Simon Willison’s Weblog

simonw/claude-skills. One of the tips I picked up from Jesse Vincent's Claude Code Superpowers post (previously) was this:

Skills are what give your agents Superpowers.

The first time they really popped up on my radar was a few weeks ago when Anthropic rolled out improved Office document creation. When the feature rolled out, I went poking around a bit -- I asked Claude to tell me all about its new skills. And it was only too happy to dish.

In that transcript Jesse asks how the skill system works and Claude replies:

Looking at the skills system, here's how it works:

Current Skills (in /mnt/skills/public/):

• docx - Word document handling
• pdf - PDF processing and form filling
• pptx - PowerPoint presentations
• xlsx - Excel spreadsheets

This looks like it's part of Claude's brand new Code Interpreter feature! I wrote about that extensively last month, but I missed that there was a /mnt/skills/public/ folder full of fascinating implementation details.

So I fired up a fresh Claude instance (fun fact: Code Interpreter also works in the Claude iOS app now, which it didn't when they first launched) and prompted:

Create a zip file of everything in your /mnt/skills folder

This worked, and gave me a .zip to download. You can run the prompt yourself here, though you'll need to enable the new feature first.

I've pushed the contents of that zip to my new simonw/claude-skills GitHub repo.

So now you can see the prompts Anthropic wrote to enable the creation and manipulation of the following files in their Claude consumer applications:

• pdf - PDF files
• docx - Microsoft Word
• pptx - Microsoft PowerPoint decks
• xlsx - Microsoft Excel

In each case the prompts spell out detailed instructions for manipulating those file types using Python, using libraries that come pre-installed on Claude's containers.

Skills are more than just prompts though: the repository also includes dozens of pre-written Python scripts for performing common operations.

pdf/scripts/fill_fillable_fields.py for example is a custom CLI tool that uses pypdf to find and then fill in a bunch of PDF form fields, specified as JSON, then render out the resulting combined PDF.

This is a really sophisticated set of tools for document manipulation, and I love that Anthropic have made those visible - presumably deliberately - to users of Claude who know how to ask for them.