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Fix LaTeX Errors Instantly — latex-fix Claude Code Skill
Stop Googling cryptic TeX log messages. latex-fix diagnoses undefined control sequences, missing dollar signs, and brace mismatches with the exact fix.
You paste your LaTeX source into the compiler and get back something like this:
! Undefined control sequence.
l.47 \textbff
{Introduction}Your first instinct is to Google it. You end up on a Stack Overflow thread from 2011, read five answers, realize none of them match your exact situation, and spend the next ten minutes staring at line 47 wondering if you accidentally hit f twice. You did. But TeX could have just told you that.
This is the daily reality of writing LaTeX without tooling. The compiler knows exactly what went wrong — it just refuses to say it in human language.
The latex-fix skill from the latex-skills pack changes that. It sits inside Claude Code and acts as an on-demand TeX error translator and fixer. You give it an error, it gives you the cause, the exact line to change, and a tip to avoid the same mistake next time.
What is the latex-fix Skill?
latex-fix is a Claude Code skill — a small, focused prompt package that gives Claude a specialized workflow for a specific task. This one handles LaTeX compilation errors.
When you invoke it, it:
- Identifies the error type from the TeX log output
- Explains what went wrong in plain English
- Shows you the exact fix as a code diff or corrected snippet
- Adds a one-line prevention note so you remember it next time
It covers the errors you actually hit in practice: undefined control sequences, missing math delimiters, mismatched environments, brace overflow, missing .sty files, and more. It also knows the difference between a fatal error that stops compilation and a cosmetic warning like Overfull \hbox that you can safely ignore — at least for now.
If you're writing LaTeX from scratch and want to avoid errors before they happen, pair this with the latex-write skill, which generates syntactically correct markup to begin with. For math-specific compilation failures, the latex-math skill goes deeper on equation environments and delimiter issues.
How to Install
The skill lives in the latex-skills repository on GitHub. Clone it, then copy the skill into your Claude Code skills directory.
Install just latex-fix:
cp -r skills/latex-fix ~/.claude/skills/Install all LaTeX skills at once:
cp -r skills/latex ~/.claude/skills/Install everything including FormatEx integration skills:
cp -r skills/latex-full ~/.claude/skills/The full pack includes skills for writing markup, handling math, managing packages, and compiling through the FormatEx API — useful if you're integrating LaTeX generation into an application. If you're building on top of FormatEx, the getting started guide walks through API setup and authentication.
After copying, restart Claude Code or reload skills. The skill is available as /latex-fix in any Claude Code session.
How it Works
The skill runs a four-step process every time you invoke it with an error message or log snippet.
Step 1 — Identify the error type
TeX errors follow consistent patterns. The exclamation mark prefix (!) signals a fatal error. The l.N notation gives you the line number. The skill parses these markers and classifies the error before doing anything else — undefined control sequence, runaway argument, missing delimiter, and so on.
Step 2 — Explain the cause in plain English
This is the part TeX refuses to do. Instead of ! Undefined control sequence, you get: "You used \textbff on line 47, which isn't a LaTeX command. You probably meant \textbf — the bold font command. The extra f made TeX treat it as an unknown macro."
No Stack Overflow required.
Step 3 — Show the exact fix
The skill outputs the corrected code directly, not a vague suggestion. For the typo above:
% Before
\textbff{Introduction}
% After
\textbf{Introduction}For more complex errors like environment mismatches or brace issues, it shows the full corrected block with context so you know exactly where to make the change.
Step 4 — Add a prevention tip
A short note on why this happens and how to catch it earlier. For command typos: "Use your editor's LaTeX autocomplete or a linter like lacheck to catch undefined commands before compilation." For missing packages: "Add \usepackage{packagename} to your preamble — see the latex-packages skill for discovery and setup."
Common Error Patterns
These are the errors the skill handles most often. If you've written LaTeX for more than a week, you've seen all of them.
| Error | What TeX Says | What It Means | Typical Fix |
|---|---|---|---|
| Undefined control sequence | ! Undefined control sequence. l.N \commandname | You used a command that doesn't exist — typo, missing package, or wrong engine | Fix the typo or add the package that defines it |
| Missing $ inserted | ! Missing $ inserted. | You used math notation outside a math environment | Wrap the content in $...$ or \(...\) |
| File not found | ! LaTeX Error: File 'package.sty' not found. | A required package isn't installed in your TeX distribution | Install the package via your package manager or TeX Live |
| Environment undefined | ! LaTeX Error: Environment envname undefined. | You called \begin{envname} for an environment that doesn't exist | Check the spelling or add the package that provides it |
| Too many braces | ! Too many }'s. | You have an extra closing brace with no matching open brace | Count braces in the surrounding block — one } too many |
| begin/end mismatch | ! LaTeX Error: \begin{X} on line N ended by \end{Y}. | Your environment open and close tags don't match | Make sure every \begin{X} has a corresponding \end{X} |
The missing dollar sign error deserves special mention because it's deceptive — TeX often reports it several lines after the actual problem. If the fix isn't obvious on the reported line, look a few lines earlier for an underscore or caret used outside math mode (_ and ^ are math-only characters in standard LaTeX).
For math environment errors specifically, the latex-math skill handles aligned equations, display math, and the common failures around amsmath environments.
Overfull hbox and Warnings — What to Ignore
Not everything in the TeX log is a crisis. The skill explicitly distinguishes fatal errors from warnings, and Overfull \hbox is the warning you'll see most.
Overfull \hbox (14.22pt too wide) in paragraph at lines 83--87This means a word or phrase didn't fit neatly into the line width, so TeX stretched it slightly and the text runs into the margin. It does not stop compilation. Your PDF still builds. The document is technically wrong by typographic standards, but it's not broken.
The skill flags these as non-fatal and gives you options: add \sloppy for the paragraph, break the URL or long word manually with \- hyphenation hints, or leave it if it's a draft. It won't tell you to panic.
Other non-fatal warnings you can usually ignore during development:
Underfull \hbox— TeX had too much space between words, opposite of overfullLaTeX Warning: Label(s) may have changed— run the compiler twice to resolve cross-referencesLaTeX Warning: Citation undefined— your.bibfile isn't linked yet
The skill categorizes each message it sees and calls out which ones require action versus which ones are cosmetic.
Engine-Specific Notes
Most LaTeX errors are universal — a missing brace is a missing brace in every engine. But a handful of errors are engine-specific, and the fix depends on what you're compiling with.
pdflatex is the default engine and the strictest about encoding. If you're using characters outside ASCII without declaring the right encoding package, you'll hit:
! Package inputenc Error: Unicode character (U+...) not set up for use with LaTeX.The fix is adding \usepackage[utf8]{inputenc} to your preamble — but this error doesn't even exist in xelatex or lualatex, which handle Unicode natively.
xelatex and lualatex have their own failure modes around font loading. If you use \setmainfont{} from the fontspec package and the font isn't installed on the system, you'll get a font-not-found error that pdflatex would never produce because it doesn't use system fonts.
The skill detects which engine you're using from the log header and tailors its diagnosis accordingly. When you're deciding between engines for your use case, the LaTeX engine guide covers the tradeoffs in detail — pdflatex for compatibility, xelatex for Unicode and system fonts, lualatex for scripting. If you're running compilation through an API rather than locally, the FormatEx engine selection docs explain how to specify the engine per request.
Engine-Aware Compilation vs. Local TeX
One thing the skill can't fix is a TeX distribution that isn't installed or is misconfigured. If pdflatex isn't in your PATH, or your TeX Live installation is partial, you'll get OS-level errors before TeX even starts. The skill diagnoses TeX errors — not shell errors.
If you're hitting infrastructure problems rather than LaTeX syntax problems, the comparison in self-hosting LaTeX vs. a compilation API might be relevant. Running TeX Live locally has a high setup cost; using an API like FormatEx eliminates the installation entirely and lets you compile from any environment with an HTTP request.
Related Skills
The latex-skills pack includes several other skills that pair well with latex-fix:
- latex-write — generates LaTeX markup from a description, reducing the errors you need to fix in the first place
- latex-math — handles math-specific errors and equation formatting
- latex-packages — identifies which package provides a command or environment, useful when
latex-fixtells you "add the package that defines this"
Using latex-write to generate and latex-fix to repair covers the full authoring loop without leaving Claude Code.
Install the Skill and Stop Reading Error Logs
TeX error messages are designed for a compiler, not a developer. The latex-fix skill translates them into language you can act on immediately — no documentation spelunking, no Stack Overflow archaeology.
# Install just latex-fix
cp -r skills/latex-fix ~/.claude/skills/
# Or install all LaTeX skills
cp -r skills/latex-full ~/.claude/skills/Get the skill pack at github.com/formatex/latex-skills.
If you're compiling LaTeX as part of an application or pipeline, FormatEx provides a REST API for PDF compilation — no TeX Live installation required, engine selection per request, and output you can stream directly to users. The getting started guide has you up and running in under ten minutes.
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