diff --git a/artifacts/api-server/src/routes/notes.ts b/artifacts/api-server/src/routes/notes.ts
index 2396faf7..2d9f99e4 100644
--- a/artifacts/api-server/src/routes/notes.ts
+++ b/artifacts/api-server/src/routes/notes.ts
@@ -303,6 +303,45 @@ async function handleListMyNotes(
   res: import("express").Response,
 ): Promise<void> {
   const userId = req.session.userId!;
+  // ?sharedFolderId=:id — read-only access to the live notes inside a
+  // folder shared WITH the caller. Spec alias for the same data the
+  // dedicated `/note-folders/:id/shared-notes` endpoint returns; kept
+  // here so clients that prefer the unified `/notes?…` shape work too.
+  const sharedFolderIdRaw = req.query.sharedFolderId;
+  if (typeof sharedFolderIdRaw === "string" && sharedFolderIdRaw.length > 0) {
+    const parsed = Number(sharedFolderIdRaw);
+    if (!Number.isInteger(parsed) || parsed <= 0) {
+      res.status(400).json({ error: "Invalid sharedFolderId" });
+      return;
+    }
+    const [share] = await db
+      .select({ folderId: noteFolderSharesTable.folderId })
+      .from(noteFolderSharesTable)
+      .where(
+        and(
+          eq(noteFolderSharesTable.folderId, parsed),
+          eq(noteFolderSharesTable.recipientUserId, userId),
+        ),
+      );
+    if (!share) {
+      res.status(403).json({ error: "Folder is not shared with you" });
+      return;
+    }
+    const [folder] = await db
+      .select({ ownerId: noteFoldersTable.userId })
+      .from(noteFoldersTable)
+      .where(eq(noteFoldersTable.id, parsed));
+    if (!folder) {
+      res.status(404).json({ error: "Folder not found" });
+      return;
+    }
+    const ownerNotes = await loadNotesWithLabels(folder.ownerId, false);
+    const filtered = ownerNotes
+      .filter((n) => n.folderId === parsed)
+      .map((n) => ({ ...n, readOnly: true as const }));
+    res.json(filtered);
+    return;
+  }
   const archived = req.query.archived === "true";
   const notes = await loadNotesWithLabels(userId, archived);
   res.json(notes);
@@ -357,14 +396,21 @@ router.patch("/notes/:id", requireAuth, async (req, res): Promise<void> => {
     return;
   }
   const { labelIds, ...data } = parsed.data;
+  // Two-step lookup so non-owners get a precise 403 (instead of an
+  // ambiguous 404). Recipients of a shared folder must never be able to
+  // mutate the owner's notes — see Task #445 acceptance criteria.
   const [existing] = await db
     .select()
     .from(notesTable)
-    .where(and(eq(notesTable.id, id.id), eq(notesTable.userId, userId)));
+    .where(eq(notesTable.id, id.id));
   if (!existing) {
     res.status(404).json({ error: "Note not found" });
     return;
   }
+  if (existing.userId !== userId) {
+    res.status(403).json({ error: "Forbidden" });
+    return;
+  }
   if (data.folderId != null && !(await userOwnsFolder(userId, data.folderId))) {
     res.status(400).json({ error: "Invalid folderId" });
     return;
@@ -395,14 +441,21 @@ router.delete("/notes/:id", requireAuth, async (req, res): Promise<void> => {
     res.status(400).json({ error: id.error });
     return;
   }
-  const result = await db
-    .delete(notesTable)
-    .where(and(eq(notesTable.id, id.id), eq(notesTable.userId, userId)))
-    .returning();
-  if (result.length === 0) {
+  // Two-step lookup so non-owners (e.g. shared-folder recipients) get
+  // a precise 403 instead of a 404 — see Task #445 acceptance criteria.
+  const [existing] = await db
+    .select({ userId: notesTable.userId })
+    .from(notesTable)
+    .where(eq(notesTable.id, id.id));
+  if (!existing) {
     res.status(404).json({ error: "Note not found" });
     return;
   }
+  if (existing.userId !== userId) {
+    res.status(403).json({ error: "Forbidden" });
+    return;
+  }
+  await db.delete(notesTable).where(eq(notesTable.id, id.id));
   res.json({ success: true });
 });
 
@@ -1278,9 +1331,11 @@ router.delete("/note-folders/:id", requireAuth, async (req, res): Promise<void>
 //
 // A folder owner can share a folder read-only with other users. Recipients see
 // the owner's CURRENT notes inside the folder (live, not snapshot). They cannot
-// edit, add, move, or delete anything; the existing per-note routes already
-// filter by `notesTable.userId = req.session.userId`, so any write attempt by a
-// recipient just 404s — no extra gating required there.
+// edit, add, move, or delete anything; the per-note write routes
+// (`PATCH/DELETE /notes/:id`, archive, checklist toggle for non-recipients)
+// look up the row by id and explicitly return 403 when the caller is not
+// the owner, so a shared-folder recipient that probes a write endpoint
+// gets a clear authorization error instead of an ambiguous 404.
 
 router.get(
   "/note-folders/shared-with-me",
@@ -1296,6 +1351,15 @@ router.get(
         ownerUsername: usersTable.username,
         ownerDisplayNameAr: usersTable.displayNameAr,
         ownerDisplayNameEn: usersTable.displayNameEn,
+        // Live count of the owner's non-archived notes in this folder so
+        // the recipient's rail can show "Folder name (12)" without a
+        // second roundtrip per row.
+        noteCount: sql<number>`(
+          SELECT COUNT(*)::int FROM ${notesTable}
+          WHERE ${notesTable.folderId} = ${noteFoldersTable.id}
+            AND ${notesTable.userId} = ${noteFoldersTable.userId}
+            AND ${notesTable.isArchived} = false
+        )`.as("note_count"),
       })
       .from(noteFolderSharesTable)
       .innerJoin(
@@ -1361,7 +1425,17 @@ router.put(
     const desired = new Set<number>();
     for (const x of body.recipientUserIds) {
       const n = Number(x);
-      if (Number.isInteger(n) && n > 0 && n !== userId) desired.add(n);
+      if (!Number.isInteger(n) || n <= 0) {
+        res.status(400).json({ error: "Invalid recipientUserIds" });
+        return;
+      }
+      // Self-share is meaningless and the spec calls for an explicit
+      // 400 rather than silently dropping the id.
+      if (n === userId) {
+        res.status(400).json({ error: "Cannot share folder with yourself" });
+        return;
+      }
+      desired.add(n);
     }
     // Validate that every desired recipient actually exists.
     if (desired.size > 0) {
diff --git a/artifacts/tx-os/src/components/notes/folders-rail.tsx b/artifacts/tx-os/src/components/notes/folders-rail.tsx
index aeb71a02..7c4f9762 100644
--- a/artifacts/tx-os/src/components/notes/folders-rail.tsx
+++ b/artifacts/tx-os/src/components/notes/folders-rail.tsx
@@ -583,6 +583,14 @@ export function FoldersRail({
                       })}
                     </span>
                   </span>
+                  <span
+                    className={`shrink-0 text-[10px] tabular-nums ${
+                      isSel ? "text-background/80" : "text-muted-foreground"
+                    }`}
+                    data-testid={`shared-folder-count-${sf.id}`}
+                  >
+                    {sf.noteCount}
+                  </span>
                 </button>
               </div>
             );
diff --git a/artifacts/tx-os/src/lib/notes-api.ts b/artifacts/tx-os/src/lib/notes-api.ts
index 2c49e9c9..a7527997 100644
--- a/artifacts/tx-os/src/lib/notes-api.ts
+++ b/artifacts/tx-os/src/lib/notes-api.ts
@@ -58,6 +58,9 @@ export interface SharedFolder {
   ownerUsername: string;
   ownerDisplayNameAr: string | null;
   ownerDisplayNameEn: string | null;
+  // Live count of the owner's non-archived notes inside this folder. Lets
+  // the rail render "Folder name (12)" without a per-row roundtrip.
+  noteCount: number;
 }
 
 // One row in a folder's share list (people the owner has shared TO).
diff --git a/artifacts/tx-os/src/pages/notes.tsx b/artifacts/tx-os/src/pages/notes.tsx
index e5235318..22162ba4 100644
--- a/artifacts/tx-os/src/pages/notes.tsx
+++ b/artifacts/tx-os/src/pages/notes.tsx
@@ -478,6 +478,7 @@ export default function NotesPage() {
           <SharedFolderView
             folderId={folderSelection.id}
             labels={labels}
+            search={search}
             onBack={() => setFolderSelection({ kind: "all" })}
           />
         )}
@@ -2964,16 +2965,35 @@ function LabelsDialog({
 function SharedFolderView({
   folderId,
   labels: _labels,
+  search,
   onBack,
 }: {
   folderId: number;
   labels: NoteLabel[];
+  search: string;
   onBack: () => void;
 }) {
   const { t, i18n } = useTranslation();
   const lang = i18n.language;
   const { data, isLoading, error } = useSharedFolderNotes(folderId);
 
+  // Recipients use the same top-bar search input as their personal notes;
+  // filter the read-only list client-side over title + content + checklist
+  // item text so the experience matches the owner's view.
+  const visibleNotes = useMemo(() => {
+    if (!data) return [];
+    const q = search.trim().toLowerCase();
+    if (!q) return data.notes;
+    return data.notes.filter((n) => {
+      if (n.title?.toLowerCase().includes(q)) return true;
+      if (n.content?.toLowerCase().includes(q)) return true;
+      if (n.kind === "checklist" && Array.isArray(n.items)) {
+        return n.items.some((it) => it.text?.toLowerCase().includes(q));
+      }
+      return false;
+    });
+  }, [data, search]);
+
   if (isLoading) return <Loading />;
 
   // 403/404 — the owner unshared the folder while we were viewing it. Show a
@@ -3030,10 +3050,14 @@ function SharedFolderView({
           {t("notes.folders.readOnly", "Read-only")}
         </span>
       </div>
-      {data.notes.length === 0 ? (
+      {visibleNotes.length === 0 ? (
         <Empty
           icon={<StickyNote size={48} />}
-          text={t("notes.folders.sharedEmpty", "This folder has no notes yet")}
+          text={
+            search.trim()
+              ? t("notes.noMatches", "No notes match your search")
+              : t("notes.folders.sharedEmpty", "This folder has no notes yet")
+          }
         />
       ) : (
         <div
@@ -3041,7 +3065,7 @@ function SharedFolderView({
           style={{ columnWidth: 260 }}
           data-testid="shared-folder-notes"
         >
-          {data.notes.map((n) => (
+          {visibleNotes.map((n) => (
             <div
               key={n.id}
               data-testid={`shared-folder-note-${n.id}`}