Can you modify http request headers in a Safari extension?

I can do this in FF and IE, and I know it doesn't exist in Chrome yet. Anybody know if you can do this in a Safari plugin? I can't find anything that says one way or another in the documentation.

Edit (November 2021): as pointed out in the comments, ParosProxy seems to no longer exist (and was last released ~2006 from what I can see). There are more modern options for debugging on Mac (outside of browser plugins on non-Safari browsers) like Proxyman. Rather than adding another list of links that might expire, I'll instead advise people to search for "debugging proxy" on their platform of choice instead.

Original Answer (2012):

The Safari "Develop" menu in advanced preferences allows you to partially customize headers (like the user agent), but it is quite limited.

However, if a particular browser or app does not allow you to alter the headers, just take it out of the equation. You can use things like Fiddler or ParosProxy (and many others) to alter the requests regardless of the application sending the request.

They also have the advantage of allowing you to make sure that you are sending the same headers regardless of the application in question and (depending on your requirements) potentially work across multiple browsers and apps without modification.

Safari has added extension support but its APIs don't let you have granular level control over Request & Response as compared to Chrome/Firefox/Edge.

To have granular level control over your Request and Response, you need setup a system wide proxy instead. Requestly Desktop App automatically does this for you and on top of that, you can do various types of modifications too like:

• Modify Request/Response Headers
• Redirect URLs
• Modify Response
• Delay Network request
• Insert Custom Scripts
• Change User-Agent

Here's an article about header modification using requestly https://requestly.io/feature/modify-request-response-headers/

Disclaimer: I work at Requestly