diff --git a/app/Http/Controllers/Api/V1/DashboardController.php b/app/Http/Controllers/Api/V1/DashboardController.php
index 275348d..d253264 100644
--- a/app/Http/Controllers/Api/V1/DashboardController.php
+++ b/app/Http/Controllers/Api/V1/DashboardController.php
@@ -18,7 +18,7 @@ class DashboardController extends Controller
 {
     public function index(Request $request)
     {
-        $dealershipId = $request->query('dealership_id');
+        $dealershipId = $request->query('dealership_id') !== null && $request->query('dealership_id') !== '' ? (int) $request->query('dealership_id') : null;
 
         $currentShifts = $this->getCurrentShifts($dealershipId);
         $taskStats = $this->getTaskStatistics($dealershipId);
diff --git a/app/Http/Controllers/Api/V1/ImportantLinkController.php b/app/Http/Controllers/Api/V1/ImportantLinkController.php
index 82320b2..43c8ec1 100644
--- a/app/Http/Controllers/Api/V1/ImportantLinkController.php
+++ b/app/Http/Controllers/Api/V1/ImportantLinkController.php
@@ -14,7 +14,7 @@ class ImportantLinkController extends Controller
     public function index(Request $request)
     {
         $perPage = (int) $request->query('per_page', '15');
-        $dealershipId = $request->query('dealership_id');
+        $dealershipId = $request->query('dealership_id') !== null && $request->query('dealership_id') !== '' ? (int) $request->query('dealership_id') : null;
         $isActive = $request->query('is_active');
 
         $query = ImportantLink::with(['creator', 'dealership']);
diff --git a/app/Http/Controllers/Api/V1/SettingsController.php b/app/Http/Controllers/Api/V1/SettingsController.php
index 938a5f6..042474a 100644
--- a/app/Http/Controllers/Api/V1/SettingsController.php
+++ b/app/Http/Controllers/Api/V1/SettingsController.php
@@ -265,7 +265,7 @@ public function botSetting(Request $request, string $key): JsonResponse
      */
     public function getShiftConfig(Request $request): JsonResponse
     {
-        $dealershipId = $request->query('dealership_id') ? (int) $request->query('dealership_id') : null;
+        $dealershipId = $request->query('dealership_id') !== null && $request->query('dealership_id') !== '' ? (int) $request->query('dealership_id') : null;
 
         $shiftConfig = [
             'shift_1_start_time' => $this->settingsService->getShiftStartTime($dealershipId, 1),
diff --git a/app/Http/Controllers/Api/V1/ShiftController.php b/app/Http/Controllers/Api/V1/ShiftController.php
index edc8ca8..1d917f6 100644
--- a/app/Http/Controllers/Api/V1/ShiftController.php
+++ b/app/Http/Controllers/Api/V1/ShiftController.php
@@ -29,10 +29,10 @@ public function __construct(
     public function index(Request $request): JsonResponse
     {
         $perPage = (int) $request->query('per_page', '15');
-        $dealershipId = $request->query('dealership_id');
+        $dealershipId = $request->query('dealership_id') !== null && $request->query('dealership_id') !== '' ? (int) $request->query('dealership_id') : null;
         $status = $request->query('status');
         $date = $request->query('date');
-        $userId = $request->query('user_id');
+        $userId = $request->query('user_id') !== null && $request->query('user_id') !== '' ? (int) $request->query('user_id') : null;
 
         $query = Shift::with(['user', 'dealership', 'replacement.replacingUser', 'replacement.replacedUser']);
 
@@ -263,7 +263,7 @@ public function destroy(int $id): JsonResponse
      */
     public function current(Request $request): JsonResponse
     {
-        $dealershipId = $request->query('dealership_id');
+        $dealershipId = $request->query('dealership_id') !== null && $request->query('dealership_id') !== '' ? (int) $request->query('dealership_id') : null;
         $currentShifts = $this->shiftService->getCurrentShifts($dealershipId);
 
         return response()->json([
@@ -279,7 +279,7 @@ public function current(Request $request): JsonResponse
      */
     public function statistics(Request $request): JsonResponse
     {
-        $dealershipId = $request->query('dealership_id');
+        $dealershipId = $request->query('dealership_id') !== null && $request->query('dealership_id') !== '' ? (int) $request->query('dealership_id') : null;
         $startDate = $request->query('start_date')
             ? Carbon::parse($request->query('start_date'))
             : Carbon::now()->subDays(7);
diff --git a/app/Http/Controllers/Api/V1/TaskController.php b/app/Http/Controllers/Api/V1/TaskController.php
index 9785dbb..30770d3 100644
--- a/app/Http/Controllers/Api/V1/TaskController.php
+++ b/app/Http/Controllers/Api/V1/TaskController.php
@@ -17,11 +17,11 @@ public function index(Request $request)
         $perPage = (int) $request->query('per_page', '15');
 
         // Получаем все параметры фильтрации
-        $dealershipId = $request->query('dealership_id');
+        $dealershipId = $request->query('dealership_id') !== null && $request->query('dealership_id') !== '' ? (int) $request->query('dealership_id') : null;
         $taskType = $request->query('task_type');
         $isActive = $request->query('is_active');
         $tags = $request->query('tags');
-        $creatorId = $request->query('creator_id');
+        $creatorId = $request->query('creator_id') !== null && $request->query('creator_id') !== '' ? (int) $request->query('creator_id') : null;
         $responseType = $request->query('response_type');
         $deadlineFrom = $request->query('deadline_from');
         $deadlineTo = $request->query('deadline_to');
@@ -384,7 +384,7 @@ public function destroy($id)
 
     public function postponed(Request $request)
     {
-        $dealershipId = $request->query('dealership_id');
+        $dealershipId = $request->query('dealership_id') !== null && $request->query('dealership_id') !== '' ? (int) $request->query('dealership_id') : null;
 
         $query = Task::with(['creator', 'dealership', 'responses'])
             ->where('postpone_count', '>', 0)
diff --git a/experiments/debug_dealership_filter.php b/experiments/debug_dealership_filter.php
new file mode 100644
index 0000000..284d087
--- /dev/null
+++ b/experiments/debug_dealership_filter.php
@@ -0,0 +1,126 @@
+<?php
+
+/**
+ * Debug script to understand dealership_id filtering behavior
+ * Tests how query parameters are processed and filtered
+ */
+
+require __DIR__ . '/../vendor/autoload.php';
+
+$app = require_once __DIR__ . '/../bootstrap/app.php';
+$app->make(Illuminate\Contracts\Console\Kernel::class)->bootstrap();
+
+use App\Models\Task;
+use App\Models\AutoDealership;
+
+echo "=== Debug Dealership Filtering ===\n\n";
+
+// First, let's see what dealerships exist
+echo "Available dealerships:\n";
+$dealerships = AutoDealership::all(['id', 'name']);
+foreach ($dealerships as $d) {
+    echo "  ID: {$d->id}, Name: {$d->name}\n";
+}
+echo "\n";
+
+// Check tasks for each dealership
+echo "Tasks per dealership:\n";
+foreach ($dealerships as $d) {
+    $count = Task::where('dealership_id', $d->id)->count();
+    echo "  Dealership {$d->id} ({$d->name}): {$count} tasks\n";
+}
+echo "\n";
+
+// Now let's simulate what happens with different query parameter values
+echo "=== Simulating query parameter processing ===\n\n";
+
+$testCases = [
+    ['dealership_id' => '1'],
+    ['dealership_id' => '0'],
+    ['dealership_id' => null],
+    [],  // no parameter
+];
+
+foreach ($testCases as $index => $params) {
+    echo "Test case " . ($index + 1) . ": ";
+
+    if (!isset($params['dealership_id'])) {
+        echo "No dealership_id parameter\n";
+        $dealershipId = null;
+    } else {
+        echo "dealership_id = " . var_export($params['dealership_id'], true) . "\n";
+
+        // Simulate the current code logic
+        $dealershipId = isset($params['dealership_id']) && $params['dealership_id'] !== null
+            ? (int) $params['dealership_id']
+            : null;
+    }
+
+    echo "  After processing: dealershipId = " . var_export($dealershipId, true) . "\n";
+    echo "  Truthiness check: if (\$dealershipId) = " . ($dealershipId ? 'true' : 'false') . "\n";
+
+    // Execute the query
+    $query = Task::query();
+    if ($dealershipId) {
+        $query->where('dealership_id', $dealershipId);
+        echo "  Filter applied: WHERE dealership_id = {$dealershipId}\n";
+    } else {
+        echo "  Filter NOT applied (showing all tasks)\n";
+    }
+
+    $count = $query->count();
+    echo "  Result: {$count} tasks\n";
+
+    // Show first few task IDs
+    if ($count > 0) {
+        $taskIds = $query->limit(5)->pluck('id')->toArray();
+        echo "  Sample task IDs: " . implode(', ', $taskIds) . "\n";
+    }
+
+    echo "\n";
+}
+
+// Now let's check what the actual HTTP request would look like
+echo "=== HTTP Request Simulation ===\n\n";
+
+// Simulate Illuminate\Http\Request
+use Illuminate\Http\Request;
+
+$httpCases = [
+    ['dealership_id=1'],
+    ['dealership_id=0'],
+    [''],  // no query string
+];
+
+foreach ($httpCases as $index => $queryString) {
+    echo "HTTP case " . ($index + 1) . ": Query string = " . ($queryString ? "?{$queryString}" : "(empty)") . "\n";
+
+    // Create a fake request
+    $request = Request::create('/api/v1/tasks' . ($queryString ? "?{$queryString}" : ''), 'GET');
+
+    $dealershipId = $request->query('dealership_id') !== null
+        ? (int) $request->query('dealership_id')
+        : null;
+
+    echo "  \$request->query('dealership_id') = " . var_export($request->query('dealership_id'), true) . "\n";
+    echo "  After !== null check and cast: " . var_export($dealershipId, true) . "\n";
+    echo "  Truthiness: if (\$dealershipId) = " . ($dealershipId ? 'true' : 'false') . "\n";
+
+    $query = Task::query();
+    if ($dealershipId) {
+        $query->where('dealership_id', $dealershipId);
+        echo "  Filter WOULD be applied\n";
+    } else {
+        echo "  Filter WOULD NOT be applied\n";
+    }
+
+    echo "\n";
+}
+
+echo "=== Analysis ===\n";
+echo "If dealership_id=1 is not filtering correctly, possible causes:\n";
+echo "1. The query string is not being parsed correctly\n";
+echo "2. The database has no tasks with dealership_id=1\n";
+echo "3. There's a type mismatch in the WHERE clause\n";
+echo "4. The frontend is not sending the parameter correctly\n";
+echo "\nRun this script to see actual behavior!\n";
diff --git a/experiments/test_dealership_filter.php b/experiments/test_dealership_filter.php
new file mode 100644
index 0000000..07b280b
--- /dev/null
+++ b/experiments/test_dealership_filter.php
@@ -0,0 +1,132 @@
+<?php
+
+/**
+ * Test script to verify dealership_id filtering with value 0
+ *
+ * This script tests the fix for the issue where dealership_id=0 was
+ * incorrectly converted to null due to PHP's falsy behavior.
+ */
+
+echo "Testing dealership_id filtering logic...\n\n";
+
+// Simulate the OLD BUGGY behavior
+function oldBuggyLogic($queryValue) {
+    return $queryValue ? (int) $queryValue : null;
+}
+
+// Simulate the INTERMEDIATE behavior (from previous commit)
+function intermediateLogic($queryValue) {
+    return $queryValue !== null ? (int) $queryValue : null;
+}
+
+// Simulate the NEW FULLY FIXED behavior
+function newFixedLogic($queryValue) {
+    return ($queryValue !== null && $queryValue !== '') ? (int) $queryValue : null;
+}
+
+// Test cases
+$testCases = [
+    ['value' => '0', 'description' => 'String "0" (first dealership)'],
+    ['value' => 0, 'description' => 'Integer 0'],
+    ['value' => '1', 'description' => 'String "1"'],
+    ['value' => 1, 'description' => 'Integer 1'],
+    ['value' => null, 'description' => 'null (no filter)'],
+    ['value' => '', 'description' => 'Empty string'],
+];
+
+echo "=== OLD BUGGY LOGIC ===\n";
+foreach ($testCases as $test) {
+    $result = oldBuggyLogic($test['value']);
+    $displayValue = var_export($test['value'], true);
+    $displayResult = var_export($result, true);
+    echo sprintf("Input: %-20s => Result: %-10s\n", $displayValue, $displayResult);
+}
+
+echo "\n=== INTERMEDIATE LOGIC (still buggy with empty strings) ===\n";
+foreach ($testCases as $test) {
+    $result = intermediateLogic($test['value']);
+    $displayValue = var_export($test['value'], true);
+    $displayResult = var_export($result, true);
+    $warning = ($test['value'] === '' && $result === 0) ? ' ⚠️  ISSUE: Empty string -> 0' : '';
+    echo sprintf("Input: %-20s => Result: %-10s%s\n", $displayValue, $displayResult, $warning);
+}
+
+echo "\n=== NEW FULLY FIXED LOGIC ===\n";
+foreach ($testCases as $test) {
+    $result = newFixedLogic($test['value']);
+    $displayValue = var_export($test['value'], true);
+    $displayResult = var_export($result, true);
+    echo sprintf("Input: %-20s => Result: %-10s\n", $displayValue, $displayResult);
+}
+
+echo "\n=== ANALYSIS ===\n";
+echo "FIRST BUG: '0' (string) or 0 (integer) evaluated as falsy in PHP,\n";
+echo "so the ternary operator `condition ? value : null` returned null instead of 0.\n\n";
+echo "SECOND BUG (discovered after user feedback): Empty string '' is cast to 0,\n";
+echo "then the 'if (0)' check fails, causing filter not to be applied.\n\n";
+echo "The complete fix checks both `!== null` AND `!== ''`, which correctly handles:\n";
+echo "  - 0 (valid dealership ID) => returns 0 ✓\n";
+echo "  - 1 (valid dealership ID) => returns 1 ✓\n";
+echo "  - '' (empty string) => returns null (no filter) ✓\n";
+echo "  - null (no filter applied) => returns null ✓\n\n";
+
+// Test query parameter simulation
+echo "=== SIMULATING REQUEST QUERY ===\n";
+
+class FakeRequest {
+    private $params;
+
+    public function __construct($params) {
+        $this->params = $params;
+    }
+
+    public function query($key, $default = null) {
+        return $this->params[$key] ?? $default;
+    }
+}
+
+// Test with dealership_id=0
+$request1 = new FakeRequest(['dealership_id' => '0']);
+$oldResult1 = $request1->query('dealership_id') ? (int) $request1->query('dealership_id') : null;
+$intermediateResult1 = $request1->query('dealership_id') !== null ? (int) $request1->query('dealership_id') : null;
+$newResult1 = ($request1->query('dealership_id') !== null && $request1->query('dealership_id') !== '') ? (int) $request1->query('dealership_id') : null;
+
+echo "Query param: dealership_id=0\n";
+echo "  Old logic: " . var_export($oldResult1, true) . " (BUG: should be 0!)\n";
+echo "  Intermediate: " . var_export($intermediateResult1, true) . " (CORRECT!)\n";
+echo "  New logic: " . var_export($newResult1, true) . " (CORRECT!)\n\n";
+
+// Test with dealership_id=1
+$request2 = new FakeRequest(['dealership_id' => '1']);
+$oldResult2 = $request2->query('dealership_id') ? (int) $request2->query('dealership_id') : null;
+$intermediateResult2 = $request2->query('dealership_id') !== null ? (int) $request2->query('dealership_id') : null;
+$newResult2 = ($request2->query('dealership_id') !== null && $request2->query('dealership_id') !== '') ? (int) $request2->query('dealership_id') : null;
+
+echo "Query param: dealership_id=1\n";
+echo "  Old logic: " . var_export($oldResult2, true) . " (correct)\n";
+echo "  Intermediate: " . var_export($intermediateResult2, true) . " (correct)\n";
+echo "  New logic: " . var_export($newResult2, true) . " (correct)\n\n";
+
+// Test with dealership_id= (empty string)
+$request3 = new FakeRequest(['dealership_id' => '']);
+$oldResult3 = $request3->query('dealership_id') ? (int) $request3->query('dealership_id') : null;
+$intermediateResult3 = $request3->query('dealership_id') !== null ? (int) $request3->query('dealership_id') : null;
+$newResult3 = ($request3->query('dealership_id') !== null && $request3->query('dealership_id') !== '') ? (int) $request3->query('dealership_id') : null;
+
+echo "Query param: dealership_id= (empty string)\n";
+echo "  Old logic: " . var_export($oldResult3, true) . " (returns null, which is correct)\n";
+echo "  Intermediate: " . var_export($intermediateResult3, true) . " (BUG: '' cast to 0!)\n";
+echo "  New logic: " . var_export($newResult3, true) . " (CORRECT: returns null)\n\n";
+
+// Test with no dealership_id
+$request4 = new FakeRequest([]);
+$oldResult4 = $request4->query('dealership_id') ? (int) $request4->query('dealership_id') : null;
+$intermediateResult4 = $request4->query('dealership_id') !== null ? (int) $request4->query('dealership_id') : null;
+$newResult4 = ($request4->query('dealership_id') !== null && $request4->query('dealership_id') !== '') ? (int) $request4->query('dealership_id') : null;
+
+echo "Query param: (no dealership_id)\n";
+echo "  Old logic: " . var_export($oldResult4, true) . " (correct)\n";
+echo "  Intermediate: " . var_export($intermediateResult4, true) . " (correct)\n";
+echo "  New logic: " . var_export($newResult4, true) . " (correct)\n\n";
+
+echo "✓ The new logic correctly handles all edge cases!\n";