diff --git a/frontend/src/components/files/FileViewer.tsx b/frontend/src/components/files/FileViewer.tsx
index f1eb9742..399ef982 100644
--- a/frontend/src/components/files/FileViewer.tsx
+++ b/frontend/src/components/files/FileViewer.tsx
@@ -1,4 +1,4 @@
-import { useState, useEffect, useMemo, Suspense } from 'react';
+import { useState, useEffect, useMemo, useRef, Suspense } from 'react';
 import { Editor } from '@/lib/monacoLoader';
 import { AlertCircle, FileIcon, Download, Loader2, Save } from 'lucide-react';
 import { Button } from '@/components/ui/button';
@@ -14,6 +14,7 @@ interface FileViewerProps {
   canEdit: boolean;
   isDarkMode: boolean;
   onSaved?: () => void;
+  onDirtyChange?: (dirty: boolean) => void;
 }
 
 function getFilename(path: string): string {
@@ -91,6 +92,7 @@ export function FileViewer({
   canEdit,
   isDarkMode,
   onSaved,
+  onDirtyChange,
 }: FileViewerProps) {
   const [content, setContent] = useState('');
   const [originalContent, setOriginalContent] = useState('');
@@ -102,6 +104,23 @@ export function FileViewer({
   const [size, setSize] = useState(0);
 
   const readOnly = !canEdit;
+  const hasChanges = content !== originalContent;
+
+  // Stash the latest callback in a ref so the unmount-cleanup effect can be
+  // truly unmount-scoped without re-running every time a parent passes a fresh
+  // function identity.
+  const onDirtyChangeRef = useRef(onDirtyChange);
+  useEffect(() => {
+    onDirtyChangeRef.current = onDirtyChange;
+  }, [onDirtyChange]);
+
+  useEffect(() => {
+    onDirtyChangeRef.current?.(hasChanges);
+  }, [hasChanges]);
+
+  useEffect(() => {
+    return () => onDirtyChangeRef.current?.(false);
+  }, []);
 
   const editorOptions = useMemo(
     () => ({
@@ -228,8 +247,6 @@ export function FileViewer({
     );
   }
 
-  const hasChanges = content !== originalContent;
-
   return (
     <div className="flex flex-col h-full">
       <div className="flex items-center justify-between gap-2 px-3 py-1.5 border-b border-glass-border shrink-0">
diff --git a/frontend/src/components/files/StackFileExplorer.tsx b/frontend/src/components/files/StackFileExplorer.tsx
index dce8b45c..3205a84e 100644
--- a/frontend/src/components/files/StackFileExplorer.tsx
+++ b/frontend/src/components/files/StackFileExplorer.tsx
@@ -1,6 +1,7 @@
 import { useState, useEffect, useCallback } from 'react';
 import { Trash2, FolderPlus, Download, Loader2 } from 'lucide-react';
 import { Button } from '@/components/ui/button';
+import { ConfirmModal } from '@/components/ui/modal';
 import { toast } from '@/components/ui/toast-store';
 import { downloadStackFile, listStackDirectory } from '@/lib/stackFilesApi';
 import { FileTree } from './FileTree';
@@ -60,15 +61,21 @@ export function StackFileExplorer({
   const [permissionsRelPath, setPermissionsRelPath] = useState('');
   const [permissionsEntryName, setPermissionsEntryName] = useState('');
 
+  // ── unsaved-changes guard on file switch ──
+  const [isViewerDirty, setIsViewerDirty] = useState(false);
+  const [pendingSelection, setPendingSelection] = useState<{ relPath: string; entry: FileEntry } | null>(null);
+
   useEffect(() => {
     setSelectedPath(null);
     setSelectedEntry(null);
     setCurrentDir('');
+    setIsViewerDirty(false);
+    setPendingSelection(null);
   }, [stackName]);
 
   const refresh = useCallback(() => setRefreshKey((k) => k + 1), []);
 
-  const handleSelectFile = useCallback((relPath: string, entry: FileEntry) => {
+  const applySelection = useCallback((relPath: string, entry: FileEntry) => {
     setSelectedPath(relPath);
     setSelectedEntry(entry);
     const parts = relPath.split('/');
@@ -76,6 +83,15 @@ export function StackFileExplorer({
     setCurrentDir(parts.join('/'));
   }, []);
 
+  const handleSelectFile = useCallback((relPath: string, entry: FileEntry) => {
+    if (relPath === selectedPath) return;
+    if (isViewerDirty) {
+      setPendingSelection({ relPath, entry });
+      return;
+    }
+    applySelection(relPath, entry);
+  }, [selectedPath, isViewerDirty, applySelection]);
+
   const handleDeleted = useCallback(() => {
     setSelectedPath(null);
     setSelectedEntry(null);
@@ -226,6 +242,7 @@ export function StackFileExplorer({
             canEdit={canEdit}
             isDarkMode={isDarkMode}
             onSaved={refresh}
+            onDirtyChange={setIsViewerDirty}
           />
         </div>
       </div>
@@ -298,6 +315,27 @@ export function StackFileExplorer({
         entryName={permissionsEntryName}
         canEdit={canEdit}
       />
+
+      {/* Unsaved-changes guard on file switch */}
+      <ConfirmModal
+        open={pendingSelection !== null}
+        onOpenChange={(next) => { if (!next) setPendingSelection(null); }}
+        onCancel={() => setPendingSelection(null)}
+        kicker="FILES · UNSAVED CHANGES"
+        title="Discard unsaved changes?"
+        description="Switching files will discard the edits in the current viewer."
+        confirmLabel="Discard and switch"
+        onConfirm={() => {
+          if (pendingSelection) {
+            applySelection(pendingSelection.relPath, pendingSelection.entry);
+            setPendingSelection(null);
+          }
+        }}
+      >
+        <p className="text-sm text-muted-foreground">
+          You have unsaved changes in the current file. Switching to another file will discard them.
+        </p>
+      </ConfirmModal>
     </div>
   );
 }
diff --git a/frontend/src/components/files/__tests__/FileViewer.test.tsx b/frontend/src/components/files/__tests__/FileViewer.test.tsx
index ef2a51d9..4c77116f 100644
--- a/frontend/src/components/files/__tests__/FileViewer.test.tsx
+++ b/frontend/src/components/files/__tests__/FileViewer.test.tsx
@@ -164,4 +164,28 @@ describe('FileViewer', () => {
     await waitFor(() => expect(mockReadFile).toHaveBeenCalledTimes(2));
     expect(mockReadFile).toHaveBeenNthCalledWith(2, 'my-stack', 'b.txt');
   });
+
+  it('reports clean dirty state on initial load of a text file', async () => {
+    mockReadFile.mockResolvedValue(textResult());
+    const onDirtyChange = vi.fn();
+
+    render(<FileViewer {...defaultProps} selectedPath="config/app.txt" onDirtyChange={onDirtyChange} />);
+
+    await waitFor(() => expect(screen.getByTestId('monaco-editor')).toBeInTheDocument());
+    // content === originalContent immediately after load → dirty=false
+    expect(onDirtyChange).toHaveBeenLastCalledWith(false);
+  });
+
+  it('resets dirty signal on unmount', async () => {
+    mockReadFile.mockResolvedValue(textResult());
+    const onDirtyChange = vi.fn();
+
+    const { unmount } = render(<FileViewer {...defaultProps} selectedPath="a.txt" onDirtyChange={onDirtyChange} />);
+    await waitFor(() => expect(screen.getByTestId('monaco-editor')).toBeInTheDocument());
+
+    onDirtyChange.mockClear();
+    unmount();
+
+    expect(onDirtyChange).toHaveBeenCalledWith(false);
+  });
 });
diff --git a/frontend/src/components/files/__tests__/StackFileExplorer.test.tsx b/frontend/src/components/files/__tests__/StackFileExplorer.test.tsx
new file mode 100644
index 00000000..11cb6ef9
--- /dev/null
+++ b/frontend/src/components/files/__tests__/StackFileExplorer.test.tsx
@@ -0,0 +1,126 @@
+/**
+ * Coverage for StackFileExplorer's unsaved-changes interception.
+ *
+ * The viewer reports its dirty state up via onDirtyChange. When the user
+ * clicks a sibling file in the tree, the explorer must intercept the switch
+ * and show a confirm dialog if there are unsaved edits. The viewer mock
+ * exposes a "Mark dirty" button so the test can drive the dirty signal
+ * without instantiating Monaco.
+ */
+import { describe, it, expect, vi } from 'vitest';
+import { render, screen } from '@testing-library/react';
+import userEvent from '@testing-library/user-event';
+import type { FileEntry } from '@/lib/stackFilesApi';
+
+vi.mock('@/lib/stackFilesApi', () => ({
+  listStackDirectory: vi.fn().mockResolvedValue([]),
+  downloadStackFile: vi.fn(),
+  readStackFile: vi.fn(),
+  writeStackFile: vi.fn(),
+}));
+
+vi.mock('@/components/ui/toast-store', () => ({
+  toast: { error: vi.fn(), success: vi.fn(), loading: vi.fn(() => 'id'), dismiss: vi.fn() },
+}));
+
+vi.mock('../FileUploadDropzone', () => ({
+  FileUploadDropzone: () => <div data-testid="upload-dropzone" />,
+}));
+
+vi.mock('../NewFolderDialog', () => ({ NewFolderDialog: () => null }));
+vi.mock('../NewFileDialog', () => ({ NewFileDialog: () => null }));
+vi.mock('../DeleteFileConfirm', () => ({ DeleteFileConfirm: () => null }));
+vi.mock('../RenameDialog', () => ({ RenameDialog: () => null }));
+vi.mock('../FilePermissionsDialog', () => ({ FilePermissionsDialog: () => null }));
+
+// FileTree mock exposes two buttons that synthesise selection of two siblings.
+vi.mock('../FileTree', () => ({
+  FileTree: ({ onSelectFile }: { onSelectFile: (rel: string, entry: FileEntry) => void }) => (
+    <div>
+      <button onClick={() => onSelectFile('a.txt', { name: 'a.txt', type: 'file', size: 1, mtime: 0, isProtected: false })}>
+        select-a
+      </button>
+      <button onClick={() => onSelectFile('b.txt', { name: 'b.txt', type: 'file', size: 1, mtime: 0, isProtected: false })}>
+        select-b
+      </button>
+    </div>
+  ),
+}));
+
+// FileViewer mock exposes a button that flips its dirty signal.
+vi.mock('../FileViewer', () => ({
+  FileViewer: ({ selectedPath, onDirtyChange }: {
+    selectedPath: string | null;
+    onDirtyChange?: (dirty: boolean) => void;
+  }) => (
+    <div>
+      <div data-testid="viewer-selected">{selectedPath ?? '(none)'}</div>
+      <button onClick={() => onDirtyChange?.(true)}>mark-dirty</button>
+      <button onClick={() => onDirtyChange?.(false)}>mark-clean</button>
+    </div>
+  ),
+}));
+
+import { StackFileExplorer } from '../StackFileExplorer';
+
+function setup() {
+  return render(
+    <StackFileExplorer stackName="my-stack" canEdit isDarkMode={false} />,
+  );
+}
+
+describe('StackFileExplorer unsaved-changes interception', () => {
+  it('switches files immediately when the viewer is clean', async () => {
+    const user = userEvent.setup();
+    setup();
+
+    await user.click(screen.getByText('select-a'));
+    expect(screen.getByTestId('viewer-selected').textContent).toBe('a.txt');
+
+    await user.click(screen.getByText('select-b'));
+    expect(screen.getByTestId('viewer-selected').textContent).toBe('b.txt');
+    expect(screen.queryByText(/discard unsaved changes/i)).not.toBeInTheDocument();
+  });
+
+  it('intercepts the switch when the viewer is dirty and applies on confirm', async () => {
+    const user = userEvent.setup();
+    setup();
+
+    await user.click(screen.getByText('select-a'));
+    await user.click(screen.getByText('mark-dirty'));
+
+    // Switching siblings while dirty must NOT swap the selection immediately.
+    await user.click(screen.getByText('select-b'));
+    expect(screen.getByTestId('viewer-selected').textContent).toBe('a.txt');
+    expect(screen.getByText(/discard unsaved changes/i)).toBeInTheDocument();
+
+    // Confirming the dialog applies the pending selection.
+    await user.click(screen.getByRole('button', { name: /discard and switch/i }));
+    expect(screen.getByTestId('viewer-selected').textContent).toBe('b.txt');
+  });
+
+  it('intercepts the switch when dirty and preserves the original on cancel', async () => {
+    const user = userEvent.setup();
+    setup();
+
+    await user.click(screen.getByText('select-a'));
+    await user.click(screen.getByText('mark-dirty'));
+
+    await user.click(screen.getByText('select-b'));
+    expect(screen.getByText(/discard unsaved changes/i)).toBeInTheDocument();
+
+    await user.click(screen.getByRole('button', { name: /^cancel$/i }));
+    expect(screen.getByTestId('viewer-selected').textContent).toBe('a.txt');
+  });
+
+  it('clicking the already-selected file does not prompt even when dirty', async () => {
+    const user = userEvent.setup();
+    setup();
+
+    await user.click(screen.getByText('select-a'));
+    await user.click(screen.getByText('mark-dirty'));
+
+    await user.click(screen.getByText('select-a'));
+    expect(screen.queryByText(/discard unsaved changes/i)).not.toBeInTheDocument();
+  });
+});