diff --git a/Sources/SnapAuth/API.swift b/Sources/SnapAuth/API.swift
index a58e96f..b27ffa2 100644
--- a/Sources/SnapAuth/API.swift
+++ b/Sources/SnapAuth/API.swift
@@ -1,4 +1,5 @@
 import Foundation
+import AuthenticationServices
 
 /// Wrapper that matches the API wire format
 ///
@@ -16,9 +17,11 @@ struct SAWrappedResponse<T>: Decodable where T: Decodable {
 
 struct SACreateRegisterOptionsRequest: Encodable {
     let user: AuthenticatingUser?
+    let upgrade: Bool
 }
 struct SACreateRegisterOptionsResponse: Decodable {
     let publicKey: PublicKeyOptions
+    let mediation: CredentialMediationRequirement
 
     struct PublicKeyOptions: Decodable {
         let rp: RPInfo
@@ -94,7 +97,7 @@ enum Transport: String, Encodable {
 
 struct SACreateAuthOptionsResponse: Decodable {
     let publicKey: PublicKeyOptions
-    // mediation
+    let mediation: CredentialMediationRequirement
 
     struct PublicKeyOptions: Decodable {
 
@@ -133,3 +136,23 @@ struct SAProcessAuthResponse: Decodable {
     let token: String
     let expiresAt: Date
 }
+
+/// https://www.w3.org/TR/credential-management-1/#mediation-requirements
+enum CredentialMediationRequirement: String, Decodable {
+    /// Default behavior: present requests in foreground if needed.
+    case optional = "optional"
+    /// Used to indicate operation should be done in the background.
+    case conditional = "conditional"
+    /// Fail if operation cannot be performed without user involvement. Unused; only present for future-proofing.
+    case silent = "silent"
+    /// Fail if operation cannot be performed with user involvement. Unused; only present for future-proofing.
+    case required = "required"
+
+    @available(iOS 18, visionOS 2.0, macOS 15.0, *)
+    var requestStyle: ASAuthorizationPlatformPublicKeyCredentialRegistrationRequest.RequestStyle {
+        if (self == .conditional) {
+            return .conditional
+        }
+        return .standard
+    }
+}
diff --git a/Sources/SnapAuth/SnapAuth+BuildRequests.swift b/Sources/SnapAuth/SnapAuth+BuildRequests.swift
index 0a07065..9b26b08 100644
--- a/Sources/SnapAuth/SnapAuth+BuildRequests.swift
+++ b/Sources/SnapAuth/SnapAuth+BuildRequests.swift
@@ -14,12 +14,30 @@ extension SnapAuth {
         if authenticators.contains(.passkey) {
             let provider = ASAuthorizationPlatformPublicKeyCredentialProvider(
                 relyingPartyIdentifier: options.publicKey.rp.id)
-            let request = provider.createCredentialRegistrationRequest(
-                challenge: challenge,
-                name: username,
-                userID: options.publicKey.user.id.data)
 
-            requests.append(request)
+            // This is a little clumsy: the conditional request API wasn't added
+            // to tvOS at all, and a simple if #available can't block an entire
+            // platform.
+            var request: ASAuthorizationPlatformPublicKeyCredentialRegistrationRequest? = nil
+            #if (os(iOS) || os(macOS) || os(visionOS))
+            if #available(iOS 18, macOS 15, visionOS 2, *) {
+                request = provider.createCredentialRegistrationRequest(
+                    challenge: challenge,
+                    name: username,
+                    userID: options.publicKey.user.id.data,
+                    requestStyle: options.mediation.requestStyle)
+            }
+            // TODO: if conditional and unsupported platform, short-circuit to ensure there's no false-positive modals
+            #endif
+            if request == nil {
+                // tvOS and previous other platforms
+                request = provider.createCredentialRegistrationRequest(
+                    challenge: challenge,
+                    name: username,
+                    userID: options.publicKey.user.id.data)
+            }
+
+            requests.append(request!)
         }
 
 #if HARDWARE_KEY_SUPPORT
diff --git a/Sources/SnapAuth/SnapAuth+Upgrades.swift b/Sources/SnapAuth/SnapAuth+Upgrades.swift
new file mode 100644
index 0000000..2de3b4c
--- /dev/null
+++ b/Sources/SnapAuth/SnapAuth+Upgrades.swift
@@ -0,0 +1,27 @@
+extension SnapAuth {
+    /// Attempts to upgrade an existing account to use passkeys by creating one
+    /// in the background.
+    ///
+    /// This should be called after a user signs in. Errors should not be
+    /// displayed to the user, though may be logged.
+    ///
+    /// - Parameters:
+    ///   - username: The username of the user, such as an email address or handle
+    ///   - displayName: The proper name of the user. If omitted, name will be used.
+    public func upgradeToPasskey(
+        username: String,
+        displayName: String? = nil
+    ) async -> SnapAuthResult {
+        if !SAAvailability.passkeyUpgrades {
+            return .failure(.unsupportedOnPlatform)
+        }
+
+        return await startRegister(
+            username: username,
+            anchor: .default,
+            displayName: displayName,
+            authenticators: [.passkey],
+            upgrade: true
+        )
+    }
+}
diff --git a/Sources/SnapAuth/SnapAuth.swift b/Sources/SnapAuth/SnapAuth.swift
index 1eefbda..be9f103 100644
--- a/Sources/SnapAuth/SnapAuth.swift
+++ b/Sources/SnapAuth/SnapAuth.swift
@@ -106,7 +106,8 @@ public class SnapAuth: NSObject { // NSObject for ASAuthorizationControllerDeleg
             username: username,
             anchor: .default,
             displayName: displayName,
-            authenticators: authenticators)
+            authenticators: authenticators,
+            upgrade: false)
     }
 
     // TODO: Only make this public if needed?
@@ -114,12 +115,13 @@ public class SnapAuth: NSObject { // NSObject for ASAuthorizationControllerDeleg
         username: String,
         anchor: ASPresentationAnchor,
         displayName: String? = nil,
-        authenticators: Set<Authenticator> = Authenticator.all
+        authenticators: Set<Authenticator> = Authenticator.all,
+        upgrade: Bool
     ) async -> SnapAuthResult {
         reset()
         self.anchor = anchor
 
-        let body = SACreateRegisterOptionsRequest(user: nil)
+        let body = SACreateRegisterOptionsRequest(user: nil, upgrade: upgrade)
         let response = await api.makeRequest(
             path: "/attestation/options",
             body: body,