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How can I intercept and modify (not resend) a request?

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Imagine that I'm on a website like example.com and I click a button. Looking at my browser's network tab, I can see that the page has sent a POST request to example.com with some data and received some data back from the server.

I want to be able to:

  • Edit the data before it is sent to the server.
  • Edit the response data before it is handled by the function.

I'm aware of Firefox's "edit and resend" feature, but I want to tamper with the very first request before it reaches the server.

I've been searching for this for hours but all of the answers appear to be outdated due to browser updates. What is the modern approach to doing this?

All Replies (2)

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Most of the results for header are designed to create and apply rules over and over rather than hack on individual requests and responses.

This is the only one I saw that can intercept a post and let you modify it before sending:

https://addons.mozilla.org/firefox/addon/http-header-live/

I didn't try it myself.

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I'm trying it out now but I don't see any options to intercept the original request. It appears to just be yet another "Edit and resend" feature like the rest. The original request still gets sent.