diff --git a/README.md b/README.md
index 549d4c9..418771a 100644
--- a/README.md
+++ b/README.md
@@ -90,6 +90,12 @@ setono_meta_conversions_api:
     cookies:
         fbp: true
         fbc: true
+        # The domain to write them on. Meta's own pixel uses the registrable domain, so set this to yours if your
+        # site is reachable on both the apex and www, or spans several subdomains. Null scopes them to the current
+        # host, which means apex and www get different cookies
+        domain: null
+        # Anything \DateTimeImmutable understands. Meta keeps these for 90 days
+        lifetime: '+90 days'
 
     # The PSR-18 http client used to send events. Defaults to Symfony's default http client, which means requests
     # to Meta show up in the profiler and honour the options you configured. Point it at a scoped client to give
diff --git a/UPGRADE.md b/UPGRADE.md
index 1a65cc0..cacb97b 100644
--- a/UPGRADE.md
+++ b/UPGRADE.md
@@ -75,6 +75,27 @@ anything to `framework.messenger`. `SendEvent` is dispatched on your application
   `?ConsentContextInterface $consentContext` and `bool $consentEnabled` / `bool $clientSideEnabled` /
   `bool $serverSideEnabled` arguments. Adapt subclasses, decorators and custom service definitions.
 
+## Cookies
+
+New `cookies` options:
+
+```yaml
+setono_meta_conversions_api:
+    cookies:
+        fbp: true
+        fbc: true
+        domain: null      # e.g. example.com
+        lifetime: '+90 days'
+```
+
+The `_fbp` and `_fbc` cookies are now only written on a 2xx or 3xx response and only when at least one pixel is
+available, because every `Set-Cookie` header makes a response uncacheable for shared caches.
+
+The subdomain index encoded in the value (the `1` in `fb.1.…`) is now derived from the domain the cookie is actually
+written on, the same way Meta's parameter builder does it, instead of always being `1`. On a host-only cookie on
+`www.example.com` the value is now `fb.2.…`. Set `cookies.domain` to your registrable domain to get `fb.1.…` and one
+cookie shared between the apex and `www`.
+
 ## The SendEvent command changed shape
 
 `SendEvent` no longer carries the `Setono\MetaConversionsApi\Event\Event` object. It carries the finished payload
diff --git a/src/Context/Fbc/QueryBasedFbcContext.php b/src/Context/Fbc/QueryBasedFbcContext.php
index 883ba2e..0f6a6bf 100644
--- a/src/Context/Fbc/QueryBasedFbcContext.php
+++ b/src/Context/Fbc/QueryBasedFbcContext.php
@@ -5,6 +5,7 @@
 namespace Setono\MetaConversionsApiBundle\Context\Fbc;
 
 use Setono\MetaConversionsApi\ValueObject\Fbc;
+use Setono\MetaConversionsApiBundle\Cookie\CookieDomain;
 use Symfony\Component\HttpFoundation\RequestStack;
 
 final class QueryBasedFbcContext implements FbcContextInterface
@@ -20,6 +21,7 @@ final class QueryBasedFbcContext implements FbcContextInterface
     public function __construct(
         private readonly FbcContextInterface $decorated,
         private readonly RequestStack $requestStack,
+        private readonly CookieDomain $cookieDomain,
     ) {
     }
 
@@ -35,6 +37,7 @@ public function getFbc(): ?Fbc
             return $this->decorated->getFbc();
         }
 
-        return new Fbc($facebookClickId);
+        // The click id is about to be written as a cookie, so it has to say which level it was set at
+        return (new Fbc($facebookClickId))->withSubdomainIndex($this->cookieDomain->subdomainIndex());
     }
 }
diff --git a/src/Context/Fbp/GeneratedFbpContext.php b/src/Context/Fbp/GeneratedFbpContext.php
index d028765..e593858 100644
--- a/src/Context/Fbp/GeneratedFbpContext.php
+++ b/src/Context/Fbp/GeneratedFbpContext.php
@@ -5,11 +5,17 @@
 namespace Setono\MetaConversionsApiBundle\Context\Fbp;
 
 use Setono\MetaConversionsApi\ValueObject\Fbp;
+use Setono\MetaConversionsApiBundle\Cookie\CookieDomain;
 
 final class GeneratedFbpContext implements FbpContextInterface
 {
+    public function __construct(private readonly CookieDomain $cookieDomain)
+    {
+    }
+
     public function getFbp(): Fbp
     {
-        return new Fbp();
+        // The generated value is about to be written as a cookie, so it has to say which level it was set at
+        return (new Fbp())->withSubdomainIndex($this->cookieDomain->subdomainIndex());
     }
 }
diff --git a/src/Cookie/CookieDomain.php b/src/Cookie/CookieDomain.php
new file mode 100644
index 0000000..92d7266
--- /dev/null
+++ b/src/Cookie/CookieDomain.php
@@ -0,0 +1,48 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Setono\MetaConversionsApiBundle\Cookie;
+
+use Symfony\Component\HttpFoundation\RequestStack;
+
+/**
+ * Decides which domain the _fbp and _fbc cookies are written on, and the subdomain index that goes with it
+ *
+ * Meta encodes the level the cookie was set at in the second segment of the value: fb.1. means example.com,
+ * fb.2. means www.example.com and fb.0. means a single label host or an ip address. Writing a host-only cookie
+ * while claiming index 1 makes the two disagree, and apex and www then end up with different cookies
+ *
+ * See https://developers.facebook.com/docs/marketing-api/conversions-api/parameters/fbp-and-fbc
+ */
+final class CookieDomain
+{
+    public function __construct(
+        private readonly RequestStack $requestStack,
+        private readonly ?string $domain = null,
+    ) {
+    }
+
+    /**
+     * The domain to write the cookies on, or null to let the browser scope them to the current host
+     */
+    public function domain(): ?string
+    {
+        return $this->domain;
+    }
+
+    /**
+     * The number of dots in the domain the cookie ends up on, which is how Meta's own parameter builder
+     * computes it (`substr_count($etldPlus1, '.')`)
+     */
+    public function subdomainIndex(): int
+    {
+        $domain = $this->domain ?? $this->requestStack->getMainRequest()?->getHost();
+        if (null === $domain || '' === $domain) {
+            return 0;
+        }
+
+        // A leading dot is the classic way of writing a domain cookie and says nothing about the level
+        return min(2, substr_count(ltrim($domain, '.'), '.'));
+    }
+}
diff --git a/src/DependencyInjection/Configuration.php b/src/DependencyInjection/Configuration.php
index 51e1ee3..076fc50 100644
--- a/src/DependencyInjection/Configuration.php
+++ b/src/DependencyInjection/Configuration.php
@@ -7,6 +7,7 @@
 use Composer\InstalledVersions;
 use Composer\Semver\VersionParser;
 use Setono\Consent\DefaultConsents;
+use Setono\MetaConversionsApiBundle\Cookie\Cookies;
 use Symfony\Component\Config\Definition\Builder\TreeBuilder;
 use Symfony\Component\Config\Definition\ConfigurationInterface;
 
@@ -74,6 +75,15 @@ public function getConfigTreeBuilder(): TreeBuilder
                 ->children()
                     ->booleanNode('fbp')->defaultTrue()->end()
                     ->booleanNode('fbc')->defaultTrue()->end()
+                    ->scalarNode('domain')
+                        ->info('The domain to write the cookies on, e.g. example.com, so the apex and www share one cookie the way Meta\'s own pixel does. Null scopes them to the current host')
+                        ->defaultNull()
+                    ->end()
+                    ->scalarNode('lifetime')
+                        ->info('Anything \DateTimeImmutable understands. Meta keeps these for 90 days')
+                        ->defaultValue(Cookies::LIFETIME)
+                        ->cannotBeEmpty()
+                    ->end()
                 ->end()
             ->end()
             ->scalarNode('http_client')
diff --git a/src/DependencyInjection/SetonoMetaConversionsApiExtension.php b/src/DependencyInjection/SetonoMetaConversionsApiExtension.php
index eb29889..49aff52 100644
--- a/src/DependencyInjection/SetonoMetaConversionsApiExtension.php
+++ b/src/DependencyInjection/SetonoMetaConversionsApiExtension.php
@@ -26,7 +26,7 @@ public function getConfiguration(array $config, ContainerBuilder $container): Co
     public function load(array $configs, ContainerBuilder $container): void
     {
         /**
-         * @var array{consent: array{enabled: bool, category: string}, client_side: array{enabled: bool}, server_side: array{enabled: bool, message_bus: string}, pixels: array<array-key, array{id: string, access_token: string}>, http_client: string, test_event_code: array{query_parameter: bool, value: string|null}, cookies: array{fbp: bool, fbc: bool}, filters: array{user_agent: list<string>}} $config
+         * @var array{consent: array{enabled: bool, category: string}, client_side: array{enabled: bool}, server_side: array{enabled: bool, message_bus: string}, pixels: array<array-key, array{id: string, access_token: string}>, http_client: string, test_event_code: array{query_parameter: bool, value: string|null}, cookies: array{fbp: bool, fbc: bool, domain: string|null, lifetime: string}, filters: array{user_agent: list<string>}} $config
          */
         $config = $this->processConfiguration($this->getConfiguration([], $container), $configs);
         // The XML format is deprecated since Symfony 7.4 and removed in 8.0. Migrate to PHP config before adding Symfony 8 support
@@ -39,6 +39,10 @@ public function load(array $configs, ContainerBuilder $container): void
         $container->setParameter('setono_meta_conversions_api.pixels', $config['pixels']);
         $container->setParameter('setono_meta_conversions_api.filters.user_agent', $config['filters']['user_agent']);
 
+        $cookieDomain = $config['cookies']['domain'];
+        $container->setParameter('setono_meta_conversions_api.cookies.domain', '' === $cookieDomain ? null : $cookieDomain);
+        $container->setParameter('setono_meta_conversions_api.cookies.lifetime', $config['cookies']['lifetime']);
+
         $testEventCode = $config['test_event_code']['value'];
         $container->setParameter('setono_meta_conversions_api.test_event_code.value', '' === $testEventCode ? null : $testEventCode);
         $container->setParameter('setono_meta_conversions_api.test_event_code.query_parameter', $config['test_event_code']['query_parameter']);
diff --git a/src/EventSubscriber/StoreFbcSubscriber.php b/src/EventSubscriber/StoreFbcSubscriber.php
index 6705b36..5b48b0c 100644
--- a/src/EventSubscriber/StoreFbcSubscriber.php
+++ b/src/EventSubscriber/StoreFbcSubscriber.php
@@ -6,6 +6,7 @@
 
 use Setono\MetaConversionsApiBundle\ConsentChecker\ConsentCheckerInterface;
 use Setono\MetaConversionsApiBundle\Context\Fbc\FbcContextInterface;
+use Setono\MetaConversionsApiBundle\Cookie\CookieDomain;
 use Setono\MetaConversionsApiBundle\Cookie\Cookies;
 use Setono\MetaConversionsApiBundle\Provider\PixelProviderInterface;
 use Symfony\Component\EventDispatcher\EventSubscriberInterface;
@@ -24,6 +25,8 @@ public function __construct(
         private readonly FbcContextInterface $fbcContext,
         private readonly ConsentCheckerInterface $consentChecker,
         private readonly PixelProviderInterface $pixelProvider,
+        private readonly CookieDomain $cookieDomain,
+        private readonly string $lifetime = Cookies::LIFETIME,
     ) {
     }
 
@@ -70,7 +73,9 @@ public function store(ResponseEvent $event): void
         $response->headers->setCookie(Cookie::create(
             Cookies::FBC,
             $fbc->value(),
-            new \DateTimeImmutable(Cookies::LIFETIME),
+            new \DateTimeImmutable($this->lifetime),
+            '/',
+            $this->cookieDomain->domain(),
         )->withHttpOnly(false));  // we need this to allow the js library to also use the cookie value
     }
 }
diff --git a/src/EventSubscriber/StoreFbpSubscriber.php b/src/EventSubscriber/StoreFbpSubscriber.php
index a006f17..dc2ca36 100644
--- a/src/EventSubscriber/StoreFbpSubscriber.php
+++ b/src/EventSubscriber/StoreFbpSubscriber.php
@@ -7,6 +7,7 @@
 use Setono\MetaConversionsApi\ValueObject\Fbp;
 use Setono\MetaConversionsApiBundle\ConsentChecker\ConsentCheckerInterface;
 use Setono\MetaConversionsApiBundle\Context\Fbp\FbpContextInterface;
+use Setono\MetaConversionsApiBundle\Cookie\CookieDomain;
 use Setono\MetaConversionsApiBundle\Cookie\Cookies;
 use Setono\MetaConversionsApiBundle\Provider\PixelProviderInterface;
 use Symfony\Component\EventDispatcher\EventSubscriberInterface;
@@ -26,6 +27,8 @@ public function __construct(
         private readonly FbpContextInterface $fbpContext,
         private readonly ConsentCheckerInterface $consentChecker,
         private readonly PixelProviderInterface $pixelProvider,
+        private readonly CookieDomain $cookieDomain,
+        private readonly string $lifetime = Cookies::LIFETIME,
     ) {
     }
 
@@ -68,7 +71,9 @@ public function store(ResponseEvent $event): void
         $response->headers->setCookie(Cookie::create(
             Cookies::FBP,
             $fbp->value(),
-            new \DateTimeImmutable(Cookies::LIFETIME),
+            new \DateTimeImmutable($this->lifetime),
+            '/',
+            $this->cookieDomain->domain(),
         )->withHttpOnly(false)); // we need this to allow the js library to also use the cookie value
     }
 
diff --git a/src/Resources/config/services/context.xml b/src/Resources/config/services/context.xml
index a68a6da..c00fb6d 100644
--- a/src/Resources/config/services/context.xml
+++ b/src/Resources/config/services/context.xml
@@ -3,6 +3,11 @@
 <container xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://symfony.com/schema/dic/services"
            xsi:schemaLocation="http://symfony.com/schema/dic/services http://symfony.com/schema/dic/services/services-1.0.xsd">
     <services>
+        <service id="Setono\MetaConversionsApiBundle\Cookie\CookieDomain">
+            <argument type="service" id="request_stack"/>
+            <argument>%setono_meta_conversions_api.cookies.domain%</argument>
+        </service>
+
         <!-- fbc contexts -->
         <service id="Setono\MetaConversionsApiBundle\Context\Fbc\FbcContextInterface"
                  alias="Setono\MetaConversionsApiBundle\Context\Fbc\CookieBasedFbcContext"/>
@@ -22,6 +27,7 @@
                  decorates="Setono\MetaConversionsApiBundle\Context\Fbc\FbcContextInterface" decoration-priority="64">
             <argument type="service" id="Setono\MetaConversionsApiBundle\Context\Fbc\QueryBasedFbcContext.inner"/>
             <argument type="service" id="request_stack"/>
+            <argument type="service" id="Setono\MetaConversionsApiBundle\Cookie\CookieDomain"/>
         </service>
 
         <!-- fbp contexts -->
@@ -29,6 +35,7 @@
                  alias="Setono\MetaConversionsApiBundle\Context\Fbp\GeneratedFbpContext"/>
 
         <service id="Setono\MetaConversionsApiBundle\Context\Fbp\GeneratedFbpContext">
+            <argument type="service" id="Setono\MetaConversionsApiBundle\Cookie\CookieDomain"/>
         </service>
 
         <service id="Setono\MetaConversionsApiBundle\Context\Fbp\CachedFbpContext"
diff --git a/src/Resources/config/services/event_subscriber.xml b/src/Resources/config/services/event_subscriber.xml
index 475ad3b..2bae05b 100644
--- a/src/Resources/config/services/event_subscriber.xml
+++ b/src/Resources/config/services/event_subscriber.xml
@@ -35,6 +35,8 @@
 
         <service id="Setono\MetaConversionsApiBundle\EventSubscriber\PopulatePixelsSubscriber">
             <argument type="service" id="Setono\MetaConversionsApiBundle\Provider\PixelProviderInterface"/>
+            <argument type="service" id="Setono\MetaConversionsApiBundle\Cookie\CookieDomain"/>
+            <argument>%setono_meta_conversions_api.cookies.lifetime%</argument>
 
             <tag name="kernel.event_subscriber"/>
         </service>
@@ -64,6 +66,8 @@
             <argument type="service" id="Setono\MetaConversionsApiBundle\Context\Fbc\FbcContextInterface"/>
             <argument type="service" id="Setono\MetaConversionsApiBundle\ConsentChecker\ConsentCheckerInterface"/>
             <argument type="service" id="Setono\MetaConversionsApiBundle\Provider\PixelProviderInterface"/>
+            <argument type="service" id="Setono\MetaConversionsApiBundle\Cookie\CookieDomain"/>
+            <argument>%setono_meta_conversions_api.cookies.lifetime%</argument>
 
             <tag name="kernel.event_subscriber"/>
         </service>
@@ -72,6 +76,8 @@
             <argument type="service" id="Setono\MetaConversionsApiBundle\Context\Fbp\FbpContextInterface"/>
             <argument type="service" id="Setono\MetaConversionsApiBundle\ConsentChecker\ConsentCheckerInterface"/>
             <argument type="service" id="Setono\MetaConversionsApiBundle\Provider\PixelProviderInterface"/>
+            <argument type="service" id="Setono\MetaConversionsApiBundle\Cookie\CookieDomain"/>
+            <argument>%setono_meta_conversions_api.cookies.lifetime%</argument>
 
             <tag name="kernel.event_subscriber"/>
         </service>
diff --git a/tests/Unit/Context/Fbc/QueryBasedFbcContextTest.php b/tests/Unit/Context/Fbc/QueryBasedFbcContextTest.php
index 37c0109..7384ae1 100644
--- a/tests/Unit/Context/Fbc/QueryBasedFbcContextTest.php
+++ b/tests/Unit/Context/Fbc/QueryBasedFbcContextTest.php
@@ -11,6 +11,7 @@
 use Setono\MetaConversionsApi\ValueObject\Fbc;
 use Setono\MetaConversionsApiBundle\Context\Fbc\FbcContextInterface;
 use Setono\MetaConversionsApiBundle\Context\Fbc\QueryBasedFbcContext;
+use Setono\MetaConversionsApiBundle\Cookie\CookieDomain;
 use Symfony\Component\HttpFoundation\Request;
 use Symfony\Component\HttpFoundation\RequestStack;
 
@@ -22,7 +23,7 @@ public function it_falls_back_to_the_decorated_context_without_a_request(): void
     {
         $fbc = new Fbc('decorated');
 
-        $context = new QueryBasedFbcContext(self::decorated($fbc), new RequestStack());
+        $context = new QueryBasedFbcContext(self::decorated($fbc), new RequestStack(), new CookieDomain(new RequestStack()));
 
         self::assertSame($fbc, $context->getFbc());
     }
@@ -31,7 +32,7 @@ public function it_falls_back_to_the_decorated_context_without_a_request(): void
     #[DataProvider('validClickIds')]
     public function it_uses_a_valid_click_id_from_the_query(string $clickId): void
     {
-        $context = new QueryBasedFbcContext(self::decorated(null), self::requestStack($clickId));
+        $context = new QueryBasedFbcContext(self::decorated(null), self::requestStack($clickId), new CookieDomain(new RequestStack()));
 
         $fbc = $context->getFbc();
 
@@ -56,7 +57,7 @@ public function it_falls_back_to_the_decorated_context_for_an_invalid_click_id(s
     {
         $decorated = new Fbc('decorated');
 
-        $context = new QueryBasedFbcContext(self::decorated($decorated), self::requestStack($clickId));
+        $context = new QueryBasedFbcContext(self::decorated($decorated), self::requestStack($clickId), new CookieDomain(new RequestStack()));
 
         self::assertSame($decorated, $context->getFbc());
     }
@@ -73,6 +74,21 @@ public static function invalidClickIds(): iterable
         yield 'dot' => ['fb.1.123.abc'];
     }
 
+    #[Test]
+    public function it_records_the_level_the_cookie_will_be_written_at(): void
+    {
+        $context = new QueryBasedFbcContext(
+            self::decorated(null),
+            self::requestStack('IwAR0rmfgHgx'),
+            new CookieDomain(new RequestStack(), 'www.example.com'),
+        );
+
+        $fbc = $context->getFbc();
+
+        self::assertInstanceOf(Fbc::class, $fbc);
+        self::assertSame(2, $fbc->getSubdomainIndex());
+    }
+
     private static function decorated(?Fbc $fbc): FbcContextInterface
     {
         return new class($fbc) implements FbcContextInterface {
diff --git a/tests/Unit/Context/Fbp/GeneratedFbpContextTest.php b/tests/Unit/Context/Fbp/GeneratedFbpContextTest.php
new file mode 100644
index 0000000..98fc33e
--- /dev/null
+++ b/tests/Unit/Context/Fbp/GeneratedFbpContextTest.php
@@ -0,0 +1,38 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Setono\MetaConversionsApiBundle\Tests\Unit\Context\Fbp;
+
+use PHPUnit\Framework\Attributes\CoversClass;
+use PHPUnit\Framework\Attributes\Test;
+use PHPUnit\Framework\TestCase;
+use Setono\MetaConversionsApiBundle\Context\Fbp\GeneratedFbpContext;
+use Setono\MetaConversionsApiBundle\Cookie\CookieDomain;
+use Symfony\Component\HttpFoundation\Request;
+use Symfony\Component\HttpFoundation\RequestStack;
+
+#[CoversClass(GeneratedFbpContext::class)]
+final class GeneratedFbpContextTest extends TestCase
+{
+    #[Test]
+    public function it_generates_a_new_fbp_every_time(): void
+    {
+        $context = new GeneratedFbpContext(new CookieDomain(new RequestStack()));
+
+        self::assertNotSame($context->getFbp()->value(), $context->getFbp()->value());
+    }
+
+    #[Test]
+    public function it_records_the_level_the_cookie_will_be_written_at(): void
+    {
+        $requestStack = new RequestStack();
+        $requestStack->push(Request::create('https://www.example.com/'));
+
+        // Host-only cookie on www.example.com, so the value has to say fb.2.
+        self::assertSame(2, (new GeneratedFbpContext(new CookieDomain($requestStack)))->getFbp()->getSubdomainIndex());
+
+        // ... but a cookie written on example.com is fb.1.
+        self::assertSame(1, (new GeneratedFbpContext(new CookieDomain($requestStack, 'example.com')))->getFbp()->getSubdomainIndex());
+    }
+}
diff --git a/tests/Unit/Cookie/CookieDomainTest.php b/tests/Unit/Cookie/CookieDomainTest.php
new file mode 100644
index 0000000..fd7a5c7
--- /dev/null
+++ b/tests/Unit/Cookie/CookieDomainTest.php
@@ -0,0 +1,77 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Setono\MetaConversionsApiBundle\Tests\Unit\Cookie;
+
+use PHPUnit\Framework\Attributes\CoversClass;
+use PHPUnit\Framework\Attributes\DataProvider;
+use PHPUnit\Framework\Attributes\Test;
+use PHPUnit\Framework\TestCase;
+use Setono\MetaConversionsApiBundle\Cookie\CookieDomain;
+use Symfony\Component\HttpFoundation\Request;
+use Symfony\Component\HttpFoundation\RequestStack;
+
+#[CoversClass(CookieDomain::class)]
+final class CookieDomainTest extends TestCase
+{
+    #[Test]
+    public function it_returns_no_domain_by_default(): void
+    {
+        self::assertNull((new CookieDomain(new RequestStack()))->domain());
+    }
+
+    #[Test]
+    public function it_returns_the_configured_domain(): void
+    {
+        self::assertSame('example.com', (new CookieDomain(new RequestStack(), 'example.com'))->domain());
+    }
+
+    /**
+     * Meta's own parameter builder computes this as the number of dots in the domain the cookie ends up on
+     */
+    #[Test]
+    #[DataProvider('configuredDomains')]
+    public function it_derives_the_subdomain_index_from_the_configured_domain(string $domain, int $expected): void
+    {
+        self::assertSame($expected, (new CookieDomain(new RequestStack(), $domain))->subdomainIndex());
+    }
+
+    /**
+     * @return iterable<string, array{string, int}>
+     */
+    public static function configuredDomains(): iterable
+    {
+        yield 'single label' => ['localhost', 0];
+        yield 'registrable domain' => ['example.com', 1];
+        yield 'leading dot' => ['.example.com', 1];
+        yield 'subdomain' => ['www.example.com', 2];
+        yield 'deep subdomain is capped' => ['shop.eu.example.com', 2];
+    }
+
+    #[Test]
+    #[DataProvider('hosts')]
+    public function it_falls_back_to_the_request_host(string $host, int $expected): void
+    {
+        $requestStack = new RequestStack();
+        $requestStack->push(Request::create('https://' . $host . '/'));
+
+        self::assertSame($expected, (new CookieDomain($requestStack))->subdomainIndex());
+    }
+
+    /**
+     * @return iterable<string, array{string, int}>
+     */
+    public static function hosts(): iterable
+    {
+        yield 'single label' => ['localhost', 0];
+        yield 'registrable domain' => ['example.com', 1];
+        yield 'subdomain' => ['www.example.com', 2];
+    }
+
+    #[Test]
+    public function it_returns_zero_without_a_request_or_a_domain(): void
+    {
+        self::assertSame(0, (new CookieDomain(new RequestStack()))->subdomainIndex());
+    }
+}
diff --git a/tests/Unit/EventSubscriber/StoreFbpSubscriberTest.php b/tests/Unit/EventSubscriber/StoreFbpSubscriberTest.php
index 3a958b5..441e47c 100644
--- a/tests/Unit/EventSubscriber/StoreFbpSubscriberTest.php
+++ b/tests/Unit/EventSubscriber/StoreFbpSubscriberTest.php
@@ -11,11 +11,13 @@
 use Setono\MetaConversionsApi\ValueObject\Fbp;
 use Setono\MetaConversionsApiBundle\ConsentChecker\ConsentCheckerInterface;
 use Setono\MetaConversionsApiBundle\Context\Fbp\FbpContextInterface;
+use Setono\MetaConversionsApiBundle\Cookie\CookieDomain;
 use Setono\MetaConversionsApiBundle\Cookie\Cookies;
 use Setono\MetaConversionsApiBundle\EventSubscriber\StoreFbpSubscriber;
 use Setono\MetaConversionsApiBundle\Provider\PixelProviderInterface;
 use Symfony\Component\HttpFoundation\Cookie;
 use Symfony\Component\HttpFoundation\Request;
+use Symfony\Component\HttpFoundation\RequestStack;
 use Symfony\Component\HttpFoundation\Response;
 use Symfony\Component\HttpKernel\Event\ResponseEvent;
 use Symfony\Component\HttpKernel\HttpKernelInterface;
@@ -123,6 +125,30 @@ public function it_renews_a_cookie_older_than_two_hours(): void
         self::assertNotNull(self::cookie($event));
     }
 
+    #[Test]
+    public function it_writes_a_host_only_cookie_by_default(): void
+    {
+        $event = self::event(new Request(), new Response());
+
+        self::subscriber()->store($event);
+
+        $cookie = self::cookie($event);
+        self::assertNotNull($cookie);
+        self::assertNull($cookie->getDomain());
+    }
+
+    #[Test]
+    public function it_writes_the_cookie_on_the_configured_domain(): void
+    {
+        $event = self::event(new Request(), new Response());
+
+        self::subscriber(domain: 'example.com')->store($event);
+
+        $cookie = self::cookie($event);
+        self::assertNotNull($cookie);
+        self::assertSame('example.com', $cookie->getDomain());
+    }
+
     private static function cookie(ResponseEvent $event): ?Cookie
     {
         foreach ($event->getResponse()->headers->getCookies() as $cookie) {
@@ -137,7 +163,7 @@ private static function cookie(ResponseEvent $event): ?Cookie
     /**
      * @param list<Pixel>|null $pixels
      */
-    private static function subscriber(?Fbp $fbp = null, bool $consentGranted = true, ?array $pixels = null): StoreFbpSubscriber
+    private static function subscriber(?Fbp $fbp = null, bool $consentGranted = true, ?array $pixels = null, ?string $domain = null): StoreFbpSubscriber
     {
         return new StoreFbpSubscriber(
             new class($fbp ?? new Fbp()) implements FbpContextInterface {
@@ -173,6 +199,7 @@ public function getPixels(): array
                     return $this->pixels;
                 }
             },
+            new CookieDomain(new RequestStack(), $domain),
         );
     }