diff --git a/apps/web/src/components/AppSidebarLayout.tsx b/apps/web/src/components/AppSidebarLayout.tsx
index 3a70390d0c4..f3cb7a0c4c0 100644
--- a/apps/web/src/components/AppSidebarLayout.tsx
+++ b/apps/web/src/components/AppSidebarLayout.tsx
@@ -1,6 +1,6 @@
 import { useAtomValue } from "@effect/atom-react";
 import * as Schema from "effect/Schema";
-import { useEffect, useState, type CSSProperties, type ReactNode } from "react";
+import { useEffect, useMemo, useState, type CSSProperties, type ReactNode } from "react";
 import { useLocation, useNavigate } from "@tanstack/react-router";
 
 import { isElectron } from "../env";
@@ -13,9 +13,10 @@ import ThreadSidebar from "./Sidebar";
 import ThreadSidebarV2 from "./SidebarV2";
 import { useSidebarStageBackdropVariant } from "./SidebarStageBackdrop";
 import {
-  resolveInitialThreadSidebarWidth,
+  resolveThreadSidebarCssWidth,
   resolveThreadSidebarMaximumWidth,
   THREAD_MAIN_CONTENT_MIN_WIDTH,
+  THREAD_SIDEBAR_DEFAULT_WIDTH,
   THREAD_SIDEBAR_MIN_WIDTH,
   THREAD_SIDEBAR_WIDTH_STORAGE_KEY,
 } from "./threadSidebarWidth";
@@ -33,13 +34,13 @@ const MACOS_TRAFFIC_LIGHTS_LEFT_INSET = "90px";
 
 function readInitialThreadSidebarWidth(): number {
   try {
-    return resolveInitialThreadSidebarWidth(
-      getLocalStorageItem(THREAD_SIDEBAR_WIDTH_STORAGE_KEY, Schema.Finite),
-      window.innerWidth,
-    );
+    const storedWidth = getLocalStorageItem(THREAD_SIDEBAR_WIDTH_STORAGE_KEY, Schema.Finite);
+    return storedWidth === null
+      ? THREAD_SIDEBAR_DEFAULT_WIDTH
+      : Math.max(THREAD_SIDEBAR_MIN_WIDTH, storedWidth);
   } catch (error) {
     console.error("Could not read persisted thread sidebar width.", error);
-    return resolveInitialThreadSidebarWidth(null, window.innerWidth);
+    return THREAD_SIDEBAR_DEFAULT_WIDTH;
   }
 }
 
@@ -109,7 +110,28 @@ export function AppSidebarLayout({ children }: { children: ReactNode }) {
   const useSidebarV2Theme = useSidebarV2 || isOnSettings;
   const isMacosDesktop = isElectron && isMacPlatform(navigator.platform);
   const [sidebarWidth, setSidebarWidth] = useState(readInitialThreadSidebarWidth);
-  const sidebarMaximumWidth = resolveThreadSidebarMaximumWidth(window.innerWidth);
+  const sidebarResizable = useMemo(
+    () => ({
+      getCssWidth: resolveThreadSidebarCssWidth,
+      maxWidth: () => resolveThreadSidebarMaximumWidth(window.innerWidth),
+      minWidth: THREAD_SIDEBAR_MIN_WIDTH,
+      shouldAcceptWidth: ({
+        currentWidth,
+        nextWidth,
+        wrapper,
+      }: {
+        currentWidth: number;
+        nextWidth: number;
+        wrapper: HTMLElement;
+      }) =>
+        nextWidth <= currentWidth ||
+        wrapper.clientWidth - nextWidth >= THREAD_MAIN_CONTENT_MIN_WIDTH,
+      storageKey: THREAD_SIDEBAR_WIDTH_STORAGE_KEY,
+      hydrateStoredWidth: false,
+      onResize: setSidebarWidth,
+    }),
+    [],
+  );
   const [isWindowFullscreen, setIsWindowFullscreen] = useState(() => {
     const getWindowFullscreenState = window.desktopBridge?.getWindowFullscreenState;
     return isMacosDesktop && typeof getWindowFullscreenState === "function"
@@ -117,7 +139,7 @@ export function AppSidebarLayout({ children }: { children: ReactNode }) {
       : false;
   });
   const sidebarProviderStyle = {
-    "--sidebar-width": `${sidebarWidth}px`,
+    "--sidebar-width": resolveThreadSidebarCssWidth(sidebarWidth),
     ...(isMacosDesktop && !isWindowFullscreen
       ? { "--workspace-controls-left": MACOS_TRAFFIC_LIGHTS_LEFT_INSET }
       : {}),
@@ -168,15 +190,7 @@ export function AppSidebarLayout({ children }: { children: ReactNode }) {
         data-app-sidebar=""
         data-sidebar-version={useSidebarV2Theme ? "v2" : "v1"}
         className="border-r border-sidebar-border bg-sidebar text-sidebar-foreground"
-        resizable={{
-          maxWidth: sidebarMaximumWidth,
-          minWidth: THREAD_SIDEBAR_MIN_WIDTH,
-          shouldAcceptWidth: ({ currentWidth, nextWidth, wrapper }) =>
-            nextWidth <= currentWidth ||
-            wrapper.clientWidth - nextWidth >= THREAD_MAIN_CONTENT_MIN_WIDTH,
-          storageKey: THREAD_SIDEBAR_WIDTH_STORAGE_KEY,
-          onResize: setSidebarWidth,
-        }}
+        resizable={sidebarResizable}
       >
         {useSidebarV2 ? <ThreadSidebarV2 /> : <ThreadSidebar />}
         <SidebarRail />
diff --git a/apps/web/src/components/threadSidebarWidth.test.ts b/apps/web/src/components/threadSidebarWidth.test.ts
index 12aef63bd5e..471fbaef25c 100644
--- a/apps/web/src/components/threadSidebarWidth.test.ts
+++ b/apps/web/src/components/threadSidebarWidth.test.ts
@@ -2,6 +2,8 @@ import { describe, expect, it } from "vite-plus/test";
 
 import {
   resolveInitialThreadSidebarWidth,
+  resolveThreadSidebarCssWidth,
+  resolveThreadSidebarMaximumWidth,
   THREAD_MAIN_CONTENT_MIN_WIDTH,
   THREAD_SIDEBAR_DEFAULT_WIDTH,
   THREAD_SIDEBAR_MIN_WIDTH,
@@ -28,6 +30,17 @@ describe("thread sidebar width", () => {
     );
   });
 
+  it("raises the maximum when the viewport grows", () => {
+    expect(resolveThreadSidebarMaximumWidth(1000)).toBe(1000 - THREAD_MAIN_CONTENT_MIN_WIDTH);
+    expect(resolveThreadSidebarMaximumWidth(1800)).toBe(1800 - THREAD_MAIN_CONTENT_MIN_WIDTH);
+  });
+
+  it("expresses the preferred width with a live viewport clamp", () => {
+    expect(resolveThreadSidebarCssWidth(400)).toBe(
+      `min(400px, max(${THREAD_SIDEBAR_MIN_WIDTH}px, calc(100vw - ${THREAD_MAIN_CONTENT_MIN_WIDTH}px)))`,
+    );
+  });
+
   it("keeps the sidebar minimum when the whole layout is narrower than its minimums", () => {
     expect(resolveInitialThreadSidebarWidth(900, 700)).toBe(THREAD_SIDEBAR_MIN_WIDTH);
   });
diff --git a/apps/web/src/components/threadSidebarWidth.ts b/apps/web/src/components/threadSidebarWidth.ts
index 93dd196e84d..1aa181106de 100644
--- a/apps/web/src/components/threadSidebarWidth.ts
+++ b/apps/web/src/components/threadSidebarWidth.ts
@@ -10,6 +10,10 @@ export function resolveThreadSidebarMaximumWidth(viewportWidth: number): number
   );
 }
 
+export function resolveThreadSidebarCssWidth(width: number): string {
+  return `min(${width}px, max(${THREAD_SIDEBAR_MIN_WIDTH}px, calc(100vw - ${THREAD_MAIN_CONTENT_MIN_WIDTH}px)))`;
+}
+
 export function resolveInitialThreadSidebarWidth(
   storedWidth: number | null,
   viewportWidth: number,
diff --git a/apps/web/src/components/ui/sidebar.test.tsx b/apps/web/src/components/ui/sidebar.test.tsx
index 904f8664772..e492ef238be 100644
--- a/apps/web/src/components/ui/sidebar.test.tsx
+++ b/apps/web/src/components/ui/sidebar.test.tsx
@@ -2,6 +2,9 @@ import { renderToStaticMarkup } from "react-dom/server";
 import { describe, expect, it } from "vite-plus/test";
 
 import {
+  clampSidebarWidthValue,
+  resolveSidebarCssWidth,
+  resolveSidebarMaximumWidth,
   SidebarMenuAction,
   SidebarMenuButton,
   SidebarMenuSubButton,
@@ -19,6 +22,25 @@ function renderSidebarButton(className?: string) {
 }
 
 describe("sidebar interactive cursors", () => {
+  it("resolves and clamps against a live maximum width", () => {
+    let maximumWidth = 360;
+    const liveMaximumWidth = () => maximumWidth;
+
+    expect(resolveSidebarMaximumWidth(liveMaximumWidth)).toBe(360);
+    expect(clampSidebarWidthValue(340, 208, liveMaximumWidth)).toBe(340);
+
+    maximumWidth = 208;
+    expect(resolveSidebarMaximumWidth(liveMaximumWidth)).toBe(208);
+    expect(clampSidebarWidthValue(340, 208, liveMaximumWidth)).toBe(208);
+  });
+
+  it("preserves a dynamic CSS width when committing a rail resize", () => {
+    const getCssWidth = (width: number) => `min(${width}px, calc(100vw - 640px))`;
+
+    expect(resolveSidebarCssWidth(340, getCssWidth)).toBe("min(340px, calc(100vw - 640px))");
+    expect(resolveSidebarCssWidth(340)).toBe("340px");
+  });
+
   it("uses mobile sheet visibility for the shared responsive state", () => {
     expect(resolveSidebarState({ isMobile: true, open: true, openMobile: false })).toBe(
       "collapsed",
diff --git a/apps/web/src/components/ui/sidebar.tsx b/apps/web/src/components/ui/sidebar.tsx
index af90a7cc65b..c11236d8d38 100644
--- a/apps/web/src/components/ui/sidebar.tsx
+++ b/apps/web/src/components/ui/sidebar.tsx
@@ -40,7 +40,9 @@ type SidebarContextProps = {
 };
 
 type SidebarResizableOptions = {
-  maxWidth?: number;
+  getCssWidth?: (width: number) => string;
+  hydrateStoredWidth?: boolean;
+  maxWidth?: number | (() => number);
   minWidth?: number;
   onResize?: (width: number) => void;
   shouldAcceptWidth?: (context: {
@@ -55,7 +57,9 @@ type SidebarResizableOptions = {
 };
 
 type SidebarResolvedResizableOptions = {
-  maxWidth: number;
+  getCssWidth?: (width: number) => string;
+  hydrateStoredWidth: boolean;
+  maxWidth: number | (() => number);
   minWidth: number;
   onResize?: (width: number) => void;
   shouldAcceptWidth?: (context: {
@@ -199,9 +203,11 @@ function Sidebar({
 
     const options = typeof resizable === "boolean" ? {} : resizable;
     return {
+      hydrateStoredWidth: options.hydrateStoredWidth ?? true,
       maxWidth: options.maxWidth ?? Number.POSITIVE_INFINITY,
       minWidth: options.minWidth ?? SIDEBAR_RESIZE_DEFAULT_MIN_WIDTH,
       storageKey: options.storageKey ?? null,
+      ...(options.getCssWidth ? { getCssWidth: options.getCssWidth } : {}),
       ...(options.onResize ? { onResize: options.onResize } : {}),
       ...(options.shouldAcceptWidth ? { shouldAcceptWidth: options.shouldAcceptWidth } : {}),
     };
@@ -343,8 +349,27 @@ function SidebarTrigger({ className, onClick, ...props }: React.ComponentProps<t
   );
 }
 
+export function resolveSidebarMaximumWidth(maxWidth: number | (() => number)): number {
+  return typeof maxWidth === "function" ? maxWidth() : maxWidth;
+}
+
+export function clampSidebarWidthValue(
+  width: number,
+  minWidth: number,
+  maxWidth: number | (() => number),
+): number {
+  return Math.max(minWidth, Math.min(width, resolveSidebarMaximumWidth(maxWidth)));
+}
+
+export function resolveSidebarCssWidth(
+  width: number,
+  getCssWidth?: (width: number) => string,
+): string {
+  return getCssWidth?.(width) ?? `${width}px`;
+}
+
 function clampSidebarWidth(width: number, options: SidebarResolvedResizableOptions): number {
-  return Math.max(options.minWidth, Math.min(width, options.maxWidth));
+  return clampSidebarWidthValue(width, options.minWidth, options.maxWidth);
 }
 
 function SidebarRail({
@@ -388,13 +413,21 @@ function SidebarRail({
       if (resizeState.rafId !== null) {
         window.cancelAnimationFrame(resizeState.rafId);
       }
+      const width = resolvedResizable
+        ? clampSidebarWidth(resizeState.width, resolvedResizable)
+        : resizeState.width;
+      // Keep caller-owned viewport clamps even when onResize does not trigger a render.
+      resizeState.wrapper.style.setProperty(
+        "--sidebar-width",
+        resolveSidebarCssWidth(width, resolvedResizable?.getCssWidth),
+      );
+      if (resolvedResizable?.storageKey && typeof window !== "undefined") {
+        setLocalStorageItem(resolvedResizable.storageKey, width, Schema.Finite);
+      }
+      resolvedResizable?.onResize?.(width);
       resizeState.transitionTargets.forEach((element) => {
         element.style.removeProperty("transition-duration");
       });
-      if (resolvedResizable?.storageKey && typeof window !== "undefined") {
-        setLocalStorageItem(resolvedResizable.storageKey, resizeState.width, Schema.Finite);
-      }
-      resolvedResizable?.onResize?.(resizeState.width);
       resizeStateRef.current = null;
       if (resizeState.rail.hasPointerCapture(pointerId)) {
         resizeState.rail.releasePointerCapture(pointerId);
@@ -556,7 +589,13 @@ function SidebarRail({
   );
 
   React.useLayoutEffect(() => {
-    if (!resolvedResizable?.storageKey || typeof window === "undefined") return;
+    if (
+      !resolvedResizable?.storageKey ||
+      !resolvedResizable.hydrateStoredWidth ||
+      typeof window === "undefined"
+    ) {
+      return;
+    }
     const rail = railRef.current;
     if (!rail) return;
     const wrapper = rail.closest<HTMLElement>("[data-slot='sidebar-wrapper']");
@@ -574,7 +613,40 @@ function SidebarRail({
     // Hydrate the CSS variable before the browser paints so a restored sidebar
     // never flashes at the default width first.
     wrapper.style.setProperty("--sidebar-width", `${clampedWidth}px`);
-    resolvedResizable.onResize?.(clampedWidth);
+    // Preserve the user's stored preference even when the current viewport
+    // temporarily applies a smaller maximum width.
+    resolvedResizable.onResize?.(storedWidth);
+  }, [resolvedResizable]);
+
+  React.useEffect(() => {
+    if (!resolvedResizable || typeof window === "undefined") return;
+
+    const suppressViewportResizeTransition = () => {
+      // Pointer moves resolve the live maximum. Do not interrupt their
+      // imperative width; stopResize clamps once more before persisting.
+      if (resizeStateRef.current) return;
+
+      const rail = railRef.current;
+      const wrapper = rail?.closest<HTMLElement>("[data-slot='sidebar-wrapper']");
+      const sidebarRoot = rail?.closest<HTMLElement>("[data-slot='sidebar']");
+      if (!wrapper || !sidebarRoot) return;
+
+      const transitionTargets = [
+        sidebarRoot.querySelector<HTMLElement>("[data-slot='sidebar-gap']"),
+        sidebarRoot.querySelector<HTMLElement>("[data-slot='sidebar-container']"),
+      ].filter((element): element is HTMLElement => element !== null);
+      transitionTargets.forEach((element) => {
+        element.style.setProperty("transition-duration", "0ms");
+      });
+      window.requestAnimationFrame(() => {
+        transitionTargets.forEach((element) => {
+          element.style.removeProperty("transition-duration");
+        });
+      });
+    };
+
+    window.addEventListener("resize", suppressViewportResizeTransition);
+    return () => window.removeEventListener("resize", suppressViewportResizeTransition);
   }, [resolvedResizable]);
 
   React.useEffect(() => {