Excel spreadsheets (xlsx) skill

The xlsx skill is how Claude produces spreadsheets that still work after you change a number. Its first rule is to write formulas, never pasted results, so a total stays a =SUM(...) that updates when inputs change.

What it does (plain English)

The skill covers opening, creating, editing and repairing .xlsx, .xlsm, .csv and .tsv files. It sets out requirements Claude must meet on every file it hands back:

• Zero formula errors. Claude may not deliver while the recalculation check reports errors.
• Formulas, not hardcoded results. Totals and ratios stay live.
• Follow your spec literally. Exact tab names, exact column headers, the formula you asked for.
• Document assumptions. Every hardcoded number gets a note saying where it came from.
• Respect existing files. When editing your workbook, Claude matches its conventions, writes only to its input cells and leaves existing formulas alone.

The key step is recalculation. The library Claude uses, openpyxl, saves formulas without calculated values, so most viewers would see blank cells. The skill’s recalc.py script opens the file in LibreOffice, computes every formula, rewrites the file and reports any #REF! or #NAME? cells by location. The skill is candid that a clean run only proves formulas evaluate, not that they are correct.

Is it for you?

Use it if you build budgets, trackers, price lists or simple financial models and want something a colleague can open in Excel and extend. It is also good at turning a messy export (wrong headers, junk rows) into a clean table.

Look elsewhere if the deliverable is really a chart image, a dashboard web page or a Google Sheet. The skill’s description says it should not trigger when the main output is a Word document, HTML report, script or Google Sheets integration.

What you can do with it

1. Clean an export. “This CSV from our CRM has merged headers and blank rows. Make it a clean xlsx with one header row and a totals row.”
2. Build a model. “Three-year revenue model: inputs on an Assumptions tab, projections on a Model tab.” The skill puts each assumption in its own labeled cell and references it, so =B5*(1+$B$6) instead of =B5*1.05.
3. Finance formatting. Unless you say otherwise, it follows a convention: blue text for hardcoded inputs, black for formulas, green for links to other sheets, yellow fill for key assumptions, negatives in parentheses, percentages stored as fractions.
4. Make a fill-in template. “Make a timesheet my team fills in.” The skill adds a short legend naming which cells to edit and one example row.
5. Repair errors. “Find why column F shows #VALUE!.” A known cause the skill lists: sheet names with spaces must be quoted in cross-sheet references.

How to set it up

claude.ai or the Claude desktop app

1. Turn on Code execution and file creation in Settings > Capabilities.
2. Check Customize > Skills: Anthropic’s document skills sit under From Anthropic & Partners.
3. Upload your file or describe the sheet, and ask for an Excel file.

Team and Enterprise members who cannot see the setting need an Owner to switch on code execution and skills under Organization settings > Capabilities.

Claude Code

1. Run /plugin marketplace add anthropics/skills
2. Run /plugin install document-skills@anthropic-agent-skills
3. Ask: “Use the xlsx skill to turn data/sales.csv into a workbook with a monthly summary tab.”

The plugin brings docx, pdf and pptx along with xlsx. Claude Code estimates the xlsx skill at roughly 330 tokens always loaded and about 3k tokens when it runs.

Watch out for

Functions it avoids on purpose. This list is checkable in the skill file and saves real confusion:

Function	What the skill does
SUMIFS, INDEX, MATCH, IFERROR, SUMPRODUCT	Preferred; work everywhere
TEXTJOIN, CONCAT, IFS, SWITCH, MAXIFS, MINIFS	Allowed, but written with an _xlfn. prefix or they show #NAME?
XLOOKUP, XMATCH, SORT, FILTER, UNIQUE, SEQUENCE	Never used; sorting and filtering happen before writing

If you specifically want XLOOKUP in your file, say so and check the result in Excel yourself.

• Macros. .xlsm files lose their macros unless Claude loads them with the right option. Keep a backup of macro workbooks.
• Reading values destroys formulas if saved. The skill warns that one openpyxl mode replaces every formula with a fixed number on save. Another reason to keep your original.
• License. Source-available, not open source. The LICENSE.txt bans copying the skill outside Anthropic’s services and redistributing it.
• Local tools. In Claude Code you need openpyxl, pandas, markitdown and LibreOffice for recalculation.

Alternatives

• Google Sheets connector if the sheet must stay in Google Drive.
• PDF skill for pulling tables out of PDFs before they go into a workbook.
• Word documents skill for the written report that goes with the numbers.