Failed to create the imposter with port 4555 and protocol http.

If you’ve ever had the following error when running tests using MbDotNet  / mountebank it can be quite frustrating.

    "errors": [
            "code": "resource conflict",
            "message": "Port 4555 is already in use"

It probably means something has not bee disposed of properly when a test has failed i.e. the imposters or the process running mountebank or perhaps you have tests running in parallel *.

Until you work out the cause it can be an annoying thing to try to resolve. How do you know what is using that port / how to free it up?

PowerShell to the rescue. Run the following to find processes running on port 4555.

netstat -ano | Select-String "4555"

Which will give you a result something like:

TCP [::]:4555 [::]:0 LISTENING 26720

Now to free up the port run:

$id = "26720"
Stop-Process $id

* If the problem causing this is tests running in parallel and you’re using XUnit you may need to add a Properties.cs file with the following content:

using Xunit;
[assembly: CollectionBehavior(DisableTestParallelization = true)]

2 thoughts on “Failed to create the imposter with port 4555 and protocol http.

    1. In a .net core project at least, that is how I stopped tests running in parallel. Just add Properties.cs in the root of your test project.

      The failures were occurring because parallel tests were trying to claim port 4555.

      We have since switched to using instead which is an option if all of your calls are made with HttpClient.

      It intercepts in memory and achieves a similar thing to MbDotNet but MUCH faster and without any such conflicts.


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