diff --git a/frontend/app/(admin)/admin/doc-control/drawings/contract/categories/page.tsx b/frontend/app/(admin)/admin/doc-control/drawings/contract/categories/page.tsx
index 7a4126a..dcab3be 100644
--- a/frontend/app/(admin)/admin/doc-control/drawings/contract/categories/page.tsx
+++ b/frontend/app/(admin)/admin/doc-control/drawings/contract/categories/page.tsx
@@ -26,7 +26,7 @@ interface Category {
 }
 
 export default function ContractCategoriesPage() {
-  const [selectedProjectId, setSelectedProjectId] = useState<number | undefined>(undefined);
+  const [selectedProjectId, setSelectedProjectId] = useState<string | undefined>(undefined);
   const { data: projects = [], isLoading: isLoadingProjects } = useProjects();
 
   const columns: ColumnDef<Category>[] = [
@@ -59,8 +59,8 @@ export default function ContractCategoriesPage() {
     <div className="flex items-center gap-4">
       <span className="text-sm font-medium">Project:</span>
       <Select
-        value={selectedProjectId?.toString() ?? ''}
-        onValueChange={(v) => setSelectedProjectId(v ? parseInt(v) : undefined)}
+        value={selectedProjectId ?? ''}
+        onValueChange={(v) => setSelectedProjectId(v || undefined)}
       >
         <SelectTrigger className="w-[300px]">
           {isLoadingProjects ? (
@@ -160,7 +160,7 @@ class MappingErrorBoundary extends React.Component<
   }
 }
 
-function CategoryMappingSection({ projectId }: { projectId: number }) {
+function CategoryMappingSection({ projectId }: { projectId: string }) {
   return (
     <div className="space-y-4">
       <h2 className="text-xl font-semibold">Category Mappings (Map Sub-categories to Categories)</h2>
@@ -174,7 +174,7 @@ function CategoryMappingSection({ projectId }: { projectId: number }) {
   );
 }
 
-function ManageMappings({ projectId }: { projectId: number }) {
+function ManageMappings({ projectId }: { projectId: string }) {
   const queryClient = useQueryClient();
   const [selectedCat, setSelectedCat] = useState<string>('');
   const [selectedSubCat, setSelectedSubCat] = useState<string>('');
@@ -210,7 +210,7 @@ function ManageMappings({ projectId }: { projectId: number }) {
   const mappings = Array.isArray(rawMappings) ? rawMappings : [];
 
   const createMutation = useMutation({
-    mutationFn: (data: { projectId: number; categoryId: number; subCategoryId: number }) =>
+    mutationFn: (data: { projectId: number | string; categoryId: number; subCategoryId: number }) =>
       drawingMasterDataService.createContractMapping(data),
     onSuccess: () => {
       queryClient.invalidateQueries({ queryKey: ['contract-mappings'] });
diff --git a/frontend/app/(admin)/admin/doc-control/drawings/contract/sub-categories/page.tsx b/frontend/app/(admin)/admin/doc-control/drawings/contract/sub-categories/page.tsx
index f5e7bfc..fc6cdc4 100644
--- a/frontend/app/(admin)/admin/doc-control/drawings/contract/sub-categories/page.tsx
+++ b/frontend/app/(admin)/admin/doc-control/drawings/contract/sub-categories/page.tsx
@@ -18,7 +18,7 @@ interface SubCategory {
 }
 
 export default function ContractSubCategoriesPage() {
-  const [selectedProjectId, setSelectedProjectId] = useState<number | undefined>(undefined);
+  const [selectedProjectId, setSelectedProjectId] = useState<string | undefined>(undefined);
   const { data: projects = [], isLoading: isLoadingProjects } = useProjects();
 
   const columns: ColumnDef<SubCategory>[] = [
@@ -51,8 +51,8 @@ export default function ContractSubCategoriesPage() {
     <div className="flex items-center gap-4">
       <span className="text-sm font-medium">Project:</span>
       <Select
-        value={selectedProjectId?.toString() ?? ''}
-        onValueChange={(v) => setSelectedProjectId(v ? parseInt(v) : undefined)}
+        value={selectedProjectId ?? ''}
+        onValueChange={(v) => setSelectedProjectId(v || undefined)}
       >
         <SelectTrigger className="w-[300px]">
           {isLoadingProjects ? (
diff --git a/frontend/app/(admin)/admin/doc-control/drawings/contract/volumes/page.tsx b/frontend/app/(admin)/admin/doc-control/drawings/contract/volumes/page.tsx
index 7d827c4..e6a2e10 100644
--- a/frontend/app/(admin)/admin/doc-control/drawings/contract/volumes/page.tsx
+++ b/frontend/app/(admin)/admin/doc-control/drawings/contract/volumes/page.tsx
@@ -24,7 +24,7 @@ interface Volume {
 }
 
 export default function ContractVolumesPage() {
-  const [selectedProjectId, setSelectedProjectId] = useState<number | undefined>(undefined);
+  const [selectedProjectId, setSelectedProjectId] = useState<string | undefined>(undefined);
   const { data: projects = [], isLoading: isLoadingProjects } = useProjects();
 
   const columns: ColumnDef<Volume>[] = [
@@ -63,8 +63,8 @@ export default function ContractVolumesPage() {
     <div className="flex items-center gap-4">
       <span className="text-sm font-medium">Project:</span>
       <Select
-        value={selectedProjectId?.toString() ?? ""}
-        onValueChange={(v) => setSelectedProjectId(v ? parseInt(v) : undefined)}
+        value={selectedProjectId ?? ''}
+        onValueChange={(v) => setSelectedProjectId(v || undefined)}
       >
         <SelectTrigger className="w-[300px]">
           {isLoadingProjects ? (
diff --git a/frontend/app/(admin)/admin/doc-control/drawings/shop/main-categories/page.tsx b/frontend/app/(admin)/admin/doc-control/drawings/shop/main-categories/page.tsx
index 7dc30d3..f32880c 100644
--- a/frontend/app/(admin)/admin/doc-control/drawings/shop/main-categories/page.tsx
+++ b/frontend/app/(admin)/admin/doc-control/drawings/shop/main-categories/page.tsx
@@ -19,7 +19,7 @@ interface MainCategory {
 }
 
 export default function ShopMainCategoriesPage() {
-  const [selectedProjectId, setSelectedProjectId] = useState<number | undefined>(undefined);
+  const [selectedProjectId, setSelectedProjectId] = useState<string | undefined>(undefined);
   const { data: projects = [], isLoading: isLoadingProjects } = useProjects();
 
   const columns: ColumnDef<MainCategory>[] = [
@@ -62,8 +62,8 @@ export default function ShopMainCategoriesPage() {
     <div className="flex items-center gap-4">
       <span className="text-sm font-medium">Project:</span>
       <Select
-        value={selectedProjectId?.toString() ?? ''}
-        onValueChange={(v) => setSelectedProjectId(v ? parseInt(v) : undefined)}
+        value={selectedProjectId ?? ''}
+        onValueChange={(v) => setSelectedProjectId(v || undefined)}
       >
         <SelectTrigger className="w-[300px]">
           {isLoadingProjects ? (
diff --git a/frontend/app/(admin)/admin/doc-control/drawings/shop/sub-categories/page.tsx b/frontend/app/(admin)/admin/doc-control/drawings/shop/sub-categories/page.tsx
index f082ce6..21191ce 100644
--- a/frontend/app/(admin)/admin/doc-control/drawings/shop/sub-categories/page.tsx
+++ b/frontend/app/(admin)/admin/doc-control/drawings/shop/sub-categories/page.tsx
@@ -19,7 +19,7 @@ interface SubCategory {
 }
 
 export default function ShopSubCategoriesPage() {
-  const [selectedProjectId, setSelectedProjectId] = useState<number | undefined>(undefined);
+  const [selectedProjectId, setSelectedProjectId] = useState<string | undefined>(undefined);
   const { data: projects = [], isLoading: isLoadingProjects } = useProjects();
 
   console.log('Projects Data:', projects);
@@ -64,8 +64,8 @@ export default function ShopSubCategoriesPage() {
     <div className="flex items-center gap-4">
       <span className="text-sm font-medium">Project:</span>
       <Select
-        value={selectedProjectId?.toString() ?? ''}
-        onValueChange={(v) => setSelectedProjectId(v ? parseInt(v) : undefined)}
+        value={selectedProjectId ?? ''}
+        onValueChange={(v) => setSelectedProjectId(v || undefined)}
       >
         <SelectTrigger className="w-[300px]">
           {isLoadingProjects ? (
diff --git a/frontend/app/(admin)/admin/doc-control/reference/tags/page.tsx b/frontend/app/(admin)/admin/doc-control/reference/tags/page.tsx
index fac1f03..be3da5b 100644
--- a/frontend/app/(admin)/admin/doc-control/reference/tags/page.tsx
+++ b/frontend/app/(admin)/admin/doc-control/reference/tags/page.tsx
@@ -14,7 +14,7 @@ export default function TagsPage() {
   });
 
   const projectOptions = [
-    { label: "Global (All Projects)", value: "" },
+    { label: "Global (All Projects)", value: "__none__" },
     ...(projectsData || []).map((p: Record<string, unknown>) => ({
       label: p.project_name || p.project_code || `Project ${p.id}`,
       value: String(p.id),
@@ -57,11 +57,9 @@ export default function TagsPage() {
 
   const formatPayload = (data: Record<string, unknown>) => {
     const payload = { ...data };
-    // Backend entity uses project_id (underscore) per ADR-017/018 schema
-    if (!payload.project_id || payload.project_id === "") {
+    // ADR-019: project_id is now a UUID string or '__none__' for global
+    if (!payload.project_id || payload.project_id === "__none__") {
         payload.project_id = null;
-    } else {
-        payload.project_id = Number(payload.project_id);
     }
     return payload;
   };
diff --git a/frontend/lib/services/drawing-master-data.service.ts b/frontend/lib/services/drawing-master-data.service.ts
index 6cd554a..543b7bb 100644
--- a/frontend/lib/services/drawing-master-data.service.ts
+++ b/frontend/lib/services/drawing-master-data.service.ts
@@ -14,7 +14,7 @@ export interface ContractVolume {
 }
 
 export interface CreateContractVolumeDto {
-  projectId: number;
+  projectId: number | string;
   volumeCode: string;
   volumeName: string;
   description?: string;
@@ -34,7 +34,7 @@ export interface ContractCategory {
 }
 
 export interface CreateContractCategoryDto {
-  projectId: number;
+  projectId: number | string;
   catCode: string;
   catName: string;
   description?: string;
@@ -54,7 +54,7 @@ export interface ContractSubCategory {
 }
 
 export interface CreateContractSubCategoryDto {
-  projectId: number;
+  projectId: number | string;
   subCatCode: string;
   subCatName: string;
   description?: string;
@@ -75,7 +75,7 @@ export interface ShopMainCategory {
 }
 
 export interface CreateShopMainCategoryDto {
-  projectId: number;
+  projectId: number | string;
   mainCategoryCode: string;
   mainCategoryName: string;
   description?: string;
@@ -97,7 +97,7 @@ export interface ShopSubCategory {
 }
 
 export interface CreateShopSubCategoryDto {
-  projectId: number;
+  projectId: number | string;
   subCategoryCode: string;
   subCategoryName: string;
   description?: string;
@@ -110,7 +110,7 @@ export interface CreateShopSubCategoryDto {
 // ===========================
 export const drawingMasterDataService = {
   // --- Contract Volumes ---
-  async getContractVolumes(projectId: number): Promise<ContractVolume[]> {
+  async getContractVolumes(projectId: number | string): Promise<ContractVolume[]> {
     const response = await apiClient.get(`/drawings/master-data/contract/volumes`, {
       params: { projectId },
     });
@@ -132,7 +132,7 @@ export const drawingMasterDataService = {
   },
 
   // --- Contract Categories ---
-  async getContractCategories(projectId: number): Promise<ContractCategory[]> {
+  async getContractCategories(projectId: number | string): Promise<ContractCategory[]> {
     const response = await apiClient.get(`/drawings/master-data/contract/categories`, {
       params: { projectId },
     });
@@ -154,7 +154,7 @@ export const drawingMasterDataService = {
   },
 
   // --- Contract Sub-categories ---
-  async getContractSubCategories(projectId: number): Promise<ContractSubCategory[]> {
+  async getContractSubCategories(projectId: number | string): Promise<ContractSubCategory[]> {
     const response = await apiClient.get(`/drawings/master-data/contract/sub-categories`, {
       params: { projectId },
     });
@@ -180,7 +180,7 @@ export const drawingMasterDataService = {
 
   // --- Contract Category Mappings ---
   async getContractMappings(
-    projectId: number,
+    projectId: number | string,
     categoryId?: number
   ): Promise<{ id: number; subCategory: ContractSubCategory; category: ContractCategory }[]> {
     const response = await apiClient.get(`/drawings/master-data/contract/mappings`, {
@@ -190,7 +190,7 @@ export const drawingMasterDataService = {
   },
 
   async createContractMapping(data: {
-    projectId: number;
+    projectId: number | string;
     categoryId: number;
     subCategoryId: number;
   }): Promise<{ id: number }> {
@@ -203,7 +203,7 @@ export const drawingMasterDataService = {
   },
 
   // --- Shop Main Categories ---
-  async getShopMainCategories(projectId: number): Promise<ShopMainCategory[]> {
+  async getShopMainCategories(projectId: number | string): Promise<ShopMainCategory[]> {
     const response = await apiClient.get(`/drawings/master-data/shop/main-categories`, {
       params: { projectId },
     });
@@ -225,7 +225,7 @@ export const drawingMasterDataService = {
   },
 
   // --- Shop Sub-categories ---
-  async getShopSubCategories(projectId: number, mainCategoryId?: number): Promise<ShopSubCategory[]> {
+  async getShopSubCategories(projectId: number | string, mainCategoryId?: number): Promise<ShopSubCategory[]> {
     const response = await apiClient.get(`/drawings/master-data/shop/sub-categories`, {
       params: { projectId, mainCategoryId },
     });