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Thanks Simon!

My tool collection [0] is inspired by yours, with a handful of differences. I'm only at 53 tools at the moment.

What I did differently:

Hosted on Cloudflare Pages. This gives you preview URLs for pull requests out the box. This might be possible with Github Pages but I haven't checked. I've used Vercel for similar projects in the past. Cloudflare seems to have the odd failed build that needs a kick from their dashboard.

Some tools can make use of Workers/Functions for backend processing and secrets. I try to keep these to a minimum but they're occasionally useful.

I have an AGENTS.md that's updated with a Github action to automatically pull in Claude-style Skills from the .skills directory. I blogged about this pattern and am still waiting for a standard to evolve [2].

I have a base stylesheet that I instruct agents to pull in. This gives a bit of consistency and also let's them use Tailwind, which they'd seem to love.

[0] https://tools.dave.engineer/

[1] https://github.com/dave1010/tools/tree/main/functions

[2] https://dave.engineer/blog/2025/11/skills-to-agents/

I was also heavy on these single-file HTML tools. My problem was not being able to sync the localstorage data between PC and smartphone. I made a paid service which did that automatically. It didn't take off unfortunately, and I am keeping it on maintenance mode because it has some amount of free users that rely on it.

For anyone interested, to achieve synchronization I basically just use the https://github.com/google/diff-match-patch lib and save the patches in a db for each version with a version id. Then there's a generic JS file that I inject to uploaded HTML files that monkey patches the localstorage methods and optimistically updates the localstorage at the same time sending the diff to the server to save to the db.

I just shipped a new one of these a few minutes ago (from my phone).

I found out about a new Python HTML parsing library - https://github.com/EmilStenstrom/justhtml - and wanted to try it out but I'm out without my laptop. So I had Claude Code for web build me a playground interface for trying it out: https://tools.simonwillison.net/justhtml

It loads the Python library using Pyodide and lets you try it out with a simple HTML UI.

The prompts I used are in this PR: https://github.com/simonw/tools/pull/156

Makes me wish there was a cross-platform equivalent to HTML Applications [1]. They were hell to develop at the time, but being able to create single-text-file tools with a browser UI, file system and network access, and app automation via COM was amazing

[1] https://en.wikipedia.org/wiki/HTML_Application

Great timing - I've taken up vibecoding and picked personal tools with simple HTML/CSS/JS/python stack as the learning ground too.

Personal tools seem like a reasonable place for happy path vibecoding given small blast radius and LLMs can do that sort of static page in front of python backend really well.

I've also been surprised how much active learning I'm doing despite specifically not look at code. Between the need to spec things out carefully (plan.md) and fast iteration loop it's been a huge boost. Having the LLM look at a plan.md and suggest improvements has lead to a lot of "oh I didn't think about that" learning on architecture and user requirements link.

Presumably much of that learning boost is because I'm a hobbyist tier programmer, guessing professionals wouldn't experience the same since they learned this via manual coding trial & error over years.

Great work, Simon -- thanks for sharing!

One tool I'd really like to see in this format is a simple "turn the background of this PNG to transparent". Models still refuse to follow the instruction to create transparent backgrounds for logos they create, and I often have to look for other tools doing this as post-processing.

It's possible that this is too complicated for the "few hundred lines of js" code envelope, though.

I'm a big fan of using this kind of thing at work. Rather than setting up a web server or packaging something into an installer, I just pop the bundled HTML file on a shared drive.

The CDN approach works, but I don't love depending on some third-party service just so your app continues working. Instead, I like using vite with vite-plugin-singlefile. This lets you package your JS and CSS into a single HTML: https://www.npmjs.com/package/vite-plugin-singlefile

Nice, I do this often enough that I created a bookmarklet to download an HTML file from clipboard after copying ChatGPT's code block.

I've also been using LLMs to create and maintain a "work assist" Chrome extension that I load unpacked from a local directory. Whenever I notice a minor pain point, I get the LLM to quickly implement a remedy. For example, I usually have several browser tabs open for Jira, and they all have the same company logo as the favicon, so my Chrome extension changes the favicon to be the issue type icon (e.g. Bug, Story, etc) when the page loads. It saves a little time when I'm looking for a specific ticket I've already opened.

Really enjoyed this, interesting read as always! It reminded me of Google Labs’ recent GenTabs project [1], and also of a recent ACM paper on user-assembled LLM-mediated tools from web content [2]. Feels like similar concepts are emerging in multiple places, all centered around lightweight intent-driven tools rather than traditional apps, which I think makes a lot of sense. Curious how this will evolve!

[1] https://labs.google/disco

[2] https://dl.acm.org/doi/10.1145/3706598.3713285

Amazing article with lots of useful info. Big kudos, Simon.

This really showcases the power of the single page apps and why web will be always ahead of native for this kind of Swiss Army Knife tools.

With LLMs, it gets ridiculously easy to “develop” (generate) those too.

The barcode font trick from valbaca is brilliant. I've done similar things with base64-encoded data URIs to make tools completely standalone - no network calls, no CDN dependencies.

One pattern I've settled into: keeping tools under ~200 lines of JS total. Past that threshold I start losing the ability to hold the whole thing in my head, and the main benefit of these tools is that you can open them in a text editor and understand everything immediately.

The CORS limitation that xnx mentions is real though. I've worked around it a few times by having tools accept paste-from-clipboard instead of fetching URLs directly. Less elegant but it keeps the tool self-contained and avoids the proxy problem simonw mentioned.

Why not introduce a single shared CSS for style consistency? Not full CSS separation, each tool could still have its local CSS.

Things like styling buttons, responsiveness, and so on are better solved once.

A good rule of thumb is: if the shared CSS fails to load, page still fully works but it might be uglier (weird fonts, etc). That's a reasonable rule for proper isolation (tools remain simple to understand, code remains reusable, etc).

I love the idea of self-contained tools, but you're already using CDNs. Having a shared CSS wouldn't hurt and actually make the tools better.

I would go as far as having a shared JS too (same idea, works if it doesn't load).

That's essentially what I did in https://alganet.github.io/spiral/ (also vibe coded).

Each spiral is mostly independent. You can go ahead and delete the shared CSS from the <head>, they still work and don't break funcionality. However, by having the shared CSS I made them consistent, made them friendly to phone users and so on.

I also started building similar tools in early 2025 and have built around 80 of them so far: https://tools.o14.ai/. Recently, I built a data viewer app (https://tools.o14.ai/excel-qa-review.html) where I can upload an Excel or CSV file containing chat queries and answers for quick manual review.

Reviewing data in Excel is painful, especially when answers are in HTML or Markdown, because you don’t get proper rendering. Building small, custom tools that reduce the friction of reviewing data makes life much easier and more pleasant. These days, I use Claude Code for Web to build most of these apps, and they are deployed on Vercel.

Pretty nice. I've been using LLMs to generate different Python and JS tools for wrangling data for ontology engineering purposes.

More recently, I've found a lot of benefit from using the extended thinking mode in GPT-5 and -5.1. It tends to provide a fully functional and complete result from a zero-shot prompt. It's as close as I've gotten to pair programming with a (significantly) more experienced coder.

One functional example of that (with 30-50% of my own coding, reprompting and reviews) is my OntoGSN [1] research prototype. After a couple of weeks of work, it can handle different integration, reasoning and extension needs of people working in assurance, at least based on how I understood them. It's an example of a human-AI collab that I'm particularly proud of.

[1] Playground at w3id.org/OntoGSN/

One thing I tend to do myself is use https://generator.jspm.io/ to produce an import map once for all base dependencies I need (there's also a CLI), then I can easily copy/paste this template and get a self-contained single-file app that still supports JSX, React, and everything else. Some people may think it's overkill, but for me it's much more productive than document.getElementById("...") everywhere.

I don't have a lot of public examples of this, but here's a larger project where I used this strategy for a relatively large app that has TypeScript annotations for easy VSCode use, Tailwind for design, and it even loads in huge libraries like the Monaco code editor etc, and it all just works quite well 100% statically:

HTML file: https://github.com/blixt/go-gittyup/blob/main/static/index.h...

Main entrypoint file: https://github.com/blixt/go-gittyup/blob/main/static/main.js

I made a heart monitor for very high heart rate https://github.com/SMUsamaShah/heart-rate which can be installed on phone as a PWA too. It could use some more polish, more features which I could do with an LLM again.

LLMs are generating app for an idea that can fit in few hundred lines of html/js. Had an idea that what if brushes were dithered in a painting tool and made a dithered painting tool. https://github.com/SMUsamaShah/dither-painter

These tools and code are ephemeral though. You don't need to use mine for example. Just ask the LLM of the time to implement the idea and in most cases it will work fine.

I love this idea! I've also been using LLMs to build tiny utilities and automations for myself, but instead of using HTML, I'm building them as:

- Shell scripts, AppleScripts, etc. that I trigger from Alfred

- Obsidian plugins

- The occasional Emacs Lisp function

They serve a similar purpose for me as OP's HTML Tools, in the sense that they let me automate a small part of my workflow that I wouldn't otherwise have automated. If I have to choose between writing AppleScript and just doing something manually, I'll pick doing something manually 100% of the time. But if I can just ask an LLM to write the automation for me and then test it in a bunch of different scenarios, the choice becomes much easier.

After reading this post, I really want to try moving some of my automations to the web. Using HTML/JS/CSS for some of these tools will let me solve a whole different set of problems. E.g. I could more easily build automations for the non-techy folks in my family instead of just keeping them to myself.

Surprised AI studio isn’t recommended here - you can go from prompt->preview->deploy in like a couple seconds.

They have a library of sample apps you can edit but I wish they included the prompts and history to build each since I generally can’t get large apps to work - after a while the I’ll just produces more bugs as complexity grows. But I’m also a bad vibe coder and never read the code so entirely my fault :)

Next evolution: UX should allow end users to build the prompt, and then the tool is generated and deployed at a random URL. Tool also keeps original prompt, so any user can customize into a new (random URL) tool page.

I also find that sticking to a single file makes coding agents perform better (fewer surgical edits, faster outputs, sensible changes, etc).

Not sure why, but the moment the file is split into files and subfolders, coding agents tend to do a lot more changes that what is absolutely necessary. That way a single html file wins!

I’ve been making stuff like this for a while (pre-LLM days). They are great little side projects that tend to be useful without getting overly complex. While I’ve vibe coded a couple just to get something done that I needed, I tend to like to write them myself still, as I enjoy the process.

> The alternative to CDNs is to use npm and have a build step for your projects. I find this reduces my productivity at hacking on individual tools and makes it harder to self-host them.

No. You can vendor these scripts & host them 1st party so you aren’t leaking data to these CDNs or risk users not actually getting the scripts. It isn’t like CDNs give you a performance boost anymore.

https://httptoolkit.com/blog/public-cdn-risks/

> "If you want to see the code and prompts, almost all of the examples in this post include a link in their footer to “view source” on GitHub. The GitHub commits usually contain either the prompt itself or a link to the transcript used to create the tool."

As if your steady stream of learning-in-public experiments and insights weren't generous enough. Seriously, massive kudos for sharing all the details.

I love this pattern. I’ve been using it myself here: https://pseudosavant.github.io/ps-web-tools/

Create PDFs from images, a Wordle hint/solver, or a classic DVD screensaver. Lots of stuff.

This was the first submission: https://news.ycombinator.com/item?id=46236333

I really like the simplicity here (maybe because it mirrors my own approach). No build chain, no node_packages, no frameworks.

I wonder if packaging the results as web components would be the next logical step.

I am building platform which makes it easy to host this kind of app but it has own set of idiosyncrasy u have to adhere to.

https://github.com/blue-monads/potatoverse

> HTML tools may not have access to server-side databases for storage but it turns out you can store a lot of state directly in the URL.

I use indexedDB for it and will use sqlite if I start to get more serious data needs.

Unrelated, but I bought a walking pad, and because I often use it at home while working on the laptop, and I wanted a nice UI to track the speed/time etc, I just asked claude to do one:

https://pastebin.com/5HRLh1G6

it does something like this

https://imgur.com/a/888BtpG

and connects through BLE

I've been building these for years. I think its a great entrypoint for new developers and can be incredibly useful at work for solving short term inconveniences or scripts that help automate some tasks.

I have a Vue3 started template I host at https://http://vue-template.spaghet.me/ and all I have to do is curl and I'm ready to go.

Showcase:

https://timer.spaghet.me/ https://colors.spaghet.me/ https://box.spaghet.me/ https://talk.spaghet.me/ https://farming.ope.cool https://stitch.ope.cool https://draw.ope.cool https://walz.ope.cool

HTML tools are a good name. I called them something like "a local html file"

One problem I solved with this was a packer needed to scan a few (10-40) ids into his barcode scanner. It was not enough where pulling up their bulk-id-uploader program but also too tedious to go to some "number to barcode" website.

Turns out, barcodes can be made from a google font!

https://fonts.google.com/specimen/Libre+Barcode+39

You can just display a number using that font. Then hooked up a for-loop that's progressed by pressing the space bar: paste in IDs, scan first, space, scan next, repeat.

If HTML tools could make network calls (CORS be damned), they could replace a huge portion of hosted apps.

I've been calling these Single-File Web Apps. I've written a couple myself:

≈ https://fuzzygraph.com ≈ https://hypervault.github.io/

In the same spirit I have started building single page nanodjango +htmx apps hosted on railway, starting with, you guessed it, a Todo list app. No login needed.

https://web-production-1fc69.up.railway.app/

To make it extra useful, they should come with REST API and swagger docs on how to use the API.

Nice. I do something similar, but with Bun + Preact, since I love that stack. I’ve found it very productive after the hurdle of the initial project setup (in which I wasted time crafting an RPC layer, etc, because I wanted to.)

I've also been creating HTML tools over the years when I couldn't find client-side only websites for PDF to SVG, CSV to Sheet, Audio to Video, Video to MP4.

I list them at https://client-side.app/

Endlessly churning out tools with vibe coding sounds quite boring to me. The world went and changed I guess.

I've done something similar for a couple tools.

I tend to make them as Python servers which serve plain html/js/css with web components. I know this is a bit more complicated than just having a single html file with inline js and css, but the tools I made were a bit too complicated for the LLMs to get just right, and separating out the logic into separate js files as web components made it easy for me to fix the logic myself. I also deliberately prompted the LLMs to avoid React because adding I didn't want to need a build step.

The only one I actually still use is the TODO app I made: https://github.com/cooljoseph1/todo-app It stores everything in a JSON file, and you can have multiple TODO lists at once by specifying that JSON file when you launch it.

I've found a lot of success building in Alpine.js but I'm surprised that most people don't do this / don't like it. It makes managing reactivity a bit easier than full vanilla JS

Have been doing the same except I do use React because its my hammer haha. But I agree if you're not used to it then pure html/cs/js make sense.

https://www.hackyexperiments.com/micro

Awesome. This is the way.

> Avoid React, or anything with a build step. The problem with React is that JSX requires a build step, which makes everything massively less convenient. I prompt “no react” and skip that whole rabbit hole entirely.

I haven’t found too many issues with loading React and Babel from a CDN. I find React easier to read than straight HTML/JS. I find it more annoying to code in but seeing what state is needed in what components is a pleasant reading experience for me with single file tools.

It's so sad that there's no wysiwyg editor besides seamonkey which is truly opensource and no electron stuff.. sigh :(

also sad, that XHTML was abandoned.

Another useful pattern for certain types of app is to include a function that saves the HTML file to your local drive/memory as a new file - for example, if the app features user inputs like writing or drawing.