diff --git a/apps/web/src/components/ChatView.tsx b/apps/web/src/components/ChatView.tsx
index 40cd1b421058..424fae3f3e5d 100644
--- a/apps/web/src/components/ChatView.tsx
+++ b/apps/web/src/components/ChatView.tsx
@@ -99,7 +99,10 @@ import { useTurnDiffSummaries } from "../hooks/useTurnDiffSummaries";
 import { useCommandPaletteStore } from "../commandPaletteStore";
 import { buildTemporaryWorktreeBranchName } from "@t3tools/shared/git";
 import { useMediaQuery } from "../hooks/useMediaQuery";
-import { RIGHT_PANEL_INLINE_LAYOUT_MEDIA_QUERY } from "../rightPanelLayout";
+import {
+  RIGHT_PANEL_INLINE_LAYOUT_MEDIA_QUERY,
+  RIGHT_PANEL_NARROW_SHEET_CLASS_NAME,
+} from "../rightPanelLayout";
 import { BranchToolbar } from "./BranchToolbar";
 import { resolveShortcutCommand, shortcutLabelForCommand } from "../keybindings";
 import PlanSidebar from "./PlanSidebar";
@@ -3532,7 +3535,11 @@ export default function ChatView(props: ChatViewProps) {
         />
       ))}
       {shouldUsePlanSidebarSheet ? (
-        <RightPanelSheet open={planSidebarOpen} onClose={closePlanSidebar}>
+        <RightPanelSheet
+          open={planSidebarOpen}
+          onClose={closePlanSidebar}
+          className={RIGHT_PANEL_NARROW_SHEET_CLASS_NAME}
+        >
           <PlanSidebar
             activePlan={activePlan}
             activeProposedPlan={sidebarProposedPlan}
diff --git a/apps/web/src/components/RightPanelSheet.tsx b/apps/web/src/components/RightPanelSheet.tsx
index ebc4aa0a698f..03fb063561d2 100644
--- a/apps/web/src/components/RightPanelSheet.tsx
+++ b/apps/web/src/components/RightPanelSheet.tsx
@@ -7,6 +7,12 @@ export function RightPanelSheet(props: {
   children: ReactNode;
   open: boolean;
   onClose: () => void;
+  /**
+   * Override the default sheet sizing classes. Use one of the named exports
+   * from `rightPanelLayout` (e.g. `RIGHT_PANEL_NARROW_SHEET_CLASS_NAME`) to
+   * pick a specific width preset; defaults to the wide variant.
+   */
+  className?: string;
 }) {
   return (
     <Sheet
@@ -21,7 +27,7 @@ export function RightPanelSheet(props: {
         side="right"
         showCloseButton={false}
         keepMounted
-        className={RIGHT_PANEL_SHEET_CLASS_NAME}
+        className={props.className ?? RIGHT_PANEL_SHEET_CLASS_NAME}
       >
         {props.children}
       </SheetPopup>
diff --git a/apps/web/src/rightPanelLayout.ts b/apps/web/src/rightPanelLayout.ts
index 5528072e57b3..0ea6c19c525b 100644
--- a/apps/web/src/rightPanelLayout.ts
+++ b/apps/web/src/rightPanelLayout.ts
@@ -1,3 +1,17 @@
 export const RIGHT_PANEL_INLINE_LAYOUT_MEDIA_QUERY = "(max-width: 980px)";
-export const RIGHT_PANEL_SHEET_CLASS_NAME =
-  "w-[min(42vw,28rem)] min-w-80 max-w-[28rem] p-0 max-[760px]:w-[min(88vw,24rem)] max-[760px]:min-w-0 wco:mt-[env(titlebar-area-height)] wco:h-[calc(100%-env(titlebar-area-height))] wco:max-h-[calc(100%-env(titlebar-area-height))]";
+
+const RIGHT_PANEL_SHEET_BASE_CLASS_NAME =
+  "p-0 wco:mt-[env(titlebar-area-height)] wco:h-[calc(100%-env(titlebar-area-height))] wco:max-h-[calc(100%-env(titlebar-area-height))]";
+
+/**
+ * Wide sheet (e.g. diff viewer) — sized for code/diff content with room for
+ * long lines on tablet-sized viewports.
+ */
+export const RIGHT_PANEL_SHEET_CLASS_NAME = `w-[min(42vw,28rem)] min-w-80 max-w-[28rem] max-[760px]:w-[min(88vw,24rem)] max-[760px]:min-w-0 ${RIGHT_PANEL_SHEET_BASE_CLASS_NAME}`;
+
+/**
+ * Narrow sheet (e.g. plan/task sidebar) — caps at 340px to match the inline
+ * sidebar width on wide viewports, preventing the sheet from covering most
+ * of the content area on viewports just below the inline-layout breakpoint.
+ */
+export const RIGHT_PANEL_NARROW_SHEET_CLASS_NAME = `w-[min(88vw,340px)] max-w-[340px] ${RIGHT_PANEL_SHEET_BASE_CLASS_NAME}`;