Built-in skills: docx, pptx, xlsx, pdf

Skills are how Cowork knows what to produce, not just what to say. In Claude Chat, skills are advisory — they shape responses in the conversation window. In Cowork, skills are operational. They govern every file Claude creates autonomously. When Cowork produces a spreadsheet, the xlsx skill determines the structure, formulas, and formatting of that file. When it produces a presentation, the pptx skill controls the layouts, speaker notes, and slide architecture.

The built-in skills are the reason you can ask for "an Excel workbook with a VLOOKUP" and get one. Ask the same thing in Chat and you get instructions for building one.

The built-in skills

Built-in skills load automatically — no setup required. Cowork reads your task description and loads the relevant skill based on what the task needs. You don't invoke them by name; they fire on description matching.

docx — produces Word documents with real document structure: table of contents, heading hierarchy, page numbers, tracked changes, and comments. Not a formatted text file — a .docx that opens in Word with the full style system intact.

pptx — produces PowerPoint presentations with speaker notes, slide layouts, and theme application. The output is a .pptx that opens in PowerPoint, with slides, not a text outline formatted to look like slides.

xlsx — produces Excel workbooks with working formulas (VLOOKUP, INDEX/MATCH, SUM across ranges), conditional formatting, multi-tab structure, and data validation where appropriate. The cells contain formulas, not static values — the workbook functions as a workbook.

pdf — handles form filling, PDF merging and splitting, and OCR (optical character recognition) for scanned documents. You can point at a scanned contract and ask for the key terms extracted; the pdf skill reads the image and returns text.

canvas-design — handles visual design outputs: diagrams, visual layouts, and design-adjacent document formats.

How skills are triggered

Skills auto-load based on description matching. Cowork reads your task and infers which skill applies. This means the output file format you specify drives skill selection:

• Ask for out/report.xlsx → xlsx skill loads
• Ask for out/brief.docx → docx skill loads
• Ask for out/launch-deck.pptx → pptx skill loads

This is why the file extension in your output specification isn't cosmetic. "Produce a spreadsheet" is ambiguous. "Produce out/report.xlsx" is not — the extension tells Cowork exactly which skill to use and what structural standards to apply.

The practical difference

Here's the concrete test. Ask Cowork for "a spreadsheet tracking project milestones with conditional formatting that highlights overdue items in red." You get a .xlsx file where:

• Column headers are in row 1
• Date columns are formatted as dates
• A conditional formatting rule applies red fill when a deadline is before today's date
• The formulas reference real cell ranges

Open the same file in Excel. Everything works. Nothing needs to be rebuilt.

Ask for the same thing in Chat. You get either a markdown table (not a spreadsheet) or step-by-step instructions for building the spreadsheet yourself. Chat can describe the structure; it cannot produce the file.

The distinction matters because the point of delegation is getting finished deliverables, not intermediate outputs you have to complete yourself. Skills are what make Cowork's outputs usable as-is rather than as starting points for manual work.