diff --git a/apps/web/src/components/ChatView.tsx b/apps/web/src/components/ChatView.tsx
index d1225065aeec..3dcfce764e96 100644
--- a/apps/web/src/components/ChatView.tsx
+++ b/apps/web/src/components/ChatView.tsx
@@ -3942,6 +3942,8 @@ export default function ChatView(props: ChatViewProps) {
         <ChatRightPanel
           sheet={shouldUsePlanSidebarSheet}
           workspaceLayoutElement={workspaceLayoutElement}
+          canvasStore={chatCanvasStore}
+          hasFloatingPreview={canvasPreview !== null}
           widthStorageKey={`t3code:preview-panel-width:${activeThreadKey}`}
           sheetAnimationDurationMs={panelAnimationsActive ? panelAnimationDurationMs : 0}
           onCloseSheet={closePreviewPanelStable}
diff --git a/apps/web/src/components/chat/ChatRightPanel.tsx b/apps/web/src/components/chat/ChatRightPanel.tsx
index 69da3a9ddccd..d3b7d473f52f 100644
--- a/apps/web/src/components/chat/ChatRightPanel.tsx
+++ b/apps/web/src/components/chat/ChatRightPanel.tsx
@@ -1,11 +1,22 @@
+import { useStore } from "zustand";
 import { usePreviewPanelInlineSize } from "../../hooks/usePreviewPanelInlineSize";
+import { useClientSettings } from "../../hooks/useSettings";
 import { RightPanelSheet } from "../RightPanelSheet";
 import { RightPanelTabs, type RightPanelTabsProps } from "../RightPanelTabs";
+import type { ChatCanvasStore } from "./canvasStore";
+import { DETAILS_CARD_CLEARANCE } from "./chatCanvasLayout";
+import {
+  resolveThreadDetailsCardLayout,
+  THREAD_DETAILS_CARD_GAP,
+  THREAD_DETAILS_CARD_WIDTH,
+} from "./threadDetailsCardLayout";
 
 type ChatRightPanelProps = Omit<RightPanelTabsProps, "mode" | "inlineSize" | "maximized"> & {
   readonly sheet: boolean;
   readonly maximized: boolean;
   readonly workspaceLayoutElement: HTMLElement | null;
+  readonly canvasStore: ChatCanvasStore;
+  readonly hasFloatingPreview: boolean;
   readonly widthStorageKey: string;
   readonly sheetAnimationDurationMs: number;
   readonly onCloseSheet: () => void;
@@ -20,6 +31,8 @@ export function ChatRightPanel(props: ChatRightPanelProps) {
   const {
     sheet,
     workspaceLayoutElement,
+    canvasStore,
+    hasFloatingPreview,
     widthStorageKey,
     sheetAnimationDurationMs,
     onCloseSheet,
@@ -27,9 +40,30 @@ export function ChatRightPanel(props: ChatRightPanelProps) {
     layoutControls,
     ...tabs
   } = props;
+  const chatWidth = useClientSettings((settings) => settings.chatWidth);
+  const autoSize = !sheet && !maximized && !hasFloatingPreview && chatWidth !== "full";
+  // Size for the card even when closed; current presentation depends on the panel's width.
+  const preferredSiblingWidth = useStore(canvasStore, ({ measurements }) => {
+    if (!autoSize) return undefined;
+    const { padding, maxChatWidth, height, findBar } = measurements;
+    const widthWithCard =
+      maxChatWidth +
+      padding +
+      DETAILS_CARD_CLEARANCE +
+      THREAD_DETAILS_CARD_WIDTH +
+      THREAD_DETAILS_CARD_GAP;
+    const card = resolveThreadDetailsCardLayout({
+      container: { width: widthWithCard, height },
+      lane: { padding, minChatWidth: maxChatWidth },
+      frame: null,
+      topInset: findBar?.bottom ?? 0,
+    });
+    return card ? widthWithCard : maxChatWidth + padding * 2;
+  });
   const inlineSize = usePreviewPanelInlineSize({
     container: workspaceLayoutElement,
     widthStorageKey,
+    preferredSiblingWidth,
   });
   if (!sheet) {
     return <RightPanelTabs {...tabs} mode="inline" maximized={maximized} inlineSize={inlineSize} />;
diff --git a/apps/web/src/components/chat/threadDetailsCardLayout.ts b/apps/web/src/components/chat/threadDetailsCardLayout.ts
index 7b5e1be8f29b..b231d5cb5ee6 100644
--- a/apps/web/src/components/chat/threadDetailsCardLayout.ts
+++ b/apps/web/src/components/chat/threadDetailsCardLayout.ts
@@ -3,7 +3,7 @@ import { DETAILS_CARD_CLEARANCE } from "./chatCanvasLayout";
 
 export const THREAD_DETAILS_CARD_GAP = 12;
 // Keep in sync with --thread-details-panel-width.
-const THREAD_DETAILS_CARD_WIDTH = 280;
+export const THREAD_DETAILS_CARD_WIDTH = 280;
 
 export function resolveThreadDetailsCardDensity(
   height: number,
diff --git a/apps/web/src/hooks/usePreviewPanelInlineSize.ts b/apps/web/src/hooks/usePreviewPanelInlineSize.ts
index 8881f5314f3a..79f4ed156e32 100644
--- a/apps/web/src/hooks/usePreviewPanelInlineSize.ts
+++ b/apps/web/src/hooks/usePreviewPanelInlineSize.ts
@@ -22,17 +22,19 @@ export function usePreviewPanelInlineSize(
     readonly enabled?: boolean | undefined;
     readonly widthStorageKey?: string | undefined;
     readonly defaultWidth?: number | undefined;
+    readonly preferredSiblingWidth?: number | undefined;
     readonly container?: HTMLElement | null | undefined;
   } = {},
 ): PreviewPanelInlineSize {
   const enabled = options.enabled ?? true;
   const container = options.container;
+  const [availableWidth, setAvailableWidth] = useState(PREVIEW_PANEL_DEFAULT_WIDTH);
   const [maxWidth, setMaxWidth] = useState(() =>
     getPreviewPanelMaxWidth(typeof window === "undefined" ? 1280 : window.innerWidth),
   );
   const { width, handlers } = useResizableWidth({
     storageKey: options.widthStorageKey ?? PREVIEW_PANEL_WIDTH_STORAGE_KEY,
-    defaultWidth: options.defaultWidth ?? PREVIEW_PANEL_DEFAULT_WIDTH,
+    defaultWidth: options.defaultWidth ?? availableWidth,
     minWidth: PREVIEW_PANEL_MIN_WIDTH,
     maxWidth,
     edge: "left",
@@ -40,10 +42,16 @@ export function usePreviewPanelInlineSize(
   const observeRow = (host: HTMLElement) => {
     const row = container ?? (enabled ? host.parentElement : null) ?? null;
     // Ref callbacks run before paint, so the width is clamped without an over-wide
-    // flash on mount. Only the derived cap is stored: the row resizes every frame
-    // of a sidebar drag, but an unchanged cap skips re-rendering the owner.
+    // flash on mount. Store only derived widths so unchanged bounds and defaults
+    // skip re-rendering the owner during a sidebar drag.
     const measure = () => {
       setMaxWidth(getPreviewPanelMaxWidth(window.innerWidth, row?.clientWidth));
+      setAvailableWidth(
+        Math.max(
+          PREVIEW_PANEL_DEFAULT_WIDTH,
+          Math.floor(row?.clientWidth ?? 0) - (options.preferredSiblingWidth ?? Infinity),
+        ),
+      );
     };
     measure();
     let frame = 0;