diff --git a/android/plugchoice/build.gradle.kts b/android/plugchoice/build.gradle.kts
index 1d89a14..60822a8 100644
--- a/android/plugchoice/build.gradle.kts
+++ b/android/plugchoice/build.gradle.kts
@@ -47,6 +47,8 @@ dependencies {
     api("androidx.activity:activity-ktx:1.12.4")
     implementation("androidx.core:core-ktx:1.17.0")
     implementation("androidx.webkit:webkit:1.15.0")
+    // auth.webSession: Google and Apple sign-in in Chrome's Auth Tab.
+    implementation("androidx.browser:browser:1.9.0")
     implementation("com.squareup.okhttp3:okhttp:4.12.0")
     implementation("org.jetbrains.kotlinx:kotlinx-coroutines-android:1.10.2")
     // camera.scanCode: the Google code scanner (Play services UI, no camera permission).
diff --git a/android/plugchoice/src/main/AndroidManifest.xml b/android/plugchoice/src/main/AndroidManifest.xml
index 2767a6b..41f7011 100644
--- a/android/plugchoice/src/main/AndroidManifest.xml
+++ b/android/plugchoice/src/main/AndroidManifest.xml
@@ -44,6 +44,13 @@
         android:name="android.hardware.bluetooth_le"
         android:required="false" />
 
+    <!-- auth.webSession: find a browser that supports Custom Tabs, to check for Auth Tab (API 30+). -->
+    <queries>
+        <intent>
+            <action android:name="android.support.customtabs.action.CustomTabsService" />
+        </intent>
+    </queries>
+
     <application>
         <!--
           configChanges: the Link UI must never reload mid-flow (it would lose its
diff --git a/android/plugchoice/src/main/java/com/plugchoice/internal/BridgeHost.kt b/android/plugchoice/src/main/java/com/plugchoice/internal/BridgeHost.kt
index ca17f30..fbcc213 100644
--- a/android/plugchoice/src/main/java/com/plugchoice/internal/BridgeHost.kt
+++ b/android/plugchoice/src/main/java/com/plugchoice/internal/BridgeHost.kt
@@ -1,6 +1,8 @@
 package com.plugchoice.internal
 
 import android.content.Context
+import android.net.Uri
+import androidx.browser.auth.AuthTabIntent
 import com.plugchoice.LinkResult
 
 /** What the bridge needs from the activity hosting it. All calls happen on the main thread. */
@@ -11,6 +13,9 @@ internal interface BridgeHost {
     /** Runtime permission request through the activity; returns permission → granted. */
     suspend fun requestPermissions(permissions: Array<String>): Map<String, Boolean>
 
+    /** `auth.webSession`: opens [url] in an Auth Tab that closes on [scheme], and waits for how it ended. */
+    suspend fun openAuthTab(url: Uri, scheme: String): AuthTabIntent.AuthResult
+
     /** `session.close`, or the shell closing on its own: dismiss the Link screen and hand [result] to the host app. */
     fun closeSession(result: LinkResult)
 
diff --git a/android/plugchoice/src/main/java/com/plugchoice/internal/LinkActivity.kt b/android/plugchoice/src/main/java/com/plugchoice/internal/LinkActivity.kt
index cd72c02..feceaa9 100644
--- a/android/plugchoice/src/main/java/com/plugchoice/internal/LinkActivity.kt
+++ b/android/plugchoice/src/main/java/com/plugchoice/internal/LinkActivity.kt
@@ -29,6 +29,7 @@ import android.widget.ImageView
 import android.widget.LinearLayout
 import android.widget.TextView
 import androidx.activity.ComponentActivity
+import androidx.browser.auth.AuthTabIntent
 import androidx.activity.SystemBarStyle
 import androidx.activity.addCallback
 import androidx.activity.enableEdgeToEdge
@@ -90,6 +91,15 @@ internal class LinkActivity : ComponentActivity() {
             pendingPermissions = null
         }
 
+    private var pendingAuthTab: CompletableDeferred<AuthTabIntent.AuthResult>? = null
+
+    // auth.webSession: the Auth Tab answers this activity directly (no intent filter).
+    private val authTabLauncher =
+        AuthTabIntent.registerActivityResultLauncher(this) { result ->
+            pendingAuthTab?.complete(result)
+            pendingAuthTab = null
+        }
+
     private val host = object : BridgeHost {
         override val hostContext: Context
             get() = this@LinkActivity
@@ -102,6 +112,13 @@ internal class LinkActivity : ComponentActivity() {
                 deferred.await()
             }
 
+        override suspend fun openAuthTab(url: Uri, scheme: String): AuthTabIntent.AuthResult {
+            val deferred = CompletableDeferred<AuthTabIntent.AuthResult>()
+            pendingAuthTab = deferred
+            AuthTabIntent.Builder().build().launch(authTabLauncher, url, scheme)
+            return deferred.await()
+        }
+
         override fun closeSession(result: LinkResult) {
             finishWith(result)
         }
diff --git a/android/plugchoice/src/main/java/com/plugchoice/internal/LinkBridge.kt b/android/plugchoice/src/main/java/com/plugchoice/internal/LinkBridge.kt
index 4674209..c8237cb 100644
--- a/android/plugchoice/src/main/java/com/plugchoice/internal/LinkBridge.kt
+++ b/android/plugchoice/src/main/java/com/plugchoice/internal/LinkBridge.kt
@@ -48,6 +48,7 @@ internal class LinkBridge(
     private val wifi: WifiController = WifiController(host, onNetworkChanged = { net.onNetworkChanged() })
     private val net: LocalNetworkClient = LocalNetworkClient(wifi, emit = this::emitEvent)
     private val scanner = CodeScanner(host)
+    private val webSignIn = WebSignIn(host)
     private val discovery = LanDiscovery(host.hostContext)
     private val sessions = HttpSessions(TlsSessionConnector)
     private val tcp = TcpSockets(emit = this::emitEvent)
@@ -60,6 +61,7 @@ internal class LinkBridge(
     )
     private val clientSecrets = ClientSecrets(scope, fetchClientSecret)
     private val bluetoothAvailable: Boolean by lazy { Transports.bluetoothAvailable(host.hostContext) }
+    private val webSessionAvailable: Boolean by lazy { WebSignIn.isAvailable(host.hostContext) }
     private var disposed = false
 
     /** Bumped for each new document: answers to the previous document's requests are dropped. */
@@ -157,6 +159,7 @@ internal class LinkBridge(
             JSONObject()
         }
         "camera.scanCode" -> scanner.scan(params)
+        "auth.webSession" -> webSignIn.signIn(params)
         "lan.discover" -> discover(LanDiscovery.parseRequest(params))
         "lan.stopDiscovery" -> {
             discovery.stop()
@@ -280,7 +283,7 @@ internal class LinkBridge(
             .put("sdkVersion", Plugchoice.SDK_VERSION)
             .put("platform", "android")
             .put("osVersion", Build.VERSION.RELEASE)
-            .put("capabilities", JSONArray(capabilities(bluetoothAvailable)))
+            .put("capabilities", JSONArray(capabilities(bluetoothAvailable, webSessionAvailable)))
             .put("lanServiceTypes", JSONObject.NULL)
     }
 
@@ -374,11 +377,13 @@ internal class LinkBridge(
         /**
          * `hello`'s capabilities (§5). No `wifi.accessory` on Android (that's AccessorySetupKit on
          * iOS); `ble` when [bluetoothAvailable] (the device has Bluetooth LE and the host kept the
-         * library's Bluetooth permissions).
+         * library's Bluetooth permissions); `auth.webSession` when [webSessionAvailable] (the
+         * browser supports Auth Tab).
          */
-        fun capabilities(bluetoothAvailable: Boolean): List<String> = buildList {
+        fun capabilities(bluetoothAvailable: Boolean, webSessionAvailable: Boolean = false): List<String> = buildList {
             addAll(listOf("wifi.join", "http.request", "ws", "session.close", "camera.scanCode", "ui.closeRequest"))
             addAll(listOf("lan.address", "lan.discover", "http.session", "auth.clientSecret"))
+            if (webSessionAvailable) add("auth.webSession")
             if (bluetoothAvailable) add("ble")
             addAll(listOf("tcp", "udp", "trust.custom"))
         }
diff --git a/android/plugchoice/src/main/java/com/plugchoice/internal/WebSignIn.kt b/android/plugchoice/src/main/java/com/plugchoice/internal/WebSignIn.kt
new file mode 100644
index 0000000..9b0a85a
--- /dev/null
+++ b/android/plugchoice/src/main/java/com/plugchoice/internal/WebSignIn.kt
@@ -0,0 +1,65 @@
+package com.plugchoice.internal
+
+import android.content.Context
+import android.net.Uri
+import androidx.browser.auth.AuthTabIntent
+import androidx.browser.customtabs.CustomTabsClient
+import org.json.JSONObject
+import java.net.URI
+import java.net.URISyntaxException
+
+/**
+ * `auth.webSession` (PROTOCOL.md §7.1): Google or Apple sign-in in Chrome's Auth Tab, since Google
+ * refuses an embedded web view. The tab closes on the `plugchoice-link` scheme and hands that URL
+ * straight back to the activity that opened it, so no intent filter is involved and apps with the
+ * SDK never compete for the scheme. Browsers without Auth Tab don't get the capability. The URL
+ * holds a one-time code: never logged.
+ */
+internal class WebSignIn(private val host: BridgeHost) {
+
+    private var showing = false
+
+    suspend fun signIn(params: JSONObject): JSONObject {
+        val value = params.optStringOrNull("url")
+        if (!isWebSessionUrl(value)) throw BridgeException.invalidParams("url must be an https URL")
+        val url = Uri.parse(value)
+        if (showing) throw BridgeException(ErrorCode.UNAVAILABLE, "a sign-in is already showing")
+        if (!isAvailable(host.hostContext)) throw BridgeException(ErrorCode.UNAVAILABLE, "the browser has no Auth Tab")
+
+        showing = true
+        try {
+            val result = host.openAuthTab(url, CALLBACK_SCHEME)
+            return when (result.resultCode) {
+                AuthTabIntent.RESULT_OK -> {
+                    val callback = result.resultUri ?: throw BridgeException(ErrorCode.UNAVAILABLE, "the sign-in returned no URL")
+                    JSONObject().put("callbackUrl", callback.toString())
+                }
+                AuthTabIntent.RESULT_CANCELED -> throw BridgeException(ErrorCode.USER_CANCELLED, "sign-in closed")
+                else -> throw BridgeException(ErrorCode.UNAVAILABLE, "sign-in failed (${result.resultCode})")
+            }
+        } finally {
+            showing = false
+        }
+    }
+
+    companion object {
+        const val CALLBACK_SCHEME = "plugchoice-link"
+
+        /** Whether the default browser (or another Custom Tabs browser) supports Auth Tab. */
+        fun isAvailable(context: Context): Boolean {
+            val browser = CustomTabsClient.getPackageName(context, null) ?: return false
+            return CustomTabsClient.isAuthTabSupported(context, browser)
+        }
+
+        /** `url`: an `https` URL with a host (core's sign-in hop). Plain JVM, so tests can run it. */
+        fun isWebSessionUrl(value: String?): Boolean {
+            if (value.isNullOrEmpty()) return false
+            val uri = try {
+                URI(value)
+            } catch (_: URISyntaxException) {
+                return false
+            }
+            return uri.scheme.equals("https", ignoreCase = true) && !uri.host.isNullOrEmpty()
+        }
+    }
+}
diff --git a/android/plugchoice/src/test/java/com/plugchoice/internal/BridgeTest.kt b/android/plugchoice/src/test/java/com/plugchoice/internal/BridgeTest.kt
index fab4ed3..ff75e7d 100644
--- a/android/plugchoice/src/test/java/com/plugchoice/internal/BridgeTest.kt
+++ b/android/plugchoice/src/test/java/com/plugchoice/internal/BridgeTest.kt
@@ -53,6 +53,24 @@ class BridgeTest {
         assertEquals(LinkBridge.capabilities(bluetoothAvailable = true) - "ble", LinkBridge.capabilities(bluetoothAvailable = false))
     }
 
+    @Test
+    fun `auth webSession only where the browser has Auth Tab`() {
+        val with = LinkBridge.capabilities(bluetoothAvailable = false, webSessionAvailable = true)
+        assertTrue("auth.webSession" in with)
+        assertEquals(with - "auth.webSession", LinkBridge.capabilities(bluetoothAvailable = false))
+    }
+
+    @Test
+    fun `a web session opens https URLs only`() {
+        assertTrue(WebSignIn.isWebSessionUrl("https://app.plugchoice.com/auth/google/link-run?ticket=t"))
+        assertFalse(WebSignIn.isWebSessionUrl(null))
+        assertFalse(WebSignIn.isWebSessionUrl(""))
+        assertFalse(WebSignIn.isWebSessionUrl("http://app.plugchoice.com/auth"))
+        assertFalse(WebSignIn.isWebSessionUrl("plugchoice-link://auth"))
+        assertFalse(WebSignIn.isWebSessionUrl("https://"))
+        assertFalse(WebSignIn.isWebSessionUrl("https://exa mple.com"))
+    }
+
     // session.close
 
     @Test
diff --git a/bridge/PROTOCOL.md b/bridge/PROTOCOL.md
index 5c3c408..8fc3bb1 100644
--- a/bridge/PROTOCOL.md
+++ b/bridge/PROTOCOL.md
@@ -1,6 +1,6 @@
 # Plugchoice bridge protocol, version 1
 
-The contract between the hosted page at `https://connect.plugchoice.com` and the native shells in this repository (`ios/`, `android/`). The page runs the flow (setting up a charger, today); the shell only does what a web page can't: join a device's Wi-Fi, find devices on the local network and talk HTTP, WebSocket, TCP and UDP to them, talk Bluetooth, scan a code with the camera, fetch the client secret from the host app, and close the screen with a result.
+The contract between the hosted page at `https://connect.plugchoice.com` and the native shells in this repository (`ios/`, `android/`). The page runs the flow (setting up a charger, today); the shell only does what a web page can't: join a device's Wi-Fi, find devices on the local network and talk HTTP, WebSocket, TCP and UDP to them, talk Bluetooth, scan a code with the camera, fetch the client secret from the host app, open the system sign-in sheet for Google and Apple, and close the screen with a result.
 
 Design rules:
 
@@ -111,6 +111,7 @@ The page sends `hello` first and waits for the answer before anything else.
 | `lan.discover` | `lan.discover`, `lan.stopDiscovery` (§9.4) | iOS: the host declares at least one `NSBonjourServices` type. Android: always |
 | `http.session` | `http.session.open`, `.request`, `.close` (§9.5) | Always |
 | `auth.clientSecret` | `auth.clientSecret` (§7) | Always |
+| `auth.webSession` | `auth.webSession` (§7.1) | iOS: always. Android: the default browser supports Auth Tab |
 | `ble` | `ble.*` (§11) | The device has Bluetooth LE, and iOS: the host has `NSBluetoothAlwaysUsageDescription`; Android: the host kept the library's Bluetooth permissions |
 | `tcp` | `tcp.open`, `tcp.write`, `tcp.close` (§9.6) | Always |
 | `udp` | `udp.exchange` (§9.7) | Always |
@@ -165,6 +166,19 @@ When the shell closes by itself, it reports the action it opened with, no `sessi
 - `clientSecretUnavailable`: the callback threw, returned an empty (or blank) string, or took longer than 30 s. The page shows its error screen with **Try again**, which calls `auth.clientSecret` again.
 - The shell never logs the secret and keeps no copy after answering.
 
+### 7.1 `auth.webSession`
+
+| Params | Result | Errors |
+|---|---|---|
+| `{ url: string }` | `{ callbackUrl: string }` | `userCancelled`, `unavailable` |
+
+- Signing in with Google or Apple. Google refuses to sign in inside an embedded web view, so the shell opens `url` (an `https` URL from core's sign-in hop) in the system's sign-in sheet, outside the page. The sheet closes when it reaches a URL with the scheme `plugchoice-link`, and `callbackUrl` is that URL, as the page reads it (core puts the result in its fragment).
+- The host app needs nothing: the sheet catches the scheme itself, and no app has to register it. Whatever comes back is worth nothing without the run's credential, which only the page holds.
+- `userCancelled`: the user closed the sheet. `unavailable`: a sheet is already showing, or the system couldn't show one. A `url` that isn't `https` is `invalidParams`.
+- iOS: `ASWebAuthenticationSession` with callback scheme `plugchoice-link`, sharing Safari's cookies (not ephemeral: someone signed in to Google in Safari picks their account in one tap). iOS asks first whether the app may use the website to sign in.
+- Android: Chrome's Auth Tab (androidx.browser `AuthTabIntent`) with the same scheme. Listed only when the default browser supports Auth Tab; elsewhere the page keeps sign-in by code. No intent filter is involved, so apps with the SDK never compete for the scheme.
+- `callbackUrl` holds a one-time code: shells never log it.
+
 ## 8. Wi-Fi
 
 ### `wifi.ensurePermissions`
@@ -417,7 +431,7 @@ Not AccessorySetupKit: declaring it for Bluetooth limits CoreBluetooth in the wh
 | `udp.exchange` | `timeoutMs` clamped to 100 ms to 30 s; `maxReplies` 1 to 1000; a datagram up to 65 507 bytes |
 | `ble.scan`, `ble.connect` | `timeoutMs` clamped to 1 s to 60 s |
 | Bluetooth request | 10 s; `ble.write` up to 512 bytes; `mtu` 23 to 517 |
-| At once | 1 `lan.discover`, 1 `ble.scan`, 1 `camera.scanCode`; 64 sessions, 16 TCP sockets, 8 Bluetooth connections |
+| At once | 1 `lan.discover`, 1 `ble.scan`, 1 `camera.scanCode`, 1 `auth.webSession`; 64 sessions, 16 TCP sockets, 8 Bluetooth connections |
 
 ## 13. Error codes
 
@@ -433,7 +447,7 @@ Not AccessorySetupKit: declaring it for Bluetooth limits CoreBluetooth in the wh
 | `undeclaredServiceType`, `busy` | `lan.discover` (`undeclaredServiceType` iOS); `busy` also `ble.scan`, `ble.connect` |
 | `unknownSession`, `tooManySessions` | `http.session.*` |
 | `unknownSocket`, `tooManySockets` | `tcp.*` |
-| `userCancelled`, `cameraPermissionDenied`, `unavailable` | `camera.scanCode`; `unavailable` also `ble.*` |
+| `userCancelled`, `cameraPermissionDenied`, `unavailable` | `camera.scanCode`; `userCancelled` and `unavailable` also `auth.webSession`; `unavailable` also `ble.*` |
 | `bluetoothOff`, `bluetoothPermissionDenied`, `unknownDevice`, `notConnected`, `unknownCharacteristic`, `notPermitted`, `tooManyConnections`, `gatt` | `ble.*` |
 
 `LinkResult.error` codes the shell sets itself: `pageLoadFailed` and `internal` (§6.3). Every other code comes from the page.
diff --git a/ios/Sources/PlugchoiceSDK/Auth/WebSignIn.swift b/ios/Sources/PlugchoiceSDK/Auth/WebSignIn.swift
new file mode 100644
index 0000000..c35f927
--- /dev/null
+++ b/ios/Sources/PlugchoiceSDK/Auth/WebSignIn.swift
@@ -0,0 +1,56 @@
+import AuthenticationServices
+import UIKit
+
+/// `auth.webSession` (PROTOCOL.md §7.1): Google or Apple sign-in in the
+/// system's sign-in sheet, since Google refuses an embedded web view. The
+/// sheet closes on the `plugchoice-link` scheme; that URL goes back to the
+/// page. It holds a one-time code: never logged.
+@MainActor
+final class WebSignIn: NSObject, ASWebAuthenticationPresentationContextProviding {
+    static let callbackScheme = "plugchoice-link"
+
+    private var showing: ASWebAuthenticationSession?
+    private weak var anchor: UIWindow?
+
+    func signIn(url: URL, from presenter: UIViewController?) async throws -> JSONObject {
+        guard showing == nil else {
+            throw BridgeError(code: "unavailable", message: "a sign-in is already showing")
+        }
+        guard let window = presenter?.viewIfLoaded?.window else {
+            throw BridgeError(code: "unavailable", message: "nothing to show the sign-in over")
+        }
+        anchor = window
+        defer { showing = nil }
+
+        let callback = try await withTaskCancellationHandler {
+            try await withCheckedThrowingContinuation { (continuation: CheckedContinuation<URL, Error>) in
+                let session = ASWebAuthenticationSession(url: url, callbackURLScheme: Self.callbackScheme) { callbackURL, error in
+                    if let callbackURL {
+                        continuation.resume(returning: callbackURL)
+                    } else if (error as? ASWebAuthenticationSessionError)?.code == .canceledLogin {
+                        continuation.resume(throwing: BridgeError(code: "userCancelled", message: "sign-in closed"))
+                    } else {
+                        continuation.resume(throwing: BridgeError(code: "unavailable", message: "sign-in failed: \(error.map { String(describing: type(of: $0)) } ?? "unknown")"))
+                    }
+                }
+                session.presentationContextProvider = self
+                // Shares Safari's cookies: someone signed in to Google there picks their account in one tap.
+                session.prefersEphemeralWebBrowserSession = false
+                showing = session
+                if !session.start() {
+                    showing = nil
+                    continuation.resume(throwing: BridgeError(code: "unavailable", message: "the sign-in sheet didn't start"))
+                }
+            }
+        } onCancel: {
+            // The page went away or the screen is closing: the session's
+            // completion answers with canceledLogin.
+            Task { @MainActor in self.showing?.cancel() }
+        }
+        return ["callbackUrl": callback.absoluteString]
+    }
+
+    nonisolated func presentationAnchor(for session: ASWebAuthenticationSession) -> ASPresentationAnchor {
+        MainActor.assumeIsolated { anchor ?? ASPresentationAnchor() }
+    }
+}
diff --git a/ios/Sources/PlugchoiceSDK/Bridge/Bridge.swift b/ios/Sources/PlugchoiceSDK/Bridge/Bridge.swift
index 3973d67..5dbcef1 100644
--- a/ios/Sources/PlugchoiceSDK/Bridge/Bridge.swift
+++ b/ios/Sources/PlugchoiceSDK/Bridge/Bridge.swift
@@ -23,7 +23,7 @@ final class Bridge {
     }
 
     weak var webView: WKWebView?
-    /// Presents the `camera.scanCode` scanner.
+    /// Presents the `camera.scanCode` scanner and the `auth.webSession` sheet.
     weak var presenter: UIViewController?
     /// `hello` was answered: the page draws its own close control and
     /// decides about native closes from now on.
@@ -38,6 +38,7 @@ final class Bridge {
     private let wifi = WifiService()
     private lazy var http = HTTPService()
     private lazy var scanner = CodeScanner()
+    private lazy var webSignIn = WebSignIn()
     private lazy var webSockets = WebSocketService { [weak self] event, params in
         self?.emit(event, params)
     }
@@ -193,6 +194,13 @@ final class Bridge {
                     ["clientSecret": try await secrets.clientSecret()]
                 }
 
+            case "auth.webSession":
+                let url = try Self.webSessionURL(params)
+                let presenter = presenter
+                run(reply) { [webSignIn, weak presenter] in
+                    try await webSignIn.signIn(url: url, from: presenter)
+                }
+
             case "camera.scanCode":
                 let request = try Self.scanRequest(params)
                 let presenter = presenter
@@ -316,7 +324,7 @@ final class Bridge {
         if LanDiscovery.isAvailable(declared: declaredServiceTypes) {
             capabilities.append("lan.discover")
         }
-        capabilities += ["http.session", "auth.clientSecret"]
+        capabilities += ["http.session", "auth.clientSecret", "auth.webSession"]
         if bluetoothAvailable {
             capabilities.append("ble")
         }
@@ -374,6 +382,15 @@ final class Bridge {
         )
     }
 
+    /// `auth.webSession`'s `url`: an `https` URL with a host (core's sign-in hop).
+    static func webSessionURL(_ params: Params) throws -> URL {
+        let string = try params.string("url")
+        guard let url = URL(string: string), url.scheme?.lowercased() == "https", url.host?.isEmpty == false else {
+            throw BridgeError.invalidParams("url must be an https URL")
+        }
+        return url
+    }
+
     static func scanRequest(_ params: Params) throws -> CodeScanner.Request {
         let formats = try params.stringArray("formats")
         guard !formats.isEmpty, formats.allSatisfy({ $0 == "qr" }) else {
diff --git a/ios/Tests/PlugchoiceSDKTests/BridgeTests.swift b/ios/Tests/PlugchoiceSDKTests/BridgeTests.swift
index 49c6604..309e3c1 100644
--- a/ios/Tests/PlugchoiceSDKTests/BridgeTests.swift
+++ b/ios/Tests/PlugchoiceSDKTests/BridgeTests.swift
@@ -29,7 +29,7 @@ final class BridgeTests: XCTestCase {
         let capabilities = try XCTUnwrap(result["capabilities"] as? [String])
         XCTAssertEqual(
             capabilities,
-            ["wifi.join", "http.request", "ws", "session.close", "ui.closeRequest", "lan.address", "http.session", "auth.clientSecret", "tcp", "udp", "trust.custom"],
+            ["wifi.join", "http.request", "ws", "session.close", "ui.closeRequest", "lan.address", "http.session", "auth.clientSecret", "auth.webSession", "tcp", "udp", "trust.custom"],
             "the test host has no NSCameraUsageDescription, NSBonjourServices, NSBluetoothAlwaysUsageDescription or NSAccessorySetupKitSupports"
         )
         XCTAssertEqual(result["lanServiceTypes"] as? [String], [], "the test host declares none")
@@ -130,6 +130,32 @@ final class BridgeTests: XCTestCase {
         }
     }
 
+    // MARK: - auth.webSession
+
+    func testWebSessionURL() throws {
+        XCTAssertEqual(
+            try Bridge.webSessionURL(Params(["url": "https://app.plugchoice.com/auth/google/link-run?ticket=t"])).absoluteString,
+            "https://app.plugchoice.com/auth/google/link-run?ticket=t"
+        )
+        XCTAssertThrowsError(try Bridge.webSessionURL(Params([:])))
+        XCTAssertThrowsError(try Bridge.webSessionURL(Params(["url": ""])))
+        XCTAssertThrowsError(try Bridge.webSessionURL(Params(["url": "http://app.plugchoice.com/auth"])))
+        XCTAssertThrowsError(try Bridge.webSessionURL(Params(["url": "plugchoice-link://auth"])))
+        XCTAssertThrowsError(try Bridge.webSessionURL(Params(["url": "https://"])))
+    }
+
+    func testWebSessionIsUnavailableWithoutAWindow() async {
+        // No presenter on screen: answer `unavailable`, never start a sheet.
+        do {
+            _ = try await WebSignIn().signIn(url: URL(string: "https://app.plugchoice.com/auth")!, from: nil)
+            XCTFail("expected unavailable")
+        } catch let error as BridgeError {
+            XCTAssertEqual(error.code, "unavailable")
+        } catch {
+            XCTFail("unexpected \(error)")
+        }
+    }
+
     // MARK: - camera.scanCode
 
     func testScanRequest() throws {