A DIAGSESSION file is a diagnostics session created by Internet Explorer, a web browser for Windows, or Visual Studio, a software development tool. It contains diagnostics data from a debug session, which allows developers to troubleshoot problems with an application, such as memory and performance issues.

The DIAGSESSION file is a .ZIP archive that contains multiple .ETL files and .XML metadata files. The ETL files store the session data that can be analyzed. You can rename the "diagsession" extension to "zip" and unzip the file in order to manually access its contents. DIAGSESSION files may be exported from F12 developer tools, which is a suite of tools available with Internet Explorer that help developers build and debug webpages. Developers can analyze their HTML code and view it the same way as the Internet Explorer Document Object model (DOM) interprets the page. You can access the F12 developer tools on any webpage by pressing the F12 key or clicking the tools button (gear icon) and then choosing "F12 tools". You can run a debug session and export the data as a DIAGSESSION file that can be imported into Microsoft Visual Studio (version 2013 and later). You can also create DIAGSESSION files in Visual Studio from the "Performance and Diagnostics" section. NOTE: The F12 developer tools suite was released with version 9 of Internet Explorer. The suite was available with Internet Explorer up until it was replaced by Edge as Microsoft's default browser in Windows 10.