What tool do you use for XML Schema validation, when it comes to your ACA file?

Summary: What tool do you use for XML Schema validation, when it comes to your ACA file?

Content (required):

We are getting an error when we try to import to the IRS, "XML Schema Validation Failed - not well formed or missing required elements", so we are thinking about running the file through an XML Schema validation tool to try to find the issue.

Any recommendations?

Version (include the version you are using, if applicable):

Code Snippet (add any code snippets that support your topic, if applicable):