diff --git a/src/features/AgentDocumentsExplorer/DocumentExplorerTree.tsx b/src/features/AgentDocumentsExplorer/DocumentExplorerTree.tsx
index 15771585f1..adafeab90a 100644
--- a/src/features/AgentDocumentsExplorer/DocumentExplorerTree.tsx
+++ b/src/features/AgentDocumentsExplorer/DocumentExplorerTree.tsx
@@ -21,6 +21,23 @@ import { isOrphanSkillBundleItem } from './types';
 import { canDropDocument } from './utils/canDrop';
 
 const SKILL_INDEX_FILENAME = 'SKILL.md';
+const FILE_TREE_HOST_TAG = 'file-tree-container';
+const RENAME_INPUT_SELECTOR = 'input[data-item-rename-input]';
+
+// pierre/trees auto-selects the full value when the rename input mounts. For
+// files with extensions (e.g. `Untitled document.md`), narrow the selection to
+// the stem so the user can type a new name without overwriting the suffix.
+const selectStemOfActiveRenameInput = (root: HTMLElement | null) => {
+  if (!root) return;
+  const host = root.querySelector(FILE_TREE_HOST_TAG);
+  const input = host?.shadowRoot?.querySelector<HTMLInputElement>(RENAME_INPUT_SELECTOR);
+  if (!input) return;
+  const value = input.value;
+  const dotIndex = value.lastIndexOf('.');
+  // Skip dotfiles and extension-less names — leave pierre's full-selection.
+  if (dotIndex <= 0) return;
+  input.setSelectionRange(0, dotIndex);
+};
 
 const styles = createStaticStyles(({ css, cssVar }) => ({
   tree: css`
@@ -53,9 +70,13 @@ const DocumentExplorerTree = memo<Props>(({ agentId, data, mutate, style }) => {
   const { t } = useTranslation(['chat', 'common']);
   const openDocument = useChatStore((s) => s.openDocument);
   const treeRef = useRef<ExplorerTreeHandle | null>(null);
+  const containerRef = useRef<HTMLDivElement | null>(null);
 
   const startInlineRename = useCallback((id: string) => {
     treeRef.current?.startRenaming(id);
+    // Match the new-file flow: leave the extension out of the selection so
+    // the user can retype only the stem.
+    requestAnimationFrame(() => selectStemOfActiveRenameInput(containerRef.current));
   }, []);
 
   const ops = useDocumentTreeOps({ agentId, data, mutate });
@@ -113,7 +134,12 @@ const DocumentExplorerTree = memo<Props>(({ agentId, data, mutate, style }) => {
   const focusNewRowForRename = useCallback((pendingId: string) => {
     // Defer past the current task so React commits the inserted row and the
     // tree adapter rebuilds its id→path map before we trigger rename.
-    setTimeout(() => treeRef.current?.startRenaming(pendingId), 0);
+    setTimeout(() => {
+      treeRef.current?.startRenaming(pendingId);
+      // After pierre's input.select() runs in its own layout effect, narrow
+      // selection to the stem so the `.md` extension stays intact.
+      requestAnimationFrame(() => selectStemOfActiveRenameInput(containerRef.current));
+    }, 0);
   }, []);
 
   const handleCreateFolder = useCallback(
@@ -233,7 +259,7 @@ const DocumentExplorerTree = memo<Props>(({ agentId, data, mutate, style }) => {
   );
 
   return (
-    <div className={styles.tree} style={{ ...style, ...treeStyleVars }}>
+    <div className={styles.tree} ref={containerRef} style={{ ...style, ...treeStyleVars }}>
       <ExplorerTree<AgentDocumentItem>
         iconsColored
         canDrag={canDrag}
diff --git a/src/features/AgentDocumentsExplorer/hooks/useDocumentTreeOps.ts b/src/features/AgentDocumentsExplorer/hooks/useDocumentTreeOps.ts
index 8778f0fba1..84159cae4d 100644
--- a/src/features/AgentDocumentsExplorer/hooks/useDocumentTreeOps.ts
+++ b/src/features/AgentDocumentsExplorer/hooks/useDocumentTreeOps.ts
@@ -18,6 +18,7 @@ interface UseDocumentTreeOpsArgs {
 }
 
 const ROOT_PATH = './';
+const DEFAULT_DOCUMENT_EXTENSION = '.md';
 
 const joinPath = (parentPath: string, segment: string) =>
   parentPath === ROOT_PATH ? `${ROOT_PATH}${segment}` : `${parentPath}/${segment}`;
@@ -112,17 +113,20 @@ export const useDocumentTreeOps = ({
   // Picks a unique filename within the given parent. Used for client-side
   // dedup of "Untitled" rows because the path-based VFS mkdir is idempotent
   // (same name re-uses the existing folder), and writeByPath in always-new
-  // mode rejects collisions outright.
+  // mode rejects collisions outright. When an extension is provided, the
+  // numeric suffix is inserted before the extension so the file keeps its
+  // type (e.g. `Untitled document 2.md`).
   const pickUniqueFilename = useCallback(
-    (parentDocumentId: string | null, baseName: string): string => {
+    (parentDocumentId: string | null, baseStem: string, extension = ''): string => {
       const siblings = dataRef.current.filter((doc) => (doc.parentId ?? null) === parentDocumentId);
       const taken = new Set(siblings.map((doc) => doc.filename));
-      if (!taken.has(baseName)) return baseName;
+      const first = `${baseStem}${extension}`;
+      if (!taken.has(first)) return first;
       for (let i = 2; i < 1000; i += 1) {
-        const candidate = `${baseName} ${i}`;
+        const candidate = `${baseStem} ${i}${extension}`;
         if (!taken.has(candidate)) return candidate;
       }
-      return `${baseName} ${Date.now()}`;
+      return `${baseStem} ${Date.now()}${extension}`;
     },
     [],
   );
@@ -183,9 +187,13 @@ export const useDocumentTreeOps = ({
         return;
       }
 
-      const baseName = t('workingPanel.resources.tree.untitledDocument');
+      const baseStem = t('workingPanel.resources.tree.untitledDocument');
       const parentDocumentId = parentId ? (byRowId.get(parentId)?.documentId ?? null) : null;
-      const pendingFilename = pickUniqueFilename(parentDocumentId, baseName);
+      const pendingFilename = pickUniqueFilename(
+        parentDocumentId,
+        baseStem,
+        DEFAULT_DOCUMENT_EXTENSION,
+      );
 
       const pending = makePendingDocument({
         agentId,
@@ -200,11 +208,13 @@ export const useDocumentTreeOps = ({
         try {
           const result =
             parentPath === ROOT_PATH
-              ? // Server's createDocument auto-deduplicates filenames at the root.
+              ? // Pass the client-picked filename (with extension) as the title so
+                // the server keeps the `.md` suffix; server dedup remains a safety
+                // net for racing clients.
                 await agentDocumentService.createDocument({
                   agentId,
                   content: '',
-                  title: baseName,
+                  title: pendingFilename,
                 })
               : await agentDocumentService.writeByPath({
                   agentId,
diff --git a/src/server/services/agentDocuments/index.test.ts b/src/server/services/agentDocuments/index.test.ts
index c767ffb9f5..e2ac9a24de 100644
--- a/src/server/services/agentDocuments/index.test.ts
+++ b/src/server/services/agentDocuments/index.test.ts
@@ -151,6 +151,37 @@ describe('AgentDocumentsService', () => {
       expect(result).toEqual({ id: 'new-doc', filename: 'note-2' });
     });
 
+    it('should append collision suffix before the filename extension', async () => {
+      mockModel.findByFilename
+        .mockResolvedValueOnce({ id: 'existing-doc' })
+        .mockResolvedValueOnce(undefined);
+      mockModel.create.mockResolvedValue({ id: 'new-doc', filename: 'Untitled document-2.md' });
+
+      const service = new AgentDocumentsService(db, userId);
+      const result = await service.createDocument('agent-1', 'Untitled document.md', 'content');
+
+      expect(mockModel.findByFilename).toHaveBeenNthCalledWith(
+        1,
+        'agent-1',
+        'Untitled document.md',
+      );
+      expect(mockModel.findByFilename).toHaveBeenNthCalledWith(
+        2,
+        'agent-1',
+        'Untitled document-2.md',
+      );
+      expect(mockModel.create).toHaveBeenCalledWith(
+        'agent-1',
+        'Untitled document-2.md',
+        'content',
+        {
+          editorData: { root: { children: [] } },
+          title: 'Untitled document.md',
+        },
+      );
+      expect(result).toEqual({ id: 'new-doc', filename: 'Untitled document-2.md' });
+    });
+
     it('should throw after too many filename collisions', async () => {
       mockModel.findByFilename.mockResolvedValue({ id: 'existing-doc' });
 
diff --git a/src/server/services/agentDocuments/index.ts b/src/server/services/agentDocuments/index.ts
index 25ff5f2644..279130a560 100644
--- a/src/server/services/agentDocuments/index.ts
+++ b/src/server/services/agentDocuments/index.ts
@@ -36,6 +36,14 @@ import {
 
 const MAX_UNIQUE_FILENAME_ATTEMPTS = 1000;
 
+const appendFilenameSuffix = (filename: string, suffix: number): string => {
+  const dotIndex = filename.lastIndexOf('.');
+
+  if (dotIndex <= 0) return `${filename}-${suffix}`;
+
+  return `${filename.slice(0, dotIndex)}-${suffix}${filename.slice(dotIndex)}`;
+};
+
 interface UpsertDocumentParams {
   agentId: string;
   content: string;
@@ -189,7 +197,7 @@ export class AgentDocumentsService {
         );
       }
 
-      filename = `${baseFilename}-${suffix}`;
+      filename = appendFilenameSuffix(baseFilename, suffix);
       suffix += 1;
     }