diff --git a/core/http/react-ui/src/pages/AgentChat.jsx b/core/http/react-ui/src/pages/AgentChat.jsx
index ae4f5c83b..13b83f2b0 100644
--- a/core/http/react-ui/src/pages/AgentChat.jsx
+++ b/core/http/react-ui/src/pages/AgentChat.jsx
@@ -8,7 +8,7 @@ import CanvasPanel from '../components/CanvasPanel'
 import ResourceCards from '../components/ResourceCards'
 import ConfirmDialog from '../components/ConfirmDialog'
 import { useAgentChat } from '../hooks/useAgentChat'
-import { relativeTime } from '../utils/format'
+import { relativeTime, normalizeTimestampMs } from '../utils/format'
 import { copyToClipboard } from '../utils/clipboard'
 
 function getLastMessagePreview(conv) {
@@ -139,8 +139,9 @@ export default function AgentChat() {
           id: nextId(),
           sender,
           content: data.content || data.message || '',
-          // Backend sends Unix milliseconds (see core/services/agents events).
-          timestamp: data.timestamp || Date.now(),
+          // Backend timestamp encoding varies by deploy mode (RFC3339 string,
+          // Unix ms, or Unix ns); normalize to JS milliseconds.
+          timestamp: normalizeTimestampMs(data.timestamp),
         }
         if (data.metadata && Object.keys(data.metadata).length > 0) {
           msg.metadata = data.metadata
diff --git a/core/http/react-ui/src/utils/format.js b/core/http/react-ui/src/utils/format.js
index b17006e08..1cf162028 100644
--- a/core/http/react-ui/src/utils/format.js
+++ b/core/http/react-ui/src/utils/format.js
@@ -12,6 +12,26 @@ export function percentColor(pct) {
   return 'var(--color-success)'
 }
 
+// normalizeTimestampMs converts a timestamp emitted by the backend into JS
+// milliseconds, regardless of its encoding. The agent SSE bridge emits the
+// json_message timestamp in three different shapes depending on deploy mode:
+// an RFC3339 string (standalone agent pool), Unix milliseconds (local
+// dispatcher), or Unix nanoseconds (older NATS path). A numeric value is
+// classified by magnitude (s / ms / us / ns) so any of them yields a sane
+// epoch. Falls back to Date.now() for null/empty/unparseable input.
+export function normalizeTimestampMs(ts) {
+  if (ts === null || ts === undefined || ts === '') return Date.now()
+  if (typeof ts === 'string') {
+    const parsed = Date.parse(ts)
+    return Number.isNaN(parsed) ? Date.now() : parsed
+  }
+  if (typeof ts !== 'number' || !Number.isFinite(ts)) return Date.now()
+  if (ts > 1e17) return Math.floor(ts / 1e6) // nanoseconds
+  if (ts > 1e14) return Math.floor(ts / 1e3) // microseconds
+  if (ts > 1e11) return ts                    // milliseconds
+  return ts * 1000                            // seconds
+}
+
 export function formatTimestamp(ts) {
   if (!ts) return '-'
   const d = new Date(ts)