diff --git a/.changeset/web-bound-dom-growth.md b/.changeset/web-bound-dom-growth.md
new file mode 100644
index 0000000000..927f545728
--- /dev/null
+++ b/.changeset/web-bound-dom-growth.md
@@ -0,0 +1,5 @@
+---
+"@moonshot-ai/kimi-code": patch
+---
+
+web: Bound DOM growth in long sessions — tail-biased truncation with expand-to-full for long tool outputs, collapsible tool bodies unmount when closed, tool groups default to collapsed (auto-open while running), and lighter scroll-follow hot paths.
diff --git a/apps/kimi-web/src/components/chat/ConversationPane.vue b/apps/kimi-web/src/components/chat/ConversationPane.vue
index 1b6b1a9e79..ea78d4f23e 100644
--- a/apps/kimi-web/src/components/chat/ConversationPane.vue
+++ b/apps/kimi-web/src/components/chat/ConversationPane.vue
@@ -805,7 +805,14 @@ const scrollKey = computed<ScrollKey>(() => {
   const thinkingLen = last?.thinking?.length ?? 0;
   const toolsLen =
     last?.tools?.reduce(
-      (n, tool) => n + tool.name.length + (tool.arg?.length ?? 0) + (tool.output?.join('').length ?? 0),
+      // Track output growth via line count + tail-line length instead of
+      // joining every tool output on each change.
+      (n, tool) =>
+        n +
+        tool.name.length +
+        (tool.arg?.length ?? 0) +
+        (tool.output?.length ?? 0) +
+        (tool.output?.at(-1)?.length ?? 0),
       0,
     ) ?? 0;
   return {
@@ -1126,7 +1133,10 @@ function rebindScrollObservers(): void {
   updatePanesScrollbarWidth();
   if (contentObserver) {
     contentObserver.disconnect();
-    if (el) contentObserver.observe(el, { childList: true, subtree: true, characterData: true });
+    // Drop characterData: per-token text updates during streaming fired
+    // this observer at delta rate; structural changes are enough, and the
+    // scroll follow is driven by the reactive scrollKey watcher anyway.
+    if (el) contentObserver.observe(el, { childList: true, subtree: true });
   }
   if (resizeObserver) {
     resizeObserver.disconnect();
diff --git a/apps/kimi-web/src/components/chat/ToolGroup.vue b/apps/kimi-web/src/components/chat/ToolGroup.vue
index 8a30af9bea..4f051814c3 100644
--- a/apps/kimi-web/src/components/chat/ToolGroup.vue
+++ b/apps/kimi-web/src/components/chat/ToolGroup.vue
@@ -1,6 +1,6 @@
 <!-- apps/kimi-web/src/components/chat/ToolGroup.vue -->
 <script setup lang="ts">
-import { computed, inject, nextTick, ref } from 'vue';
+import { computed, inject, nextTick, ref, watch } from 'vue';
 import { useI18n } from 'vue-i18n';
 import ToolCall from './ToolCall.vue';
 import { toolStackKey, toolStackPosition } from '../chatTurnRendering';
@@ -25,14 +25,23 @@ const emit = defineEmits<{
   openAgent: [toolCallId: string];
 }>();
 
-const open = ref(true);
-
 const count = computed(() => props.tools.length);
 const aggregateStatus = computed<'running' | 'error' | 'done'>(() => {
   if (props.tools.some((t) => t.tool.status === 'running')) return 'running';
   if (props.tools.some((t) => t.tool.status === 'error')) return 'error';
   return 'done';
 });
+
+// kimi-ui: groups default to collapsed (they used to default open, mounting
+// every tool call body of every past turn). A group with tools still running
+// opens itself so live execution stays visible.
+const open = ref(aggregateStatus.value === 'running');
+// A collapsed group must also reopen when a running tool is appended LATER
+// (e.g. a completed stack from a refresh gains a new in-flight tool) — the
+// initial ref only captures mount-time state.
+watch(aggregateStatus, (status) => {
+  if (status === 'running') open.value = true;
+});
 const { t } = useI18n();
 
 const statusLabel = computed(() => {
@@ -70,7 +79,8 @@ function onHeadClick(): void {
       <Icon class="tg-car" name="chevron-right" size="sm" />
     </button>
     <div class="tool-group-body" :class="{ open }" :inert="!open">
-      <div class="tool-group-body-inner">
+      <!-- kimi-ui: unmount when collapsed (same DOM-bloat fix as ToolRow). -->
+      <div v-if="open" class="tool-group-body-inner">
         <ToolCall
           v-for="(item, si) in tools"
           :key="toolStackKey(item)"
diff --git a/apps/kimi-web/src/components/chat/ToolRow.vue b/apps/kimi-web/src/components/chat/ToolRow.vue
index 65d8a3bd5a..28879e9993 100644
--- a/apps/kimi-web/src/components/chat/ToolRow.vue
+++ b/apps/kimi-web/src/components/chat/ToolRow.vue
@@ -74,7 +74,9 @@ function onHeadClick(): void {
       <Icon v-if="expandable" class="car" :name="open ? 'chevron-down' : 'chevron-right'" size="sm" />
     </div>
     <div class="bb" :class="{ open }" :inert="!open">
-      <div class="bb-pad">
+      <!-- kimi-ui: unmount the body when closed — collapsed tool rows used to
+           keep every output line in the DOM (grid 0fr only hid them visually). -->
+      <div v-if="open" class="bb-pad">
         <slot />
       </div>
     </div>
diff --git a/apps/kimi-web/src/components/chat/tool-calls/ToolOutputBlock.vue b/apps/kimi-web/src/components/chat/tool-calls/ToolOutputBlock.vue
index 3f7059556e..55e5833fc2 100644
--- a/apps/kimi-web/src/components/chat/tool-calls/ToolOutputBlock.vue
+++ b/apps/kimi-web/src/components/chat/tool-calls/ToolOutputBlock.vue
@@ -1,24 +1,42 @@
 <!-- Shared line-oriented tool output block. Keeps long outputs to a readable
-     viewport while preserving the tool card's normal typography. -->
+     viewport while preserving the tool card's normal typography.
+     kimi-ui: outputs beyond TRUNCATE_LINE_COUNT render tail-biased (the last
+     N lines, where errors/results live) behind an expand button, so huge logs
+     no longer put thousands of divs in the DOM. -->
 <script setup lang="ts">
-import { computed } from 'vue';
+import { computed, ref } from 'vue';
+import { useI18n } from 'vue-i18n';
 
 const OUTPUT_SCROLL_LINE_COUNT = 50;
+// Default render window for long outputs (tail-biased).
+const TRUNCATE_LINE_COUNT = 80;
 
 const props = defineProps<{
   lines?: string[];
   emptyText?: string;
 }>();
 
+const { t } = useI18n();
+const expanded = ref(false);
+
 const outputLines = computed(() => props.lines ?? []);
-const isScrollable = computed(() => outputLines.value.length > OUTPUT_SCROLL_LINE_COUNT);
+const truncated = computed(
+  () => !expanded.value && outputLines.value.length > TRUNCATE_LINE_COUNT,
+);
+const visibleLines = computed(() =>
+  truncated.value ? outputLines.value.slice(-TRUNCATE_LINE_COUNT) : outputLines.value,
+);
+const isScrollable = computed(() => visibleLines.value.length > OUTPUT_SCROLL_LINE_COUNT);
 const outputStyle = { '--tool-output-visible-lines': String(OUTPUT_SCROLL_LINE_COUNT) };
 </script>
 
 <template>
   <div class="bb-code tool-output-block" :class="{ scroll: isScrollable }" :style="outputStyle">
     <div v-if="outputLines.length === 0 && emptyText" class="bb-empty">{{ emptyText }}</div>
-    <div v-for="(line, i) in outputLines" :key="i">{{ line }}</div>
+    <button v-if="truncated" class="truncate-toggle" type="button" @click="expanded = true">
+      {{ t('tools.output.showAll', { count: outputLines.length }) }}
+    </button>
+    <div v-for="(line, i) in visibleLines" :key="i">{{ line }}</div>
   </div>
 </template>
 
@@ -39,4 +57,20 @@ const outputStyle = { '--tool-output-visible-lines': String(OUTPUT_SCROLL_LINE_C
   color: var(--color-text-muted);
   font-style: italic;
 }
+.truncate-toggle {
+  display: block;
+  width: 100%;
+  margin: 0 0 var(--space-2);
+  padding: var(--space-1) 0;
+  border: none;
+  border-bottom: 1px dashed var(--color-line);
+  background: transparent;
+  color: var(--color-accent);
+  font: var(--text-xs) var(--font-ui);
+  cursor: pointer;
+  text-align: center;
+}
+.truncate-toggle:hover {
+  text-decoration: underline;
+}
 </style>
diff --git a/apps/kimi-web/src/i18n/locales/en/tools.ts b/apps/kimi-web/src/i18n/locales/en/tools.ts
index 13cc18211f..3148b81917 100644
--- a/apps/kimi-web/src/i18n/locales/en/tools.ts
+++ b/apps/kimi-web/src/i18n/locales/en/tools.ts
@@ -53,6 +53,9 @@ export default {
     error: 'failed',
     done: 'done',
   },
+  output: {
+    showAll: 'Show all {count} lines',
+  },
   ask: {
     dismissed: 'Dismissed',
     answer: '{count} answer',
diff --git a/apps/kimi-web/src/i18n/locales/zh/tools.ts b/apps/kimi-web/src/i18n/locales/zh/tools.ts
index f9560e3da9..8561299410 100644
--- a/apps/kimi-web/src/i18n/locales/zh/tools.ts
+++ b/apps/kimi-web/src/i18n/locales/zh/tools.ts
@@ -53,6 +53,9 @@ export default {
     error: '有失败',
     done: '已完成',
   },
+  output: {
+    showAll: '显示全部 {count} 行',
+  },
   ask: {
     dismissed: '已忽略',
     answer: '{count} 个回答',