diff --git a/README.md b/README.md
index 903af97..0719363 100644
--- a/README.md
+++ b/README.md
@@ -53,6 +53,10 @@ echo $payment->state;     // "initial"
 echo $payment->state()?->name; // PaymentState enum (or null for an unknown value)
 ```
 
+Fields the Quickpay API unconditionally requires (verified against the live API) are required
+constructor arguments — `orderId` and `currency` here, `amount` on links and operations. Every other
+field is optional and simply omitted from the request JSON when unset.
+
 ### Payment link flow (redirect the customer to the payment window)
 
 The recommended way to take a payment is to create the payment, create a link for it, then redirect
@@ -211,7 +215,7 @@ use Setono\Quickpay\Exception\QuickpayException;
 use Setono\Quickpay\Exception\ValidationException;
 
 try {
-    $client->payments()->create(new CreatePaymentRequest(orderId: 'dup', currency: 'DKK'));
+    $client->payments()->create(new CreatePaymentRequest(orderId: 'dup-order-1', currency: 'DKK'));
 } catch (ValidationException $e) {
     $e->getMessageText();       // Quickpay's "message"
     $e->getErrorCode();         // Quickpay's "error_code"
diff --git a/src/Client/Client.php b/src/Client/Client.php
index 4f8eefb..94d8f70 100644
--- a/src/Client/Client.php
+++ b/src/Client/Client.php
@@ -308,14 +308,10 @@ private static function camelToSnake(string $key): string
      *
      * @throws MalformedResponseException if the body is not valid JSON or does not decode to an array
      */
-    private static function decodeJson(RequestInterface $request, ResponseInterface $response, bool $allowEmpty = false): array
+    private static function decodeJson(RequestInterface $request, ResponseInterface $response): array
     {
         $body = (string) $response->getBody();
 
-        if ($allowEmpty && '' === trim($body)) {
-            return [];
-        }
-
         // Strip query + fragment so consumer-supplied secrets don't land in exception messages.
         $sanitizedUri = $request->getUri()->withQuery('')->withFragment('');
         $context = sprintf(' [%s %s]', $request->getMethod(), (string) $sanitizedUri);
diff --git a/src/Client/Endpoint/PaymentsEndpoint.php b/src/Client/Endpoint/PaymentsEndpoint.php
index 48589e2..ff67e33 100644
--- a/src/Client/Endpoint/PaymentsEndpoint.php
+++ b/src/Client/Endpoint/PaymentsEndpoint.php
@@ -40,7 +40,7 @@ public function create(CreatePaymentRequest $request): Payment
     }
 
     /**
-     * PUT `/payments/{id}`.
+     * PATCH `/payments/{id}`.
      */
     public function updatePayment(int $id, UpdatePaymentRequest $request): Payment
     {
diff --git a/tests/Callback/CallbackTest.php b/tests/Callback/CallbackTest.php
index 1a02e49..8bf9527 100644
--- a/tests/Callback/CallbackTest.php
+++ b/tests/Callback/CallbackTest.php
@@ -176,7 +176,10 @@ public function it_does_not_map_a_non_payment_resource_to_a_payment(): void
         self::assertFalse($callback->isPayment());
         self::assertSame(['id' => 42, 'state' => 'active'], $callback->toArray());
 
+        // Assert the MESSAGE too: without it, removing the type guard would still end in an
+        // InvalidCallbackException via the shape-mismatch path and the guard would go untested.
         $this->expectException(InvalidCallbackException::class);
+        $this->expectExceptionMessage('not a payment');
         $callback->payment();
     }
 
diff --git a/tests/Client/ClientTest.php b/tests/Client/ClientTest.php
index 3d0cc53..a8a85dd 100644
--- a/tests/Client/ClientTest.php
+++ b/tests/Client/ClientTest.php
@@ -4,6 +4,7 @@
 
 namespace Setono\Quickpay\Client;
 
+use Nyholm\Psr7\Factory\Psr17Factory;
 use PHPUnit\Framework\Attributes\DataProvider;
 use PHPUnit\Framework\Attributes\Test;
 use Setono\Quickpay\Exception\ConflictException;
@@ -139,6 +140,66 @@ public static function statusCodeProvider(): iterable
         yield '500' => [500, InternalServerErrorException::class];
         yield '503' => [503, InternalServerErrorException::class];
         yield '418' => [418, UnexpectedStatusCodeException::class];
+        yield '302' => [302, UnexpectedStatusCodeException::class];
+    }
+
+    #[Test]
+    public function it_throws_when_the_body_is_valid_json_but_not_an_array(): void
+    {
+        $http = (new ScriptedHttpClient())->on(self::BASE . '/ping', '"pong"');
+
+        $this->expectException(MalformedResponseException::class);
+        $this->expectExceptionMessage('Expected decoded response body to be an array but got string');
+
+        $this->client($http)->ping();
+    }
+
+    #[Test]
+    public function it_does_not_set_a_content_type_on_get_requests(): void
+    {
+        $http = (new ScriptedHttpClient())->on(self::BASE . '/ping', self::fixture('ping.json'));
+
+        $this->client($http)->ping();
+
+        self::assertFalse($http->sentRequests[0]->hasHeader('Content-Type'));
+    }
+
+    #[Test]
+    public function it_preserves_a_preset_content_type(): void
+    {
+        $http = (new ScriptedHttpClient())->on(self::BASE . '/ping', self::fixture('ping.json'));
+
+        $request = (new Psr17Factory())->createRequest('POST', self::BASE . '/ping')
+            ->withHeader('Content-Type', 'application/custom+json');
+        $this->client($http)->request($request);
+
+        self::assertSame('application/custom+json', $http->sentRequests[0]->getHeaderLine('Content-Type'));
+    }
+
+    #[Test]
+    public function it_allows_an_absolute_url_regardless_of_casing(): void
+    {
+        // RFC 3986 hosts are case-insensitive — the host-pinning guard must not reject the
+        // Quickpay host just because it is written in upper case, and the port guard must resolve
+        // the default port from the lowercased SCHEME too. (The PSR-7 implementation then
+        // normalizes scheme + host to lower case and drops the default port on the wire.)
+        $http = (new ScriptedHttpClient())->on(self::BASE . '/ping', self::fixture('ping.json'));
+
+        $this->client($http)->get('HTTPS://API.QUICKPAY.NET:443/ping');
+
+        self::assertSame(self::BASE . '/ping', (string) $http->sentRequests[0]->getUri());
+    }
+
+    #[Test]
+    public function it_allows_an_explicit_default_port_on_the_api_host(): void
+    {
+        // An explicit :443 matches the https default, so the port guard must not reject it. (The
+        // PSR-7 implementation then drops the redundant default port on the wire.)
+        $http = (new ScriptedHttpClient())->on(self::BASE . '/ping', self::fixture('ping.json'));
+
+        $this->client($http)->get(self::BASE . ':443/ping');
+
+        self::assertSame(self::BASE . '/ping', (string) $http->sentRequests[0]->getUri());
     }
 
     #[Test]
diff --git a/tests/Client/Endpoint/PaymentsEndpointTest.php b/tests/Client/Endpoint/PaymentsEndpointTest.php
index f601c34..52a7070 100644
--- a/tests/Client/Endpoint/PaymentsEndpointTest.php
+++ b/tests/Client/Endpoint/PaymentsEndpointTest.php
@@ -4,8 +4,10 @@
 
 namespace Setono\Quickpay\Client\Endpoint;
 
+use CuyZ\Valinor\Mapper\MappingError;
 use PHPUnit\Framework\Attributes\Test;
 use Setono\Quickpay\Enum\PaymentState;
+use Setono\Quickpay\Exception\MappingException;
 use Setono\Quickpay\QuickpayTestCase;
 use Setono\Quickpay\Request\Payment\AuthorizePaymentRequest;
 use Setono\Quickpay\Request\Payment\BasketItem;
@@ -259,6 +261,52 @@ public function it_overrides_the_client_wide_synchronized_default_per_call(): vo
         self::assertSame(self::BASE . '/payments/1234/capture', (string) $http->sentRequests[0]->getUri());
     }
 
+    #[Test]
+    public function it_throws_a_mapping_exception_when_a_2xx_body_does_not_fit_the_dto(): void
+    {
+        // Valinor is strict: a single mis-typed field fails the WHOLE resource mapping (the `$raw`
+        // fallback only protects fields the SDK does not type). `id` cannot cast to int here.
+        $http = (new ScriptedHttpClient())->on(
+            self::BASE . '/payments/1234',
+            '{"id":"nope","order_id":"o-1","currency":"DKK","state":"new","merchant_id":1}',
+        );
+
+        try {
+            $this->client($http)->payments()->getById(1234);
+            self::fail('Expected a MappingException.');
+        } catch (MappingException $e) {
+            self::assertStringContainsString('Could not map response body to', $e->getMessage());
+            self::assertStringContainsString('[GET https://api.quickpay.net/payments/1234]', $e->getMessage());
+            self::assertInstanceOf(MappingError::class, $e->getPrevious());
+        }
+    }
+
+    #[Test]
+    public function it_maps_both_supported_date_formats(): void
+    {
+        $http = (new ScriptedHttpClient())->on(
+            self::BASE . '/payments/1234',
+            '{"id":1234,"order_id":"o-1","currency":"DKK","state":"new","merchant_id":1,'
+            . '"created_at":"2018-10-17T13:25:44Z","updated_at":"2018-10-17T13:25:44.557Z"}',
+        );
+
+        $payment = $this->client($http)->payments()->getById(1234);
+
+        self::assertSame('2018-10-17 13:25:44 +00:00', $payment->createdAt?->format('Y-m-d H:i:s P'));
+        self::assertSame('2018-10-17 13:25:44.557000', $payment->updatedAt?->format('Y-m-d H:i:s.u'));
+    }
+
+    #[Test]
+    public function it_maps_a_202_accepted_operation_response(): void
+    {
+        // Async operations answer 202 Accepted; the body is still the full payment resource.
+        $http = (new ScriptedHttpClient())->on(self::BASE . '/payments/1234/capture', self::fixture('payment.json'), 202);
+
+        $payment = $this->client($http)->payments()->capture(1234, new CaptureRequest(1000));
+
+        self::assertSame(1234, $payment->id);
+    }
+
     #[Test]
     public function it_lists_payments(): void
     {