diff --git a/web/src/components/ChatBlockList.tsx b/web/src/components/ChatBlockList.tsx
index 35838f4e..7fee5f91 100644
--- a/web/src/components/ChatBlockList.tsx
+++ b/web/src/components/ChatBlockList.tsx
@@ -3,6 +3,7 @@ import type { MessageStatus } from '@/types/api'
 import type { ApiClient } from '@/api/client'
 import type { SessionMetadataSummary } from '@/types/api'
 import { MarkdownRenderer } from '@/components/MarkdownRenderer'
+import { LazyRainbowText } from '@/components/LazyRainbowText'
 import { ToolCard } from '@/components/ToolCard/ToolCard'
 
 function ErrorIcon() {
@@ -99,7 +100,7 @@ export function ChatBlockList(props: {
                         <div key={`user:${block.id}`} className={userBubbleClass}>
                             <div className="flex items-end gap-2">
                                 <div className="flex-1">
-                                    <MarkdownRenderer content={block.text} />
+                                    <LazyRainbowText text={block.text} />
                                 </div>
                                 {status ? (
                                     <div className="shrink-0 self-end pb-0.5">
diff --git a/web/src/components/LazyRainbowText.tsx b/web/src/components/LazyRainbowText.tsx
new file mode 100644
index 00000000..d5f557f3
--- /dev/null
+++ b/web/src/components/LazyRainbowText.tsx
@@ -0,0 +1,104 @@
+import React, { useRef, useState, useEffect, useMemo } from 'react'
+import { MarkdownRenderer } from '@/components/MarkdownRenderer'
+
+const ULTRATHINK_PATTERN = /\b(ultrathink)\b/gi
+
+// Each letter gets a different delay for wave effect
+function RainbowWord({ word, baseKey }: { word: string; baseKey: number }) {
+    const totalLetters = word.length
+    const cycleDuration = 2 // seconds for sparkle to travel across all letters
+
+    return (
+        <span>
+            {word.split('').map((letter, i) => {
+                // Each letter has a different delay to create wave effect
+                const colorDelay = (i / totalLetters) * 2 // stagger rainbow colors
+                const sparkleDelay = (i / totalLetters) * cycleDuration // sparkle wave
+
+                return (
+                    <span
+                        key={`${baseKey}-${i}`}
+                        className="rainbow-letter"
+                        style={{
+                            animationDelay: `${-colorDelay}s, ${-sparkleDelay}s`,
+                        }}
+                    >
+                        {letter}
+                    </span>
+                )
+            })}
+        </span>
+    )
+}
+
+// Process text string to wrap "ultrathink" with RainbowWord
+function processTextForRainbow(text: string): React.ReactNode {
+    ULTRATHINK_PATTERN.lastIndex = 0
+    const parts: React.ReactNode[] = []
+    let lastIndex = 0
+    let match: RegExpExecArray | null
+
+    while ((match = ULTRATHINK_PATTERN.exec(text)) !== null) {
+        if (match.index > lastIndex) {
+            parts.push(text.slice(lastIndex, match.index))
+        }
+        parts.push(<RainbowWord key={match.index} word={match[1]} baseKey={match.index} />)
+        lastIndex = match.index + match[0].length
+    }
+
+    if (lastIndex < text.length) {
+        parts.push(text.slice(lastIndex))
+    }
+
+    return <>{parts}</>
+}
+
+// Process React children to apply rainbow to text nodes
+function processChildrenForRainbow(children: React.ReactNode): React.ReactNode {
+    return React.Children.map(children, (child) => {
+        if (typeof child === 'string') {
+            return processTextForRainbow(child)
+        }
+        return child
+    })
+}
+
+export function LazyRainbowText({ text }: { text: string }) {
+    const ref = useRef<HTMLDivElement>(null)
+    const [hasBeenVisible, setHasBeenVisible] = useState(false)
+
+    useEffect(() => {
+        const el = ref.current
+        if (!el) return
+
+        const observer = new IntersectionObserver(
+            ([entry]) => {
+                if (entry.isIntersecting) {
+                    setHasBeenVisible(true)
+                }
+            },
+            { rootMargin: '100px' }
+        )
+
+        observer.observe(el)
+        return () => observer.disconnect()
+    }, [])
+
+    // Quick check: if no ultrathink, just render markdown
+    const hasUltrathink = text.toLowerCase().includes('ultrathink')
+
+    const rainbowComponents = useMemo(() => ({
+        p: ({ children }: { children?: React.ReactNode }) => (
+            <p>{processChildrenForRainbow(children)}</p>
+        ),
+    }), [])
+
+    return (
+        <div ref={ref}>
+            <MarkdownRenderer
+                content={text}
+                components={hasUltrathink && hasBeenVisible ? rainbowComponents : undefined}
+            />
+        </div>
+    )
+}
diff --git a/web/src/components/MarkdownRenderer.tsx b/web/src/components/MarkdownRenderer.tsx
index 24fe24c6..99f9a310 100644
--- a/web/src/components/MarkdownRenderer.tsx
+++ b/web/src/components/MarkdownRenderer.tsx
@@ -1,4 +1,4 @@
-import ReactMarkdown from 'react-markdown'
+import ReactMarkdown, { Components } from 'react-markdown'
 import remarkGfm from 'remark-gfm'
 import { CodeBlock } from '@/components/CodeBlock'
 
@@ -6,36 +6,42 @@ function getLanguageFromClassName(className?: string): string | null {
     if (!className) return null
     for (const token of className.split(/\s+/g)) {
         if (token.startsWith('language-')) {
-            const language = token.slice('language-'.length)
-            return language || null
+            return token.slice('language-'.length) || null
         }
     }
     return null
 }
 
-export function MarkdownRenderer({ content }: { content: string }) {
+const defaultComponents: Components = {
+    code({ className, children, ...props }) {
+        const language = getLanguageFromClassName(className) || 'text'
+        const code = String(children).replace(/\n$/, '')
+
+        if (language !== 'text' || code.includes('\n')) {
+            return (
+                <CodeBlock
+                    code={code}
+                    language={language}
+                    showCopyButton={false}
+                />
+            )
+        }
+
+        return <code className={className} {...props}>{children}</code>
+    },
+}
+
+interface MarkdownRendererProps {
+    content: string
+    components?: Components
+}
+
+export function MarkdownRenderer({ content, components }: MarkdownRendererProps) {
     return (
         <div className="markdown-content text-sm">
             <ReactMarkdown
                 remarkPlugins={[remarkGfm]}
-                components={{
-                    code({ className, children, ...props }) {
-                        const language = getLanguageFromClassName(className) || 'text'
-                        const code = String(children).replace(/\n$/, '')
-
-                        if (language !== 'text' || code.includes('\n')) {
-                            return (
-                                <CodeBlock
-                                    code={code}
-                                    language={language}
-                                    showCopyButton={false}
-                                />
-                            )
-                        }
-
-                        return <code className={className} {...props}>{children}</code>
-                    }
-                }}
+                components={{ ...defaultComponents, ...components }}
             >
                 {content}
             </ReactMarkdown>
diff --git a/web/src/index.css b/web/src/index.css
index ea4ca3b4..9dc0aa19 100644
--- a/web/src/index.css
+++ b/web/src/index.css
@@ -85,3 +85,30 @@ body {
 .markdown-content ul, .markdown-content ol { padding-left: 1.5rem; margin: 0.5rem 0; }
 .markdown-content table { border-collapse: collapse; width: 100%; }
 .markdown-content th, .markdown-content td { border: 1px solid var(--app-border); padding: 0.25rem 0.5rem; }
+
+/* Ultrathink rainbow sparkle effect - per-letter wave animation */
+@keyframes rainbow-wave {
+  0% { color: #ff5555; }      /* red */
+  14% { color: #ffaa00; }     /* orange */
+  28% { color: #ffff55; }     /* yellow */
+  42% { color: #55ff55; }     /* green */
+  57% { color: #55ffff; }     /* cyan */
+  71% { color: #5555ff; }     /* blue */
+  85% { color: #ff55ff; }     /* magenta/pink */
+  100% { color: #ff5555; }    /* red */
+}
+
+@keyframes sparkle-wave {
+  0%, 100% {
+    filter: brightness(1);
+  }
+  50% {
+    filter: brightness(0.7);
+  }
+}
+
+.rainbow-letter {
+  display: inline-block;
+  animation: rainbow-wave 2s linear infinite, sparkle-wave 2s ease-in-out infinite;
+  font-weight: 600;
+}