Cloud SQL Auth Proxy sidecar is always available on 127.0.0.1:1433 when running on Cloud Run, and therefore your .NET application should use that address, rather than the Cloud SQL instance name or external IP. Although your credential, IAM roles and add-cloudsql-instances flag are valid, you will receive the error of server not found by using the incorrect DataSource. A change of the connection string to reference 127.0.0.1,1433 and a pull of DB_USER, DB_PASS and DB_NAME as environment variables or via Secret Manager solves the problem, as the proxy passes the traffic over to your SQL Server instance.