diff --git a/frontend/src/__tests__/components/ModelViewerModal.test.tsx b/frontend/src/__tests__/components/ModelViewerModal.test.tsx
index e41eadab0..62c0e4bdb 100644
--- a/frontend/src/__tests__/components/ModelViewerModal.test.tsx
+++ b/frontend/src/__tests__/components/ModelViewerModal.test.tsx
@@ -29,12 +29,13 @@ vi.mock('../../components/GcodeViewer', () => ({
   ),
 }));
 
-vi.mock('../../utils/slicer', () => ({
+// Only the protocol-handler launch is stubbed — it would navigate the jsdom
+// window. Everything else in the module is a pure predicate, so keep the real
+// implementations: re-declaring them here would let the file-type rule these
+// tests assert on drift away from the one the component actually runs.
+vi.mock('../../utils/slicer', async (importOriginal) => ({
+  ...(await importOriginal<typeof import('../../utils/slicer')>()),
   openInSlicer: vi.fn(),
-  resolveDesktopSlicer: vi.fn(
-    (openInSlicer?: string, preferredSlicer?: string) =>
-      (openInSlicer ?? preferredSlicer ?? 'bambu_studio') as 'bambu_studio' | 'orcaslicer',
-  ),
 }));
 
 const mockCapabilities = {
diff --git a/frontend/src/__tests__/pages/FileManagerPage.test.tsx b/frontend/src/__tests__/pages/FileManagerPage.test.tsx
index 080230936..a07de5309 100644
--- a/frontend/src/__tests__/pages/FileManagerPage.test.tsx
+++ b/frontend/src/__tests__/pages/FileManagerPage.test.tsx
@@ -12,12 +12,13 @@ import { setAuthToken } from '../../api/client';
 import { http, HttpResponse } from 'msw';
 import { server } from '../mocks/server';
 
-vi.mock('../../utils/slicer', () => ({
+// Only the protocol-handler launch is stubbed — it would navigate the jsdom
+// window. Everything else in the module is a pure predicate, so keep the real
+// implementations: isSliceableFilename decides which rows even offer the
+// action these tests click.
+vi.mock('../../utils/slicer', async (importOriginal) => ({
+  ...(await importOriginal<typeof import('../../utils/slicer')>()),
   openInSlicer: vi.fn(),
-  resolveDesktopSlicer: vi.fn(
-    (openInSlicer?: string, preferredSlicer?: string) =>
-      (openInSlicer ?? preferredSlicer ?? 'bambu_studio') as 'bambu_studio' | 'orcaslicer',
-  ),
 }));
 
 vi.mock('../../components/SliceModal', () => ({
@@ -1239,11 +1240,12 @@ describe('FileManagerPage', () => {
       expect(within(card).queryByText('Slice')).not.toBeInTheDocument();
     });
 
-    // Permission gating is the security-relevant half of the slice action:
-    // the in-app API path needs library:upload (the permission the backend
-    // enforces on the slicer-token endpoint), the desktop handoff mirrors the
-    // backend's ownership check and needs library:read_all / library:read_own
-    // (legacy library:read also accepted).
+    // Permission gating is the security-relevant half of the slice action: the
+    // in-app API path needs library:upload, and the desktop handoff mirrors the
+    // ownership check the slicer-token endpoint runs — library:read_all or
+    // library:read_own. The legacy library:read is deliberately not accepted;
+    // it satisfies neither the token endpoint nor the folder listing that gets
+    // a user to this page at all.
     const mockAuthUser = (permissions: string[]) => {
       setAuthToken('test-token', 'session');
       server.use(
@@ -1322,6 +1324,26 @@ describe('FileManagerPage', () => {
       expect(sliceItem).not.toBeDisabled();
     });
 
+    it('does not accept the legacy library:read for the desktop handoff', async () => {
+      // require_ownership_permission(LIBRARY_READ_ALL, LIBRARY_READ_OWN) does no
+      // legacy expansion, so this group 403s on the slicer-token endpoint.
+      // Enabling the item would offer an action the server refuses, and the
+      // failure would look like "no slicer installed" once the fallback URL is
+      // handed over.
+      mockAuthUser(['library:read']);
+      const user = userEvent.setup();
+      render(<FileManagerPage />);
+
+      await waitFor(() => expect(screen.getByText('bracket.stl')).toBeInTheDocument());
+
+      const card = await openMenu(user, 'bracket.stl');
+      const sliceItem = within(card).getByText('Slice').closest('button');
+      expect(sliceItem).toBeDisabled();
+
+      await user.click(sliceItem!);
+      expect(openInSlicer).not.toHaveBeenCalled();
+    });
+
     it('slices from the list-view button when the slicer API is disabled', async () => {
       const user = userEvent.setup();
       render(<FileManagerPage />);
diff --git a/frontend/src/components/ModelViewerModal.tsx b/frontend/src/components/ModelViewerModal.tsx
index 2d91a81e0..9f12a0953 100644
--- a/frontend/src/components/ModelViewerModal.tsx
+++ b/frontend/src/components/ModelViewerModal.tsx
@@ -7,7 +7,7 @@ import { GcodeViewer } from './GcodeViewer';
 import { Button } from './Button';
 import { api, withStreamToken } from '../api/client';
 import { useToast } from '../contexts/ToastContext';
-import { openInSlicer, resolveDesktopSlicer, type SlicerType } from '../utils/slicer';
+import { isSliceableFileType, openInSlicer, resolveDesktopSlicer, type SlicerType } from '../utils/slicer';
 import type { ArchivePlatesResponse, LibraryFilePlatesResponse, PlateMetadata } from '../types/plates';
 
 type ViewTab = '3d' | 'gcode';
@@ -373,11 +373,11 @@ export function ModelViewerModal({ archiveId, libraryFileId, title, fileType, on
   }, [isDraggingDivider, dividerHeight, minPlateHeight, minViewerPx, minViewerRatio]);
 
   // Which file types can be handed to a desktop slicer via the URL protocol
-  // handler — and sliced in-app via the sidecar. Kept in step with
-  // `isSliceableFilename()` in FileManagerPage so a file's card-menu "Slice"
-  // and its 3D-preview slicer button never disagree on the same type.
-  const normalizedFileType = (fileType || '').toLowerCase();
-  const slicerReadyType = ['3mf', 'stl', 'step', 'stp'].includes(normalizedFileType);
+  // handler — and sliced in-app via the sidecar. Shares its list with
+  // `isSliceableFilename()`, which the File Manager's card menu and list row
+  // use, so a file's "Slice" action and its 3D-preview slicer button can no
+  // longer disagree about the same file.
+  const slicerReadyType = isSliceableFileType(fileType);
   const canOpenInSlicer = isLibrary ? slicerReadyType : true;
 
   // When the user has the in-app Slicer API enabled (Settings → Workflow →
diff --git a/frontend/src/pages/FileManagerPage.tsx b/frontend/src/pages/FileManagerPage.tsx
index c3425bac7..1fa82151a 100644
--- a/frontend/src/pages/FileManagerPage.tsx
+++ b/frontend/src/pages/FileManagerPage.tsx
@@ -74,7 +74,7 @@ import { usePageFileDrop } from '../hooks/usePageFileDrop';
 import { useAuth } from '../contexts/AuthContext';
 import { formatDuration, parseUTCDate, formatDate } from '../utils/date';
 import { formatFileSize } from '../utils/file';
-import { openInSlicer, resolveDesktopSlicer, type SlicerType } from '../utils/slicer';
+import { isSliceableFilename, openInSlicer, resolveDesktopSlicer, type SlicerType } from '../utils/slicer';
 
 type SortField = 'name' | 'date' | 'size' | 'type' | 'prints';
 type SortDirection = 'asc' | 'desc';
@@ -743,14 +743,6 @@ function isSlicedFilename(filename: string): boolean {
   return lower.endsWith('.gcode') || lower.endsWith('.gcode.3mf');
 }
 
-// Files that can be fed to the slicer sidecar (model geometry inputs).
-// Excludes .gcode.* (already sliced) and any other non-model formats.
-function isSliceableFilename(filename: string): boolean {
-  const lower = filename.toLowerCase();
-  if (lower.endsWith('.gcode') || lower.endsWith('.gcode.3mf')) return false;
-  return lower.endsWith('.stl') || lower.endsWith('.3mf') || lower.endsWith('.step') || lower.endsWith('.stp');
-}
-
 // File Card
 interface FileCardProps {
   file: LibraryFileListItem;
@@ -1172,16 +1164,26 @@ export function FileManagerPage() {
     }
   }, [preferredSlicer, showToast, t]);
 
-  // Slice permission: API mode needs upload rights, desktop handoff is a download.
-  // The handoff mirrors the backend's ownership check on the slicer-token
-  // endpoint (library:read_all / library:read_own); `library:read` is a legacy
-  // permission default groups don't carry, so requiring it would disable the
-  // handoff for Operators and Viewers.
+  // Slice permission: API mode needs upload rights, the desktop handoff is a
+  // download. Each mirrors what the backend enforces on the endpoint that
+  // branch actually calls, so the UI never offers an action the server refuses.
+  //
+  // Deliberately NOT accepting the legacy `library:read` on the handoff branch.
+  // It looks like the safe back-compat term to include, but the slicer-token
+  // endpoint gates on require_ownership_permission(LIBRARY_READ_ALL,
+  // LIBRARY_READ_OWN), and neither that dependency nor User.has_permission
+  // expands the legacy name — so a group holding only `library:read` gets a 403
+  // there. It cannot reach this page to find out either: GET /library/folders
+  // gates on the same pair. Accepting it here would only enable a menu item
+  // that fails, and the `library:read` -> `library:read_own` migration in
+  // core/database.py runs only over the groups named in DEFAULT_GROUPS, so a
+  // custom role that still carries it is genuinely stuck rather than silently
+  // upgraded.
   const canSlice = useCallback(() => {
     if (settings?.use_slicer_api) {
       return hasPermission('library:upload');
     }
-    return hasAnyPermission('library:read_all', 'library:read_own', 'library:read');
+    return hasAnyPermission('library:read_all', 'library:read_own');
   }, [settings?.use_slicer_api, hasPermission, hasAnyPermission]);
   const { data: folders, isLoading: foldersLoading } = useQuery({
     queryKey: ['library-folders'],
diff --git a/frontend/src/utils/slicer.ts b/frontend/src/utils/slicer.ts
index ad6a71f5a..b1fd719e3 100644
--- a/frontend/src/utils/slicer.ts
+++ b/frontend/src/utils/slicer.ts
@@ -41,6 +41,43 @@ export function resolveDesktopSlicer(
   return openInSlicer ?? preferredSlicer ?? 'bambu_studio';
 }
 
+/**
+ * File types a slicer can be handed — both by the desktop URI handler and by
+ * the in-app sidecar. Source geometry only: a sliced file is an output, and
+ * neither slicer has anything to do with one.
+ *
+ * Lives here rather than beside either caller because both the File Manager
+ * (which has a filename) and the 3D preview (which has a `LibraryFile.file_type`)
+ * decide the same thing about the same file. They used to hold separate lists,
+ * and the two disagreed — a card menu offered a desktop handoff for an STL
+ * whose own 3D preview showed "Open in Slicer" greyed out.
+ */
+export const SLICEABLE_FILE_TYPES = ['3mf', 'stl', 'step', 'stp'] as const;
+
+/**
+ * Does a `LibraryFile.file_type` name a sliceable source file?
+ *
+ * The backend stores compound extensions whole — a sliced 3MF classifies as
+ * `gcode.3mf`, not `3mf` (`classify_file_type` in `api/routes/library.py`) — so
+ * membership alone is enough to exclude sliced output here.
+ */
+export function isSliceableFileType(fileType?: string | null): boolean {
+  const normalized = (fileType || '').toLowerCase();
+  return (SLICEABLE_FILE_TYPES as readonly string[]).includes(normalized);
+}
+
+/**
+ * Does a filename name a sliceable source file?
+ *
+ * Checked against the name rather than a stored type, so the compound
+ * extensions have to be ruled out explicitly: `.gcode.3mf` ends with `.3mf`.
+ */
+export function isSliceableFilename(filename: string): boolean {
+  const lower = filename.toLowerCase();
+  if (lower.endsWith('.gcode') || lower.endsWith('.gcode.3mf')) return false;
+  return SLICEABLE_FILE_TYPES.some((ext) => lower.endsWith(`.${ext}`));
+}
+
 /**
  * Detect the user's operating system
  */