diff --git a/launcher/AV-Live-Body/Sources/AVLiveBody/USBClient.swift b/launcher/AV-Live-Body/Sources/AVLiveBody/USBClient.swift
new file mode 100644
index 0000000..e85ab63
--- /dev/null
+++ b/launcher/AV-Live-Body/Sources/AVLiveBody/USBClient.swift
@@ -0,0 +1,49 @@
+import Foundation
+
+/// Transport abstraction over the usbmuxd Unix socket. The real
+/// implementation wraps a `socket(AF_UNIX)`; tests inject a mock.
+protocol MuxTransport {
+    func send(_ data: Data)
+    func receivePacket() -> Data?
+    func close()
+}
+
+/// usbmux client: device discovery + connect-to-port. After a
+/// successful `connect`, the same transport carries the raw tunneled
+/// byte stream from the device.
+final class USBClient {
+    private let transport: MuxTransport
+    private var tag: UInt32 = 0
+
+    init(transport: MuxTransport) {
+        self.transport = transport
+    }
+
+    func listDevices() -> [Int] {
+        tag += 1
+        transport.send(USBMuxProtocol.encode(
+            plist: ["MessageType": "ListDevices"], tag: tag))
+        guard let reply = transport.receivePacket(),
+              let plist = USBMuxProtocol.decode(reply),
+              let list = plist["DeviceList"] as? [[String: Any]]
+        else { return [] }
+        return list.compactMap { $0["DeviceID"] as? Int }
+    }
+
+    /// Returns true once the transport is tunneled to `port` on the
+    /// device. usbmux wants the TCP port in big-endian order.
+    func connect(deviceID: Int, port: UInt16) -> Bool {
+        tag += 1
+        let swapped = Int((port << 8) | (port >> 8))
+        transport.send(USBMuxProtocol.encode(plist: [
+            "MessageType": "Connect",
+            "DeviceID": deviceID,
+            "PortNumber": swapped,
+        ], tag: tag))
+        guard let reply = transport.receivePacket(),
+              let plist = USBMuxProtocol.decode(reply),
+              let number = plist["Number"] as? Int
+        else { return false }
+        return number == 0
+    }
+}
diff --git a/launcher/AV-Live-Body/Sources/AVLiveBody/USBMuxProtocol.swift b/launcher/AV-Live-Body/Sources/AVLiveBody/USBMuxProtocol.swift
index 15ef05a..40c17f6 100644
--- a/launcher/AV-Live-Body/Sources/AVLiveBody/USBMuxProtocol.swift
+++ b/launcher/AV-Live-Body/Sources/AVLiveBody/USBMuxProtocol.swift
@@ -28,7 +28,8 @@ enum USBMuxProtocol {
         for i in 0..<4 { d.append(UInt8((v >> (8 * i)) & 0xFF)) }
     }
 
-    static func readLE32(_ d: Data, _ offset: Int) -> UInt32 {
+    static func readLE32(_ d: Data, _ offset: Int) -> UInt32? {
+        guard offset >= 0, d.count >= offset + 4 else { return nil }
         let b = [UInt8](d)
         var v: UInt32 = 0
         for i in 0..<4 { v |= UInt32(b[offset + i]) << (8 * i) }
diff --git a/launcher/AV-Live-Body/Tests/AVLiveBodyTests/USBClientTests.swift b/launcher/AV-Live-Body/Tests/AVLiveBodyTests/USBClientTests.swift
new file mode 100644
index 0000000..c663931
--- /dev/null
+++ b/launcher/AV-Live-Body/Tests/AVLiveBodyTests/USBClientTests.swift
@@ -0,0 +1,49 @@
+import XCTest
+@testable import AVLiveBody
+
+/// In-memory stand-in for the usbmuxd Unix socket.
+final class MockMuxTransport: MuxTransport {
+    var sent: [Data] = []
+    var canned: [Data] = []
+    func send(_ data: Data) { sent.append(data) }
+    func receivePacket() -> Data? {
+        canned.isEmpty ? nil : canned.removeFirst()
+    }
+    func close() {}
+}
+
+final class USBClientTests: XCTestCase {
+    func testListDevicesParsesDeviceIDs() {
+        let mock = MockMuxTransport()
+        mock.canned = [USBMuxProtocol.encode(plist: [
+            "DeviceList": [
+                ["DeviceID": 42,
+                 "Properties": ["ConnectionType": "USB"]],
+            ]], tag: 0)]
+        let client = USBClient(transport: mock)
+        let devices = client.listDevices()
+        XCTAssertEqual(devices, [42])
+    }
+
+    func testConnectSendsConnectRequest() {
+        let mock = MockMuxTransport()
+        mock.canned = [USBMuxProtocol.encode(
+            plist: ["MessageType": "Result", "Number": 0], tag: 0)]
+        let client = USBClient(transport: mock)
+        let ok = client.connect(deviceID: 42, port: 7000)
+        XCTAssertTrue(ok)
+        let req = USBMuxProtocol.decode(mock.sent.last!)
+        XCTAssertEqual(req?["MessageType"] as? String, "Connect")
+        XCTAssertEqual(req?["DeviceID"] as? Int, 42)
+        XCTAssertEqual(req?["PortNumber"] as? Int,
+                       Int((UInt16(7000) << 8) | (UInt16(7000) >> 8)))
+    }
+
+    func testConnectFailsOnNonZeroResult() {
+        let mock = MockMuxTransport()
+        mock.canned = [USBMuxProtocol.encode(
+            plist: ["MessageType": "Result", "Number": 3], tag: 0)]
+        let client = USBClient(transport: mock)
+        XCTAssertFalse(client.connect(deviceID: 1, port: 7000))
+    }
+}
diff --git a/launcher/AV-Live-Body/Tests/AVLiveBodyTests/USBMuxProtocolTests.swift b/launcher/AV-Live-Body/Tests/AVLiveBodyTests/USBMuxProtocolTests.swift
index 53fc351..7fbf2c9 100644
--- a/launcher/AV-Live-Body/Tests/AVLiveBodyTests/USBMuxProtocolTests.swift
+++ b/launcher/AV-Live-Body/Tests/AVLiveBodyTests/USBMuxProtocolTests.swift
@@ -6,7 +6,7 @@ final class USBMuxProtocolTests: XCTestCase {
         let body: [String: Any] = ["MessageType": "ListDevices"]
         let packet = USBMuxProtocol.encode(plist: body, tag: 3)
         XCTAssertGreaterThan(packet.count, 16)
-        XCTAssertEqual(Int(USBMuxProtocol.readLE32(packet, 0)),
+        XCTAssertEqual(USBMuxProtocol.readLE32(packet, 0).map(Int.init),
                        packet.count)
         XCTAssertEqual(USBMuxProtocol.readLE32(packet, 4), 1)
         XCTAssertEqual(USBMuxProtocol.readLE32(packet, 8), 8)