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@@ -375,6 +375,58 @@ val productDetailsParams = BillingFlowParams.ProductDetailsParams.newBuilder()
 | `DEFERRED` | Deferred, no charge |
 | `KEEP_EXISTING` | Keep existing payment schedule (8.1+) |
 
+## External Payments Program (8.3+)
+
+Billing Library 8.3 (December 2025) added support for the External Payments program (Japan-only, as of launch). Developers enrolled in the program can offer alternative payment methods alongside Google Play billing.
+
+### Enable Developer Billing Option
+
+```kotlin
+// During BillingClient setup
+val billingClient = BillingClient.newBuilder(context)
+    .setListener(purchasesUpdatedListener)
+    .enablePendingPurchases()
+    .enableAutoServiceReconnection()
+    .enableDeveloperBillingOption(
+        DeveloperBillingOptionParams.newBuilder()
+            .setDeveloperProvidedBillingListener(developerBillingListener)
+            .build()
+    )
+    .build()
+```
+
+### DeveloperProvidedBillingListener
+
+```kotlin
+val developerBillingListener = DeveloperProvidedBillingListener {
+    userInitiatedBillingDetails ->
+    // User chose the developer-provided billing flow.
+    // Launch your external payment UI here.
+}
+```
+
+### Launch Purchase with External Payments Option
+
+```kotlin
+val params = BillingFlowParams.newBuilder()
+    .setProductDetailsParamsList(listOf(productDetailsParams))
+    .setBillingOption(BillingOption.EXTERNAL_PAYMENTS)  // 8.3+
+    .build()
+
+billingClient.launchBillingFlow(activity, params)
+```
+
+### Key Types (8.3+)
+
+| Type | Purpose |
+|------|---------|
+| `DeveloperBillingOptionParams` | Configures developer-billing support on `BillingClient` |
+| `DeveloperProvidedBillingListener` | Callback when user picks developer-provided billing |
+| `DeveloperProvidedBillingDetails` | Billing details to report back for reconciliation |
+| `BillingOption.EXTERNAL_PAYMENTS` | Purchase-flow flag requesting external payments |
+
+> **OpenIAP Note**: Exposed through the Android-specific `AlternativeBilling*` surface in OpenIAP. Enrolment with Google Play's External Payments program is required; availability is currently restricted to Japan. The Horizon flavor does NOT implement this.
+
 ## Best Practices
 
 1. **Always acknowledge purchases** within 3 days or they will be refunded
 
@@ -8,7 +8,7 @@
 Meta Horizon provides IAP functionality for Quest VR applications. There are two main integration paths:
 
 1. **Platform SDK IAP** - Native Horizon IAP APIs
-2. **Billing Compatibility SDK** - Google Play Billing Library compatible wrapper
+2. **Billing Compatibility SDK** - Google Play Billing Library-compatible wrapper
 
 ## Version Compatibility Matrix
 
@@ -37,7 +37,7 @@ When writing shared code for both Play and Horizon flavors:
 
 ### APIs Only in Billing 8.x (DO NOT use in shared code)
 
-- `enableAutoServiceReconnection()` - Auto reconnect feature (8.0+)
+- `enableAutoServiceReconnection()` - Auto-reconnect feature (8.0+)
 - Product-level status codes in `queryProductDetailsAsync()` response (8.0+)
 - One-time products with multiple offers (8.0+)
 - Sub-response codes in `BillingResult` (8.0+)
@@ -192,11 +192,12 @@ Mark consumable item as used (required for re-purchase).
 interface VerifyPurchaseHorizonOptions {
   userId: string;      // Horizon user ID
   sku: string;         // Product SKU
-  appId: string;       // Horizon App ID
-  appSecret: string;   // Horizon App Secret
+  accessToken: string; // Format: "OC|APP_ID|APP_SECRET"
 }
 ```
 
+> **OpenIAP Note**: The GraphQL schema takes a single `accessToken` formatted as `OC|APP_ID|APP_SECRET` rather than separate `appId` / `appSecret` fields. Build the token server-side and pass it as one string.
+
 ### VerifyPurchaseResultHorizon
 
 ```typescript
 
@@ -7,24 +7,27 @@ This document provides external API reference for Apple's StoreKit 2 framework.
 | Feature | iOS Version | Description |
 |---------|-------------|-------------|
 | Win-back offers | iOS 18.0 | Re-engage churned subscribers |
-| Consumable transaction history | iOS 18.0 | History includes finished consumables |
-| Billing issue messages | iOS 18.0 | Automatic billing issue notifications via StoreKit Message |
+| `eligibleWinBackOfferIDs` | iOS 18.0 | Query win-back offer eligibility before purchase |
+| Consumable transaction history | iOS 18.0 | Opt-in via `SK2ConsumableTransactionHistory` Info.plist key |
+| StoreKit `Message` API | iOS 18.0 | Listener for billing issues, win-back, price increase, generic |
 | UI context for purchases | iOS 18.2 | Required for proper payment sheet display |
-| External purchase notice | iOS 18.2 | `presentExternalPurchaseNoticeSheetIOS` |
+| External purchase notice | iOS 17.4 | `ExternalPurchase.presentNoticeSheet()` |
 | `appTransactionID` | iOS 18.4 | Globally unique app transaction identifier (back-deployed to iOS 15) |
 | `originalPlatform` | iOS 18.4 | Original purchase platform (back-deployed to iOS 15) |
-| `Offer.Period` | iOS 18.4 | Offer period information |
-| `advancedCommerceInfo` | iOS 18.4 | Advanced Commerce API data |
-| Expanded offer codes | iOS 18.4 | For consumables/non-consumables |
+| `Transaction.offerPeriod` | iOS 18.4 | Offer period information on Transaction |
+| `Transaction.advancedCommerceInfo` | iOS 18.4 | Advanced Commerce API data on Transaction |
+| `Transaction.appTransactionID` | iOS 18.4 | Per-Apple-Account identifier on Transaction |
+| Expanded offer codes | iOS 18.4 | Offer codes for consumables/non-consumables |
 | JWS promotional offers | WWDC 2025 | New `promotionalOffer` purchase option with JWS format |
 | `introductoryOfferEligibility` | WWDC 2025 | Set eligibility via purchase option |
+| `SubscriptionStatus` by Transaction ID | WWDC 2025 | `status(for: transactionID:)` |
 
 ### WWDC 2025 Updates
 
-- **SubscriptionStatus by Transaction ID**: Query subscription status using any transaction ID
-- **JWS-based promotional offers**: New `promotionalOffer` purchase option with compact JWS string
-- **Introductory offer eligibility**: Override eligibility check with `introductoryOfferEligibility` purchase option
-- Both new purchase options are back-deployed to iOS 15
+- **SubscriptionStatus by Transaction ID**: `SubscriptionInfo.Status.status(for: transactionID:)` accepts any transaction ID, not just SKU.
+- **JWS-based promotional offers**: New `promotionalOffer` purchase option with compact JWS string.
+- **Introductory offer eligibility**: Override eligibility check with `introductoryOfferEligibility` purchase option.
+- Both new purchase options are back-deployed to iOS 15.
 
 ## appAccountToken
 
@@ -218,11 +221,16 @@ let result = try await product.purchase(options: [
 
 ### Checking Eligibility
 
+Discover eligible win-back offers before purchase via `Product.SubscriptionInfo.eligibleWinBackOfferIDs` (iOS 18+):
+
 ```swift
-// Win-back offers are available in subscription.promotionalOffers
-// with type == .winBack
-let winBackOffers = product.subscription?.promotionalOffers.filter {
-    $0.type == .winBack
+let status = try await product.subscription?.status.first
+guard let renewalInfo = try status?.renewalInfo.payloadValue else { return }
+
+// iOS 18+: offer IDs the current Apple Account is eligible for
+let eligibleIDs = renewalInfo.eligibleWinBackOfferIDs
+let eligibleOffers = (product.subscription?.promotionalOffers ?? []).filter {
+    $0.type == .winBack && eligibleIDs.contains($0.id ?? "")
 }
 ```
 
@@ -272,6 +280,25 @@ let originalPlatform = appTransaction.originalPlatform   // Original purchase pl
 - Works with Family Sharing (each family member gets unique ID)
 - Back-deployed to iOS 15
 
+## Transaction Updates (iOS 18.4+)
+
+iOS 18.4 added three new read-only properties to `Transaction` (not just `AppTransaction`):
+
+```swift
+let transaction: Transaction
+
+// iOS 18.4+ — all back-deployed to iOS 15
+let txAppTransactionID = transaction.appTransactionID        // Apple Account identifier
+let offerPeriod = transaction.offerPeriod                    // Offer.Period?
+let advancedCommerce = transaction.advancedCommerceInfo      // AdvancedCommerceInfo?
+```
+
+| Property | Type | Notes |
+|----------|------|-------|
+| `appTransactionID` | String | Mirrors AppTransaction's identifier |
+| `offerPeriod` | Offer.Period? | Phase of the promotional/intro offer |
+| `advancedCommerceInfo` | AdvancedCommerceInfo? | Present for Advanced Commerce SKUs only |
+
 ## Advanced Commerce API (iOS 18.4+)
 
 For apps with large product catalogs:
@@ -283,7 +310,61 @@ if let advancedInfo = product.advancedCommerceInfo {
 }
 ```
 
-## External Purchase Support (iOS 18.2+)
+## StoreKit Message API (iOS 18+)
+
+Listen for App Store–generated messages (billing issues, win-back offers, price increases, generic).
+
+```swift
+// Somewhere near app launch
+Task {
+    for await message in Message.messages {
+        switch message.reason {
+        case .billingIssue:
+            // Show UI when user is ready; display from message.display(in:)
+            break
+        case .winBackOffer:
+            break
+        case .priceIncrease:
+            break
+        case .generic:
+            break
+        @unknown default:
+            break
+        }
+    }
+}
+```
+
+| Reason | Trigger |
+|--------|---------|
+| `.billingIssue` | User has an unresolved billing problem on a subscription |
+| `.priceIncrease` | Price change that requires user consent |
+| `.winBackOffer` | User is eligible for a win-back offer |
+| `.generic` | All other system-initiated messages |
+
+> **OpenIAP Note**: To be surfaced by the cross-platform event layer — see `event.graphql` additions for message events.
+
+## SubscriptionStatus by Transaction ID (WWDC 2025)
+
+```swift
+// WWDC 2025: look up status using any transactionID, not just a SKU
+let status = try await Product.SubscriptionInfo.Status.status(for: transactionID)
+```
+
+## Consumable Transaction History (iOS 18+)
+
+By default, `Transaction.all` omits finished consumables. Opt in by adding this key to **Info.plist**:
+
+```xml
+<key>SK2ConsumableTransactionHistory</key>
+<true/>
+```
+
+With the key set, finished consumable transactions are included in `Transaction.all` and `Transaction.currentEntitlements`.
+
+## External Purchase Support (iOS 17.4+)
+
+`ExternalPurchase.presentNoticeSheet()` / `ExternalPurchase.open(url:)` ship on iOS 17.4+. The follow-on custom-link APIs (`ExternalPurchaseCustomLink.isEligible`, `showNotice(type:)`, `token(for:)`) are iOS 18.1+.
 
 ### Present External Purchase Notice
 
 
@@ -0,0 +1,136 @@
+---
+title: Sandbox E2E — subscriptionBillingIssue
+audience: contributors, release QA
+---
+
+# Sandbox E2E: `subscriptionBillingIssue`
+
+The `subscriptionBillingIssue` event requires live store signals that cannot be produced from a unit-test JVM. This document captures the exact sandbox procedure for both platforms so any reviewer can reproduce.
+
+All code paths verified by local compile + Horizon Robolectric unit test:
+
+```bash
+cd packages/google
+./gradlew :openiap:compilePlayDebugKotlin
+./gradlew :openiap:compileHorizonDebugKotlin
+./gradlew :openiap:testHorizonDebugUnitTest   # Robolectric no-op assertion
+
+cd ../apple
+swift build && swift test                     # 87 tests
+
+cd ../../libraries/kmp-iap
+./gradlew :library:compileDebugKotlinAndroid
+
+cd ../react-native-iap && yarn typecheck
+cd ../expo-iap && bun run tsc --noEmit
+cd ../flutter_inapp_purchase && flutter analyze
+```
+
+---
+
+## iOS (StoreKit 2 sandbox)
+
+**Prereqs**
+
+- Physical iOS device running **iOS 18.0 or later** (the `Message.Reason.billingIssue` API is iOS 18+ / Mac Catalyst 18+; the iOS Simulator does not deliver StoreKit Messages).
+- A sandbox Apple ID enrolled in App Store Connect → Users and Access → Sandbox Testers.
+- An auto-renewable subscription product configured on App Store Connect, and the Example project's `subscriptionIds` list pointing at it (`dev.hyo.martie.premium` by default).
+
+**Step-by-step**
+
+1. Sign the device out of its production Apple ID. Sign the sandbox tester into **Settings → App Store → Sandbox Account**.
+2. Open the Example app:
+   - `packages/apple/Example/OpenIapExample.xcodeproj` — run the `OpenIapExample` scheme.
+3. In-app: navigate to the **Subscription Flow** screen and subscribe to `dev.hyo.martie.premium`.
+4. Force a billing issue on the **device** (requires iOS 16+ / iPadOS 16+):
+   - Go to **Settings → Developer → Sandbox Account → Manage → Account Settings**.
+   - Disable the **Allow Purchases & Renewals** setting.
+   - This causes all in-app purchases to fail and auto-renewable subscriptions to stop renewing.
+   - The setting applies to all devices the sandbox account signs in to.
+   - Reference: <https://developer.apple.com/documentation/storekit/testing-failing-subscription-renewals-and-in-app-purchases#Configure-the-sandbox-environment-to-simulate-billing-issues>.
+5. Wait for the next renewal cycle (Renewal Rate = 5 minutes → wait ~5 min). The renewal fails, and StoreKit delivers `Message.Reason.billingIssue` when the app is in the foreground.
+6. To simulate the user fixing the issue, re-enable **Allow Purchases & Renewals**.
+7. Expected UI: the orange "Subscription needs attention" banner appears at the top of the Subscription Flow screen. Tapping **Fix payment method** opens `SKPaymentQueue` / `showManageSubscriptions`.
+
+**What success looks like**
+
+- Console logs:
+
+  ```text
+  🔔 [MessageListener] billingIssue received
+  Emitting subscriptionBillingIssue: dev.hyo.martie.premium
+  ```
+
+- Banner visible on `SubscriptionFlowScreen`.
+- `Transaction.currentEntitlements` shows the affected subscription in `.inBillingRetryPeriod` or `.inGracePeriod`.
+
+**If nothing fires**
+
+- iOS < 18 — silent no-op by design (confirm with `#available` trace in logs).
+- tvOS / watchOS / native macOS (non-Catalyst) / visionOS build — silent no-op by design (StoreKit.Message API is iOS / Mac Catalyst only).
+- App not foregrounded when the message is posted — StoreKit delivers on next `Message.messages` await; bring the app to foreground.
+
+---
+
+## Android (Play Billing 8.1+ sandbox)
+
+**Prereqs**
+
+- Physical Android device (or emulator with Play Store) running the Play flavor of the Example app:
+  `packages/google/Example` → run with product flavor **play**.
+- A Play Console sandbox tester account on the device.
+- A subscription product configured in the Play Console, matching `subscriptionSkus` in `SubscriptionFlowScreen.kt`.
+
+**Step-by-step**
+
+1. Install the Example APK (`./gradlew :Example:installPlayDebug`).
+2. Sign in with the sandbox tester account in the Play Store app.
+3. Subscribe to a test subscription in the Example app.
+4. Force a suspension:
+   - In the **Google Play Store → Payment methods**, remove all payment methods for the sandbox account, OR
+   - Use Play Console → **Subscriptions → Test suspensions** (requires appropriate Play Console role). Reference: <https://developer.android.com/google/play/billing/subscriptions#suspended>.
+5. Wait for Play's renewal cycle. When Play suspends the subscription, the next `getAvailablePurchases` or `onPurchasesUpdated` will include the purchase with `isSuspended == true`.
+6. Return to the Example app. The banner fires once per session per affected purchase (deduped by `purchaseToken`).
+
+**What success looks like**
+
+- `logcat` shows:
+
+  ```text
+  D OpenIapModule: onPurchasesUpdated isSuspended=true ...
+  D Example: subscriptionBillingIssue fired for sku=...
+  ```
+
+- Banner visible on `SubscriptionFlowScreen`.
+- Tapping **Fix payment method** launches `deepLinkToSubscriptions` which routes to Play's subscription center.
+
+**Horizon flavor (do NOT attempt)**
+
+- The Horizon flavor's `addSubscriptionBillingIssueListener` is a documented no-op. Verified by
+  `SubscriptionBillingIssueHorizonNoOpTest` (Robolectric, runs on CI). There is no sandbox path on Horizon because the Billing Compatibility SDK 1.1.1 targets Play Billing 7.0 which does not expose `Purchase.isSuspended`.
+
+---
+
+## Cross-library smoke (optional)
+
+Use `libraries-versions.jsonc` to point example apps at the local monorepo sources (already `"local"` by default), then verify each downstream library surfaces the event:
+
+| Library | Check |
+|---------|-------|
+| react-native-iap | `useIAP({ onSubscriptionBillingIssue: p => console.log(p) })` fires the callback. `subscriptionBillingIssueListener()` also fires independently. |
+| expo-iap | `subscriptionBillingIssueListener((p) => console.log(p))` fires via expo event emitter. |
+| flutter_inapp_purchase | `iap.subscriptionBillingIssueListener.listen(...)` emits the Purchase. |
+| godot-iap | `godot_iap.subscription_billing_issue.connect(...)` emits the Dictionary payload. |
+| kmp-iap | `kmpIapInstance.subscriptionBillingIssueListener.collect {...}` emits in the Flow. |
+
+---
+
+## Automated coverage matrix
+
+| Layer | Mechanism | Status |
+|-------|-----------|--------|
+| Horizon no-op guarantee | Robolectric unit test (`SubscriptionBillingIssueHorizonNoOpTest`) | Runs on CI |
+| Play-flavor compile of listener surface | `compilePlayDebugKotlin` | Runs on CI |
+| Apple Swift test fakes implement protocol | `swift test` | Runs on CI |
+| Downstream types synced | Gen check by each library's typecheck task | Runs on CI |
+| Live sandbox behavior (iOS 18 message + Play suspended) | Manual, this document | Release QA |