diff --git a/docs/solutions/ui-bugs/tablet-keyboard-viewport-mode-flip.md b/docs/solutions/ui-bugs/tablet-keyboard-viewport-mode-flip.md
index c2034fc565..e1b80a123d 100644
--- a/docs/solutions/ui-bugs/tablet-keyboard-viewport-mode-flip.md
+++ b/docs/solutions/ui-bugs/tablet-keyboard-viewport-mode-flip.md
@@ -42,6 +42,8 @@ Keep the exported `MOBILE_MEDIA_QUERY` string unchanged for listener compatibili
 
 This preserves landscape-phone behavior while preventing keyboard-driven height shrink from changing tablet/desktop viewport mode. Direct breakpoint consumers (`Board`, `WorkflowNodeEditor`, and `SessionTerminal`) should subscribe to `MOBILE_MEDIA_QUERY` for reactivity but recompute state with `isMobileViewport()` rather than reading `.matches` as the final decision.
 
+ChatView also keeps a defense-in-depth CSS guard from FN-6210: `.chat-sidebar` has a non-mobile `max-width` matching `CHAT_SIDEBAR_MAX_WIDTH`, with the mobile media rule overriding it back to `100%`. That guard bounds the sidebar even if viewport-mode state is temporarily wrong and the inline sidebar width is removed.
+
 ## Regression coverage
 
 Cover the invariant rather than the single repro:
@@ -52,3 +54,4 @@ Cover the invariant rather than the single repro:
 - Portrait phone width stays `mobile` regardless of height.
 - Undefined/zero `window.screen` does not throw and falls back to width-only detection.
 - Component-local mobile hooks such as `SessionTerminal` also use the guarded predicate.
+- ChatView sidebar CSS remains bounded at the sidebar max width during simulated viewport-mode flicker while the keyboard is open.
diff --git a/packages/dashboard/app/components/ChatView.css b/packages/dashboard/app/components/ChatView.css
index 558488c7c5..f2d97623ef 100644
--- a/packages/dashboard/app/components/ChatView.css
+++ b/packages/dashboard/app/components/ChatView.css
@@ -14,6 +14,7 @@
 /* Sidebar */
 .chat-sidebar {
   min-width: 0;
+  max-width: 500px; /* FN-6210: defensive cap — matches CHAT_SIDEBAR_MAX_WIDTH */
   border-right: 1px solid var(--border);
   display: flex;
   flex-direction: column;
@@ -1725,6 +1726,7 @@
   .chat-sidebar {
     width: 100%;
     min-width: 100%;
+    max-width: 100%; /* FN-6210: mobile sidebar spans full viewport */
     height: 100%;
     max-height: none;
     border-right: none;
diff --git a/packages/dashboard/app/components/__tests__/ChatView.mobile-render.test.tsx b/packages/dashboard/app/components/__tests__/ChatView.mobile-render.test.tsx
index d5e60b10c7..638c9de5b7 100644
--- a/packages/dashboard/app/components/__tests__/ChatView.mobile-render.test.tsx
+++ b/packages/dashboard/app/components/__tests__/ChatView.mobile-render.test.tsx
@@ -386,6 +386,59 @@ describe("FN-5997 mobile chat message pane rendering", () => {
     }
   });
 
+  it("keeps sidebar width bounded even if viewport mode flickers to mobile during keyboard-open on tablet", async () => {
+    const restoreMatchMedia = mockViewportMode("tablet");
+    const originalScreenDescriptor = Object.getOwnPropertyDescriptor(window, "screen");
+    const visualViewport = mockVisualViewport({ width: 900, height: 1112 });
+    try {
+      setupChat({
+        sessions: [activeSession],
+        filteredSessions: [activeSession],
+        activeSession,
+      });
+      await renderWithCss(<ChatView projectId="proj-123" addToast={vi.fn()} />);
+
+      const sidebar = getSidebar();
+      expect(sidebar).not.toHaveClass("chat-sidebar--hidden");
+      expect(sidebar.style.width).toBe("280px");
+
+      const input = screen.getByTestId("chat-input") as HTMLTextAreaElement;
+      await act(async () => {
+        input.focus();
+      });
+      await setVisualViewportHeight(visualViewport, 400);
+
+      // Simulate the FN-6213 bug scenario where viewport mode transiently
+      // resolves to mobile on a tablet while the keyboard has shrunk height.
+      Object.defineProperty(window, "screen", { configurable: true, value: { width: 390, height: 844 } });
+      restoreMatchMedia.mockImplementation((query: string) => ({
+        matches:
+          query.includes("max-width: 768px") ||
+          query.includes("max-height: 480px"),
+        media: query,
+        onchange: null,
+        addListener: vi.fn(),
+        removeListener: vi.fn(),
+        addEventListener: vi.fn(),
+        removeEventListener: vi.fn(),
+        dispatchEvent: vi.fn(),
+      }));
+      await act(async () => {
+        window.dispatchEvent(new Event("resize"));
+      });
+
+      await waitFor(() => expect(sidebar.style.width).toBe(""));
+      const maxWidth = parseInt(getComputedStyle(sidebar).maxWidth, 10);
+      expect(maxWidth).toBeLessThanOrEqual(500);
+      expect(sidebar.offsetWidth).toBeLessThanOrEqual(500);
+    } finally {
+      restoreMatchMedia.mockRestore();
+      if (originalScreenDescriptor) {
+        Object.defineProperty(window, "screen", originalScreenDescriptor);
+      }
+    }
+  });
+
   it("keeps the desktop sidebar fixed even if visualViewport shrinks while the composer is focused", async () => {
     const restoreMatchMedia = mockViewportMode("desktop");
     const visualViewport = mockVisualViewport({ width: 1280, height: 900 });