Claude's Excel Skill: Built In Already? A Real Report-and-Update Test

If you want Claude to build an Excel report with real formulas and a chart, then update it with next month's numbers, do you need to install the xlsx skill? This guide runs a real test: one folder with nothing installed, one folder with Anthropic's document-skills plugin. The surprise: the folder with nothing installed used the Excel skill anyway, because Claude Code already ships Anthropic's Office skills and calls them when needed.

Tested on 12 October 2026 with Claude Code 2.1.296 (Opus 5.5, high effort) on Linux, auto mode on, signed in with a Claude Max account. The sales figures are sample data and the three branch names are fictional. Each setup ran once, so this is a hands-on observation, not a statistic. Not tested with a Free account.

Watch the real test run (Vietnamese narration): https://youtu.be/WMiJE17Pghs

The test: two folders, two tasks

ban-hang.csv has 18 rows: monthly revenue and order counts from 2026-04 to 2026-09 for three branches (Cầu Giấy, Hai Bà Trưng, Long Biên). Each folder did two tasks, with prompts written in Vietnamese:

1. Build the report: a bao-cao.xlsx with a Data sheet holding the raw figures; a Summary sheet with total revenue and total orders per branch, each branch's share of revenue, and average revenue per order; plus a column chart of monthly revenue for the three branches. No follow-up questions.
2. Add October from thang-10.csv (3 rows): append to the Data sheet, update the summary and the chart, keep all existing formatting, save as bao-cao-t10.xlsx.

The khong-skill folder had nothing installed. The co-skill folder had the document-skills plugin, and its prompts started with "Use the xlsx skill." Every run was a fresh Claude Code session.

Nothing installed, and Claude still loaded the xlsx skill

The first thing Claude did in the empty folder was load a skill:

anthropic-skills:xlsx was not something we installed. Claude Code's config folder has ~/.claude/skills/synced/…/ with Anthropic's skills already in it: xlsx, docx, pptx, pdf, skill-creator and a few more. Claude Code syncs these to the machine after you sign in with a Claude account. The xlsx skill triggers whenever a spreadsheet is the input or output, so the prompt does not need to name it.

That is the opposite of the previous test with the frontend-design skill, which Claude did not use until the prompt named it. The built-in Excel skill was called right away, in both tasks.

The result: real formulas, not pasted numbers

In under two minutes, typing included, Claude wrote bao-cao.xlsx. Before reporting back, it recalculated the formulas with LibreOffice and checked the totals against its own sums from the CSV.

What makes the file useful is inside it:

• Totals are formulas, such as =SUMIFS('Dữ liệu'!$D$2:$D$19,'Dữ liệu'!$B$2:$B$19,$A5). Change a raw number and the summary and chart follow.
• Division by zero is guarded: average revenue per order is =IF(C5=0,0,B5/C5).
• Raw inputs are in blue text, formulas in black, a common convention in financial workbooks.
• A column chart of monthly revenue, one color per branch.

This matches what the xlsx skill's SKILL.md requires: formulas instead of precomputed numbers, zero formula errors, and a full LibreOffice recalculation before handing the file over.

Adding October: formatting kept, formulas extended

The harder task is adding rows to an existing file without breaking its formatting or chart. In the empty folder, Claude loaded anthropic-skills:xlsx again on its own.

One command failed midway; Claude read the error and carried on. It inspected the workbook's internal structure, added three October rows to the Data sheet, extended the formula ranges to row 22, changed the period line to 2026-04 to 2026-10, added October to the chart, and recalculated with LibreOffice. The original bao-cao.xlsx was left untouched.

A quick check: Cầu Giấy earned 664,824,000 in the first six months and 91,476,000 in October, 756,300,000 in total, exactly the figure in the new report.

With document-skills installed: the same result

The second folder installed document-skills from the anthropics/skills repository:

claude plugin install document-skills@anthropic-agent-skills --scope project

The prompt started with "Use the xlsx skill." and the terminal showed Skill(document-skills:xlsx). Claude worked almost the same way: write the file, recalculate, export a PDF preview to check it. After the October update, the two summaries matched:

Revenue, orders, share and average per order are identical for every branch. Only the layout differs: the plugin version put the chart to the right of the tables and added a note explaining the calculations.

Takeaways: making Excel files with Claude Code

1. No extra install needed for Excel. Signing in to Claude Code with a Claude account gave us the xlsx skill; installing document-skills made no difference in this test.
2. Watch for the Skill(...) line in the terminal to see whether Claude used a skill. If it is missing and you want to be sure, name the skill in the prompt.
3. Spell out the structure: sheet names, columns, formulas, chart type. When updating, add "keep all existing formatting" and save to a new file so the original survives.
4. Open the file and check a few numbers before sending it. The xlsx skill's own SKILL.md warns that formulas without errors are not necessarily right: a range off by one row still gives wrong numbers.

New to skills? Read What Are Claude Skills? Install One From GitHub and Test the PowerPoint Skill. To write your own, see Create a Claude Skill With skill-creator. If you have Excel through Microsoft 365, compare with using Copilot in Excel.

FAQ

Does Claude Code come with an Excel skill?

In this test, yes. Signed in with a Claude Max account, Claude Code had a ~/.claude/skills/synced/ folder with Anthropic's xlsx, docx, pptx and pdf skills, and called the xlsx skill automatically for a spreadsheet task. We have not tested a Free account.

Do I need document-skills for Claude to make Excel files?

Not for the report in this guide. With and without document-skills, the numbers, formulas and chart were the same; only the layout differed. The plugin helps if your machine has no built-in xlsx skill (no Skill(...) line appears when you work on a spreadsheet).

Does Claude's Excel file contain formulas or just numbers?

Real formulas. Totals use SUMIFS pointing at the Data sheet, and averages guard against division by zero. That is also a requirement in the xlsx skill's SKILL.md.

How can I tell whether Claude is using a skill?

Look at the terminal: a Skill(skill-name) line with "Successfully loaded skill" appears when Claude uses one. Without that line, Claude is working from general knowledge.