diff --git a/.changeset/web-unbounded-panel-width.md b/.changeset/web-unbounded-panel-width.md
new file mode 100644
index 0000000000..42748324df
--- /dev/null
+++ b/.changeset/web-unbounded-panel-width.md
@@ -0,0 +1,5 @@
+---
+"@moonshot-ai/kimi-code": minor
+---
+
+Allow the web sidebar and detail panel to be resized up to the available viewport width, keeping their resize handles reachable on narrow windows.
diff --git a/apps/kimi-web/src/App.vue b/apps/kimi-web/src/App.vue
index beffa8e337..64474fc7d6 100644
--- a/apps/kimi-web/src/App.vue
+++ b/apps/kimi-web/src/App.vue
@@ -115,22 +115,6 @@ function onGlobalKeydown(e: KeyboardEvent): void {
   }
 }
 
-// ---------------------------------------------------------------------------
-// Layout: resizable session column. ResizeHandle owns the column width (with
-// localStorage persistence); we mirror it here to drive the App grid.
-// ---------------------------------------------------------------------------
-const {
-  SIDEBAR_WIDTH_KEY,
-  SIDEBAR_DEFAULT,
-  SIDEBAR_MIN,
-  SIDEBAR_MAX,
-  sessionColWidth,
-  sidebarCollapsed,
-  sideWidth,
-  loadSidebarCollapsed,
-  toggleSidebarCollapse,
-} = useSidebarLayout();
-
 // ---------------------------------------------------------------------------
 // Unified right-side detail layer. Only one detail is open at a time. The
 // shared `detailTarget` ref lives here so the file-preview and detail-panel
@@ -152,6 +136,28 @@ const {
   revealPreviewFile,
 } = useFilePreview({ client, detailTarget });
 
+// True while the right-side slot is actually occupied, so the sidebar reserves
+// room for it and the conversation can never be squeezed. Keyed off detailTarget
+// (the real occupant) rather than previewTarget, which can stay set after the
+// panel is hidden.
+const previewOpen = computed(() => detailTarget.value !== null);
+
+// ---------------------------------------------------------------------------
+// Layout: resizable session column. ResizeHandle owns the column width (with
+// localStorage persistence); we mirror it here to drive the App grid.
+// ---------------------------------------------------------------------------
+const {
+  SIDEBAR_WIDTH_KEY,
+  SIDEBAR_DEFAULT,
+  SIDEBAR_MIN,
+  sidebarMax,
+  sessionColWidth,
+  sidebarCollapsed,
+  sideWidth,
+  loadSidebarCollapsed,
+  toggleSidebarCollapse,
+} = useSidebarLayout({ previewOpen });
+
 // ---------------------------------------------------------------------------
 // Unified right-side detail layer (thinking / compaction / agent / diff / side
 // chat) plus the preview-panel width. Only one detail is open at a time.
@@ -160,8 +166,9 @@ const {
   PREVIEW_WIDTH_KEY,
   PREVIEW_MIN,
   previewDefaultWidth,
-  previewMaxWidth,
+  previewMax,
   previewWidth,
+  previewPanelWidth,
   thinkingPanelText,
   thinkingVisible,
   openThinkingPanel,
@@ -359,7 +366,9 @@ function handleCommand(cmd: string): void {
   if (cmd === '/btw' || cmd.startsWith('/btw ')) {
     const arg = cmd.slice('/btw'.length).trim();
     if (!arg && client.sideChatVisible.value) {
-      client.closeSideChat();
+      // Use the detail-layer close so detailTarget is cleared too; the bare
+      // client.closeSideChat() only hides the panel and leaves detailTarget set.
+      closeSideChat();
     } else {
       void openSideChatTab(arg || undefined);
     }
@@ -527,13 +536,13 @@ function openPr(url: string): void {
       v-else
       class="app"
       :class="{ mobile: isMobile, 'sidebar-collapsed': sidebarCollapsed && !isMobile }"
-      :style="{ '--side-w': sideWidth + 'px', '--preview-w': previewWidth + 'px' }"
+      :style="{ '--side-w': sideWidth + 'px', '--preview-w': previewPanelWidth + 'px' }"
     >
     <!-- Desktop navigation: workspace rail + resizable session column. -->
     <template v-if="!isMobile">
       <Sidebar
         v-show="!sidebarCollapsed"
-        :col-width="sessionColWidth"
+        :col-width="sideWidth"
         :active-workspace="client.visibleWorkspace.value"
         :active-workspace-id="client.activeWorkspaceId.value"
         :sessions="client.sessionsForView.value"
@@ -561,7 +570,7 @@ function openPr(url: string): void {
         :storage-key="SIDEBAR_WIDTH_KEY"
         :default-width="SIDEBAR_DEFAULT"
         :min="SIDEBAR_MIN"
-        :max="SIDEBAR_MAX"
+        :max="sidebarMax"
         @update:width="sessionColWidth = $event"
       />
       <div v-if="sidebarCollapsed" class="sidebar-rail">
@@ -686,7 +695,7 @@ function openPr(url: string): void {
       :storage-key="PREVIEW_WIDTH_KEY"
       :default-width="previewDefaultWidth"
       :min="PREVIEW_MIN"
-      :max="previewMaxWidth"
+      :max="previewMax"
       reverse
       :aria-label="t('layout.resizePreviewAria')"
       @update:width="previewWidth = $event"
diff --git a/apps/kimi-web/src/components/ResizeHandle.vue b/apps/kimi-web/src/components/ResizeHandle.vue
index 1c670aefc2..39ff4a25cd 100644
--- a/apps/kimi-web/src/components/ResizeHandle.vue
+++ b/apps/kimi-web/src/components/ResizeHandle.vue
@@ -33,7 +33,9 @@ const { width, dragging, onPointerDown } = useResizable({
   storageKey: props.storageKey,
   defaultWidth: props.defaultWidth,
   min: props.min,
-  max: props.max,
+  // Pass a getter so the cap stays reactive: a viewport-derived max can grow
+  // after the handle mounts and the next drag will use the new limit.
+  max: () => props.max,
   reverse: props.reverse,
 });
 
diff --git a/apps/kimi-web/src/composables/useDetailPanel.ts b/apps/kimi-web/src/composables/useDetailPanel.ts
index a119d5ed56..11c41fb16b 100644
--- a/apps/kimi-web/src/composables/useDetailPanel.ts
+++ b/apps/kimi-web/src/composables/useDetailPanel.ts
@@ -5,11 +5,12 @@ import { computed, ref, watch, type Ref } from 'vue';
 import type { AgentMember } from '../types';
 import type { DetailTarget } from './useFilePreview';
 import type { useKimiWebClient } from './useKimiWebClient';
+import { clampPanelWidth, panelMaxWidth, useViewportWidth } from './useViewportWidth';
 
 type KimiWebClient = ReturnType<typeof useKimiWebClient>;
 
 const PREVIEW_WIDTH_KEY = 'kimi-web.file-preview-width';
-const PREVIEW_MIN = 320;
+export const PREVIEW_MIN = 320;
 
 export interface UseDetailPanelOptions {
   client: KimiWebClient;
@@ -30,23 +31,33 @@ export function useDetailPanel({
   // ---------------------------------------------------------------------------
   // Panel width helpers
   // ---------------------------------------------------------------------------
-  function previewAreaWidth(): number {
-    if (typeof window === 'undefined') return PREVIEW_MIN * 2;
-    return Math.max(0, window.innerWidth - sideWidth.value);
-  }
+  const { viewportWidth } = useViewportWidth();
+
+  // Area available to the right of the sidebar (conversation + preview).
+  const previewAreaWidth = computed(() =>
+    Math.max(0, viewportWidth.value - sideWidth.value),
+  );
+
+  // Largest preview width that still leaves the conversation pane usable.
+  const previewMax = computed(() =>
+    panelMaxWidth(previewAreaWidth.value, PREVIEW_MIN, PREVIEW_MIN),
+  );
 
   function clampPreviewWidth(width: number): number {
-    const max = Math.max(PREVIEW_MIN, previewAreaWidth() - PREVIEW_MIN);
-    return Math.min(max, Math.max(PREVIEW_MIN, Math.round(width)));
+    return clampPanelWidth(Math.round(width), PREVIEW_MIN, previewMax.value);
   }
 
   function defaultPreviewWidth(): number {
-    return clampPreviewWidth(previewAreaWidth() / 2);
+    return clampPreviewWidth(previewAreaWidth.value / 2);
   }
 
   const previewDefaultWidth = computed(() => defaultPreviewWidth());
-  const previewMaxWidth = computed(() => Math.max(PREVIEW_MIN, previewAreaWidth() - PREVIEW_MIN));
   const previewWidth = ref(previewDefaultWidth.value);
+  // Rendered width, clamped to the current cap so a restored width or a window
+  // shrink can never push the resize handle off-screen.
+  const previewPanelWidth = computed(() =>
+    clampPanelWidth(previewWidth.value, PREVIEW_MIN, previewMax.value),
+  );
 
   // ---------------------------------------------------------------------------
   // Thinking panel
@@ -227,8 +238,9 @@ export function useDetailPanel({
     PREVIEW_WIDTH_KEY,
     PREVIEW_MIN,
     previewDefaultWidth,
-    previewMaxWidth,
+    previewMax,
     previewWidth,
+    previewPanelWidth,
     thinkingPanelText,
     thinkingVisible,
     openThinkingPanel,
diff --git a/apps/kimi-web/src/composables/useResizable.ts b/apps/kimi-web/src/composables/useResizable.ts
index 41c9d8d60d..4317ba0f91 100644
--- a/apps/kimi-web/src/composables/useResizable.ts
+++ b/apps/kimi-web/src/composables/useResizable.ts
@@ -4,7 +4,7 @@
 // up pointer events (pointerdown/move/up with capture, no text-selection while
 // dragging). Used by the sidebar session column drag handle.
 
-import { onBeforeUnmount, ref, type Ref } from 'vue';
+import { onBeforeUnmount, ref, toValue, type MaybeRefOrGetter, type Ref } from 'vue';
 import { safeGetString, safeSetString } from '../lib/storage';
 
 export interface UseResizableOptions {
@@ -14,8 +14,9 @@ export interface UseResizableOptions {
   defaultWidth: number;
   /** Smallest allowed width (px). */
   min: number;
-  /** Largest allowed width (px). */
-  max: number;
+  /** Largest allowed width (px). Accepts a ref/getter so a cap derived from the
+   *  viewport keeps working as the window is resized after the handle mounts. */
+  max: MaybeRefOrGetter<number>;
   /** True when dragging right should shrink the controlled width. */
   reverse?: boolean;
 }
@@ -57,7 +58,7 @@ export function useResizable(options: UseResizableOptions): UseResizable {
 
   function clamp(value: number): number {
     if (!Number.isFinite(value)) return defaultWidth;
-    return Math.min(max, Math.max(min, Math.round(value)));
+    return Math.min(toValue(max), Math.max(min, Math.round(value)));
   }
 
   const width = ref<number>(clamp(readStored(storageKey) ?? defaultWidth));
@@ -107,7 +108,10 @@ export function useResizable(options: UseResizableOptions): UseResizable {
     event.preventDefault();
     dragging.value = true;
     startX = event.clientX;
-    startWidth = width.value;
+    // The stored width can exceed the current cap (e.g. after the window narrows
+    // or a side panel opens). Clamp the drag start so the handle responds
+    // immediately instead of first covering an invisible delta.
+    startWidth = clamp(width.value);
     activeEl = event.currentTarget as HTMLElement;
     activePointerId = event.pointerId;
     // Suppress text selection / show a resize cursor for the whole drag.
diff --git a/apps/kimi-web/src/composables/useSidebarLayout.ts b/apps/kimi-web/src/composables/useSidebarLayout.ts
index a05e1c080d..f568cb6a50 100644
--- a/apps/kimi-web/src/composables/useSidebarLayout.ts
+++ b/apps/kimi-web/src/composables/useSidebarLayout.ts
@@ -2,21 +2,45 @@
 // Layout: resizable session column. ResizeHandle owns the column width (with
 // localStorage persistence); we mirror it here to drive the App grid.
 
-import { computed, ref } from 'vue';
+import { computed, ref, toValue, type MaybeRefOrGetter } from 'vue';
 import { safeGetString, safeSetString, STORAGE_KEYS } from '../lib/storage';
+import { PREVIEW_MIN } from './useDetailPanel';
+import { clampPanelWidth, panelMaxWidth, useViewportWidth } from './useViewportWidth';
 
 const SIDEBAR_WIDTH_KEY = STORAGE_KEYS.sidebarWidth;
 const SIDEBAR_COLLAPSED_KEY = STORAGE_KEYS.sidebarCollapsed;
 const SIDEBAR_DEFAULT = 270;
 const SIDEBAR_MIN = 170;
-const SIDEBAR_MAX = 420;
 const SIDEBAR_COLLAPSED_WIDTH = 36;
+// Minimum width kept for the conversation pane. The sidebar is capped so the
+// conversation keeps at least this much room, which also guarantees the sidebar
+// resize handle and collapse button stay inside the viewport even when a width
+// saved on a wider display is restored on a narrower one.
+const CONVERSATION_MIN = 320;
 
-export function useSidebarLayout() {
+export interface UseSidebarLayoutOptions {
+  /** True while the right-side detail/preview panel is open, so the sidebar
+   *  reserves room for it in addition to the conversation pane. */
+  previewOpen?: MaybeRefOrGetter<boolean>;
+}
+
+export function useSidebarLayout(options: UseSidebarLayoutOptions = {}) {
+  const { viewportWidth } = useViewportWidth();
   const sessionColWidth = ref(SIDEBAR_DEFAULT);
   const sidebarCollapsed = ref(false);
+
+  // Largest sidebar width that still leaves the conversation pane usable. When
+  // the right-side panel is open, also reserves its minimum width so the
+  // conversation column can never be squeezed to nothing.
+  const sidebarMax = computed(() => {
+    const reserve = CONVERSATION_MIN + (toValue(options.previewOpen) ? PREVIEW_MIN : 0);
+    return panelMaxWidth(viewportWidth.value, SIDEBAR_MIN, reserve);
+  });
+
   const sideWidth = computed(() =>
-    sidebarCollapsed.value ? SIDEBAR_COLLAPSED_WIDTH : sessionColWidth.value,
+    sidebarCollapsed.value
+      ? SIDEBAR_COLLAPSED_WIDTH
+      : clampPanelWidth(sessionColWidth.value, SIDEBAR_MIN, sidebarMax.value),
   );
 
   function loadSidebarCollapsed(): void {
@@ -44,7 +68,7 @@ export function useSidebarLayout() {
     SIDEBAR_WIDTH_KEY,
     SIDEBAR_DEFAULT,
     SIDEBAR_MIN,
-    SIDEBAR_MAX,
+    sidebarMax,
     sessionColWidth,
     sidebarCollapsed,
     sideWidth,
diff --git a/apps/kimi-web/src/composables/useViewportWidth.ts b/apps/kimi-web/src/composables/useViewportWidth.ts
new file mode 100644
index 0000000000..8271fd1d4b
--- /dev/null
+++ b/apps/kimi-web/src/composables/useViewportWidth.ts
@@ -0,0 +1,50 @@
+// apps/kimi-web/src/composables/useViewportWidth.ts
+// Shared reactive viewport width for the resizable layout panels. A single
+// window resize listener backs every consumer, so panels can cap themselves to
+// the current window size without each wiring up their own listener.
+
+import { onBeforeUnmount, onMounted, ref } from 'vue';
+
+const viewportWidth = ref(typeof window === 'undefined' ? 0 : window.innerWidth);
+let subscribers = 0;
+let listening = false;
+
+function update(): void {
+  viewportWidth.value = window.innerWidth;
+}
+
+function startListening(): void {
+  if (listening || typeof window === 'undefined') return;
+  window.addEventListener('resize', update);
+  listening = true;
+  update();
+}
+
+function stopListening(): void {
+  if (!listening || typeof window === 'undefined') return;
+  window.removeEventListener('resize', update);
+  listening = false;
+}
+
+/** Largest a panel may grow while keeping `reserve` px free for the rest of the
+ *  layout. Never drops below the panel's own `min`. */
+export function panelMaxWidth(available: number, min: number, reserve: number): number {
+  return Math.max(min, available - reserve);
+}
+
+/** Clamp a panel's chosen width into its allowed [min, max] range. */
+export function clampPanelWidth(width: number, min: number, max: number): number {
+  return Math.min(max, Math.max(min, width));
+}
+
+export function useViewportWidth() {
+  onMounted(() => {
+    subscribers += 1;
+    startListening();
+  });
+  onBeforeUnmount(() => {
+    subscribers = Math.max(0, subscribers - 1);
+    if (subscribers === 0) stopListening();
+  });
+  return { viewportWidth };
+}