Show HN: Cursor for Userscripts

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

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mrandish an hour ago | parent | next [-]

Cool! Just this week I've been playing around using web chatbots to write userscript. So far, I've just been cutting/pasting code from ViolentMonkey's basic edit window - which is a clunky workflow and a real editor like VSCode would be great. Even though the script is quite short, I've already found I definitely need auto-rollback due to AI regressions. So this sounds very useful!

I use Firefox (on Win) but I guess it might be worth using Chrome just for write/test iteration and then copy over to FF. Looking just now it appears ViolentMonkey on Chrome died in the Manifest V3 apocalypse but is still on Edge.

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rahimnathwani 5 hours ago | parent | prev | next [-]

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.

	▲	mifydev 5 hours ago \| parent [-]
		That's a good point, I'll add support for other models shortly.

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Akranazon 6 hours ago | parent | prev | next [-]

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.

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mifydev 6 hours ago | parent [-]

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.

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Akranazon 6 hours ago | parent [-]

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?

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mifydev 5 hours ago | parent [-]

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.

	▲	Akranazon 4 hours ago \| parent [-]
		Oh neat. So the edit tool is like a convenient API/wrapper for it to eg add HTML to some element? I guess theoretically that can also be achieved through Bash as well, but the tool fits closer to an interface we know exiting agents are good at.

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cxr 3 hours ago | parent | prev | next [-]

<https://web.archive.org/web/20120601101804/https://blog.mozi...>

<https://sites.coecis.cornell.edu/leshed/files/2015/12/coscri...>

<https://ofb.net/~tlau//research/papers/p97-lin.pdf>

<https://github.com/jeffnichols-ibm/coscripter-extension>

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Esophagus4 6 hours ago | parent | prev | next [-]

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.

	▲	Zekio 5 hours ago \| parent \| next [-]
		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
	▲	mifydev 6 hours ago \| parent \| prev [-]
		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.

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_false 4 hours ago | parent | prev [-]

Love the decision to edit DOM directly. More LLM tools should carefully consider their training environments instead of treating LLMs like AI Gods.