The mysterious .dev.localhost checkbox

When creating a new ASP.NET Core project in Visual Studio, there's a checkbox labeled 'Use the .dev.localhost TLD in the application URL.' Starting with .NET 10, this feature gives each local project a unique subdomain (e.g., myapp.dev.localhost:7099) instead of sharing the same localhost address differentiated only by port. Benefits include easier app identification in the browser, isolated cookie/storage scopes per project, and out-of-the-box HTTPS support via the existing ASP.NET Core dev certificate which already includes .dev.localhost as a SAN. Kestrel binds only to the loopback address and continues to serve plain localhost simultaneously. The main gotcha is Safari on macOS, which doesn't auto-resolve .localhost names, requiring fallback to plain localhost.

Source: https://bartwullems.blogspot.com/2026/07/the-mysterious-devlocalhost-checkbox.html. 8sync News only summarizes and links out; content copyright belongs to the authors and original sources.