Cloudflare response header transform rules

January 23, 2026 ยท View on GitHub

I serve Python files from my tools.simonwillison.net subdomain, which is a GitHub Pages site that's served via Cloudflare. For example:

https://tools.simonwillison.net/python/q3_tts.py

This is meant to be used with uv as described here:

uv run https://tools.simonwillison.net/python/q3_tts.py \
  'I am a pirate, give me your gold!' \
  -i 'gruff voice' -o pirate.wav

By default files with .py extensions are served with a content-type: application/octet-stream header. This means browsers will download them rather than displaying them directly to the user.

For code that you're going to execute on your machine it's nice to be able to preview it in your browser first!

Here's that default content-type header:

~ % curl -I 'https://tools.simonwillison.net/python/q3_tts.py'
HTTP/2 200 
date: Fri, 23 Jan 2026 14:52:54 GMT
content-type: application/octet-stream

I can't control how GitHub Pages serves files, but since the site is behind Cloudflare I can fix the problem there instead.

The area to look for here is called "Rules". Within that area the "Create rule" button has an option to create a "Response Header Transform Rule":

Rules Overview page. The Create rule menu is open showing a list of different rule types, including Redirect Rule, URL Rewrite Rule, Request Header Transform rule and Response Header Transform Rule.

I created a rule that looks like this:

Edit HTTP Response Header Transform Rule form - the rule name is set to tools.simonwillison.net .py files to text/plain, it has a Custom filter expression for the incoming requests matching Hostname equals tools.simonwillison.net and URI Path ends with .py - this shows an expression preview of http.host eq "tools.simonwillison.net" and ends_with(http.request.uri.path, ""py")) - then it uses Set static of content-type to text/plain; charset=utf-8

The response header transform rule

My new rule has the following settings:

  • Rule name: tools.simonwillison.net .py files to text/plain
  • When incoming requests match: Custom filter expression
  • Filter expression:
    • Hostname equals tools.simonwillison.net
    • AND URI Path ends with .py
  • Then:
    • Set static content-type to text/plain; charset=utf-8

I clicked 'Deploy' and within a few seconds saw this result:

~ % curl -I 'https://tools.simonwillison.net/python/q3_tts.py'
HTTP/2 200 
date: Fri, 23 Jan 2026 14:52:58 GMT
content-type: text/plain; charset=utf-8

And now https://tools.simonwillison.net/python/q3_tts.py displays the Python code directly in my browser.