That generally requires a reverse proxy or content-switch. That reverse proxy will then have to lie to one or both apps about their URLs, etc... Microsoft is r

That generally requires a reverse proxy or content-switch. That reverse proxy will then have to lie to one or both apps about their URLs, etc... Microsoft is recommending YARP for this: https://learn.microsoft.com/en-us/events/dotnetconf-2022/migrate-your-legacy-aspnet-projects-to-aspnet-core-incrementally-with-yarp https://learn.microsoft.com/en-us/events/dotnetconf-2022/mig... That sounds okay, but you'll hit all sorts of fun technical challenges. Good look making WCF client certificate authentication work through this! There are also performance gotchas with buffering, etc... There are not-atypical scenarios where during a migration, an app might be behind 4+ reverse proxies, all of which are different products. E.g.: CDN -> App Gateway WAF v2 -> Kubernetes Ingress -> YARP -> Legacy App. You'll quickly discover all sorts of fun interactions between these and 15-year-old ASP.NET code!