Bug 293576 – declarativeNetRequest modifyHeaders doesn't support custom headers

• Component: WebKit Extensions
• Status: RESOLVED DUPLICATE

tl;dr: Safari ignores rules for `modifyHeaders` when making `fetch` request from MV3 extension. However, headers are successfully modified when making `chrome.runtime.sendMessage` request. 

 I can reproduce this bug in Safari 18.5 and Safari Technology Preview Release 219. This bug can't be reproduced in the latest Chrome or Firefox. 



I have following `manifest.json` (MV3 extension):
```
"permissions": ["storage", "declarativeNetRequestWithHostAccess"],
  "host_permissions": ["http://localhost/*"],
  "declarative_net_request": {
    "rule_resources": [
      {
        "id": "modify-origin",
        "enabled": true,
        "path": "rules.json"
      }
    ]
  },

```

`rules.json`:
```
[
    {
      "id": 1,
      "priority": 1,
      "action": {
        "type": "modifyHeaders",
        "requestHeaders": [
          {
            "header": "Origin",
            "operation": "set",
            "value": "http://localhost"
          }
        ]
      },
      "condition": {
        "urlFilter": "http://localhost:11434/*",
        "resourceTypes": ["xmlhttprequest"]
      }
    }
  ]
  


```

and `test-script.js`:
```
...
const response = await fetch('http://localhost:11434/api/generate', {
        method: 'POST',
        headers: { 'Content-Type': 'application/json' },
        body: params
    });
    console.log(response)

    const response2 = await new Promise((resolve, reject) => {
        chrome.runtime.sendMessage({
            type: "ollama-fetch",
            url: "http://localhost:11434/api/generate",
            body: params
        }, (response) => {
            if (response && response.success) {
                resolve(response.data);
            } else {
                reject(response ? response.error : "No response");
            }
        });
    });
    console.log(response2)
...

```

`fetch` request will end up ignoring `modifyHeaders` rule, but it works in other browsers.

[Radar WebKit Bug Importer]

<rdar://problem/152126602>

[Timothy Hatcher]

*** This bug has been marked as a duplicate of bug 290922 ***