diff --git a/frontend/src/features/resources/resources-constants.ts b/frontend/src/features/resources/resources-constants.ts
index 74d7fb1df..4d4af40c4 100644
--- a/frontend/src/features/resources/resources-constants.ts
+++ b/frontend/src/features/resources/resources-constants.ts
@@ -20,3 +20,13 @@ export const MAX_UPLOAD_TOTAL_SIZE_MB = 2 * 1024;
 
 /** Mirrors `resources.MaxUploadFiles`. */
 export const MAX_UPLOAD_FILES_PER_REQUEST = 1000;
+
+/**
+ * Above this many entries, mounting the FileManager tree unfiltered risks
+ * freezing the page: the tree has no row virtualization, so building and
+ * rendering tens of thousands of DOM nodes on first paint blocks the main
+ * thread (see https://github.com/vxcontrol/pentagi/issues/403). Past this
+ * size, the Resources page prompts the user to search instead of eagerly
+ * rendering the whole library.
+ */
+export const LARGE_LIBRARY_PROMPT_THRESHOLD = 5000;
diff --git a/frontend/src/features/resources/resources-utils.test.ts b/frontend/src/features/resources/resources-utils.test.ts
new file mode 100644
index 000000000..038fcd1c8
--- /dev/null
+++ b/frontend/src/features/resources/resources-utils.test.ts
@@ -0,0 +1,19 @@
+import { describe, expect, it } from 'vitest';
+
+import { shouldPromptToSearchLargeLibrary } from './resources-utils';
+
+describe('shouldPromptToSearchLargeLibrary', () => {
+    it('returns false when the count is at or below the threshold', () => {
+        expect(shouldPromptToSearchLargeLibrary(5000, false, 5000)).toBe(false);
+        expect(shouldPromptToSearchLargeLibrary(100, false, 5000)).toBe(false);
+    });
+
+    it('returns true when the count exceeds the threshold and no query is active', () => {
+        expect(shouldPromptToSearchLargeLibrary(5001, false, 5000)).toBe(true);
+        expect(shouldPromptToSearchLargeLibrary(100000, false, 5000)).toBe(true);
+    });
+
+    it('returns false once a search query is active, regardless of count', () => {
+        expect(shouldPromptToSearchLargeLibrary(100000, true, 5000)).toBe(false);
+    });
+});
diff --git a/frontend/src/features/resources/resources-utils.ts b/frontend/src/features/resources/resources-utils.ts
index c4510eede..e037373c5 100644
--- a/frontend/src/features/resources/resources-utils.ts
+++ b/frontend/src/features/resources/resources-utils.ts
@@ -48,3 +48,14 @@ export const buildResourcesDownloadHref = (files: readonly FileNode[]): string =
 };
 
 export const pluralizeItems = (count: number): string => (count === 1 ? 'item' : 'items');
+
+/**
+ * True when the library is large enough that rendering it unfiltered risks
+ * freezing the page (see `LARGE_LIBRARY_PROMPT_THRESHOLD`), and no search
+ * query is active yet to narrow the result set down.
+ */
+export const shouldPromptToSearchLargeLibrary = (
+    resourceCount: number,
+    hasActiveQuery: boolean,
+    threshold: number,
+): boolean => !hasActiveQuery && resourceCount > threshold;
diff --git a/frontend/src/pages/resources/resources.tsx b/frontend/src/pages/resources/resources.tsx
index 7f0cc957f..b32b5c60d 100644
--- a/frontend/src/pages/resources/resources.tsx
+++ b/frontend/src/pages/resources/resources.tsx
@@ -44,10 +44,16 @@ import { Empty, EmptyDescription, EmptyHeader, EmptyMedia, EmptyTitle } from '@/
 import { FileDropZone } from '@/components/ui/file-drop-zone';
 import { InputGroup, InputGroupAddon, InputGroupButton, InputGroupInput } from '@/components/ui/input-group';
 import { Spinner } from '@/components/ui/spinner';
+import { LARGE_LIBRARY_PROMPT_THRESHOLD } from '@/features/resources/resources-constants';
 import { ResourcesCopyDialog } from '@/features/resources/resources-copy-dialog';
 import { ResourcesMkdirDialog } from '@/features/resources/resources-mkdir-dialog';
 import { ResourcesMoveDialog } from '@/features/resources/resources-move-dialog';
-import { buildResourcesDownloadHref, pluralizeItems, toFileNode } from '@/features/resources/resources-utils';
+import {
+    buildResourcesDownloadHref,
+    pluralizeItems,
+    shouldPromptToSearchLargeLibrary,
+    toFileNode,
+} from '@/features/resources/resources-utils';
 import { useResourcesDelete } from '@/features/resources/use-resources-delete';
 import { useResourcesMove } from '@/features/resources/use-resources-move';
 import { useResourcesSearch } from '@/features/resources/use-resources-search';
@@ -433,6 +439,30 @@ function Resources() {
         </Empty>
     );
 
+    // Above LARGE_LIBRARY_PROMPT_THRESHOLD entries, the FileManager tree (which
+    // isn't virtualized) can freeze the page on first paint. Skip mounting it
+    // until the user narrows the list down with a search query.
+    const isLibraryTooLargeToBrowse = shouldPromptToSearchLargeLibrary(
+        resources.length,
+        Boolean(search.debouncedQuery.trim()),
+        LARGE_LIBRARY_PROMPT_THRESHOLD,
+    );
+
+    const largeLibraryState = (
+        <Empty>
+            <EmptyHeader>
+                <EmptyMedia variant="icon">
+                    <Search />
+                </EmptyMedia>
+                <EmptyTitle>Large resource library</EmptyTitle>
+                <EmptyDescription>
+                    This library has {resources.length.toLocaleString()} entries — too many to browse at once. Search
+                    above to narrow it down.
+                </EmptyDescription>
+            </EmptyHeader>
+        </Empty>
+    );
+
     // Error surface only when there's no data — a failed background refetch must not blank a working list.
     if (error && !hasResources) {
         return (
@@ -544,25 +574,29 @@ function Resources() {
                     </DropdownMenu>
                 </div>
 
-                <FileManager
-                    actions={fileManagerActions}
-                    bulkActions={fileManagerBulkActions}
-                    className="min-h-0 flex-1"
-                    columns={{
-                        isModifiedVisible: viewOptions.modified,
-                        isSizeVisible: viewOptions.size,
-                    }}
-                    emptyAreaActions={fileManagerEmptyAreaActions}
-                    emptyState={noResourcesState}
-                    files={fileNodes}
-                    isFoldersFirst={viewOptions.foldersFirst}
-                    isLoading={isInitialLoading}
-                    labels={fileManagerLabels}
-                    onExternalFileDrop={handleExternalFileDrop}
-                    onMoveItems={handleMoveItems}
-                    onOpen={handleOpenFile}
-                    search={{ emptyState: noMatchesState, query: search.debouncedQuery }}
-                />
+                {isLibraryTooLargeToBrowse ? (
+                    largeLibraryState
+                ) : (
+                    <FileManager
+                        actions={fileManagerActions}
+                        bulkActions={fileManagerBulkActions}
+                        className="min-h-0 flex-1"
+                        columns={{
+                            isModifiedVisible: viewOptions.modified,
+                            isSizeVisible: viewOptions.size,
+                        }}
+                        emptyAreaActions={fileManagerEmptyAreaActions}
+                        emptyState={noResourcesState}
+                        files={fileNodes}
+                        isFoldersFirst={viewOptions.foldersFirst}
+                        isLoading={isInitialLoading}
+                        labels={fileManagerLabels}
+                        onExternalFileDrop={handleExternalFileDrop}
+                        onMoveItems={handleMoveItems}
+                        onOpen={handleOpenFile}
+                        search={{ emptyState: noMatchesState, query: search.debouncedQuery }}
+                    />
+                )}
 
                 <ResourcesMkdirDialog
                     defaultParentPath={mkdirParentOverride ?? ''}