Hello Rik,

We provided insights in the linked blog post (about the possibilities for Fiddler Classic) and through our internal communication (for Fiddler Everywhere), but I will also repost my answers here so that the community can have access to them as well.

Fiddler Classic

Fiddler Classic might create a new connection if the client didn't receive a response and the server closes or resets the connection. See this forum thread on how to explicitly configure Fiddler (through FiddlerScript) to set the retry mode. For example:

CONFIG.RetryOnReceiveFailure = RetryMode.Never.

Fiddler Everywhere

No, Fiddler Everywhere is not making automatic retries explicitly. 

Unless the .NET http-client is being explicitly set to disable keep-alive then its default configuration tries to reuse connections.

What Fiddler does change is explained in the following section of the blog post:

Fiddler maintains a “connection pool” of idle keep-alive connections to the server. When the a client request comes in, this pool is first checked to determine if an existing connection is available on which the request can be sent. Even if the client specifies a Connection: close request header, that only causes Fiddler to close the client’s connection after the response is sent—the server connection is returned to the pool (unless it too disabled keep-alive).

The above means that even if your client application is disabling keep-alive (through connection: close), Fiddler will still reuse the open connection to the server from the so called connection pool. So you could expect different behavior in case the http-client is explicitly set to close the connections before the Fiddler's "connection pool" does it (which remains open for approximately 200 seconds).