FoxyProxy: Content Script Fetch() Requests Not Proxied?

Hey guys! Ever faced a head-scratching issue where your browser extension's content script fetch() requests seem to ignore your proxy settings? You're not alone! This is a tricky problem, and we're going to dive deep into understanding why this happens and how to troubleshoot it. We'll specifically address the scenario where FoxyProxy logs the requests but doesn't apply proxy rules, even though manual fetch() calls work perfectly. Let's get started!

Understanding the Problem: Content Script fetch() and Proxies

So, the core issue here is that fetch() requests originating from a content script within a browser extension are not being proxied as expected. You've configured your proxy (likely using FoxyProxy) and set rules, but these requests are bypassing the proxy server. This can be super frustrating, especially when other requests, like those made directly from the browser console, are correctly routed through the proxy. You might be wondering why this discrepancy exists. Let’s dissect the usual suspects and common misconfigurations that can lead to this behavior.

One of the key things to understand is how browser extensions, especially content scripts, interact with the browser's proxy settings. Content scripts operate within the context of a webpage, which means they have access to the DOM and can execute JavaScript code as if it were part of the website itself. However, this also means they are subject to the browser's security restrictions and proxy configurations. When a content script makes a fetch() request, the browser needs to determine whether that request should be proxied based on the configured rules. If the browser fails to correctly interpret these rules for content script requests, it leads to the problem we’re tackling today.

Another critical aspect is the asynchronous nature of fetch() requests. When you initiate a fetch() call, it doesn't immediately return a response. Instead, it returns a Promise that resolves when the response is available. This asynchronous behavior can sometimes interact unexpectedly with proxy configurations, especially if the proxy rules are dependent on certain request properties that aren't immediately available at the time the request is initiated. For instance, if a proxy rule depends on the request's URL or headers, and these are modified asynchronously by the content script, the proxy might not correctly match the rule.

Furthermore, the logging behavior of FoxyProxy gives us valuable clues. The fact that the requests appear in the logs with empty proxy fields (no Document URL, Proxy Title, etc.) indicates that FoxyProxy is seeing the requests but not processing them according to the configured rules. This suggests that the requests are being recognized as HTTP requests, but the information needed to match them against proxy rules is either missing or not being correctly interpreted. This could be due to various factors, such as the timing of the request, the specific headers being sent, or even subtle differences in how content script requests are handled compared to manual requests from the console.

Diagnosing the FoxyProxy and Content Script Issue: Why Your fetch() Requests Aren't Being Proxied

Okay, so let's get into the nitty-gritty of diagnosing why your fetch() requests from content scripts aren't being proxied, even when FoxyProxy logs them. This situation is like a detective story, and we're going to follow the clues to find the culprit. Here's a breakdown of the common causes and how to investigate them:

1. Incorrect Proxy Rules: This is the most common suspect. Double-check your FoxyProxy rules. Are they correctly configured to match the URLs or domains you're trying to fetch() from your content script? Pay close attention to wildcard usage (like ://*reddit.com/*) and make sure they are doing what you intend. Sometimes a subtle typo or an incorrect wildcard can cause the rules to fail. Remember, proxy rules are often matched in a specific order, so ensure the most specific rules are at the top of your list.

2. Private Window Configuration: You mentioned this is happening in a private window, which is a crucial detail. Ensure your proxy settings are enabled for private windows. In FoxyProxy's options, there's usually a setting to allow the proxy to function in private browsing mode. If this setting is not enabled, your requests from private windows will bypass the proxy, regardless of your other rules. This is a frequent oversight, so it's worth a careful check.

3. Content Script Injection Timing: This is a more nuanced issue. When does your content script inject itself into the page? If it injects too late, after the initial fetch() requests are made, the proxy rules might not apply to those early requests. Some websites initiate requests very early in their lifecycle, even before extensions have fully loaded. Try adjusting your content script's injection timing in your manifest file (using run_at with values like `