diff --git a/Sources/Portside/AppModel.swift b/Sources/Portside/AppModel.swift
index ba4e592..b303fe9 100644
--- a/Sources/Portside/AppModel.swift
+++ b/Sources/Portside/AppModel.swift
@@ -57,8 +57,10 @@ final class AppModel {
     }
 
     /// Returns a URL reachable from other devices on the Wi-Fi. Servers bound
-    /// to all interfaces are reachable directly; loopback-bound ones get a relay.
-    func startSharing(_ server: DevServer) -> ActiveShare? {
+    /// to all interfaces are reachable directly; loopback-bound ones get a
+    /// relay. `ShareProxy`'s init can block briefly waiting for its listener,
+    /// so — like `PortScanner.scan()` — it's built off the main actor.
+    func startSharing(_ server: DevServer) async -> ActiveShare? {
         if let existing = shares[server.port] { return existing }
         guard let ip = lanAddress else { return nil }
 
@@ -67,9 +69,20 @@ final class AppModel {
             shares[server.port] = share
             return share
         }
-        guard let relay = try? ShareProxy(targetPort: server.port), relay.listenPort > 0 else {
-            return nil
+
+        let targetPort = server.port
+        let relay = await Task.detached(priority: .utility) {
+            try? ShareProxy(targetPort: targetPort)
+        }.value
+
+        // Another call could have started (or stopped) a share for this port
+        // while we were suspended constructing the relay above.
+        if let existing = shares[server.port] {
+            relay?.stop()
+            return existing
         }
+        guard let relay, relay.listenPort > 0 else { return nil }
+
         relays[server.port] = relay
         let share = ActiveShare(url: URL(string: "http://\(ip):\(relay.listenPort)")!, isRelayed: true)
         shares[server.port] = share
diff --git a/Sources/Portside/CLI.swift b/Sources/Portside/CLI.swift
index 3e9388d..ff5d334 100644
--- a/Sources/Portside/CLI.swift
+++ b/Sources/Portside/CLI.swift
@@ -55,7 +55,7 @@ enum CLI {
             print("Relaying 127.0.0.1:\(port) -> http://\(ip):\(proxy.listenPort)  (Ctrl-C to stop)")
             dispatchMain()
         } catch {
-            print("error: \(error)")
+            print("error: \(error.localizedDescription)")
             exit(1)
         }
     }
diff --git a/Sources/Portside/UI/QRPanel.swift b/Sources/Portside/UI/QRPanel.swift
index f98783d..8ec4bb0 100644
--- a/Sources/Portside/UI/QRPanel.swift
+++ b/Sources/Portside/UI/QRPanel.swift
@@ -67,6 +67,6 @@ struct QRPanel: View {
         )
         .padding(.horizontal, 10)
         .padding(.bottom, 6)
-        .onAppear { share = model.startSharing(server) }
+        .task { share = await model.startSharing(server) }
     }
 }
diff --git a/Sources/PortsideCore/ShareProxy.swift b/Sources/PortsideCore/ShareProxy.swift
index 251db16..6195761 100644
--- a/Sources/PortsideCore/ShareProxy.swift
+++ b/Sources/PortsideCore/ShareProxy.swift
@@ -1,6 +1,17 @@
 import Foundation
 import Network
 
+public enum ShareProxyError: LocalizedError, Sendable, Equatable {
+    case invalidPort(Int)
+
+    public var errorDescription: String? {
+        switch self {
+        case .invalidPort(let port):
+            return "Invalid port \(port): must be between 1 and 65535."
+        }
+    }
+}
+
 /// A TCP relay that listens on every interface and pipes bytes to a target
 /// port on loopback. This is what lets a phone on the same Wi-Fi reach a dev
 /// server that bound only to 127.0.0.1 — the relay is reachable from the LAN,
@@ -9,18 +20,26 @@ import Network
 public final class ShareProxy: @unchecked Sendable {
 
     public let targetPort: Int
+    private let targetEndpoint: NWEndpoint.Port
     private let listener: NWListener
     private let queue = DispatchQueue(label: "portside.proxy")
     private let lock = NSLock()
     private var connections: [ObjectIdentifier: (NWConnection, NWConnection)] = [:]
+    private var stopped = false
 
     public var listenPort: Int {
         Int(listener.port?.rawValue ?? 0)
     }
 
-    /// Starts immediately on an OS-assigned port.
+    /// Starts immediately on an OS-assigned port. Blocks the calling thread
+    /// briefly (up to 2s) while the listener comes up — callers on the main
+    /// actor must dispatch this off-main, the same way `PortScanner.scan()` is.
     public init(targetPort: Int) throws {
+        guard (1...65535).contains(targetPort), let endpoint = NWEndpoint.Port(rawValue: UInt16(targetPort)) else {
+            throw ShareProxyError.invalidPort(targetPort)
+        }
         self.targetPort = targetPort
+        self.targetEndpoint = endpoint
         let params = NWParameters.tcp
         params.allowLocalEndpointReuse = true
         listener = try NWListener(using: params, on: .any)
@@ -38,9 +57,14 @@ public final class ShareProxy: @unchecked Sendable {
         listener.stateUpdateHandler = nil
     }
 
+    deinit {
+        stop()
+    }
+
     public func stop() {
         listener.cancel()
         lock.lock()
+        stopped = true
         let open = connections.values
         connections.removeAll()
         lock.unlock()
@@ -51,12 +75,15 @@ public final class ShareProxy: @unchecked Sendable {
     }
 
     private func relay(_ inbound: NWConnection) {
-        let outbound = NWConnection(
-            host: "127.0.0.1",
-            port: NWEndpoint.Port(rawValue: UInt16(targetPort))!,
-            using: .tcp
-        )
+        let outbound = NWConnection(host: "127.0.0.1", port: targetEndpoint, using: .tcp)
+
         lock.lock()
+        guard !stopped else {
+            lock.unlock()
+            inbound.cancel()
+            outbound.cancel()
+            return
+        }
         connections[ObjectIdentifier(inbound)] = (inbound, outbound)
         lock.unlock()
 
diff --git a/Tests/PortsideCoreTests/ShareProxyTests.swift b/Tests/PortsideCoreTests/ShareProxyTests.swift
new file mode 100644
index 0000000..4e5a01a
--- /dev/null
+++ b/Tests/PortsideCoreTests/ShareProxyTests.swift
@@ -0,0 +1,23 @@
+import XCTest
+@testable import PortsideCore
+
+final class ShareProxyTests: XCTestCase {
+
+    func testRejectsPortZero() {
+        XCTAssertThrowsError(try ShareProxy(targetPort: 0)) { error in
+            XCTAssertEqual(error as? ShareProxyError, .invalidPort(0))
+        }
+    }
+
+    func testRejectsNegativePort() {
+        XCTAssertThrowsError(try ShareProxy(targetPort: -1)) { error in
+            XCTAssertEqual(error as? ShareProxyError, .invalidPort(-1))
+        }
+    }
+
+    func testRejectsPortAboveValidRange() {
+        XCTAssertThrowsError(try ShareProxy(targetPort: 70000)) { error in
+            XCTAssertEqual(error as? ShareProxyError, .invalidPort(70000))
+        }
+    }
+}