $…$ or an equation env.
• Missing \begin{document} → a stray character before the preamble ended.
• Bib: Citation 'key' undefined / stale .bbl → add the entry to the .bib (don't remove the \cite), then recompile (in Papyrus the app's compile runs bibtex and the extra passes; outside it, run bibtex/biber and two more pdflatex passes).
• There's no line here to end → a \\ on an otherwise-empty line.
• Overfull \hbox, Underfull, most Warning: lines are WARNINGS, not errors — do NOT "fix" them by hacking spacing (see papyrus-writing: never hack margins).

Apply the smallest fix at the identified line, recompile (in Papyrus the app recompiles when your turn ends only while the author has the Papyrus page open — it is a page-scoped effect, and nothing server-side compiles — so on the next turn check that the .log is NEWER than your edit before believing it, and ask the author to press Cmd+S when it is not; outside it, rerun the compiler), and confirm THAT error is gone before moving on. Report what broke and exactly what you changed.