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Ask HN: Browser extension vs. native app for structured form filling?
I’m working on a project called Injectless — a browser extension that allows websites to explicitly declare which data they are allowed to inject into external sites, fully controlled by the user.

Note: This post was translated to English using AI. My native language is Spanish.

The Problem:

Users of SaaS apps (accounting, project management, etc.) often need to repeatedly copy data into external forms (government portals, client systems, etc.). Today this is a tedious, fully manual process.

My Current Solution

A browser extension where:

- Websites expose an injectless.json declaring which fields they can fill and on which domains

- The user explicitly installs the integration (one-click opt-in)

- When visiting an allowed site, the extension offers to “paste” each field

The Doubt

A friend suggested that instead of a browser extension, this should be a native app (similar to KeePassXC or Espanso) that:

- Works in any browser without installing multiple extensions

- Pastes sequences of fields using TAB (simpler, more universal)

- Works even outside the browser

- Avoids extension permissions, CSP issues, Shadow DOM, etc.

My Concerns About a Native App

- Mobile: Browser extensions do work on mobile (Safari iOS, Firefox Android). Native apps would face heavy sandboxing restrictions

- UX: The extension popup can show exactly which fields are available for the current page. A native app would be more “blind”

- Context: The extension knows which page you’re on and can automatically validate allowed domains

The Question

What seems more valuable / practical?

A) Browser extension (current approach) — more context, mobile support, clearer UX

B) Native app like Espanso/KeePassXC — more universal, single install, simpler

C) Both — native app as a base + optional extension as a companion for better UX

Has anyone worked on something similar?

What trade-offs might I be missing?

Thanks!

These guys solve same problem https://www.getmagical.com/ worth researching their experience
Thanks for the link; I wasn't familiar with Magical.

From what I can see, Magical focuses on general autofill, while Injectless is exploring a more declarative, least-privilege approach, where each site explicitly defines which fields it can expose and on which domains, with domain-level validation and explicit user control.

Even so, comparing UX, adoption, and technical trade-offs with products like this is exactly the kind of signal I was looking for.

They have pr had that as one of their initial features. Recently thy pivoted away from that to more ai and corporate use cases
Since what you're building is similar to a password manager, you should probably do it the way they do. A browser extension + a native app for mobile. Mobile platforms have autofill APIs that password managers use to fill forms.
I think this would probably be the closest comparison.

The password manager model seems like the most realistic way to achieve a good user experience on mobile devices while maintaining strong context and domain validation on desktops.

One difference I'm exploring is that Injectless is intentionally declarative and website-based (sites publish what they can inject, rather than the tool heuristically completing everything).

I appreciate the perspective.