diff --git a/src/app/(dashboard)/inbox/page.tsx b/src/app/(dashboard)/inbox/page.tsx
index 16ee965..7ec3895 100644
--- a/src/app/(dashboard)/inbox/page.tsx
+++ b/src/app/(dashboard)/inbox/page.tsx
@@ -12,6 +12,10 @@ import { toast } from "sonner";
 import { WifiOff } from "lucide-react";
 import { cn } from "@/lib/utils";
 
+// Remembers the agent's show/hide choice for the desktop contact panel
+// across reloads and sessions (device-scoped, like the theme prefs).
+const CONTACT_PANEL_STORAGE_KEY = "wacrm:inbox:contact-panel-open";
+
 export default function InboxPage() {
   const router = useRouter();
   const searchParams = useSearchParams();
@@ -39,6 +43,36 @@ export default function InboxPage() {
    */
   const [resyncToken, setResyncToken] = useState(0);
 
+  /**
+   * Whether the desktop contact sidebar (tags / deals / notes) is shown.
+   * Defaults to `true` (the historical behaviour) and is restored from
+   * localStorage after mount. We deliberately do NOT read localStorage in
+   * the initializer: the server renders with `true`, so reading a stored
+   * `false` synchronously would produce a hydration mismatch. The effect
+   * below reconciles to the stored value right after mount instead.
+   */
+  const [contactPanelOpen, setContactPanelOpen] = useState(true);
+  useEffect(() => {
+    try {
+      const stored = localStorage.getItem(CONTACT_PANEL_STORAGE_KEY);
+      if (stored !== null) setContactPanelOpen(stored === "true");
+    } catch {
+      // localStorage can throw in private-browsing / sandboxed contexts.
+    }
+  }, []);
+
+  const handleToggleContactPanel = useCallback(() => {
+    setContactPanelOpen((prev) => {
+      const next = !prev;
+      try {
+        localStorage.setItem(CONTACT_PANEL_STORAGE_KEY, String(next));
+      } catch {
+        // Persistence is best-effort; ignore storage failures.
+      }
+      return next;
+    });
+  }, []);
+
   // Fire the deep-link auto-select exactly once per URL — subsequent
   // list refreshes (realtime, manual refetch) must not snap the user
   // back to the deep-linked conversation if they've already clicked
@@ -570,13 +604,20 @@ export default function InboxPage() {
             onBack={handleCloseConversation}
             resyncToken={resyncToken}
             onRefresh={handleManualRefresh}
+            contactPanelOpen={contactPanelOpen}
+            onToggleContactPanel={handleToggleContactPanel}
           />
         </div>
 
-        {/* Right panel: Contact sidebar — desktop only. */}
-        <div className="hidden lg:block">
-          <ContactSidebar contact={activeContact} />
-        </div>
+        {/* Right panel: Contact sidebar — desktop only, and only when the
+            agent hasn't collapsed it via the thread-header toggle (#258).
+            On mobile it's always hidden (the `lg:block` below), so the
+            toggle — which is itself desktop-only — never affects it. */}
+        {contactPanelOpen && (
+          <div className="hidden lg:block">
+            <ContactSidebar contact={activeContact} />
+          </div>
+        )}
       </div>
     </div>
   );
diff --git a/src/components/inbox/message-thread.tsx b/src/components/inbox/message-thread.tsx
index 46053f5..770c2d4 100644
--- a/src/components/inbox/message-thread.tsx
+++ b/src/components/inbox/message-thread.tsx
@@ -21,6 +21,8 @@ import {
   Clock,
   ArrowLeft,
   RefreshCw,
+  PanelRightOpen,
+  PanelRightClose,
 } from "lucide-react";
 import { format, isToday, isYesterday, differenceInHours } from "date-fns";
 import { Badge } from "@/components/ui/badge";
@@ -92,6 +94,15 @@ interface MessageThreadProps {
    * working; the button is only rendered when this is provided.
    */
   onRefresh?: () => void;
+  /**
+   * Desktop-only contact-panel toggle. The page owns the open/closed
+   * state (it's the one that renders the sidebar), so the thread just
+   * reflects it and asks the page to flip it. Both optional so existing
+   * callers keep working; the toggle button only renders when
+   * `onToggleContactPanel` is wired up.
+   */
+  contactPanelOpen?: boolean;
+  onToggleContactPanel?: () => void;
 }
 
 function formatDateSeparator(dateStr: string): string {
@@ -148,6 +159,8 @@ export function MessageThread({
   onBack,
   resyncToken = 0,
   onRefresh,
+  contactPanelOpen,
+  onToggleContactPanel,
 }: MessageThreadProps) {
   const { user } = useAuth();
   const [loading, setLoading] = useState(false);
@@ -839,6 +852,33 @@ export function MessageThread({
         </div>
 
         <div className="flex items-center gap-2">
+          {/* Contact-panel toggle — desktop only. The contact sidebar
+              eats a chunk of horizontal width that crowds the thread on
+              smaller laptops; this lets agents reclaim it when they just
+              want to read and reply. Hidden on mobile, where the sidebar
+              never renders as a permanent panel anyway. Issue #258. */}
+          {onToggleContactPanel && (
+            <button
+              type="button"
+              onClick={onToggleContactPanel}
+              aria-label={
+                contactPanelOpen ? "Hide contact panel" : "Show contact panel"
+              }
+              aria-pressed={contactPanelOpen}
+              title={contactPanelOpen ? "Hide contact" : "Show contact"}
+              className={cn(
+                "hidden h-7 w-7 items-center justify-center rounded-md transition-colors hover:bg-muted hover:text-foreground lg:inline-flex",
+                contactPanelOpen ? "text-primary" : "text-muted-foreground",
+              )}
+            >
+              {contactPanelOpen ? (
+                <PanelRightClose className="h-4 w-4" />
+              ) : (
+                <PanelRightOpen className="h-4 w-4" />
+              )}
+            </button>
+          )}
+
           {/* Manual refresh — forces a refetch of the messages + the
               conversation list (the parent bumps its resyncToken). Useful
               when realtime missed an event or the agent just wants to be