diff --git a/core/lib/Security/Event/SecurityEvent.php b/core/lib/Security/Event/SecurityEvent.php
index ccfaee3..fd9d150 100644
--- a/core/lib/Security/Event/SecurityEvent.php
+++ b/core/lib/Security/Event/SecurityEvent.php
@@ -9,7 +9,7 @@ use KTXF\Event\Event;
 /**
  * Security-specific event for authentication and access control events
  */
-class SecurityEvent extends Event
+final class SecurityEvent extends Event
 {
     // Event names
     public const AUTH_SUCCESS = 'security.auth.success';
@@ -35,15 +35,6 @@ class SecurityEvent extends Event
     public const FIREWALL_RULE_REMOVED = 'security.firewall.rule.removed';
     public const FIREWALL_SETTINGS_UPDATED = 'security.firewall.settings.updated';
 
-    private ?string $ipAddress = null;
-    private ?string $deviceFingerprint = null;
-    private ?string $userAgent = null;
-    private ?string $requestPath = null;
-    private ?string $requestMethod = null;
-    private ?string $userId = null;
-    private ?string $reason = null;
-    private int $severity = self::SEVERITY_INFO;
-
     // Severity levels
     public const SEVERITY_DEBUG = 0;
     public const SEVERITY_INFO = 1;
@@ -51,11 +42,25 @@ class SecurityEvent extends Event
     public const SEVERITY_ERROR = 3;
     public const SEVERITY_CRITICAL = 4;
 
-    public function __construct(string $name, array $data = [])
-    {
-        parent::__construct($name, $data);
+    private readonly int $severity;
 
-        $this->severity = self::getSeverityForEvent($name);
+    public function __construct(
+        string $name,
+        array $data = [],
+        ?string $tenantId = null,
+        ?string $identityId = null,
+        private readonly ?string $ipAddress = null,
+        private readonly ?string $deviceFingerprint = null,
+        private readonly ?string $userAgent = null,
+        private readonly ?string $requestPath = null,
+        private readonly ?string $requestMethod = null,
+        private readonly ?string $userId = null,
+        private readonly ?string $reason = null,
+        ?int $severity = null,
+    ) {
+        parent::__construct($name, $data, $tenantId, $identityId);
+
+        $this->severity = $severity ?? self::getSeverityForEvent($name);
     }
 
     /**
@@ -65,13 +70,30 @@ class SecurityEvent extends Event
         string $name,
         ?string $ipAddress = null,
         ?string $deviceFingerprint = null,
-        array $data = []
+        array $data = [],
+        ?string $tenantId = null,
+        ?string $identityId = null,
+        ?string $userAgent = null,
+        ?string $requestPath = null,
+        ?string $requestMethod = null,
+        ?string $userId = null,
+        ?string $reason = null,
+        ?int $severity = null,
     ): self {
-        $event = new self($name, $data);
-        $event->ipAddress = $ipAddress;
-        $event->deviceFingerprint = $deviceFingerprint;
-
-        return $event;
+        return new self(
+            $name,
+            $data,
+            $tenantId,
+            $identityId,
+            $ipAddress,
+            $deviceFingerprint,
+            $userAgent,
+            $requestPath,
+            $requestMethod,
+            $userId,
+            $reason,
+            $severity,
+        );
     }
 
     /**
@@ -81,15 +103,26 @@ class SecurityEvent extends Event
         string $ipAddress,
         ?string $deviceFingerprint = null,
         ?string $userId = null,
-        ?string $reason = null
+        ?string $reason = null,
+        ?string $tenantId = null,
+        ?string $identityId = null,
+        ?string $userAgent = null,
+        ?string $requestPath = null,
+        ?string $requestMethod = null,
     ): self {
-        $event = self::create(self::AUTH_FAILURE, $ipAddress, $deviceFingerprint, [
-            'userId' => $userId,
-            'reason' => $reason,
-        ]);
-        $event->userId = $userId;
-        $event->reason = $reason;
-        return $event;
+        return self::create(
+            self::AUTH_FAILURE,
+            $ipAddress,
+            $deviceFingerprint,
+            ['userId' => $userId, 'reason' => $reason],
+            $tenantId,
+            $identityId,
+            $userAgent,
+            $requestPath,
+            $requestMethod,
+            $userId,
+            $reason,
+        );
     }
 
     /**
@@ -98,13 +131,17 @@ class SecurityEvent extends Event
     public static function authSuccess(
         string $ipAddress,
         ?string $deviceFingerprint = null,
-        string $userId = null
+        ?string $userId = null,
+        ?string $tenantId = null,
     ): self {
-        $event = self::create(self::AUTH_SUCCESS, $ipAddress, $deviceFingerprint, [
-            'userId' => $userId,
-        ]);
-        $event->userId = $userId;
-        return $event;
+        return self::create(
+            self::AUTH_SUCCESS,
+            $ipAddress,
+            $deviceFingerprint,
+            ['userId' => $userId],
+            tenantId: $tenantId,
+            userId: $userId,
+        );
     }
 
     /**
@@ -113,18 +150,16 @@ class SecurityEvent extends Event
     public static function bruteForceDetected(
         string $ipAddress,
         int $failureCount,
-        int $windowSeconds
+        int $windowSeconds,
+        ?string $tenantId = null,
     ): self {
-        $event = self::create(self::BRUTE_FORCE_DETECTED, $ipAddress, null, [
-            'failureCount' => $failureCount,
-            'windowSeconds' => $windowSeconds,
-        ]);
-        $event->reason = sprintf(
-            '%d failed attempts in %d seconds',
-            $failureCount,
-            $windowSeconds
+        return self::create(
+            self::BRUTE_FORCE_DETECTED,
+            $ipAddress,
+            data: ['failureCount' => $failureCount, 'windowSeconds' => $windowSeconds],
+            tenantId: $tenantId,
+            reason: sprintf('%d failed attempts in %d seconds', $failureCount, $windowSeconds),
         );
-        return $event;
     }
 
     /**
@@ -134,20 +169,21 @@ class SecurityEvent extends Event
         string $ipAddress,
         int $requestCount,
         int $windowSeconds,
-        ?string $endpoint = null
+        ?string $endpoint = null,
+        ?string $tenantId = null,
     ): self {
-        $event = self::create(self::RATE_LIMIT_EXCEEDED, $ipAddress, null, [
-            'requestCount' => $requestCount,
-            'windowSeconds' => $windowSeconds,
-            'endpoint' => $endpoint,
-        ]);
-        $event->requestPath = $endpoint;
-        $event->reason = sprintf(
-            '%d requests in %d seconds',
-            $requestCount,
-            $windowSeconds
+        return self::create(
+            self::RATE_LIMIT_EXCEEDED,
+            $ipAddress,
+            data: [
+                'requestCount' => $requestCount,
+                'windowSeconds' => $windowSeconds,
+                'endpoint' => $endpoint,
+            ],
+            tenantId: $tenantId,
+            requestPath: $endpoint,
+            reason: sprintf('%d requests in %d seconds', $requestCount, $windowSeconds),
         );
-        return $event;
     }
 
     /**
@@ -158,15 +194,19 @@ class SecurityEvent extends Event
         ?string $deviceFingerprint = null,
         ?string $ruleId = null,
         ?string $ruleScope = null,
-        ?string $reason = null
+        ?string $reason = null,
+        ?string $tenantId = null,
+        ?string $identityId = null,
     ): self {
-        $event = self::create(self::ACCESS_DENIED, $ipAddress, $deviceFingerprint, [
-            'ruleId' => $ruleId,
-            'ruleScope' => $ruleScope,
-            'reason' => $reason,
-        ]);
-        $event->reason = $reason;
-        return $event;
+        return self::create(
+            self::ACCESS_DENIED,
+            $ipAddress,
+            $deviceFingerprint,
+            ['ruleId' => $ruleId, 'ruleScope' => $ruleScope, 'reason' => $reason],
+            $tenantId,
+            $identityId,
+            reason: $reason,
+        );
     }
 
     /**
@@ -202,86 +242,39 @@ class SecurityEvent extends Event
         return $this->ipAddress;
     }
 
-    public function setIpAddress(?string $ipAddress): self
-    {
-        $this->ipAddress = $ipAddress;
-        return $this;
-    }
-
     public function getDeviceFingerprint(): ?string
     {
         return $this->deviceFingerprint;
     }
 
-    public function setDeviceFingerprint(?string $deviceFingerprint): self
-    {
-        $this->deviceFingerprint = $deviceFingerprint;
-        return $this;
-    }
-
     public function getUserAgent(): ?string
     {
         return $this->userAgent;
     }
 
-    public function setUserAgent(?string $userAgent): self
-    {
-        $this->userAgent = $userAgent;
-        return $this;
-    }
-
     public function getRequestPath(): ?string
     {
         return $this->requestPath;
     }
 
-    public function setRequestPath(?string $requestPath): self
-    {
-        $this->requestPath = $requestPath;
-        return $this;
-    }
-
     public function getRequestMethod(): ?string
     {
         return $this->requestMethod;
     }
 
-    public function setRequestMethod(?string $requestMethod): self
-    {
-        $this->requestMethod = $requestMethod;
-        return $this;
-    }
-
     public function getUserId(): ?string
     {
         return $this->userId;
     }
 
-    public function setUserId(?string $userId): self
-    {
-        $this->userId = $userId;
-        return $this;
-    }
-
     public function getReason(): ?string
     {
         return $this->reason;
     }
 
-    public function setReason(?string $reason): self
-    {
-        $this->reason = $reason;
-        return $this;
-    }
-
     public function getSeverity(): int
     {
         return $this->severity;
     }
 
-    public function setSeverity(int $severity): self
-    {
-        $this->severity = $severity;
-        return $this;
-    }
 }
diff --git a/core/lib/Service/FirewallRuleManager.php b/core/lib/Service/FirewallRuleManager.php
index fc59a17..5bc03e0 100644
--- a/core/lib/Service/FirewallRuleManager.php
+++ b/core/lib/Service/FirewallRuleManager.php
@@ -254,8 +254,13 @@ final class FirewallRuleManager
             $origin
         );
 
-        $event = new SecurityEvent(SecurityEvent::DEVICE_BLOCKED, ['device' => $fingerprint, 'reason' => $reason]);
-        $event->setDeviceFingerprint($fingerprint)->setReason($reason)->setTenantId($scope->tenantId);
+        $event = new SecurityEvent(
+            SecurityEvent::DEVICE_BLOCKED,
+            ['device' => $fingerprint, 'reason' => $reason],
+            tenantId: $scope->tenantId,
+            deviceFingerprint: $fingerprint,
+            reason: $reason,
+        );
         $this->events->dispatch($event);
 
         return $rule;
@@ -499,8 +504,13 @@ final class FirewallRuleManager
         string $ipAddress,
         ?string $reason
     ): void {
-        $event = new SecurityEvent($name, ['ip' => $ipAddress, 'reason' => $reason]);
-        $event->setIpAddress($ipAddress)->setReason($reason)->setTenantId($scope->tenantId);
+        $event = new SecurityEvent(
+            $name,
+            ['ip' => $ipAddress, 'reason' => $reason],
+            tenantId: $scope->tenantId,
+            ipAddress: $ipAddress,
+            reason: $reason,
+        );
         $this->events->dispatch($event);
     }
 
@@ -511,20 +521,23 @@ final class FirewallRuleManager
         array $change = []
     ): void
     {
-        $event = new SecurityEvent($name, [
-            'ruleId' => $rule->getId(),
-            'ruleScope' => $rule->getScope(),
-            'ruleType' => $rule->getType(),
-            'ruleAction' => $rule->getAction(),
-            'ruleValue' => $rule->getValue(),
-            'reason' => $rule->getReason(),
-            'origin' => $rule->getMetadata()['origin'] ?? self::ORIGIN_MANUAL,
-            'expiresAt' => $rule->getExpiresAt()?->format(\DateTimeInterface::ATOM),
-            ...($rule->getMetadata() ?? []),
-            ...$change,
-        ]);
-        $event->setTenantId($rule->getTenantId())
-            ->setIdentityId($actorId ?? $rule->getCreatedBy());
+        $event = new SecurityEvent(
+            $name,
+            [
+                'ruleId' => $rule->getId(),
+                'ruleScope' => $rule->getScope(),
+                'ruleType' => $rule->getType(),
+                'ruleAction' => $rule->getAction(),
+                'ruleValue' => $rule->getValue(),
+                'reason' => $rule->getReason(),
+                'origin' => $rule->getMetadata()['origin'] ?? self::ORIGIN_MANUAL,
+                'expiresAt' => $rule->getExpiresAt()?->format(\DateTimeInterface::ATOM),
+                ...($rule->getMetadata() ?? []),
+                ...$change,
+            ],
+            tenantId: $rule->getTenantId(),
+            identityId: $actorId ?? $rule->getCreatedBy(),
+        );
         $this->events->dispatch($event);
     }
 }
diff --git a/core/lib/Service/FirewallService.php b/core/lib/Service/FirewallService.php
index 94d4275..6f42dc7 100644
--- a/core/lib/Service/FirewallService.php
+++ b/core/lib/Service/FirewallService.php
@@ -139,7 +139,6 @@ class FirewallService
             return;
         }
 
-        $event->setTenantId($tenantId);
         $log = $this->securityLog($event);
         if ($log === null || !$this->store->createLogOnce($log)) {
             return;
@@ -195,8 +194,12 @@ class FirewallService
         int $blockDuration
     ): void {
         // Publish brute force event
-        $event = SecurityEvent::bruteForceDetected($ipAddress, $failureCount, $windowSeconds);
-        $event->setTenantId($tenantId);
+        $event = SecurityEvent::bruteForceDetected(
+            $ipAddress,
+            $failureCount,
+            $windowSeconds,
+            $tenantId,
+        );
         $this->events->dispatch($event);
 
         $this->rules->blockIp(
@@ -308,9 +311,9 @@ class FirewallService
             $deviceFingerprint,
             $rule->getId(),
             $rule->getScope(),
-            $rule->getReason()
+            $rule->getReason(),
+            $this->tenantContext->identifier(),
         );
-        $event->setTenantId($this->tenantContext->identifier());
         $this->events->dispatch($event);
     }
 
diff --git a/core/lib/Service/FirewallSettingsService.php b/core/lib/Service/FirewallSettingsService.php
index c9d0b79..2fec583 100644
--- a/core/lib/Service/FirewallSettingsService.php
+++ b/core/lib/Service/FirewallSettingsService.php
@@ -52,13 +52,17 @@ final class FirewallSettingsService
         $tenant->setConfiguration($configuration);
         $this->tenants->deposit($tenant);
 
-        $event = new SecurityEvent(SecurityEvent::FIREWALL_SETTINGS_UPDATED, [
-            'changeReason' => $reason,
-            'changeOrigin' => FirewallRuleManager::ORIGIN_MANUAL,
-            'previous' => $previous,
-            'current' => $current,
-        ]);
-        $event->setTenantId($tenantId)->setIdentityId($actorId);
+        $event = new SecurityEvent(
+            SecurityEvent::FIREWALL_SETTINGS_UPDATED,
+            [
+                'changeReason' => $reason,
+                'changeOrigin' => FirewallRuleManager::ORIGIN_MANUAL,
+                'previous' => $previous,
+                'current' => $current,
+            ],
+            tenantId: $tenantId,
+            identityId: $actorId,
+        );
         $this->events->dispatch($event);
 
         return $current;
diff --git a/shared/lib/Event/Event.php b/shared/lib/Event/Event.php
index 1ea6c96..daa34dd 100644
--- a/shared/lib/Event/Event.php
+++ b/shared/lib/Event/Event.php
@@ -10,16 +10,18 @@ namespace KTXF\Event;
 class Event
 {
     private bool $propagationStopped = false;
-    private array $data = [];
-    private float $timestamp;
-    private string $eventId;
-    private ?string $tenantId = null;
-    private ?string $identityId = null;
+    private readonly array $data;
+    private readonly float $timestamp;
+    private readonly string $eventId;
 
     public function __construct(
         private readonly string $name,
-        array $data = []
+        array $data = [],
+        private readonly ?string $tenantId = null,
+        private readonly ?string $identityId = null,
     ) {
+        self::validateData($data);
+
         $this->data = $data;
         $this->timestamp = microtime(true);
         $this->eventId = bin2hex(random_bytes(16));
@@ -41,15 +43,6 @@ class Event
         return $this->data[$key] ?? $default;
     }
 
-    /**
-     * Set a data value
-     */
-    public function set(string $key, mixed $value): self
-    {
-        $this->data[$key] = $value;
-        return $this;
-    }
-
     /**
      * Check if a data key exists
      */
@@ -111,15 +104,6 @@ class Event
         return $this->tenantId;
     }
 
-    /**
-     * Set tenant ID for multi-tenant context
-     */
-    public function setTenantId(?string $tenantId): self
-    {
-        $this->tenantId = $tenantId;
-        return $this;
-    }
-
     /**
      * Get identity ID (user who triggered the event)
      */
@@ -128,12 +112,18 @@ class Event
         return $this->identityId;
     }
 
-    /**
-     * Set identity ID
-     */
-    public function setIdentityId(?string $identityId): self
+    private static function validateData(array $data): void
     {
-        $this->identityId = $identityId;
-        return $this;
+        foreach ($data as $value) {
+            if (is_array($value)) {
+                self::validateData($value);
+                continue;
+            }
+            if ($value !== null && !is_scalar($value)) {
+                throw new \InvalidArgumentException(
+                    'Event data must contain only scalar, null, or array values.',
+                );
+            }
+        }
     }
 }
diff --git a/tests/php/Unit/Event/EventTest.php b/tests/php/Unit/Event/EventTest.php
new file mode 100644
index 0000000..6609c58
--- /dev/null
+++ b/tests/php/Unit/Event/EventTest.php
@@ -0,0 +1,41 @@
+<?php
+
+declare(strict_types=1);
+
+namespace KTXT\Unit\Event;
+
+use KTXF\Event\Event;
+use PHPUnit\Framework\Attributes\Test;
+use PHPUnit\Framework\Attributes\TestDox;
+use PHPUnit\Framework\TestCase;
+
+final class EventTest extends TestCase
+{
+    #[Test]
+    #[TestDox('Event data and context are supplied during construction')]
+    public function constructsCompleteEventState(): void
+    {
+        $event = new Event(
+            'test.event',
+            ['nested' => ['value' => 'original']],
+            'tenant-a',
+            'identity-a',
+        );
+
+        $copy = $event->getData();
+        $copy['nested']['value'] = 'changed';
+
+        self::assertSame('original', $event->get('nested')['value']);
+        self::assertSame('tenant-a', $event->getTenantId());
+        self::assertSame('identity-a', $event->getIdentityId());
+    }
+
+    #[Test]
+    #[TestDox('Event data rejects mutable object references')]
+    public function rejectsMutablePayloadValues(): void
+    {
+        $this->expectException(\InvalidArgumentException::class);
+
+        new Event('test.event', ['mutable' => new \stdClass()]);
+    }
+}
diff --git a/tests/php/Unit/Event/SecurityEventTest.php b/tests/php/Unit/Event/SecurityEventTest.php
index ac04c8f..3d0b901 100644
--- a/tests/php/Unit/Event/SecurityEventTest.php
+++ b/tests/php/Unit/Event/SecurityEventTest.php
@@ -34,13 +34,35 @@ final class SecurityEventTest extends TestCase
     }
 
     #[Test]
-    #[TestDox('Explicit severity can override the event type default')]
+    #[TestDox('Construction can override the event type default severity')]
     public function allowsSeverityOverride(): void
     {
-        $event = new SecurityEvent(SecurityEvent::AUTH_FAILURE);
-
-        $event->setSeverity(SecurityEvent::SEVERITY_CRITICAL);
+        $event = new SecurityEvent(
+            SecurityEvent::AUTH_FAILURE,
+            severity: SecurityEvent::SEVERITY_CRITICAL,
+        );
 
         self::assertSame(SecurityEvent::SEVERITY_CRITICAL, $event->getSeverity());
     }
+
+    #[Test]
+    #[TestDox('Event state exposes no mutation methods')]
+    public function exposesNoMutationMethods(): void
+    {
+        foreach ([
+            'set',
+            'setTenantId',
+            'setIdentityId',
+            'setIpAddress',
+            'setDeviceFingerprint',
+            'setUserAgent',
+            'setRequestPath',
+            'setRequestMethod',
+            'setUserId',
+            'setReason',
+            'setSeverity',
+        ] as $method) {
+            self::assertFalse(method_exists(SecurityEvent::class, $method));
+        }
+    }
 }
diff --git a/tests/php/Unit/Service/FirewallServiceTest.php b/tests/php/Unit/Service/FirewallServiceTest.php
index 95c9f7b..28df6ef 100644
--- a/tests/php/Unit/Service/FirewallServiceTest.php
+++ b/tests/php/Unit/Service/FirewallServiceTest.php
@@ -215,9 +215,9 @@ class FirewallServiceTest extends TestCase
             null,
             'tenant-rule',
             FirewallRuleObject::SCOPE_TENANT,
-            'Tenant block'
+            'Tenant block',
+            'tenant-a',
         );
-        $event->setTenantId('tenant-a');
 
         $this->service->logSecurityEvent($event);
     }
@@ -275,9 +275,9 @@ class FirewallServiceTest extends TestCase
             '203.0.113.10',
             101,
             60,
-            '/login'
+            '/login',
+            'tenant-a',
         );
-        $event->setTenantId('tenant-a');
 
         $this->service->logSecurityEvent($event);
     }
@@ -300,11 +300,11 @@ class FirewallServiceTest extends TestCase
             \KTXC\Security\Event\SecurityEvent::SUSPICIOUS_ACTIVITY,
             '203.0.113.20',
             null,
-            ['detector' => 'payload-signature']
+            ['detector' => 'payload-signature'],
+            tenantId: 'tenant-a',
+            requestPath: '/admin',
+            requestMethod: 'POST',
         );
-        $event->setTenantId('tenant-a')
-            ->setRequestPath('/admin')
-            ->setRequestMethod('POST');
 
         $this->service->logSecurityEvent($event);
     }
@@ -329,9 +329,9 @@ class FirewallServiceTest extends TestCase
                 'ruleId' => 'rule-123',
                 'ruleScope' => FirewallRuleObject::SCOPE_SYSTEM,
                 'origin' => FirewallRuleManager::ORIGIN_MANUAL,
-            ]
+            ],
+            identityId: 'operator',
         );
-        $event->setIdentityId('operator');
 
         $this->service->logSecurityEvent($event);
     }
@@ -351,9 +351,10 @@ class FirewallServiceTest extends TestCase
             ->willReturnArgument(0);
         $event = new \KTXC\Security\Event\SecurityEvent(
             \KTXC\Security\Event\SecurityEvent::FIREWALL_SETTINGS_UPDATED,
-            ['changeReason' => 'Tighten controls']
+            ['changeReason' => 'Tighten controls'],
+            tenantId: 'tenant-a',
+            identityId: 'operator',
         );
-        $event->setTenantId('tenant-a')->setIdentityId('operator');
 
         $this->service->logSecurityEvent($event);
     }
@@ -376,8 +377,10 @@ class FirewallServiceTest extends TestCase
             ->willReturn(4);
         $this->events->expects($this->never())->method('dispatch');
 
-        $event = \KTXC\Security\Event\SecurityEvent::authFailure('203.0.113.10');
-        $event->setTenantId('tenant-a');
+        $event = \KTXC\Security\Event\SecurityEvent::authFailure(
+            '203.0.113.10',
+            tenantId: 'tenant-a',
+        );
         $this->service->handleAuthFailure($event);
 
         self::assertSame(8, $this->currentConfiguration->firewall()->maxAuthFailures());
@@ -400,8 +403,10 @@ class FirewallServiceTest extends TestCase
             ->with('tenant-a', '203.0.113.10', 300)
             ->willReturn(0);
 
-        $event = \KTXC\Security\Event\SecurityEvent::authFailure('203.0.113.10');
-        $event->setTenantId('tenant-a');
+        $event = \KTXC\Security\Event\SecurityEvent::authFailure(
+            '203.0.113.10',
+            tenantId: 'tenant-a',
+        );
         $this->service->handleAuthFailure($event);
     }
 
@@ -454,8 +459,10 @@ class FirewallServiceTest extends TestCase
                 }
             });
 
-        $event = \KTXC\Security\Event\SecurityEvent::authFailure('203.0.113.10');
-        $event->setTenantId('tenant-event');
+        $event = \KTXC\Security\Event\SecurityEvent::authFailure(
+            '203.0.113.10',
+            tenantId: 'tenant-event',
+        );
         $this->service->handleAuthFailure($event);
 
         self::assertSame(['tenant-event', 'tenant-event', 'tenant-event'], $publishedTenants);