diff --git a/CONTEXT.md b/CONTEXT.md
index 75b482c..3af5d86 100644
--- a/CONTEXT.md
+++ b/CONTEXT.md
@@ -10,6 +10,8 @@ A requested change of media, playback state, position, or quality that may still
 **Cast playback transition**:
 A playback transition that transfers playback between this Mac and a Cast receiver. Connection alone does not transfer playback; the destination must acknowledge the handoff, and interruption must preserve the latest playback intent and acknowledged position.
 
+Discovery supplies receiver addresses to both Network.framework and the native media relay's POSIX resolver. IPv4 addresses must omit interface scope suffixes; IPv6 addresses retain their scopes.
+
 **Playback intent**:
 The user's latest requested playing or paused state, which can differ from the state playback has acknowledged.
 
diff --git a/Checks/cast-session.swift b/Checks/cast-session.swift
index 4c1bcce..ea12b4c 100644
--- a/Checks/cast-session.swift
+++ b/Checks/cast-session.swift
@@ -412,6 +412,16 @@ struct CastSessionCheck {
         let fallbackID = discovery.devices.first { $0.name == prefix + "-fallback" }!.id
         let device = ownDevices()[0]
         require(device.port == second.port?.rawValue, "Discovery preserves the advertised Cast port")
+        for device in discovery.devices {
+            guard case .ipv4 = NWEndpoint.Host(device.host) else { continue }
+            var hints = addrinfo()
+            hints.ai_family = AF_INET
+            hints.ai_socktype = SOCK_DGRAM
+            var addresses: UnsafeMutablePointer<addrinfo>?
+            let result = getaddrinfo(device.host, "8009", &hints, &addresses)
+            if let addresses { freeaddrinfo(addresses) }
+            require(result == 0, "Discovered IPv4 receiver resolves through the native relay's POSIX API")
+        }
         let connection = NWConnection(host: NWEndpoint.Host(device.host),
                                       port: NWEndpoint.Port(rawValue: device.port)!, using: .tcp)
         defer { connection.cancel() }
diff --git a/Sources/App/CastSession.swift b/Sources/App/CastSession.swift
index 359572f..8e0ff27 100644
--- a/Sources/App/CastSession.swift
+++ b/Sources/App/CastSession.swift
@@ -115,9 +115,17 @@ final class CastDiscovery: ObservableObject {
                       let value = String(data: data, encoding: .utf8), !value.isEmpty else { return nil }
                 return value
             }
+            // Network.framework scopes IPv4 to an interface; POSIX getaddrinfo
+            // rejects that suffix. IPv6 scope identifiers must remain intact.
+            let address: String
+            if case .ipv4 = host {
+                address = String(host.debugDescription.prefix { $0 != "%" })
+            } else {
+                address = host.debugDescription
+            }
             resolved[endpoint] = CastDevice(id: text("id") ?? domain + type + name,
                                             name: text("fn") ?? name,
-                                            host: host.debugDescription, port: port.rawValue)
+                                            host: address, port: port.rawValue)
         }
         services[endpoint]?.timeout?.cancel()
         services[endpoint]?.timeout = nil