Has this been reported before?

• I have checked for existing reports of this issue

Repro steps

Hello, recently (in the last two weeks 1.20.1) I noticed a HTTP response integrity checker was added.

However, in our professional work, we often encounter situations where responses don't always comply with RFCs.

I have an application that responds with two X-Content-Type-Options header values.
Like:

HTTP/2 200 OK
Date: Wed, 27 Aug 2025 15:45:34 GMT

X-Content-Type-Options: nosniff
...
X-Xss-Protection: 0
X-Content-Type-Options: nosniff

Expected: I want to have control of the responses, I don't care if they come malformed, also burp handle responses as expected.

I try to apply a rule, to remove headers from "Transform & Mock" but looks HTTP Toolkit block before.

It is important to note that before this "integrity change" I was able to work fine.

Logs
last-run.log

How often does this bug happen?

Every time

The desktop OS you're using

Mac OS 15.3

Details of other apps/devices

APIs

Error screenshot

Any other info?

No response