diff --git a/.changeset/web-code-block-font-fix.md b/.changeset/web-code-block-font-fix.md
new file mode 100644
index 0000000000..630f644595
--- /dev/null
+++ b/.changeset/web-code-block-font-fix.md
@@ -0,0 +1,5 @@
+---
+"@moonshot-ai/kimi-code": patch
+---
+
+web: Fix chat code blocks rendering in the proportional UI font at the wrong size after the markdown renderer upgrade, and align the loading fallback with the highlighted block so the upgrade no longer shifts layout.
diff --git a/apps/kimi-web/package.json b/apps/kimi-web/package.json
index 87d0a2041c..7e0d79ed06 100644
--- a/apps/kimi-web/package.json
+++ b/apps/kimi-web/package.json
@@ -18,13 +18,13 @@
     "@xterm/addon-fit": "^0.11.0",
     "@xterm/xterm": "^6.0.0",
     "katex": "^0.17.0",
-    "markstream-vue": "^1.0.9-beta.1",
+    "markstream-vue": "1.0.9-beta.1",
     "mermaid": "^11.15.0",
     "monaco-editor": "^0.55.1",
     "shiki": "^4.3.0",
-    "stream-diffs": "^0.0.2",
-    "stream-markdown": "^0.0.16",
-    "stream-monaco": "^0.0.49",
+    "stream-diffs": "0.0.2",
+    "stream-markdown": "0.0.16",
+    "stream-monaco": "0.0.49",
     "vue": "^3.5.35",
     "vue-i18n": "^11.4.5"
   },
diff --git a/apps/kimi-web/src/components/chat/Markdown.vue b/apps/kimi-web/src/components/chat/Markdown.vue
index 25a9936c05..267520c637 100644
--- a/apps/kimi-web/src/components/chat/Markdown.vue
+++ b/apps/kimi-web/src/components/chat/Markdown.vue
@@ -332,9 +332,17 @@ const CODE_DARK_THEME = 'github-dark';
 // Chat code blocks show no gutter: line numbers eat 3+ characters of reading
 // width and every chat block starts at line 1 anyway. stream-diffs derives its
 // gutter from `lineNumbers === false` (boolean, not monaco's 'off' string).
-// This rides inside codeBlockProps because markstream 1.0.7 only forwards the
+// This rides inside codeBlockProps because markstream only forwards the
 // top-level codeBlockMonacoOptions to the 'monaco' renderer kind — the 'shiki'
 // kind's props object omits it, while codeBlockProps reach the same component.
+//
+// fontSize / fontFamily matter as much as the gutter: since 1.0.9 the shiki
+// renderer (stream-diffs) applies these options as inline styles on the code
+// container (`applyEditorStyles`), so CSS overrides cannot beat them — the
+// values must be passed here. fontSize mirrors --text-sm (13px). `padding`
+// is ignored by the settled renderer (it draws inside a shadow root; its
+// vertical padding comes from `--diffs-gap-block` below) but sets the loading
+// fallback's inline padding, keeping the fallback → settled swap stable.
 const codeBlockProps = {
   showHeader: true,
   showCopyButton: true,
@@ -343,7 +351,12 @@ const codeBlockProps = {
   showCollapseButton: false,
   showFontSizeButtons: false,
   loading: false,
-  monacoOptions: { lineNumbers: false },
+  monacoOptions: {
+    lineNumbers: false,
+    fontSize: 13,
+    fontFamily: 'var(--font-mono)',
+    padding: { top: 12, bottom: 12 },
+  },
 };
 
 // Root cause for the "large session turns into code skeletons" failure:
@@ -629,9 +642,9 @@ function copyDiff(code: string, idx: number) {
   font-family: var(--font-ui);
 }
 /* Copy button — mirrors the §03 IconButton: muted glyph, sunken hover, soft
-   radius, and the shared focus ring. markstream renders its own button, so we
-   restyle it in place instead of swapping in the IconButton primitive. */
-.md :deep(.code-block-header .copy-button),
+   radius, and the shared focus ring. markstream renders its own button (the
+   `.code-action-btn` class since 1.0.9 — the old `.copy-button` is gone), so
+   we restyle it in place instead of swapping in the IconButton primitive. */
 .md :deep(.code-block-header .code-action-btn) {
   color: var(--color-text-muted);
   background: transparent;
@@ -641,24 +654,49 @@ function copyDiff(code: string, idx: number) {
   transition: background var(--duration-base) var(--ease-out),
     color var(--duration-base) var(--ease-out);
 }
-.md :deep(.code-block-header .copy-button:hover),
 .md :deep(.code-block-header .code-action-btn:hover) {
   background: var(--color-surface-sunken);
   color: var(--color-text);
 }
-.md :deep(.code-block-header .copy-button:focus-visible),
 .md :deep(.code-block-header .code-action-btn:focus-visible) {
   outline: none;
   box-shadow: var(--p-focus-ring);
 }
-.md :deep(.code-block-header .copy-button *),
 .md :deep(.code-block-header .code-action-btn *) {
   pointer-events: none;
 }
-.md :deep(.code-block-content),
+/* The code body wrapper was renamed in 1.0.9: `.code-block-content` is gone,
+   the shiki (stream-diffs) block now mounts under `.code-block-shell-content`. */
+.md :deep(.code-block-shell-content),
 .md :deep(.markstream-pre) {
   background: var(--color-surface-sunken);
 }
+/* Pierre renders the highlighted code inside a shadow root. Its native gap
+   variable inherits through that boundary; the Monaco `padding` option above
+   does not, but it sets the loading fallback's inline padding — both layers
+   end up at 12px. Line height stays relative (1.65) so every rendering path
+   shares the token ratio without pinning a px value into inline styles. */
+.md :deep(.code-editor-container) {
+  line-height: 1.65;
+  --diffs-gap-block: var(--space-3);
+}
+.md :deep(.code-editor-container diffs-container) {
+  --diffs-line-height: 1.65em;
+}
+/* Loading/streaming fallback <pre>: upstream hardcodes show-line-numbers on
+   it while the settled stream-diffs block honors lineNumbers:false — the
+   gutter (and its reserved padding) popping in and out on every load is a
+   visible flash. Hide the fallback gutter, pin its left inset to the settled
+   per-line padding (1ch), and force the shared 1.65 line height over the
+   inline 1.5×-font-size default upstream stamps on the pre, so the
+   fallback → highlighted swap is layout-stable. */
+.md :deep(.code-pre-fallback > .markstream-pre__line-numbers) {
+  display: none;
+}
+.md :deep(.code-block-container .code-pre-fallback) {
+  padding-left: 1ch;
+  line-height: 1.65 !important;
+}
 .md :deep(.code-block-container pre:not(.code-pre-fallback):not(.markstream-pre--line-numbers)),
 .md :deep(.markstream-pre:not(.code-pre-fallback):not(.markstream-pre--line-numbers)) {
   margin: 0;
@@ -676,8 +714,8 @@ function copyDiff(code: string, idx: number) {
 }
 .md :deep(.markstream-pre),
 .md :deep(.code-pre-fallback),
-.md :deep(.code-block-content pre:not(.shiki)),
-.md :deep(.code-block-content pre:not(.shiki) code) {
+.md :deep(.code-block-shell-content pre:not(.shiki)),
+.md :deep(.code-block-shell-content pre:not(.shiki) code) {
   color: var(--color-text);
 }
 
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index 980988e3bf..72d8deb208 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -218,7 +218,7 @@ importers:
         specifier: ^0.17.0
         version: 0.17.0
       markstream-vue:
-        specifier: ^1.0.9-beta.1
+        specifier: 1.0.9-beta.1
         version: 1.0.9-beta.1(katex@0.17.0)(mermaid@11.15.0)(stream-diffs@0.0.2(@shikijs/themes@4.3.0)(react-dom@19.2.5(react@19.2.5))(react@19.2.5)(vue@3.5.35(typescript@6.0.2)))(stream-markdown@0.0.16(react@19.2.5)(shiki@4.3.0)(vue@3.5.35(typescript@6.0.2)))(stream-monaco@0.0.49(monaco-editor@0.55.1))(vue-i18n@11.4.5(vue@3.5.35(typescript@6.0.2)))(vue@3.5.35(typescript@6.0.2))
       mermaid:
         specifier: ^11.15.0
@@ -230,13 +230,13 @@ importers:
         specifier: ^4.3.0
         version: 4.3.0
       stream-diffs:
-        specifier: ^0.0.2
+        specifier: 0.0.2
         version: 0.0.2(@shikijs/themes@4.3.0)(react-dom@19.2.5(react@19.2.5))(react@19.2.5)(vue@3.5.35(typescript@6.0.2))
       stream-markdown:
-        specifier: ^0.0.16
+        specifier: 0.0.16
         version: 0.0.16(react@19.2.5)(shiki@4.3.0)(vue@3.5.35(typescript@6.0.2))
       stream-monaco:
-        specifier: ^0.0.49
+        specifier: 0.0.49
         version: 0.0.49(monaco-editor@0.55.1)
       vue:
         specifier: ^3.5.35