web config error: Unrecognized attribute 'xmlns:xdt'. Note that attribute names are case-sensitive

Ensure that the line throwing the error:

<configuration xmlns:xdt="http://schemas.microsoft.com/XML-Document-Transform">

Is not in your primary web.config file. This is the line for a web.config transformation file. Basically, when you create a new project, you will get a Web.config file with one or more sub web.config files named according to the build profiles for your solution such as:

Web.config
    Web.Release.config
    Web.Debug.config

Your parent web.config file should start with <configuration> and none of the xmlns:xdt stuff. Your sub web.config files (those used for transforming your parent web.config file) should start with that line.

If you are unfamiliar with web.config transformations, this allows VS to automatically change elements of your configuration depending on the build type of the project (used for changing connection strings, turning debug on and off, etc).

Also, please check the blog postings at AppHarbor for instructions on dealing with Web.config transformations. Having never used AppHarbor, it appears you may need to make some minor project changes on your end for the transformations to work correctly. Lastly, you may need to ensure that your web.config transformation files are indeed being included in the build/deployment to app harbor

I would check that you have the following line within the top of your transform file.

<?xml version="1.0"?>

If you confirm that is there, I would also check that the following is not within your actual transformed web.config.

<configuration xmlns:xdt="http://schemas.microsoft.com/XML-Document-Transform">

I have found the web essentials extension invaluable for resolving errors like this before I deploy.

You can simply right-click your transform file and select preview from Visual Studio and if you encounter any problems you can investigate/fix locally.

Web essentials 2012

http://visualstudiogallery.msdn.microsoft.com/07d54d12-7133-4e15-becb-6f451ea3bea6

Tommy's very clear description of the transformation process allowed me to solve a very stubborn problem. I have an ASP.NET MVC 4 project that was giving an error when I was trying to publish from Visual Studio 2012. I kept getting the error

Unrecognized attribute 'xmlns:xdt'. Note that attribute names are case-sensitive.

The error said it was in the web.config file, but it actually isn’t there. It was in the Web.Release.Config. I tried many of the recommended solutions in Web.config transformation: Unrecognized attribute 'xmlns:xdt'. Note that attribute names are case-sensitive

I removed the contents of the obj directory, and that worked once or twice and then stopped working. I removed the attribute from Web.Release.Config,

xmlns:xdt=http://schemas.microsoft.com/XML-Document-Transform

but that gave me errors. I tried

This is kind of a workaround, but you may add the following line to your pre-build commands:
del $(ProjectDir)obj\* /F /S /Q

Right click your project > Properties > Build Events > Pre-build.

That worked once, but not a second time.

I tried the longer version of the above solution

del "$(ProjectDir)obj\*" /F /Q
del "$(ProjectDir)obj\$(ConfigurationName)\AspnetCompileMerge\*" /F /S /Q
del "$(ProjectDir)obj\$(ConfigurationName)\CSAutoParameterize\*" /F /S /Q
del "$(ProjectDir)obj\$(ConfigurationName)\Package\*" /F /S /Q
del "$(ProjectDir)obj\$(ConfigurationName)\ProfileTransformWebConfig\*" /F /S /Q
del "$(ProjectDir)obj\$(ConfigurationName)\TempPE\*" /F /S /Q
del "$(ProjectDir)obj\$(ConfigurationName)\TransformWebConfig\*" /F /S /Q

but that gave me a lot of errors. The site doctor had an interesting looking solution, but it required downloading pre-release software from GitHub and I didn’t want to take a chance with that if I didn’t have to. http://blogs.thesitedoctor.co.uk/tim/Trackback.aspx?guid=7988f7bc-947c-4134-ab52-af22770b639c Then I found Tommy's article, and it gave me an idea. I deleted this xmlns:xdt=http://schemas.microsoft.com/XML-Document-Transform from this line

<configuration xmlns:xdt="http://schemas.microsoft.com/XML-Document-Transform">

When I did that, I had to delete

xdt:Transform="RemoveAttributes(debug)"

from this line

<compilation xdt:Transform="RemoveAttributes(debug)" />

This worked for me. It probably isn’t a great idea to remove the transformation that seems to remove the debug attributes when you publish/release your project, but it seems to fix the problem.