diff --git a/composer.json b/composer.json
index d0a00ae..7514ccd 100644
--- a/composer.json
+++ b/composer.json
@@ -12,6 +12,7 @@
     "require": {
         "php": ">=8.1",
         "composer/semver": "^3.0",
+        "psr/log": "^1.1 || ^2.0 || ^3.0",
         "setono/bot-detection-bundle": "^1.7",
         "setono/consent-contracts": "^1.1",
         "setono/meta-conversions-api-php-sdk": "^1.1",
diff --git a/src/Context/Fbc/CookieBasedFbcContext.php b/src/Context/Fbc/CookieBasedFbcContext.php
index b4f7814..e41f118 100644
--- a/src/Context/Fbc/CookieBasedFbcContext.php
+++ b/src/Context/Fbc/CookieBasedFbcContext.php
@@ -4,13 +4,31 @@
 
 namespace Setono\MetaConversionsApiBundle\Context\Fbc;
 
+use Psr\Log\LoggerInterface;
+use Psr\Log\NullLogger;
 use Setono\MetaConversionsApi\ValueObject\Fbc;
 use Symfony\Component\HttpFoundation\RequestStack;
 
 final class CookieBasedFbcContext implements FbcContextInterface
 {
-    public function __construct(private readonly RequestStack $requestStack)
-    {
+    /**
+     * Matches the _fbc cookie as Meta writes it today: fb.<subdomain index>.<creation time>.<click id> with an
+     * optional trailing appendix segment.
+     *
+     * This is deliberately more lenient than Fbc::fromString() in the SDK, which only accepts alphanumeric click ids
+     * and exactly four segments. Real click ids are base64url and contain - and _, and Meta's own parameter builder
+     * (facebook/capi-param-builder-php) appends a 2 or 8 character appendix, so the strict pattern rejects cookies
+     * that the browser pixel writes
+     */
+    private const COOKIE_PATTERN = '/^fb\.([012])\.(\d{13})\.([A-Za-z0-9_-]+)(?:\.([A-Za-z0-9_-]{2,8}))?$/';
+
+    private readonly LoggerInterface $logger;
+
+    public function __construct(
+        private readonly RequestStack $requestStack,
+        ?LoggerInterface $logger = null,
+    ) {
+        $this->logger = $logger ?? new NullLogger();
     }
 
     public function getFbc(): ?Fbc
@@ -20,14 +38,30 @@ public function getFbc(): ?Fbc
             return null;
         }
 
-        $fbc = $request->cookies->get('_fbc');
-        if (is_string($fbc)) {
-            try {
-                return Fbc::fromString($fbc);
-            } catch (\InvalidArgumentException) {
-            }
+        $cookie = $request->cookies->get('_fbc');
+        if (!is_string($cookie) || '' === $cookie) {
+            return null;
         }
 
-        return null;
+        if (1 !== preg_match(self::COOKIE_PATTERN, $cookie, $matches)) {
+            $this->logger->debug('The _fbc cookie value "{value}" could not be parsed and is ignored', ['value' => $cookie]);
+
+            return null;
+        }
+
+        try {
+            return (new Fbc($matches[3]))
+                ->withSubdomainIndex((int) $matches[1])
+                ->withCreationTime((int) $matches[2])
+            ;
+        } catch (\InvalidArgumentException $e) {
+            // The creation time is in the future or predates Facebook
+            $this->logger->debug('The _fbc cookie value "{value}" was rejected: {message}', [
+                'value' => $cookie,
+                'message' => $e->getMessage(),
+            ]);
+
+            return null;
+        }
     }
 }
diff --git a/src/Resources/config/services/context.xml b/src/Resources/config/services/context.xml
index a68a6da..ed7abc7 100644
--- a/src/Resources/config/services/context.xml
+++ b/src/Resources/config/services/context.xml
@@ -9,6 +9,9 @@
 
         <service id="Setono\MetaConversionsApiBundle\Context\Fbc\CookieBasedFbcContext">
             <argument type="service" id="request_stack"/>
+            <argument type="service" id="logger" on-invalid="null"/>
+
+            <tag name="monolog.logger" channel="setono_meta_conversions_api"/>
         </service>
 
         <service id="Setono\MetaConversionsApiBundle\Context\Fbc\CachedFbcContext"
diff --git a/tests/Unit/Context/Fbc/CookieBasedFbcContextTest.php b/tests/Unit/Context/Fbc/CookieBasedFbcContextTest.php
new file mode 100644
index 0000000..91c86de
--- /dev/null
+++ b/tests/Unit/Context/Fbc/CookieBasedFbcContextTest.php
@@ -0,0 +1,107 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Setono\MetaConversionsApiBundle\Tests\Unit\Context\Fbc;
+
+use PHPUnit\Framework\Attributes\CoversClass;
+use PHPUnit\Framework\Attributes\DataProvider;
+use PHPUnit\Framework\Attributes\Test;
+use PHPUnit\Framework\TestCase;
+use Setono\MetaConversionsApi\ValueObject\Fbc;
+use Setono\MetaConversionsApiBundle\Context\Fbc\CookieBasedFbcContext;
+use Symfony\Component\HttpFoundation\Request;
+use Symfony\Component\HttpFoundation\RequestStack;
+
+#[CoversClass(CookieBasedFbcContext::class)]
+final class CookieBasedFbcContextTest extends TestCase
+{
+    #[Test]
+    public function it_returns_null_without_a_request(): void
+    {
+        self::assertNull((new CookieBasedFbcContext(new RequestStack()))->getFbc());
+    }
+
+    #[Test]
+    public function it_returns_null_without_a_cookie(): void
+    {
+        self::assertNull((new CookieBasedFbcContext(self::requestStack(null)))->getFbc());
+    }
+
+    #[Test]
+    #[DataProvider('cookies')]
+    public function it_parses_the_cookie(string $cookie, string $expectedClickId, int $expectedSubdomainIndex, int $expectedCreationTime): void
+    {
+        $fbc = (new CookieBasedFbcContext(self::requestStack($cookie)))->getFbc();
+
+        self::assertInstanceOf(Fbc::class, $fbc);
+        self::assertSame($expectedClickId, $fbc->getClickId());
+        self::assertSame($expectedSubdomainIndex, $fbc->getSubdomainIndex());
+        self::assertSame($expectedCreationTime, $fbc->getCreationTime());
+    }
+
+    /**
+     * @return iterable<string, array{string, string, int, int}>
+     */
+    public static function cookies(): iterable
+    {
+        yield 'alphanumeric click id' => [
+            'fb.1.1657051589577.IwAR0rmfgHgxjdKoEopat9y2SPzyjGgfHm9AhdqygToWvarP59nPq15T07MiA',
+            'IwAR0rmfgHgxjdKoEopat9y2SPzyjGgfHm9AhdqygToWvarP59nPq15T07MiA',
+            1,
+            1657051589577,
+        ];
+
+        // Real click ids are base64url, so they contain - and _
+        yield 'base64url click id' => [
+            'fb.1.1657051589577.IwZXh0bgNhZW0CMTAAAR-uK_5w',
+            'IwZXh0bgNhZW0CMTAAAR-uK_5w',
+            1,
+            1657051589577,
+        ];
+
+        // Meta's own parameter builder appends a 2 or 8 character appendix
+        yield 'appendix v1' => [
+            'fb.2.1657051589577.IwAR0rmfgHgx.AQ',
+            'IwAR0rmfgHgx',
+            2,
+            1657051589577,
+        ];
+
+        yield 'appendix v2' => [
+            'fb.0.1657051589577.IwAR0rmfgHgx.AbCdEfGh',
+            'IwAR0rmfgHgx',
+            0,
+            1657051589577,
+        ];
+    }
+
+    #[Test]
+    #[DataProvider('invalidCookies')]
+    public function it_returns_null_for_an_unparseable_cookie(string $cookie): void
+    {
+        self::assertNull((new CookieBasedFbcContext(self::requestStack($cookie)))->getFbc());
+    }
+
+    /**
+     * @return iterable<string, array{string}>
+     */
+    public static function invalidCookies(): iterable
+    {
+        yield 'empty' => [''];
+        yield 'garbage' => ['not-an-fbc-cookie'];
+        yield 'wrong prefix' => ['xx.1.1657051589577.abc'];
+        yield 'invalid subdomain index' => ['fb.3.1657051589577.abc'];
+        yield 'short creation time' => ['fb.1.165705158.abc'];
+        yield 'missing click id' => ['fb.1.1657051589577.'];
+        yield 'creation time in the future' => ['fb.1.9999999999999.abc'];
+    }
+
+    private static function requestStack(?string $cookie): RequestStack
+    {
+        $requestStack = new RequestStack();
+        $requestStack->push(new Request([], [], [], null === $cookie ? [] : ['_fbc' => $cookie]));
+
+        return $requestStack;
+    }
+}