diff --git a/packages/ui/src/components/message-preview.tsx b/packages/ui/src/components/message-preview.tsx
new file mode 100644
index 00000000..b8e153fb
--- /dev/null
+++ b/packages/ui/src/components/message-preview.tsx
@@ -0,0 +1,27 @@
+import type { Component } from "solid-js"
+import MessageItem from "./message-item"
+import type { MessageRecord } from "../stores/message-v2/types"
+import type { MessageInfo } from "../types/message"
+
+interface MessagePreviewProps {
+  record: MessageRecord
+  messageInfo?: MessageInfo
+  instanceId: string
+  sessionId: string
+}
+
+const MessagePreview: Component<MessagePreviewProps> = (props) => {
+  return (
+    <div class="message-preview">
+      <MessageItem
+        record={props.record}
+        messageInfo={props.messageInfo}
+        instanceId={props.instanceId}
+        sessionId={props.sessionId}
+        parts={props.record.partIds.map((id) => props.record.parts[id]?.data).filter((part): part is NonNullable<typeof part> => Boolean(part))}
+      />
+    </div>
+  )
+}
+
+export default MessagePreview
diff --git a/packages/ui/src/components/message-section.tsx b/packages/ui/src/components/message-section.tsx
index 24c46c61..b6b4aedb 100644
--- a/packages/ui/src/components/message-section.tsx
+++ b/packages/ui/src/components/message-section.tsx
@@ -502,6 +502,8 @@ export default function MessageSection(props: MessageSectionProps) {
               segments={timelineSegments()}
               onSegmentClick={handleTimelineSegmentClick}
               activeMessageId={activeMessageId()}
+              instanceId={props.instanceId}
+              sessionId={props.sessionId}
             />
           </div>
         </Show>
diff --git a/packages/ui/src/components/message-timeline.tsx b/packages/ui/src/components/message-timeline.tsx
index e8e7a132..79d4eae7 100644
--- a/packages/ui/src/components/message-timeline.tsx
+++ b/packages/ui/src/components/message-timeline.tsx
@@ -1,4 +1,6 @@
-import { For, createEffect, onCleanup, type Component } from "solid-js"
+import { For, Show, createEffect, createMemo, createSignal, onCleanup, type Component } from "solid-js"
+import MessagePreview from "./message-preview"
+import { messageStoreBus } from "../stores/message-v2/bus"
 import type { ClientPart } from "../types/message"
 import type { MessageRecord } from "../stores/message-v2/types"
 import { buildRecordDisplayData } from "../stores/message-v2/record-display-cache"
@@ -17,10 +19,12 @@ interface MessageTimelineProps {
   segments: TimelineSegment[]
   onSegmentClick?: (segment: TimelineSegment) => void
   activeMessageId?: string | null
+  instanceId: string
+  sessionId: string
 }
 
 const SEGMENT_LABELS: Record<TimelineSegmentType, string> = {
-  user: "User",
+  user: "You",
   assistant: "Asst",
   tool: "Tool",
 }
@@ -204,6 +208,10 @@ export function buildTimelineSegments(instanceId: string, record: MessageRecord)
 
 const MessageTimeline: Component<MessageTimelineProps> = (props) => {
   const buttonRefs = new Map<string, HTMLButtonElement>()
+  const store = () => messageStoreBus.getOrCreate(props.instanceId)
+  const [hoveredSegment, setHoveredSegment] = createSignal<TimelineSegment | null>(null)
+  const [tooltipCoords, setTooltipCoords] = createSignal<{ top: number; left: number }>({ top: 0, left: 0 })
+  let hoverTimer: number | null = null
  
   const registerButtonRef = (segmentId: string, element: HTMLButtonElement | null) => {
     if (element) {
@@ -213,6 +221,36 @@ const MessageTimeline: Component<MessageTimelineProps> = (props) => {
     }
   }
  
+  const clearHoverTimer = () => {
+    if (hoverTimer !== null && typeof window !== "undefined") {
+      window.clearTimeout(hoverTimer)
+      hoverTimer = null
+    }
+  }
+ 
+  const handleMouseEnter = (segment: TimelineSegment, event: MouseEvent) => {
+    if (typeof window === "undefined") return
+    clearHoverTimer()
+    const target = event.currentTarget as HTMLButtonElement
+    hoverTimer = window.setTimeout(() => {
+      const rect = target.getBoundingClientRect()
+      const preferredTop = rect.top + rect.height / 2
+      const clampedTop = Math.min(window.innerHeight - 220, Math.max(16, preferredTop - 110))
+      const tooltipWidth = 360
+      const preferredLeft = rect.right + 12
+      const clampedLeft = Math.min(window.innerWidth - tooltipWidth - 16, preferredLeft)
+      setTooltipCoords({ top: clampedTop, left: clampedLeft })
+      setHoveredSegment(segment)
+    }, 200)
+  }
+ 
+  const handleMouseLeave = () => {
+    clearHoverTimer()
+    setHoveredSegment(null)
+  }
+ 
+  onCleanup(() => clearHoverTimer())
+ 
   createEffect(() => {
     const activeId = props.activeMessageId
     if (!activeId) return
@@ -230,6 +268,15 @@ const MessageTimeline: Component<MessageTimelineProps> = (props) => {
     })
   })
  
+  const previewData = createMemo(() => {
+    const segment = hoveredSegment()
+    if (!segment) return null
+    const record = store().getMessage(segment.messageId)
+    if (!record) return null
+    const info = store().getMessageInfo(segment.messageId)
+    return { record, info }
+  })
+ 
   return (
     <div class="message-timeline" role="navigation" aria-label="Message timeline">
       <For each={props.segments}>
@@ -241,9 +288,10 @@ const MessageTimeline: Component<MessageTimelineProps> = (props) => {
               ref={(el) => registerButtonRef(segment.id, el)}
               type="button"
               class={`message-timeline-segment message-timeline-${segment.type} ${isActive() ? "message-timeline-segment-active" : ""}`}
-              title={segment.tooltip}
               aria-current={isActive() ? "true" : undefined}
               onClick={() => props.onSegmentClick?.(segment)}
+              onMouseEnter={(event) => handleMouseEnter(segment, event)}
+              onMouseLeave={handleMouseLeave}
             >
               <span class="message-timeline-label message-timeline-label-full">{segment.label}</span>
               <span class="message-timeline-label message-timeline-label-short">{SEGMENT_SHORT_LABELS[segment.type]}</span>
@@ -251,9 +299,21 @@ const MessageTimeline: Component<MessageTimelineProps> = (props) => {
           )
         }}
       </For>
+      <Show when={previewData()}>
+        {(data) => (
+          <div class="message-timeline-tooltip" style={{ top: `${tooltipCoords().top}px`, left: `${tooltipCoords().left}px` }}>
+            <MessagePreview
+              record={data().record}
+              messageInfo={data().info}
+              instanceId={props.instanceId}
+              sessionId={props.sessionId}
+            />
+          </div>
+        )}
+      </Show>
     </div>
   )
 }
-
-
+ 
 export default MessageTimeline
+
diff --git a/packages/ui/src/styles/messaging/message-timeline.css b/packages/ui/src/styles/messaging/message-timeline.css
index 1261fcc6..fddc19f8 100644
--- a/packages/ui/src/styles/messaging/message-timeline.css
+++ b/packages/ui/src/styles/messaging/message-timeline.css
@@ -149,4 +149,23 @@
   }
 }
 
+.message-timeline-tooltip {
+  position: fixed;
+  z-index: 1000;
+  pointer-events: none;
+}
 
+.message-preview {
+  width: 360px;
+  max-height: 420px;
+  overflow: auto;
+  border-radius: 8px;
+  border: 1px solid var(--border-base);
+  background-color: var(--surface-base);
+  box-shadow: var(--panel-shadow, 0 12px 32px rgba(0, 0, 0, 0.25));
+  padding: 0.75rem;
+}
+
+.message-preview .message-item-base {
+  font-size: 0.85rem;
+}