diff --git a/README.md b/README.md
index cb7524a..4d9dc93 100644
--- a/README.md
+++ b/README.md
@@ -157,6 +157,63 @@ Each order has `status` (`processing` = current month, still collecting; `pendin
 paid; `completed`; `cancelled`), the totals, `payment_attempts`, `next_payment_attempt_at` (null =
 no more automatic retries) and `last_payment_error`.
 
+### Checkout sessions (the user confirms several subscriptions at once)
+
+Instead of checking subscriptions out itself, an app can let the user confirm them on Anikeen ID,
+all of them in one step: a checkout session bundles new subscriptions (or existing drafts of the
+app, `['id' => …]`) of one user. The user sees every item with its price and the total of the first
+period, confirms (all subscriptions start, or none) or cancels, and returns to `return_url`.
+Open sessions expire after 24 hours with their drafts. Trusted apps can still check out directly;
+sessions are opt-in.
+
+Example: the Console's builder ("Jetzt kaufen") with several products and coupon prices:
+
+```php
+use Anikeen\Id\Exceptions\ApiException;
+
+$session = $request->user()->appCheckoutSessions()->create([   // app token; or ->checkoutSessions() with the user's token
+    'return_url' => route('builder.done'),                      // same origin as one of your redirect URIs
+    'metadata' => ['cart' => $cart->id],                        // returned as is
+    'idempotency_key' => "cart:{$cart->id}",                    // retries return the same session
+    'subscriptions' => $cart->items->map(fn ($item) => [
+        'group' => $team->id,
+        'name' => $item->product->name,
+        'description' => $item->label,
+        'unit' => $item->unit,                                  // hour, day, week, month, year
+        'price' => $item->priceAfterCoupon(),                   // net, per unit
+        'payload' => ['cart_item' => $item->id],
+    ])->all(),
+]);
+
+$cart->update(['checkout_session_id' => $session->id, 'subscription_ids' => $session->subscriptionIds()]);
+
+return redirect($session->confirmationUrl());
+// later: https://console.test/builder/done?status=completed&checkout_session=01K…
+```
+
+Provision when the webhook arrives (or after checking the session through the API; never trust the
+query string):
+
+```php
+use Anikeen\Id\Webhooks\EventType;
+
+match ($event['type']) {
+    EventType::CHECKOUT_COMPLETED => Provisioner::provision($event['data']['object']['subscriptions']), // all running
+    EventType::CHECKOUT_CANCELLED, EventType::CHECKOUT_EXPIRED => Cart::release($event['data']['object']['metadata']['cart'] ?? null),
+    default => null,
+};
+
+app(AnikeenId::class)->appCheckoutSessions()->find($id)?->isCompleted(); // any user's session
+$session->refresh()->subscriptions();  // AppSubscription objects
+$session->cancel();                    // withdraw an open session and its drafts (e.g. the cart changed)
+```
+
+While a session is open, its subscriptions can't be checked out or repriced by the app (409
+`in_checkout_session` / `not_changeable`). The session's `status` is `open`, `completed`,
+`cancelled` or `expired` (`isOpen()`, `isCompleted()`, `isCancelled()` for cancelled or expired).
+`subscription.activated` / `subscription.cancelled` are still sent per subscription, before the
+`checkout.*` event of the session.
+
 ### Selling as a third-party app (user confirms on Anikeen ID)
 
 Apps that are not trusted can create orders and subscriptions for the signed-in user (user token),
@@ -222,12 +279,13 @@ points to the same page. Trusted apps keep checking out directly.
 
 ### Webhooks
 
-Anikeen ID sends `order.*` and `subscription.*` events to the webhook endpoints of your app (set up
+Anikeen ID sends `order.*`, `subscription.*` and `checkout.*` events to the webhook endpoints of your app (set up
 in the Anikeen ID admin panel, which also shows the signing secret `whsec_…`). Verify each request
 with the raw body and answer 2xx within 10 seconds; failed deliveries are retried for about three days.
 
 ```php
 use Anikeen\Id\Exceptions\SignatureVerificationException;
+use Anikeen\Id\Webhooks\EventType;
 use Anikeen\Id\Webhooks\Webhook;
 
 Route::post('webhooks/anikeen', function (Request $request) {
@@ -238,8 +296,9 @@ Route::post('webhooks/anikeen', function (Request $request) {
     }
 
     match ($event['type']) {
-        'order.completed' => Shop::fulfil($event['data']['object']['id']),
-        'subscription.cancelled' => Panel::suspend($event['data']['object']['id']),
+        EventType::ORDER_COMPLETED => Shop::fulfil($event['data']['object']['id']),
+        EventType::SUBSCRIPTION_CANCELLED => Panel::suspend($event['data']['object']['id']),
+        EventType::CHECKOUT_COMPLETED => Panel::provision($event['data']['object']['subscription_ids']),
         default => null,
     };
 
@@ -471,6 +530,7 @@ public function shippingPriceList(): Result
 
 ```php
 public function appOrders(): AppOrders
+public function appCheckoutSessions(): AppCheckoutSessions
 public function createOrderForUser(string $userId, array $attributes): Result
 public function createTransactionForUser(string $userId, array $attributes): Result
 ```
@@ -573,6 +633,8 @@ public function getBillable(): Billable
 ```php
 public function subscriptions(): Subscriptions
 public function appSubscriptions(): AppSubscriptions
+public function appCheckoutSessions(): AppCheckoutSessions
+public function checkoutSessions(): CheckoutSessions
 public function appOrders(): AppOrders
 public function setBillable(Billable|Model $billable): self
 public function getBillable(): Billable
diff --git a/README.stub b/README.stub
index 6fdf071..5522a5f 100644
--- a/README.stub
+++ b/README.stub
@@ -157,6 +157,63 @@ Each order has `status` (`processing` = current month, still collecting; `pendin
 paid; `completed`; `cancelled`), the totals, `payment_attempts`, `next_payment_attempt_at` (null =
 no more automatic retries) and `last_payment_error`.
 
+### Checkout sessions (the user confirms several subscriptions at once)
+
+Instead of checking subscriptions out itself, an app can let the user confirm them on Anikeen ID,
+all of them in one step: a checkout session bundles new subscriptions (or existing drafts of the
+app, `['id' => …]`) of one user. The user sees every item with its price and the total of the first
+period, confirms (all subscriptions start, or none) or cancels, and returns to `return_url`.
+Open sessions expire after 24 hours with their drafts. Trusted apps can still check out directly;
+sessions are opt-in.
+
+Example: the Console's builder ("Jetzt kaufen") with several products and coupon prices:
+
+```php
+use Anikeen\Id\Exceptions\ApiException;
+
+$session = $request->user()->appCheckoutSessions()->create([   // app token; or ->checkoutSessions() with the user's token
+    'return_url' => route('builder.done'),                      // same origin as one of your redirect URIs
+    'metadata' => ['cart' => $cart->id],                        // returned as is
+    'idempotency_key' => "cart:{$cart->id}",                    // retries return the same session
+    'subscriptions' => $cart->items->map(fn ($item) => [
+        'group' => $team->id,
+        'name' => $item->product->name,
+        'description' => $item->label,
+        'unit' => $item->unit,                                  // hour, day, week, month, year
+        'price' => $item->priceAfterCoupon(),                   // net, per unit
+        'payload' => ['cart_item' => $item->id],
+    ])->all(),
+]);
+
+$cart->update(['checkout_session_id' => $session->id, 'subscription_ids' => $session->subscriptionIds()]);
+
+return redirect($session->confirmationUrl());
+// later: https://console.test/builder/done?status=completed&checkout_session=01K…
+```
+
+Provision when the webhook arrives (or after checking the session through the API; never trust the
+query string):
+
+```php
+use Anikeen\Id\Webhooks\EventType;
+
+match ($event['type']) {
+    EventType::CHECKOUT_COMPLETED => Provisioner::provision($event['data']['object']['subscriptions']), // all running
+    EventType::CHECKOUT_CANCELLED, EventType::CHECKOUT_EXPIRED => Cart::release($event['data']['object']['metadata']['cart'] ?? null),
+    default => null,
+};
+
+app(AnikeenId::class)->appCheckoutSessions()->find($id)?->isCompleted(); // any user's session
+$session->refresh()->subscriptions();  // AppSubscription objects
+$session->cancel();                    // withdraw an open session and its drafts (e.g. the cart changed)
+```
+
+While a session is open, its subscriptions can't be checked out or repriced by the app (409
+`in_checkout_session` / `not_changeable`). The session's `status` is `open`, `completed`,
+`cancelled` or `expired` (`isOpen()`, `isCompleted()`, `isCancelled()` for cancelled or expired).
+`subscription.activated` / `subscription.cancelled` are still sent per subscription, before the
+`checkout.*` event of the session.
+
 ### Selling as a third-party app (user confirms on Anikeen ID)
 
 Apps that are not trusted can create orders and subscriptions for the signed-in user (user token),
@@ -185,12 +242,13 @@ points to the same page. Trusted apps keep checking out directly.
 
 ### Webhooks
 
-Anikeen ID sends `order.*` and `subscription.*` events to the webhook endpoints of your app (set up
+Anikeen ID sends `order.*`, `subscription.*` and `checkout.*` events to the webhook endpoints of your app (set up
 in the Anikeen ID admin panel, which also shows the signing secret `whsec_…`). Verify each request
 with the raw body and answer 2xx within 10 seconds; failed deliveries are retried for about three days.
 
 ```php
 use Anikeen\Id\Exceptions\SignatureVerificationException;
+use Anikeen\Id\Webhooks\EventType;
 use Anikeen\Id\Webhooks\Webhook;
 
 Route::post('webhooks/anikeen', function (Request $request) {
@@ -201,8 +259,9 @@ Route::post('webhooks/anikeen', function (Request $request) {
     }
 
     match ($event['type']) {
-        'order.completed' => Shop::fulfil($event['data']['object']['id']),
-        'subscription.cancelled' => Panel::suspend($event['data']['object']['id']),
+        EventType::ORDER_COMPLETED => Shop::fulfil($event['data']['object']['id']),
+        EventType::SUBSCRIPTION_CANCELLED => Panel::suspend($event['data']['object']['id']),
+        EventType::CHECKOUT_COMPLETED => Panel::provision($event['data']['object']['subscription_ids']),
         default => null,
     };
 
diff --git a/src/Id/Concerns/IsCheckoutSession.php b/src/Id/Concerns/IsCheckoutSession.php
new file mode 100644
index 0000000..05fb49c
--- /dev/null
+++ b/src/Id/Concerns/IsCheckoutSession.php
@@ -0,0 +1,49 @@
+<?php
+
+namespace Anikeen\Id\Concerns;
+
+/**
+ * Reading a checkout session (see AppCheckoutSession and CheckoutSession).
+ */
+trait IsCheckoutSession
+{
+    /**
+     * Where the user confirms (or cancels) all of the session's subscriptions, or null once the
+     * session is no longer open. The user then returns to the session's `return_url` with
+     * `status` (completed, cancelled or expired) and `checkout_session` (the id). Check the status
+     * through the API or the checkout.* webhooks; don't trust the query string.
+     */
+    public function confirmationUrl(): ?string
+    {
+        return $this->confirmation_url ?: null;
+    }
+
+    public function isOpen(): bool
+    {
+        return $this->status === self::STATUS_OPEN;
+    }
+
+    /**
+     * Confirmed: all of its subscriptions are running.
+     */
+    public function isCompleted(): bool
+    {
+        return $this->status === self::STATUS_COMPLETED;
+    }
+
+    /**
+     * Cancelled or expired: all of its drafts were cancelled.
+     */
+    public function isCancelled(): bool
+    {
+        return in_array($this->status, [self::STATUS_CANCELLED, self::STATUS_EXPIRED], true);
+    }
+
+    /**
+     * @return list<string>
+     */
+    public function subscriptionIds(): array
+    {
+        return array_values((array) ($this->subscription_ids ?? []));
+    }
+}
diff --git a/src/Id/Concerns/ManagesSubscriptions.php b/src/Id/Concerns/ManagesSubscriptions.php
index 446c89b..33c78b4 100644
--- a/src/Id/Concerns/ManagesSubscriptions.php
+++ b/src/Id/Concerns/ManagesSubscriptions.php
@@ -2,8 +2,10 @@
 
 namespace Anikeen\Id\Concerns;
 
+use Anikeen\Id\Resources\AppCheckoutSessions;
 use Anikeen\Id\Resources\AppOrders;
 use Anikeen\Id\Resources\AppSubscriptions;
+use Anikeen\Id\Resources\CheckoutSessions;
 use Anikeen\Id\Resources\Subscriptions;
 use Throwable;
 
@@ -37,6 +39,24 @@ trait ManagesSubscriptions
         return new AppSubscriptions($this);
     }
 
+    /**
+     * Checkout sessions for the user, created with the app's own token (trusted apps only, scope
+     * subscriptions:client): the user confirms several subscriptions at once on Anikeen ID.
+     */
+    public function appCheckoutSessions(): AppCheckoutSessions
+    {
+        return new AppCheckoutSessions($this);
+    }
+
+    /**
+     * Checkout sessions with the user's own access token (any app): the user confirms several
+     * subscriptions at once on Anikeen ID.
+     */
+    public function checkoutSessions(): CheckoutSessions
+    {
+        return new CheckoutSessions($this);
+    }
+
     /**
      * The user's orders that bill the app's subscriptions, read with the app's own token
      * (trusted apps only, scope orders:client), e.g. ->pending() for unpaid ones.
@@ -45,4 +65,4 @@ trait ManagesSubscriptions
     {
         return new AppOrders($this);
     }
-}
\ No newline at end of file
+}
diff --git a/src/Id/Concerns/ManagesUserBilling.php b/src/Id/Concerns/ManagesUserBilling.php
index e675647..f5d91ad 100644
--- a/src/Id/Concerns/ManagesUserBilling.php
+++ b/src/Id/Concerns/ManagesUserBilling.php
@@ -2,6 +2,7 @@
 
 namespace Anikeen\Id\Concerns;
 
+use Anikeen\Id\Resources\AppCheckoutSessions;
 use Anikeen\Id\Resources\AppOrders;
 use Anikeen\Id\Result;
 use Throwable;
@@ -19,6 +20,15 @@ trait ManagesUserBilling
         return new AppOrders();
     }
 
+    /**
+     * Checkout sessions of the app for any user, e.g. to look one up from a checkout.* webhook
+     * (create() then needs a user_id).
+     */
+    public function appCheckoutSessions(): AppCheckoutSessions
+    {
+        return new AppCheckoutSessions();
+    }
+
     /**
      * Create an order for a user (VAT from the billing address). With `checkout`, it is paid right
      * away: with the given `transaction` (a payment captured elsewhere) or by charging the user's
diff --git a/src/Id/Resources/AppCheckoutSession.php b/src/Id/Resources/AppCheckoutSession.php
new file mode 100644
index 0000000..380e721
--- /dev/null
+++ b/src/Id/Resources/AppCheckoutSession.php
@@ -0,0 +1,97 @@
+<?php
+
+namespace Anikeen\Id\Resources;
+
+use Anikeen\Id\Concerns\IsCheckoutSession;
+use Anikeen\Id\Exceptions\ApiException;
+use Anikeen\Id\Support\AppRequest;
+use JsonSerializable;
+
+/**
+ * A checkout session created with the app's own token, see AppCheckoutSessions.
+ *
+ * @property string $id
+ * @property string $user_id
+ * @property ?string $oauth_client_id
+ * @property string $status open, completed, cancelled or expired
+ * @property ?string $return_url
+ * @property ?object $metadata
+ * @property ?string $confirmation_url
+ * @property list<string> $subscription_ids
+ * @property list<object> $subscriptions
+ * @property ?string $expires_at
+ * @property ?string $completed_at
+ * @property ?string $cancelled_at
+ * @property ?string $created_at
+ */
+class AppCheckoutSession implements JsonSerializable
+{
+    use IsCheckoutSession;
+
+    public const STATUS_OPEN = 'open';
+    public const STATUS_COMPLETED = 'completed';
+    public const STATUS_CANCELLED = 'cancelled';
+    public const STATUS_EXPIRED = 'expired';
+
+    public function __construct(protected object $data)
+    {
+    }
+
+    public function __get(string $name): mixed
+    {
+        return $this->data->{$name} ?? null;
+    }
+
+    public function __isset(string $name): bool
+    {
+        return isset($this->data->{$name});
+    }
+
+    /**
+     * The session's subscriptions (drafts until the user confirms).
+     *
+     * @return list<AppSubscription>
+     */
+    public function subscriptions(): array
+    {
+        return array_map(fn (object $subscription) => new AppSubscription($subscription), array_values((array) ($this->data->subscriptions ?? [])));
+    }
+
+    /**
+     * Withdraw an open session (e.g. the cart changed): it and its drafts are cancelled.
+     *
+     * @throws ApiException 409 "checkout_session_completed", "checkout_session_cancelled", "checkout_session_expired"
+     */
+    public function cancel(): self
+    {
+        return $this->send('POST', '/cancel');
+    }
+
+    /**
+     * @throws ApiException
+     */
+    public function refresh(): self
+    {
+        return $this->send('GET', '');
+    }
+
+    public function toArray(): array
+    {
+        return (array) $this->data;
+    }
+
+    public function jsonSerialize(): array
+    {
+        return $this->toArray();
+    }
+
+    /**
+     * @throws ApiException
+     */
+    private function send(string $method, string $action): self
+    {
+        $this->data = AppRequest::send($method, sprintf('v1/client/checkout-sessions/%s%s', $this->id, $action))->data;
+
+        return $this;
+    }
+}
diff --git a/src/Id/Resources/AppCheckoutSessions.php b/src/Id/Resources/AppCheckoutSessions.php
new file mode 100644
index 0000000..f4661f0
--- /dev/null
+++ b/src/Id/Resources/AppCheckoutSessions.php
@@ -0,0 +1,79 @@
+<?php
+
+namespace Anikeen\Id\Resources;
+
+use Anikeen\Id\Contracts\Billable;
+use Anikeen\Id\Exceptions\ApiException;
+use Anikeen\Id\Support\AppRequest;
+use Illuminate\Database\Eloquent\Model;
+use InvalidArgumentException;
+
+/**
+ * Checkout sessions created with the app's own token (trusted apps, scope `subscriptions:client`):
+ * several subscriptions of one user that the user confirms together on Anikeen ID, instead of the
+ * app checking them out itself. For one user ($user->appCheckoutSessions()) or, to look sessions
+ * up e.g. from a webhook, for all users (app(AnikeenId::class)->appCheckoutSessions()).
+ * Nothing is requested until a method is called.
+ */
+class AppCheckoutSessions
+{
+    public function __construct(protected Billable|Model|null $billable = null)
+    {
+    }
+
+    /**
+     * Open a checkout session with new subscriptions (their attributes, like
+     * AppSubscriptions::create()) and/or existing drafts of the app (`['id' => …]`).
+     *
+     * @param array{
+     *     subscriptions: list<array{id: string}|array{
+     *         group?: null|string,
+     *         name: string,
+     *         description?: null|string,
+     *         unit: string,
+     *         price: float|string,
+     *         vat_rate?: float,
+     *         payload?: null|array,
+     *         ends_at?: null|string
+     *     }>,
+     *     return_url?: null|string,
+     *     metadata?: null|array,
+     *     idempotency_key?: null|string,
+     *     user_id?: string
+     * } $attributes return_url must share scheme, host and port with one of the app's redirect
+     *               URIs; the user comes back to it with `status` and `checkout_session`. A
+     *               repeated idempotency_key returns the first session instead of a new one.
+     *
+     * @throws ApiException 422 validation (e.g. return_url), 409 "idempotency_key_conflict"
+     */
+    public function create(array $attributes): AppCheckoutSession
+    {
+        if ($this->billable) {
+            $attributes['user_id'] = (new AppSubscriptions($this->billable))->userId();
+        } elseif (empty($attributes['user_id'])) {
+            throw new InvalidArgumentException('A checkout session needs a user: use $user->appCheckoutSessions() or pass user_id.');
+        }
+
+        return new AppCheckoutSession(AppRequest::send('POST', 'v1/client/checkout-sessions', $attributes)->data);
+    }
+
+    /**
+     * A session of the app; null when it doesn't exist (or belongs to another user).
+     *
+     * @throws ApiException
+     */
+    public function find(string $id): ?AppCheckoutSession
+    {
+        try {
+            $session = new AppCheckoutSession(AppRequest::send('GET', sprintf('v1/client/checkout-sessions/%s', $id))->data);
+        } catch (ApiException $e) {
+            if ($e->isNotFound()) {
+                return null;
+            }
+
+            throw $e;
+        }
+
+        return !$this->billable || $session->user_id === (new AppSubscriptions($this->billable))->userId() ? $session : null;
+    }
+}
diff --git a/src/Id/Resources/AppSubscription.php b/src/Id/Resources/AppSubscription.php
index dba35fe..03f3377 100644
--- a/src/Id/Resources/AppSubscription.php
+++ b/src/Id/Resources/AppSubscription.php
@@ -29,6 +29,7 @@ use JsonSerializable;
  * @property int $failed_renewal_retries
  * @property ?array $payload
  * @property ?string $idempotency_key
+ * @property ?string $checkout_session_id the checkout session the user confirms it in, if any
  */
 class AppSubscription implements JsonSerializable
 {
diff --git a/src/Id/Resources/CheckoutSession.php b/src/Id/Resources/CheckoutSession.php
new file mode 100644
index 0000000..5fa0450
--- /dev/null
+++ b/src/Id/Resources/CheckoutSession.php
@@ -0,0 +1,57 @@
+<?php
+
+namespace Anikeen\Id\Resources;
+
+use Anikeen\Id\Concerns\HasBillable;
+use Anikeen\Id\Concerns\IsCheckoutSession;
+use Throwable;
+
+/**
+ * A checkout session created with the user's access token, see CheckoutSessions.
+ *
+ * @property string $id
+ * @property string $user_id
+ * @property ?string $oauth_client_id
+ * @property string $status open, completed, cancelled or expired
+ * @property ?string $return_url
+ * @property ?object $metadata
+ * @property ?string $confirmation_url
+ * @property list<string> $subscription_ids
+ * @property list<object> $subscriptions
+ * @property ?string $expires_at
+ * @property ?string $completed_at
+ * @property ?string $cancelled_at
+ * @property ?string $created_at
+ */
+class CheckoutSession extends BaseResource
+{
+    use HasBillable;
+    use IsCheckoutSession;
+
+    public const STATUS_OPEN = 'open';
+    public const STATUS_COMPLETED = 'completed';
+    public const STATUS_CANCELLED = 'cancelled';
+    public const STATUS_EXPIRED = 'expired';
+
+    /**
+     * Withdraw an open session: it and its drafts are cancelled.
+     *
+     * @throws Throwable
+     */
+    public function cancel(): self
+    {
+        return (new self(fn () => $this->billable->anikeenId()
+            ->request('POST', sprintf('v1/checkout-sessions/%s/cancel', $this->id))))
+            ->setBillable($this->billable);
+    }
+
+    /**
+     * @throws Throwable
+     */
+    public function refresh(): self
+    {
+        return (new self(fn () => $this->billable->anikeenId()
+            ->request('GET', sprintf('v1/checkout-sessions/%s', $this->id))))
+            ->setBillable($this->billable);
+    }
+}
diff --git a/src/Id/Resources/CheckoutSessions.php b/src/Id/Resources/CheckoutSessions.php
new file mode 100644
index 0000000..b77c08a
--- /dev/null
+++ b/src/Id/Resources/CheckoutSessions.php
@@ -0,0 +1,58 @@
+<?php
+
+namespace Anikeen\Id\Resources;
+
+use Anikeen\Id\Concerns\HasBillable;
+use Anikeen\Id\Contracts\Billable;
+use Anikeen\Id\Exceptions\ResourceException;
+use Illuminate\Database\Eloquent\Model;
+use Throwable;
+
+/**
+ * Checkout sessions of the signed-in user, with the user's access token (any app): several
+ * subscriptions the user confirms together on Anikeen ID. Nothing is requested until a method is
+ * called.
+ */
+class CheckoutSessions
+{
+    use HasBillable;
+
+    public function __construct(Billable|Model $billable)
+    {
+        $this->billable = $billable;
+    }
+
+    /**
+     * Open a checkout session with new subscriptions and/or existing drafts of the app
+     * (`['id' => …]`); see AppCheckoutSessions::create() for the attributes (no user_id here,
+     * vat_rate only for trusted apps).
+     *
+     * @throws Throwable
+     */
+    public function create(array $attributes): CheckoutSession
+    {
+        return (new CheckoutSession(fn () => $this->billable->anikeenId()
+            ->request('POST', 'v1/checkout-sessions', $attributes)))
+            ->setBillable($this->billable);
+    }
+
+    /**
+     * A session the app created for the user; null when it doesn't exist.
+     *
+     * @throws Throwable
+     */
+    public function find(string $id): ?CheckoutSession
+    {
+        try {
+            return (new CheckoutSession(fn () => $this->billable->anikeenId()
+                ->request('GET', sprintf('v1/checkout-sessions/%s', $id))))
+                ->setBillable($this->billable);
+        } catch (ResourceException $e) {
+            if ($e->getCode() === 404) {
+                return null;
+            }
+
+            throw $e;
+        }
+    }
+}
diff --git a/src/Id/Webhooks/EventType.php b/src/Id/Webhooks/EventType.php
new file mode 100644
index 0000000..e4b8b44
--- /dev/null
+++ b/src/Id/Webhooks/EventType.php
@@ -0,0 +1,46 @@
+<?php
+
+namespace Anikeen\Id\Webhooks;
+
+/**
+ * The `type` of webhook events Anikeen ID sends, e.g.
+ * `match ($event['type']) { EventType::CHECKOUT_COMPLETED => …, … }`.
+ */
+final class EventType
+{
+    public const ORDER_CREATED = 'order.created';
+    public const ORDER_COMPLETED = 'order.completed';
+    public const ORDER_PAYMENT_FAILED = 'order.payment_failed';
+    public const ORDER_CANCELLED = 'order.cancelled';
+    public const ORDER_SHIPPED = 'order.shipped';
+
+    public const SUBSCRIPTION_CREATED = 'subscription.created';
+    public const SUBSCRIPTION_ACTIVATED = 'subscription.activated';
+    public const SUBSCRIPTION_PAUSED = 'subscription.paused';
+    public const SUBSCRIPTION_RESUMED = 'subscription.resumed';
+    public const SUBSCRIPTION_CANCELLED = 'subscription.cancelled';
+    public const SUBSCRIPTION_EXPIRED = 'subscription.expired';
+
+    /** The user confirmed a checkout session: all of its subscriptions are running (sent after their subscription.activated). */
+    public const CHECKOUT_COMPLETED = 'checkout.completed';
+
+    /** The user or the app cancelled a checkout session; its drafts were cancelled. */
+    public const CHECKOUT_CANCELLED = 'checkout.cancelled';
+
+    /** A checkout session was not confirmed within 24 hours; its drafts were cancelled. */
+    public const CHECKOUT_EXPIRED = 'checkout.expired';
+
+    public const TEST = 'webhook.test';
+
+    /**
+     * @return list<string>
+     */
+    public static function all(): array
+    {
+        return array_values((new \ReflectionClass(self::class))->getConstants());
+    }
+
+    private function __construct()
+    {
+    }
+}
diff --git a/tests/Id/CheckoutSessionsTest.php b/tests/Id/CheckoutSessionsTest.php
new file mode 100644
index 0000000..06635f8
--- /dev/null
+++ b/tests/Id/CheckoutSessionsTest.php
@@ -0,0 +1,163 @@
+<?php
+
+namespace Anikeen\Id\Tests;
+
+use Anikeen\Id\AnikeenId;
+use Anikeen\Id\Billable;
+use Anikeen\Id\Exceptions\ApiException;
+use Anikeen\Id\Resources\AppCheckoutSession;
+use Anikeen\Id\Resources\AppSubscription;
+use Anikeen\Id\Resources\CheckoutSession;
+use Anikeen\Id\Webhooks\EventType;
+use Anikeen\Id\Webhooks\Webhook;
+use Illuminate\Database\Eloquent\Model;
+use InvalidArgumentException;
+
+class CheckoutSessionsTest extends TestCase
+{
+    private function owner(): Model
+    {
+        $owner = new class extends Model {
+            use Billable;
+
+            protected $guarded = [];
+        };
+        $owner->anikeen_id = '01JUSER';
+        $owner->anikeen_id_access_token = 'user-token';
+
+        return $owner;
+    }
+
+    private function sessionData(array $overrides = []): array
+    {
+        return ['data' => [
+            'object' => 'checkout_session',
+            'id' => '01JCS',
+            'user_id' => '01JUSER',
+            'status' => 'open',
+            'return_url' => 'https://console.test/builder/done',
+            'metadata' => ['cart' => '42'],
+            'confirmation_url' => 'https://id.anikeen.com/checkout/sessions/01JCS',
+            'subscription_ids' => ['01JS1', '01JS2'],
+            'subscriptions' => [
+                ['id' => '01JS1', 'user_id' => '01JUSER', 'name' => 'Cloud Server S', 'active' => false, 'payload' => ['product' => 'server-s']],
+                ['id' => '01JS2', 'user_id' => '01JUSER', 'name' => 'Backup', 'active' => false],
+            ],
+            ...$overrides,
+        ]];
+    }
+
+    private function request(int $index): string
+    {
+        $request = $this->sent[$index]['request'];
+
+        return $request->getMethod() . ' ' . $request->getUri()->getPath() . ' ' . $request->getHeaderLine('Authorization');
+    }
+
+    public function test_a_trusted_app_opens_a_session_with_its_own_token(): void
+    {
+        $this->respond(201, $this->sessionData());
+
+        $session = $this->owner()->appCheckoutSessions()->create([
+            'return_url' => 'https://console.test/builder/done',
+            'idempotency_key' => 'cart:42',
+            'subscriptions' => [
+                ['group' => 'team-1', 'name' => 'Cloud Server S', 'unit' => 'hour', 'price' => '0.0137', 'payload' => ['product' => 'server-s']],
+                ['id' => '01JS2'],
+            ],
+        ]);
+
+        $this->assertSame('POST /api/v1/client/checkout-sessions Bearer app-token', $this->request(0));
+        $this->assertSame('01JUSER', $this->sentJson(0)['user_id']);
+        $this->assertSame([['group' => 'team-1', 'name' => 'Cloud Server S', 'unit' => 'hour', 'price' => '0.0137', 'payload' => ['product' => 'server-s']], ['id' => '01JS2']], $this->sentJson(0)['subscriptions']);
+
+        $this->assertInstanceOf(AppCheckoutSession::class, $session);
+        $this->assertSame('https://id.anikeen.com/checkout/sessions/01JCS', $session->confirmationUrl());
+        $this->assertTrue($session->isOpen());
+        $this->assertSame(['01JS1', '01JS2'], $session->subscriptionIds());
+        $this->assertContainsOnlyInstancesOf(AppSubscription::class, $session->subscriptions());
+        $this->assertSame('server-s', $session->subscriptions()[0]->payload->product);
+        $this->assertSame('42', $session->metadata->cart);
+    }
+
+    public function test_the_app_finds_refreshes_and_cancels_sessions(): void
+    {
+        $this->respond(200, $this->sessionData(['status' => 'completed', 'confirmation_url' => null]));
+        $this->respond(200, $this->sessionData(['user_id' => 'someone-else']));
+        $this->respond(404, ['message' => 'Checkout session not found.', 'error' => 'checkout_session_not_found']);
+        $this->respond(200, $this->sessionData());
+        $this->respond(200, $this->sessionData(['status' => 'cancelled', 'confirmation_url' => null]));
+        $this->respond(409, ['message' => 'The checkout session is cancelled.', 'error' => 'checkout_session_cancelled']);
+
+        $sessions = $this->owner()->appCheckoutSessions();
+
+        $completed = $sessions->find('01JCS');
+        $this->assertTrue($completed->isCompleted());
+        $this->assertNull($completed->confirmationUrl());
+        $this->assertNull($sessions->find('01JCS'), 'another user\'s session');
+        $this->assertNull($sessions->find('unknown'));
+
+        $session = app(AnikeenId::class)->appCheckoutSessions()->find('01JCS'); // e.g. from a webhook
+        $session->cancel();
+        $this->assertTrue($session->isCancelled());
+        $this->assertSame('POST /api/v1/client/checkout-sessions/01JCS/cancel Bearer app-token', $this->request(4));
+
+        try {
+            $session->cancel();
+            $this->fail('Expected an ApiException.');
+        } catch (ApiException $e) {
+            $this->assertTrue($e->isConflict());
+            $this->assertSame('checkout_session_cancelled', $e->error);
+        }
+    }
+
+    public function test_without_a_billable_the_user_id_is_required(): void
+    {
+        $this->expectException(InvalidArgumentException::class);
+
+        app(AnikeenId::class)->appCheckoutSessions()->create(['subscriptions' => [['id' => '01JS1']]]);
+    }
+
+    public function test_any_app_opens_a_session_with_the_users_token(): void
+    {
+        $this->respond(201, $this->sessionData());
+        $this->respond(200, $this->sessionData(['status' => 'expired', 'confirmation_url' => null]));
+        $this->respond(404, ['message' => 'Checkout session not found.', 'error' => 'checkout_session_not_found']);
+
+        $user = $this->owner();
+        $session = $user->checkoutSessions()->create(['subscriptions' => [['name' => 'Pro', 'unit' => 'month', 'price' => 4.99]]]);
+
+        $this->assertSame('POST /api/v1/checkout-sessions Bearer user-token', $this->request(0));
+        $this->assertArrayNotHasKey('user_id', $this->sentJson(0));
+        $this->assertInstanceOf(CheckoutSession::class, $session);
+        $this->assertSame('https://id.anikeen.com/checkout/sessions/01JCS', $session->confirmationUrl());
+        $this->assertSame(['01JS1', '01JS2'], $session->subscriptionIds());
+
+        $expired = $user->checkoutSessions()->find('01JCS');
+        $this->assertTrue($expired->isCancelled());
+        $this->assertSame(CheckoutSession::STATUS_EXPIRED, $expired->status);
+        $this->assertNull($user->checkoutSessions()->find('unknown'));
+    }
+
+    public function test_nothing_is_requested_until_a_method_is_called(): void
+    {
+        $this->owner()->checkoutSessions();
+        $this->owner()->appCheckoutSessions();
+
+        $this->assertCount(0, $this->sent);
+    }
+
+    public function test_checkout_webhooks_carry_the_session(): void
+    {
+        $payload = json_encode(['id' => '01JEV', 'type' => 'checkout.completed', 'created' => time(), 'data' => ['object' => $this->sessionData(['status' => 'completed'])['data']]]);
+        $timestamp = time();
+        $signature = 't=' . $timestamp . ',v1=' . hash_hmac('sha256', "{$timestamp}.{$payload}", 'whsec_test');
+
+        $event = Webhook::constructEvent($payload, $signature, 'whsec_test');
+
+        $this->assertSame(EventType::CHECKOUT_COMPLETED, $event['type']);
+        $this->assertSame(['01JS1', '01JS2'], $event['data']['object']['subscription_ids']);
+        $this->assertContains('checkout.expired', EventType::all());
+        $this->assertContains('subscription.activated', EventType::all());
+    }
+}