Appending URI

That's due to an optimization in the caching of variables. The value of [HTTP::uri] is stored in a cache and when you re-assign the value with the "HTTP::uri value" command, the cached value for the [HTTP::uri] command is not updated. The URI will get modified, but the value returned from [HTTP::uri] is the original value. That issue has been around for a long time, maybe someday it'll get fixed.

In the meantime, if you want to verify your changes with logging, I'd suggest you use a temporary variable.

when HTTP_REQUEST {
  log local0. "in HTTP_REQUEST"
  log local0. "Original: [HTTP::uri]."
  set uri "/prefix[HTTP::uri]/what_the/"
  log local0. "After mapping: $uri"
  HTTP::uri $uri
}

You can verify this by looking at your backend webserver logs.

-Joe