diff --git a/src/Resources/config/services.php b/src/Resources/config/services.php
index 1a8e5920..1582dbca 100644
--- a/src/Resources/config/services.php
+++ b/src/Resources/config/services.php
@@ -89,6 +89,7 @@
 use Swag\AgenticCommerce\Ucp\Checkout\CheckoutWebhookUrlGuard;
 use Swag\AgenticCommerce\Ucp\Checkout\DoctrineDbalCheckoutCompletionStore;
 use Swag\AgenticCommerce\Ucp\Checkout\Payment\AbstractCompletionPaymentApplier;
+use Swag\AgenticCommerce\Ucp\Checkout\Payment\PaymentInstrumentResolver;
 use Swag\AgenticCommerce\Ucp\Checkout\Payment\UnappliedCompletionPayment;
 use Swag\AgenticCommerce\Ucp\Command\SeedSmokeCatalogCommand;
 use Swag\AgenticCommerce\Ucp\Config\DoctrineDbalUcpConfigRepository;
@@ -330,6 +331,11 @@
     $services->set(ShopwareInvoicePaymentHandler::class)
         ->tag('ucp_sdk.payment_handler');
 
+    // The documented seam to turn the instrument the agent presented into the
+    // concrete payment method a provider can charge. Registered as a service so
+    // a provider plugin can alias or decorate it; see docs/completion-payment.md.
+    $services->set(PaymentInstrumentResolver::class);
+
     // Scheduled tasks.
     $services->set(CleanupExpiredOAuthTokensTask::class)
         ->tag('shopware.scheduled.task');
diff --git a/src/Ucp/Checkout/Payment/PaymentInstrumentResolver.php b/src/Ucp/Checkout/Payment/PaymentInstrumentResolver.php
new file mode 100644
index 00000000..994ac48e
--- /dev/null
+++ b/src/Ucp/Checkout/Payment/PaymentInstrumentResolver.php
@@ -0,0 +1,57 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Swag\AgenticCommerce\Ucp\Checkout\Payment;
+
+use Shopware\Core\Framework\Log\Package;
+use Ucp\Sdk\Exception\ValidationException;
+use Ucp\Sdk\Model\Checkout\PaymentInstrument;
+use Ucp\Sdk\Model\RequestContext;
+use Ucp\Sdk\Service\PaymentHandlerRegistryInterface;
+
+/**
+ * Resolves a UCP payment instrument to the concrete Shopware payment method a
+ * provider can charge, without deciding who charges or how.
+ *
+ * This is the "intended path" docs/completion-payment.md names but which every
+ * earlier release left unimplemented: the SDK's
+ * PaymentHandlerInterface::prepareInstrument() existed and
+ * ShopwareInvoicePaymentHandler implemented it, yet nothing ever called it. A
+ * payment integration had to reimplement handler lookup + prepareInstrument
+ * for itself -- or skip it and fall back to the sales channel default, which is
+ * how an agent's instrument used to disappear without trace.
+ *
+ * It is deliberately rail- and provider-agnostic: it only turns the instrument
+ * into the pair (paymentMethodId, token) the handler declares. Deciding whether
+ * to switch the order to that method, refusing, or settling on-chain stays with
+ * the caller's applier, exactly as the seam intends.
+ *
+ * @internal
+ */
+#[Package('checkout')]
+final class PaymentInstrumentResolver
+{
+    public function __construct(
+        private readonly PaymentHandlerRegistryInterface $paymentHandlerRegistry,
+    ) {
+    }
+
+    /**
+     * @return array{paymentMethodId: string, token: string, displayLast4?: string, displayBrand?: string}
+     */
+    public function prepare(
+        PaymentInstrument $instrument,
+        RequestContext $context,
+    ): array {
+        $handler = $this->paymentHandlerRegistry->find($instrument->handlerId);
+        if (null === $handler) {
+            throw new ValidationException(\sprintf(
+                'No UCP payment handler is registered for id "%s". The business must publish a payment handler for it before an agent can settle a completion with it.',
+                $instrument->handlerId,
+            ));
+        }
+
+        return $handler->prepareInstrument($instrument, $context);
+    }
+}
diff --git a/tests/Unit/PaymentInstrumentResolverTest.php b/tests/Unit/PaymentInstrumentResolverTest.php
new file mode 100644
index 00000000..d79f5046
--- /dev/null
+++ b/tests/Unit/PaymentInstrumentResolverTest.php
@@ -0,0 +1,83 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Swag\AgenticCommerce\Tests\Unit;
+
+use PHPUnit\Framework\Attributes\CoversClass;
+use PHPUnit\Framework\Attributes\Test;
+use PHPUnit\Framework\TestCase;
+use Swag\AgenticCommerce\Ucp\Checkout\Payment\PaymentInstrumentResolver;
+use Ucp\Sdk\Contract\PaymentHandlerInterface;
+use Ucp\Sdk\Exception\ValidationException;
+use Ucp\Sdk\Model\Checkout\PaymentInstrument;
+use Ucp\Sdk\Model\RequestContext;
+use Ucp\Sdk\Service\PaymentHandlerRegistryInterface;
+
+/**
+ * Pins the documented seam: a UCP payment instrument resolves, through the
+ * handler that published it, to the concrete payment method a provider can
+ * charge. This was the "intended path" of docs/completion-payment.md that no
+ * release ever wired up, so a payment integration had to reimplement handler
+ * lookup + prepareInstrument for itself.
+ *
+ * The resolver deliberately does NOT decide who charges or how. It only turns
+ * the instrument into the (paymentMethodId, token) pair the handler declares;
+ * switching the order or settling stays with the caller's applier.
+ */
+#[CoversClass(PaymentInstrumentResolver::class)]
+final class PaymentInstrumentResolverTest extends TestCase
+{
+    #[Test]
+    public function itResolvesTheInstrumentThroughItsHandler(): void
+    {
+        $handler = $this->createMock(PaymentHandlerInterface::class);
+        $handler->method('prepareInstrument')->willReturn([
+            'paymentMethodId' => 'pm-nano',
+            'token' => '',
+        ]);
+
+        $registry = $this->createMock(PaymentHandlerRegistryInterface::class);
+        $registry->method('find')->with('com.nano.xno')->willReturn($handler);
+
+        $resolved = (new PaymentInstrumentResolver($registry))->prepare(
+            new PaymentInstrument('crypto', 'com.nano.xno', ['network' => 'nano:mainnet']),
+            new RequestContext('shop.example'),
+        );
+
+        self::assertSame('pm-nano', $resolved['paymentMethodId']);
+    }
+
+    #[Test]
+    public function itPassesTheInstrumentAndContextThroughToPrepareInstrument(): void
+    {
+        $instrument = new PaymentInstrument('crypto', 'com.nano.xno', ['network' => 'nano:mainnet']);
+        $context = new RequestContext('shop.example');
+
+        $handler = $this->createMock(PaymentHandlerInterface::class);
+        $handler->expects(self::once())
+            ->method('prepareInstrument')
+            ->with($instrument, $context)
+            ->willReturn(['paymentMethodId' => 'pm-nano', 'token' => '']);
+        $handler->method('supportsTokenization')->willReturn(false);
+
+        $registry = $this->createMock(PaymentHandlerRegistryInterface::class);
+        $registry->method('find')->with('com.nano.xno')->willReturn($handler);
+
+        (new PaymentInstrumentResolver($registry))->prepare($instrument, $context);
+    }
+
+    #[Test]
+    public function itRefusesWhenTheHandlerIsNotPublished(): void
+    {
+        $registry = $this->createMock(PaymentHandlerRegistryInterface::class);
+        $registry->method('find')->willReturn(null);
+
+        $this->expectException(ValidationException::class);
+
+        (new PaymentInstrumentResolver($registry))->prepare(
+            new PaymentInstrument('crypto', 'com.never.published'),
+            new RequestContext('shop.example'),
+        );
+    }
+}