diff --git a/.gitignore b/.gitignore
index 990331b..ad588fe 100644
--- a/.gitignore
+++ b/.gitignore
@@ -13,6 +13,7 @@ composer.phar
 
 # PHPUnit cache
 .phpunit.result.cache
+.phpunit.cache/
 .vscode/launch.json
 
 .idea/*
diff --git a/lib/Model/DevCycleUser.php b/lib/Model/DevCycleUser.php
index d63c195..83cc789 100644
--- a/lib/Model/DevCycleUser.php
+++ b/lib/Model/DevCycleUser.php
@@ -713,18 +713,35 @@ public function toHeaderValue(): string
     public static function FromEvaluationContext(EvaluationContext $context): DevCycleUser
     {
         $user = new DevCycleUser();
-        if ($context->getTargetingKey() === null && $context->getAttributes()->get("user_id") === null) {
-            throw new \InvalidArgumentException('targetingKey or user_id is missing from EvaluationContext');
-        }
-        if ($context->getAttributes()->get("user_id") !== null) {
-            $userId = $context->getAttributes()->get("user_id");
-        } else {
+        
+        // Priority order: targetingKey -> user_id -> userId
+        $userId = null;
+        $userIdSource = null;
+        
+        if ($context->getTargetingKey() !== null) {
             $userId = $context->getTargetingKey();
+            $userIdSource = 'targetingKey';
+        } elseif ($context->getAttributes()->get("user_id") !== null) {
+            $userId = $context->getAttributes()->get("user_id");
+            $userIdSource = 'user_id';
+        } elseif ($context->getAttributes()->get("userId") !== null) {
+            $userId = $context->getAttributes()->get("userId");
+            $userIdSource = 'userId';
+        }
+        
+        if ($userId === null) {
+            throw new \InvalidArgumentException('targetingKey, user_id, or userId is missing from EvaluationContext');
+        }
+        
+        if (!ValueTypeValidator::isString($userId)) {
+            throw new \InvalidArgumentException("User ID field '{$userIdSource}' must be a string value, got " . gettype($userId));
         }
+        
         $user->setUserId($userId);
 
         foreach ($context->getAttributes()->toArray() as $key => $value) {
-            if ($key === 'user_id' || $key === 'targetingKey') {
+            // Always skip all user ID field types to avoid duplication in custom data
+            if ($key === 'targetingKey' || $key === 'user_id' || $key === 'userId') {
                 continue;
             }
             switch ($key) {
diff --git a/test/Api/OpenFeatureTest.php b/test/Api/OpenFeatureTest.php
index 006272b..205d793 100644
--- a/test/Api/OpenFeatureTest.php
+++ b/test/Api/OpenFeatureTest.php
@@ -27,22 +27,121 @@ final class OpenFeatureTest extends TestCase
 
     public function testEvaluationContextTargetingKey()
     {
-
+        // Test targetingKey only
         $evalContextTK = new EvaluationContext(targetingKey: "test");
         $user = DevCycleUser::FromEvaluationContext($evalContextTK);
         self::assertEquals("test", $user->getUserId(), 'User ID not properly passed through/set');
 
+        // Test user_id only
         $attributes = new Attributes(array("user_id" => "test"));
         $evaluationContext = new EvaluationContext(attributes: $attributes);
         $user = DevCycleUser::FromEvaluationContext($evaluationContext);
         self::assertEquals("test", $user->getUserId(), 'User ID not properly passed through/set');
 
+        // Test userId only
+        $attributes = new Attributes(array("userId" => "test"));
+        $evaluationContext = new EvaluationContext(attributes: $attributes);
+        $user = DevCycleUser::FromEvaluationContext($evaluationContext);
+        self::assertEquals("test", $user->getUserId(), 'User ID not properly passed through/set');
+
+        // Test priority: targetingKey should take precedence over user_id
         $attributes = new Attributes(array("user_id" => "test"));
         $evaluationContext = new EvaluationContext(targetingKey: "test2", attributes: $attributes);
         $user = DevCycleUser::FromEvaluationContext($evaluationContext);
-        self::assertEquals("test", $user->getUserId(), 'User ID not properly passed through/set');
+        self::assertEquals("test2", $user->getUserId(), 'targetingKey should take precedence over user_id');
+
+        // Test priority: targetingKey should take precedence over userId
+        $attributes = new Attributes(array("userId" => "test"));
+        $evaluationContext = new EvaluationContext(targetingKey: "test2", attributes: $attributes);
+        $user = DevCycleUser::FromEvaluationContext($evaluationContext);
+        self::assertEquals("test2", $user->getUserId(), 'targetingKey should take precedence over userId');
+
+        // Test priority: user_id should take precedence over userId
+        $attributes = new Attributes(array("user_id" => "test", "userId" => "test2"));
+        $evaluationContext = new EvaluationContext(attributes: $attributes);
+        $user = DevCycleUser::FromEvaluationContext($evaluationContext);
+        self::assertEquals("test", $user->getUserId(), 'user_id should take precedence over userId');
 
+        // Test all three present: targetingKey should win
+        $attributes = new Attributes(array("user_id" => "test", "userId" => "test2"));
+        $evaluationContext = new EvaluationContext(targetingKey: "test3", attributes: $attributes);
+        $user = DevCycleUser::FromEvaluationContext($evaluationContext);
+        self::assertEquals("test3", $user->getUserId(), 'targetingKey should take precedence over all other user ID fields');
     }
+
+    public function testUserIdFieldsNotInCustomData()
+    {
+        // Test that when user_id is used as main user ID, it doesn't appear in custom data
+        $attributes = new Attributes(array("user_id" => "test-user", "other_field" => "value"));
+        $evaluationContext = new EvaluationContext(attributes: $attributes);
+        $user = DevCycleUser::FromEvaluationContext($evaluationContext);
+        self::assertEquals("test-user", $user->getUserId(), 'user_id should be used as main user ID');
+        self::assertArrayNotHasKey("user_id", $user->getCustomData(), 'user_id should not appear in custom data when used as main user ID');
+        self::assertArrayHasKey("other_field", $user->getCustomData(), 'other fields should still appear in custom data');
+
+        // Test that when userId is used as main user ID, it doesn't appear in custom data
+        $attributes = new Attributes(array("userId" => "test-user", "other_field" => "value"));
+        $evaluationContext = new EvaluationContext(attributes: $attributes);
+        $user = DevCycleUser::FromEvaluationContext($evaluationContext);
+        self::assertEquals("test-user", $user->getUserId(), 'userId should be used as main user ID');
+        self::assertArrayNotHasKey("userId", $user->getCustomData(), 'userId should not appear in custom data when used as main user ID');
+        self::assertArrayHasKey("other_field", $user->getCustomData(), 'other fields should still appear in custom data');
+
+        // Test that all user ID fields are always excluded from custom data
+        $attributes = new Attributes(array("user_id" => "test-user-id", "userId" => "test-userId", "other_field" => "value"));
+        $evaluationContext = new EvaluationContext(targetingKey: "targeting-key", attributes: $attributes);
+        $user = DevCycleUser::FromEvaluationContext($evaluationContext);
+        self::assertEquals("targeting-key", $user->getUserId(), 'targetingKey should be used as main user ID');
+        self::assertArrayNotHasKey("user_id", $user->getCustomData(), 'user_id should never appear in custom data');
+        self::assertArrayNotHasKey("userId", $user->getCustomData(), 'userId should never appear in custom data');
+        self::assertArrayHasKey("other_field", $user->getCustomData(), 'other fields should still appear in custom data');
+
+        // Test that all user ID fields are excluded even when not used as main user ID
+        $attributes = new Attributes(array("user_id" => "test-user-id", "userId" => "test-userId", "other_field" => "value"));
+        $evaluationContext = new EvaluationContext(attributes: $attributes);
+        $user = DevCycleUser::FromEvaluationContext($evaluationContext);
+        self::assertEquals("test-user-id", $user->getUserId(), 'user_id should be used as main user ID');
+        self::assertArrayNotHasKey("user_id", $user->getCustomData(), 'user_id should never appear in custom data');
+        self::assertArrayNotHasKey("userId", $user->getCustomData(), 'userId should never appear in custom data');
+        self::assertArrayHasKey("other_field", $user->getCustomData(), 'other fields should still appear in custom data');
+    }
+
+    public function testMissingUserIdThrowsException()
+    {
+        // Test that missing all user ID fields throws exception
+        $attributes = new Attributes(array("other_field" => "value"));
+        $evaluationContext = new EvaluationContext(attributes: $attributes);
+        
+        $this->expectException(\InvalidArgumentException::class);
+        $this->expectExceptionMessage('targetingKey, user_id, or userId is missing from EvaluationContext');
+        
+        DevCycleUser::FromEvaluationContext($evaluationContext);
+    }
+
+    public function testInvalidUserIdTypeThrowsException()
+    {
+        // Test that non-string user_id throws string validation exception
+        $attributes = new Attributes(array("user_id" => array("invalid", "array"), "other_field" => "value"));
+        $evaluationContext = new EvaluationContext(attributes: $attributes);
+        
+        $this->expectException(\InvalidArgumentException::class);
+        $this->expectExceptionMessage("User ID field 'user_id' must be a string value, got array");
+        
+        DevCycleUser::FromEvaluationContext($evaluationContext);
+    }
+
+    public function testInvalidUserIdFieldTypeThrowsException()
+    {
+        // Test that non-string userId throws string validation exception with correct field name
+        $attributes = new Attributes(array("userId" => true, "other_field" => "value"));
+        $evaluationContext = new EvaluationContext(attributes: $attributes);
+        
+        $this->expectException(\InvalidArgumentException::class);
+        $this->expectExceptionMessage("User ID field 'userId' must be a string value, got boolean");
+        
+        DevCycleUser::FromEvaluationContext($evaluationContext);
+    }
+
     public function testEvaluationContext()
     {
         $attributes = new Attributes(array(