diff --git a/packages/ui/src/components/chat/MarkdownRendererImpl.tsx b/packages/ui/src/components/chat/MarkdownRendererImpl.tsx
index 35667011..943e91c6 100644
--- a/packages/ui/src/components/chat/MarkdownRendererImpl.tsx
+++ b/packages/ui/src/components/chat/MarkdownRendererImpl.tsx
@@ -845,17 +845,29 @@ const useMorphdomMarkdown = ({
       // to the trailing (growing) block instead of the whole message.
       blocks.forEach((block, index) => {
         let el = existing[index];
+        let isNewBlock = false;
         if (!el) {
           el = document.createElement('div');
           el.setAttribute('data-md-block', '');
           el.style.display = 'contents';
           target.appendChild(el);
+          isNewBlock = true;
         }
         if (el.getAttribute('data-md-id') === block.id) return;
 
         const temp = document.createElement('div');
         temp.innerHTML = block.html;
         decorateMarkdown(temp, ctx);
+        if (isNewBlock && streaming && index > 0) {
+          // A freshly committed block enters with a short reveal. The class
+          // goes on the block's children — the wrapper is display:contents
+          // and cannot animate — and the transform never changes layout, so
+          // row measurement stays exact. Skipped for the first block so a
+          // full initial render does not shimmer.
+          for (const child of Array.from(temp.children)) {
+            child.classList.add('oc-md-block-enter');
+          }
+        }
         const hadMermaidBlock = shouldRefreshMermaidViewers(el);
         const tempHasMermaidBlock = shouldRefreshMermaidViewers(temp);
         morphdom(el, temp, {
diff --git a/packages/ui/src/components/chat/markdown/markdownCore.ts b/packages/ui/src/components/chat/markdown/markdownCore.ts
index 822a3168..5d43f798 100644
--- a/packages/ui/src/components/chat/markdown/markdownCore.ts
+++ b/packages/ui/src/components/chat/markdown/markdownCore.ts
@@ -178,9 +178,11 @@ type MarkdownBlock = {
   raw: string;
   src: string;
   mode: 'full' | 'live';
-  // When false, skip syntax highlighting for this block. Set for the actively
-  // streaming open code fence so we don't re-tokenize a growing block ~40x/sec
-  // (O(n^2)); it highlights once the fence closes and becomes a stable block.
+  // When false, skip syntax highlighting for this block. Block-level commit
+  // feeds the open fence whole lines at the throttle cadence (<=10/sec), so a
+  // partial fence highlights too and streamed code arrives colored; only a
+  // very large open fence falls back to plain text until it closes, keeping
+  // the repeated worker re-tokenization bounded.
   highlight: boolean;
 };
 
@@ -201,6 +203,11 @@ const hasOpenFence = (raw: string): boolean => {
   return !new RegExp(`^[\\t ]{0,3}${char}{${size},}[\\t ]*$`).test(last);
 };
 
+// Above this, re-highlighting the still-open fence on every committed line
+// costs more than the colored preview is worth; the block highlights in one
+// pass when the fence closes.
+const OPEN_FENCE_HIGHLIGHT_LINE_LIMIT = 300;
+
 const heal = (text: string): string => {
   try {
     return remend(text, { linkMode: 'text-only' });
@@ -250,11 +257,13 @@ const streamBlocks = (text: string, live: boolean): MarkdownBlock[] => {
     const raw = token.raw ?? '';
     const isLast = i === tail;
     const openFence = token.type === 'code' && hasOpenFence(raw);
+    const openFenceHighlight = openFence
+      && raw.split('\n').length <= OPEN_FENCE_HIGHLIGHT_LINE_LIMIT;
     blocks.push({
       raw,
       src: openFence ? raw : heal(raw),
       mode: isLast ? 'live' : 'full',
-      highlight: !openFence,
+      highlight: !openFence || openFenceHighlight,
     });
   }
 
diff --git a/packages/ui/src/index.css b/packages/ui/src/index.css
index 1ba7e73c..4b631c6b 100644
--- a/packages/ui/src/index.css
+++ b/packages/ui/src/index.css
@@ -1378,6 +1378,29 @@ html:not(.dark) .chat-scroll {
   }
 }
 
+/* A block committed mid-stream enters with a short fade-and-rise; the
+   transform is paint-only, so virtualized row measurement is unaffected. */
+@keyframes oc-md-block-enter {
+  from {
+    opacity: 0;
+    transform: translateY(4px);
+  }
+  to {
+    opacity: 1;
+    transform: none;
+  }
+}
+
+.oc-md-block-enter {
+  animation: oc-md-block-enter 140ms ease-out both;
+}
+
+@media (prefers-reduced-motion: reduce) {
+  .oc-md-block-enter {
+    animation: none;
+  }
+}
+
 /* While streaming defers the per-line gutter markup, hold its horizontal
    footprint (2rem column + 0.75rem gap) so the finished pass only fills in
    the numbers instead of shifting every code line. */