diff --git a/api/assign_ticket.php b/api/assign_ticket.php
index 1046909..28ced6c 100644
--- a/api/assign_ticket.php
+++ b/api/assign_ticket.php
@@ -1,11 +1,7 @@
 <?php
 
 require_once __DIR__ . '/bootstrap.php';
-require_once dirname(__DIR__) . '/models/TicketModel.php';
-require_once dirname(__DIR__) . '/models/AuditLogModel.php';
-require_once dirname(__DIR__) . '/models/UserModel.php';
-require_once dirname(__DIR__) . '/helpers/NotificationHelper.php';
-require_once dirname(__DIR__) . '/helpers/SynapseHelper.php';
+require_once dirname(__DIR__) . '/services/AssignmentService.php';
 
 // Get request data
 $data = json_decode(file_get_contents('php://input'), true);
@@ -15,79 +11,13 @@ if (!is_array($data)) {
     exit;
 }
 
-$ticketIdRaw = isset($data['ticket_id']) ? trim((string)$data['ticket_id']) : '';
-$assignedTo = $data['assigned_to'] ?? null;
+// Validation, permission check, audit log, notification and stats-cache
+// invalidation live in AssignmentService so the MCP assign_ticket tool runs
+// the same code path.
+$result = AssignmentService::assign($conn, $currentUser, $data);
 
-if (!ctype_digit($ticketIdRaw) || (int)$ticketIdRaw <= 0) {
-    http_response_code(400);
-    echo json_encode(['success' => false, 'error' => 'Ticket ID required']);
-    exit;
-}
-$ticketId = $ticketIdRaw;
-
-$ticketModel = new TicketModel($conn);
-$auditLogModel = new AuditLogModel($conn);
-$userModel = new UserModel($conn);
-
-// Verify ticket exists and user can access it
-$ticket = $ticketModel->getTicketById($ticketId);
-if (!$ticket || !$ticketModel->canUserAccessTicket($ticket, $currentUser)) {
-    http_response_code(404);
-    echo json_encode(['success' => false, 'error' => 'Ticket not found']);
-    exit;
-}
-
-// Authorization: only admins or the ticket creator/assignee can reassign
-if (!$isAdmin && (int)$ticket['created_by'] !== (int)$userId && (int)$ticket['assigned_to'] !== (int)$userId) {
-    http_response_code(403);
-    echo json_encode(['success' => false, 'error' => 'Permission denied']);
-    exit;
-}
-
-if ($assignedTo === null || $assignedTo === '') {
-    // Unassign ticket
-    $success = $ticketModel->unassignTicket($ticketId, $userId);
-    if ($success) {
-        $auditLogModel->log($userId, 'unassign', 'ticket', $ticketId);
-    }
-} else {
-    // Validate assigned_to is a valid user ID
-    $assignedTo = (int)$assignedTo;
-    $targetUser = $userModel->getUserById($assignedTo);
-    if (!$targetUser) {
-        http_response_code(400);
-        echo json_encode(['success' => false, 'error' => 'Invalid user ID']);
-        exit;
-    }
-
-    // Assign ticket
-    $success = $ticketModel->assignTicket($ticketId, $assignedTo, $userId);
-    if ($success) {
-        $auditLogModel->log($userId, 'assign', 'ticket', $ticketId, ['assigned_to' => $assignedTo]);
-
-        if (!empty($GLOBALS['config']['MATRIX_NOTIFY_ASSIGNMENTS'])) {
-            $changedByDisplay = $currentUser['display_name'] ?? $currentUser['username'] ?? null;
-            $assigneeName     = $targetUser['display_name'] ?? $targetUser['username'] ?? null;
-            $assigneeMatrix   = isset($targetUser['username'])
-                                ? SynapseHelper::resolveUsername($targetUser['username'])
-                                : null;
-            NotificationHelper::sendAssignmentNotification(
-                $ticketId,
-                $ticket['title'] ?? "Ticket #{$ticketId}",
-                $assigneeName,
-                $assigneeMatrix,
-                $changedByDisplay,
-                $ticket['visibility'] ?? 'public'
-            );
-        }
-    }
-}
-
-if (!$success) {
-    http_response_code(500);
-    apiRespond(['success' => false, 'error' => 'Failed to update ticket assignment']);
-} else {
-    require_once dirname(__DIR__) . '/models/StatsModel.php';
-    (new StatsModel($conn))->invalidateCache();
-    apiRespond(['success' => true]);
+if (!empty($result['http_status'])) {
+    http_response_code($result['http_status']);
+    unset($result['http_status']);
 }
+apiRespond($result);
diff --git a/services/AssignmentService.php b/services/AssignmentService.php
new file mode 100644
index 0000000..706fcba
--- /dev/null
+++ b/services/AssignmentService.php
@@ -0,0 +1,98 @@
+<?php
+
+/**
+ * Assigning / unassigning a ticket: access check, permission (admin, creator,
+ * or current assignee), audit log, optional Matrix assignment notification,
+ * stats-cache invalidation.
+ *
+ * Shared by the web UI (api/assign_ticket.php) and the MCP assign_ticket tool
+ * so both run one code path (tinker_tickets#111). Extracted verbatim from
+ * assign_ticket.php. Returns result arrays; failures carry 'http_status'.
+ */
+
+require_once dirname(__DIR__) . '/models/TicketModel.php';
+require_once dirname(__DIR__) . '/models/AuditLogModel.php';
+require_once dirname(__DIR__) . '/models/UserModel.php';
+require_once dirname(__DIR__) . '/models/StatsModel.php';
+require_once dirname(__DIR__) . '/helpers/NotificationHelper.php';
+require_once dirname(__DIR__) . '/helpers/SynapseHelper.php';
+
+class AssignmentService
+{
+    /**
+     * @param array $currentUser Authenticated user row (user_id, username, display_name, is_admin, ...)
+     * @param array $data        ticket_id, assigned_to (user_id; null/'' to unassign)
+     */
+    public static function assign(mysqli $conn, array $currentUser, array $data): array
+    {
+        $userId = $currentUser['user_id'];
+        $isAdmin = $currentUser['is_admin'] ?? false;
+
+        $ticketIdRaw = isset($data['ticket_id']) ? trim((string)$data['ticket_id']) : '';
+        $assignedTo = $data['assigned_to'] ?? null;
+
+        if (!ctype_digit($ticketIdRaw) || (int)$ticketIdRaw <= 0) {
+            return ['success' => false, 'error' => 'Ticket ID required', 'http_status' => 400];
+        }
+        $ticketId = $ticketIdRaw;
+
+        $ticketModel = new TicketModel($conn);
+        $auditLogModel = new AuditLogModel($conn);
+        $userModel = new UserModel($conn);
+
+        // Verify ticket exists and user can access it
+        $ticket = $ticketModel->getTicketById($ticketId);
+        if (!$ticket || !$ticketModel->canUserAccessTicket($ticket, $currentUser)) {
+            return ['success' => false, 'error' => 'Ticket not found', 'http_status' => 404];
+        }
+
+        // Authorization: only admins or the ticket creator/assignee can reassign
+        if (!$isAdmin && (int)$ticket['created_by'] !== (int)$userId && (int)$ticket['assigned_to'] !== (int)$userId) {
+            return ['success' => false, 'error' => 'Permission denied', 'http_status' => 403];
+        }
+
+        if ($assignedTo === null || $assignedTo === '') {
+            // Unassign ticket
+            $success = $ticketModel->unassignTicket($ticketId, $userId);
+            if ($success) {
+                $auditLogModel->log($userId, 'unassign', 'ticket', $ticketId);
+            }
+        } else {
+            // Validate assigned_to is a valid user ID
+            $assignedTo = (int)$assignedTo;
+            $targetUser = $userModel->getUserById($assignedTo);
+            if (!$targetUser) {
+                return ['success' => false, 'error' => 'Invalid user ID', 'http_status' => 400];
+            }
+
+            // Assign ticket
+            $success = $ticketModel->assignTicket($ticketId, $assignedTo, $userId);
+            if ($success) {
+                $auditLogModel->log($userId, 'assign', 'ticket', $ticketId, ['assigned_to' => $assignedTo]);
+
+                if (!empty($GLOBALS['config']['MATRIX_NOTIFY_ASSIGNMENTS'])) {
+                    $changedByDisplay = $currentUser['display_name'] ?? $currentUser['username'] ?? null;
+                    $assigneeName     = $targetUser['display_name'] ?? $targetUser['username'] ?? null;
+                    $assigneeMatrix   = isset($targetUser['username'])
+                                        ? SynapseHelper::resolveUsername($targetUser['username'])
+                                        : null;
+                    NotificationHelper::sendAssignmentNotification(
+                        $ticketId,
+                        $ticket['title'] ?? "Ticket #{$ticketId}",
+                        $assigneeName,
+                        $assigneeMatrix,
+                        $changedByDisplay,
+                        $ticket['visibility'] ?? 'public'
+                    );
+                }
+            }
+        }
+
+        if (!$success) {
+            return ['success' => false, 'error' => 'Failed to update ticket assignment', 'http_status' => 500];
+        }
+
+        (new StatsModel($conn))->invalidateCache();
+        return ['success' => true];
+    }
+}