Simon Willison’s Weblog

Introducing gisthost.github.io

1st January 2026

I am a huge fan of gistpreview.github.io, the site by Leon Huang that lets you append ?GIST_id to see a browser-rendered version of an HTML page that you have saved to a Gist. The last commit was ten years ago and I needed a couple of small changes so I’ve forked it and deployed an updated version at gisthost.github.io.

Some background on gistpreview

The genius thing about gistpreview.github.io is that it’s a core piece of GitHub infrastructure, hosted and cost-covered entirely by GitHub, that wasn’t built with any involvement from GitHub at all.

To understand how it works we need to first talk about Gists.

Any file hosted in a GitHub Gist can be accessed via a direct URL that looks like this:

https://gist.githubusercontent.com/simonw/d168778e8e62f65886000f3f314d63e3/raw/79e58f90821aeb8b538116066311e7ca30c870c9/index.html

That URL is served with a few key HTTP headers:

Content-Type: text/plain; charset=utf-8
X-Content-Type-Options: nosniff

These ensure that every file is treated by browsers as plan text, so HTML file will not be rendered even by older browsers that attempt to guess the content type based on the content.

Via: 1.1 varnish
Cache-Control: max-age=300
X-Served-By: cache-sjc1000085-SJC

These confirm that the file is sever via GitHub’s caching CDN, which means I don’t feel guilty about linking to them for potentially high traffic scenarios.

Access-Control-Allow-Origin: *

This is my favorite HTTP header! It means I can hit these files with a fetch() call from any domain on the internet, which is fantastic for building HTML tools that do useful things with content hosted in a Gist.

The one big catch is that Content-Type header. It means you can’t use a Gist to serve HTML files that people can view.

That’s where gistpreview comes in. The gistpreview.github.io site belongs to the dedicated gistpreview GitHub organization, and is served out of the github.com/gistpreview/gistpreview.github.io repository by GitHub Pages.

It’s not much code. The key functionality is this snippet of JavaScript from main.js:

fetch('https://api.github.com/gists/' + gistId)
.then(function (res) {
  return res.json().then(function (body) {
    if (res.status === 200) {
      return body;
    }
    console.log(res, body); // debug
    throw new Error('Gist <strong>' + gistId + '</strong>, ' + body.message.replace(/\(.*\)/, ''));
  });
})
.then(function (info) {
  if (fileName === '') {
    for (var file in info.files) {
      // index.html or the first file
      if (fileName === '' || file === 'index.html') {
        fileName = file;
      }
    }
  }
  if (info.files.hasOwnProperty(fileName) === false) {
    throw new Error('File <strong>' + fileName + '</strong> is not exist');
  }
  var content = info.files[fileName].content;
  document.write(content);
})

This chain of promises fetches the Gist content from the GitHub API, finds the section of that JSON corresponding to the requested file name and then outputs it to the page like this:

document.write(content);

This is smart. Injecting the content using document.body.innerHTML = content would fail to execute inline scripts. Using document.write() causes the browser to treat the HTML as if it was directly part of the parent page.

That’s pretty much the whole trick! Read the Gist ID from the query string, fetch the content via the JSON API and document.write() it into the page.

Here’s a demo:

https://gistpreview.github.io/?d168778e8e62f65886000f3f314d63e3

Fixes for gisthost.github.io

I forked gistpreview to add two new features:

1. A workaround for Substack mangling the URLs
2. The ability to serve larger files that get truncated in the JSON API

I also removed some dependencies (jQuery and Bootstrap and an old fetch() polyfill) and inlined the JavaScript into a single index.html file.

The Substack issue was small but frustrating. If you email out a link to a gistpreview page via Substack it modifies the URL to look like this:

https://gistpreview.github.io/?f40971b693024fbe984a68b73cc283d2=&utm_source=substack&utm_medium=email

This breaks gistpreview because it treats f40971b693024fbe984a68b73cc283d2=&utm_source... as the Gist ID.

The fix is to read everything up to that equals sign. I submitted a PR for that back in November.

The second issue around truncated files was reported against my claude-code-transcripts project a few days ago.

That project provides a CLI tool for exporting HTML rendered versions of Claude Code sessions. It includes a --gist option which uses the gh CLI tool to publish the resulting HTML to a Gist and returns a gistpreview URL that the user can share.

These exports can get pretty big, and some of the resulting HTML was past the size limit of what comes back from the Gist API.

As of claude-code-transcripts 0.5 the --gist option now publishes to gisthost.github.io instead, fixing both bugs.

Here’s the Claude Code transcript that refactored Gist Host to remove those dependencies, which I published to Gist Host using the following command:

uvx claude-code-transcripts web --gist