diff --git a/internal/gateway/methods/chat.go b/internal/gateway/methods/chat.go
index 3843c465..41e233fa 100644
--- a/internal/gateway/methods/chat.go
+++ b/internal/gateway/methods/chat.go
@@ -60,7 +60,16 @@ func (m *ChatMethods) handleSend(ctx context.Context, client *gateway.Client, re
 	}
 
 	if params.AgentID == "" {
-		params.AgentID = "default"
+		// Extract agent key from session key (format: "agent:{key}:{rest}")
+		// so resuming an existing session routes to the correct agent.
+		if params.SessionKey != "" {
+			if agentKey, _ := sessions.ParseSessionKey(params.SessionKey); agentKey != "" {
+				params.AgentID = agentKey
+			}
+		}
+		if params.AgentID == "" {
+			params.AgentID = "default"
+		}
 	}
 
 	loop, err := m.agents.Get(params.AgentID)
diff --git a/ui/web/src/pages/chat/chat-page.tsx b/ui/web/src/pages/chat/chat-page.tsx
index 98cd4fa1..3ba0ae28 100644
--- a/ui/web/src/pages/chat/chat-page.tsx
+++ b/ui/web/src/pages/chat/chat-page.tsx
@@ -10,7 +10,7 @@ import { ChatInput } from "@/components/chat/chat-input";
 import { useChatSessions } from "./hooks/use-chat-sessions";
 import { useChatMessages } from "./hooks/use-chat-messages";
 import { useChatSend } from "./hooks/use-chat-send";
-import { isOwnSession } from "@/lib/session-key";
+import { isOwnSession, parseSessionKey } from "@/lib/session-key";
 
 export function ChatPage() {
   const { sessionKey: urlSessionKey } = useParams<{ sessionKey: string }>();
@@ -18,7 +18,13 @@ export function ChatPage() {
   const connected = useAuthStore((s) => s.connected);
   const userId = useAuthStore((s) => s.userId);
 
-  const [agentId, setAgentId] = useState("default");
+  const [agentId, setAgentId] = useState(() => {
+    if (urlSessionKey) {
+      const { agentId: parsed } = parseSessionKey(urlSessionKey);
+      if (parsed) return parsed;
+    }
+    return "default";
+  });
   const [sessionKey, setSessionKey] = useState(urlSessionKey ?? "");
 
   const {
@@ -78,10 +84,15 @@ export function ChatPage() {
 
   const handleSessionSelect = useCallback(
     (key: string) => {
+      // Sync agentId from session key to ensure correct routing
+      const { agentId: parsed } = parseSessionKey(key);
+      if (parsed && parsed !== agentId) {
+        setAgentId(parsed);
+      }
       setSessionKey(key);
       navigate(`/chat/${encodeURIComponent(key)}`);
     },
-    [navigate],
+    [navigate, agentId],
   );
 
   const handleAgentChange = useCallback(