Show HN: Cursor for Userscripts

Posted by mifydev 3 hours ago

Counter26Comment9OpenOriginal

I’ve been experimenting with embedding an Claude Code/Cursor-style coding agent directly into the browser.

At a high level, the agent generates and maintains userscripts and CSS that are re-applied on page load. Rather than just editing DOM via JS in console the agent is treating the page, and the DOM as a file.

The models are often trained in RL sandboxes with full access to the filesystem and bash, so they are really good at using it. So to make the agent behave well, I've simulated this environment.

The whole state of a page and scripts is implemented as a virtual filesystem hacked on top of browser.local storage. URL is mapped to directories, and the agent starts inside this directory. It has the tools to read/edit files, grep around and a fake bash command that is just used for running scripts and executing JS code.

I've tested only with Opus 4.5 so far, and it works pretty reliably. The state of the file system can be synced to the real filesystem, although because Firefox doesn't support Filesystem API, you need to manually import the fs contents first.

This agent is really useful for extracting things to CSV, but it's also can be used for fun.

Demo: https://x.com/ichebykin/status/2015686974439608607

Comments

Comment by rahimnathwani 29 minutes ago

It would be cool if you could make this work with Gemini Flash, with keys from AI Studio. I imagine that would expand the set of people who would try it out, because they could use 'free' keys and not worry about unexpected bills.

Comment by mifydev 7 minutes ago

That's a good point, I'll add support for other models shortly.

Comment by Akranazon 1 hour ago

I'm working on a version of this, https://www.quillmonkey.com/ so you got ahead of me. I imagine there are many versions of this coming. Interesting what set of tools you went with.

Comment by mifydev 1 hour ago

Oh that's cool! I've just used wxt to pack extension for firefox and chrome and just used typescript and plain anthropic api. My goal is to make this run fully inside the browser, without any helper binaries, like I've seen with others.

Comment by Akranazon 52 minutes ago

Your project seems pretty close to where mine was a couple weeks ago, where I was focused on a BYOK solution (user-entered Anthropic API key). I saw there was another similar extension already released in the app store (RobotMonkey) which hooks up to their own backend service, and offers subscriptions. For my project, I think that's the right way to go.

It's funny what details about our designs are similar through accident. And what other things are completely different. I can show you my design potentially.

Representing websites in a virtual filesystem is creative and definitely makes it easier for the agent to collect information about the page. But I'm confused between the `Bash` and the `Edit` tools. It seems like one uses the chrome executeScript API, and the other updates the file system. But if it's just doing file writes, are those edits visible in the browser, and persistent across sessions?

Comment by mifydev 8 minutes ago

Backend service is definitely way to go if you want to serve models for the user.

So Bash and Edit tools are a bit weird, Bash tool is essentially JS execution, and Edit tool automatically generates a script that performs the edits on the page. These tools are needed for the model to explore the page, whatever it does at the end it creates a separate script that will be applied on the page load.

Comment by Esophagus4 1 hour ago

Awesome! So the agent has access to the DOM/JS running in the browser?

That’s one of my biggest headaches writing user scripts currently: I write the script in an IDE with Claude then copy it to the browser / manually test it in the browser, then copy the results back to Claude or tell it what went wrong.

Looking forward to trying this.

Comment by Zekio 43 minutes ago

to my knowledge all the major userscript extensions, at least allow watching for file changes so you don't have to copy it manually, so you can just refresh the page to test

Comment by mifydev 1 hour ago

Yup, full access to DOM! Still needs a lot of optimizations, but the trick is that the agent reads the DOM as file, so it can grep parts of it naturally.