diff --git a/ui/web/src/stores/use-chat-messages-store.ts b/ui/web/src/stores/use-chat-messages-store.ts
index 98ee7239..bc271a9d 100644
--- a/ui/web/src/stores/use-chat-messages-store.ts
+++ b/ui/web/src/stores/use-chat-messages-store.ts
@@ -1,119 +1,101 @@
 import { create } from "zustand";
 import type { ChatMessage } from "@/types/chat";
 
+const MAX_CACHED_SESSIONS = 25;
+
 interface SessionMessages {
   messages: ChatMessage[];
   streamText: string | null;
   thinkingText: string | null;
   isRunning: boolean;
+  lastAccessedAt: number;
 }
 
 interface ChatMessagesState {
-  // Map of sessionKey -> session message state
   sessions: Record<string, SessionMessages>;
-
-  // Set messages for a specific session
   setSessionMessages: (sessionKey: string, messages: ChatMessage[]) => void;
-
-  // Update session messages incrementally
   updateSessionMessages: (sessionKey: string, updater: (prev: ChatMessage[]) => ChatMessage[]) => void;
-
-  // Set streaming text for a session
   setSessionStream: (sessionKey: string, streamText: string | null) => void;
-
-  // Set thinking text for a session
   setSessionThinking: (sessionKey: string, thinkingText: string | null) => void;
-
-  // Set running state for a session
   setSessionRunning: (sessionKey: string, isRunning: boolean) => void;
 }
 
+// Evict oldest idle sessions when cache exceeds limit.
+// Never evict sessions with an active run (isRunning).
+function evictStale(sessions: Record<string, SessionMessages>): Record<string, SessionMessages> {
+  const keys = Object.keys(sessions);
+  if (keys.length <= MAX_CACHED_SESSIONS) return sessions;
+
+  const evictable = keys
+    .filter((k) => !sessions[k]?.isRunning)
+    .sort((a, b) => (sessions[a]?.lastAccessedAt ?? 0) - (sessions[b]?.lastAccessedAt ?? 0));
+
+  const toRemove = evictable.slice(0, keys.length - MAX_CACHED_SESSIONS);
+  if (toRemove.length === 0) return sessions;
+
+  const next = { ...sessions };
+  for (const k of toRemove) delete next[k];
+  return next;
+}
+
+function touchSession(existing: SessionMessages | undefined, patch: Partial<SessionMessages>): SessionMessages {
+  return {
+    messages: patch.messages ?? existing?.messages ?? [],
+    streamText: patch.streamText !== undefined ? patch.streamText : (existing?.streamText ?? null),
+    thinkingText: patch.thinkingText !== undefined ? patch.thinkingText : (existing?.thinkingText ?? null),
+    isRunning: patch.isRunning !== undefined ? patch.isRunning : (existing?.isRunning ?? false),
+    lastAccessedAt: Date.now(),
+  };
+}
+
 export const useChatMessagesStore = create<ChatMessagesState>((set) => ({
   sessions: {},
 
-  setSessionMessages: (sessionKey: string, messages: ChatMessage[]) => {
+  setSessionMessages: (sessionKey, messages) => {
+    set((state) => ({
+      sessions: evictStale({
+        ...state.sessions,
+        [sessionKey]: touchSession(state.sessions[sessionKey], { messages }),
+      }),
+    }));
+  },
+
+  updateSessionMessages: (sessionKey, updater) => {
     set((state) => {
-      const existing = state.sessions[sessionKey];
+      const current = state.sessions[sessionKey]?.messages ?? [];
       return {
-        sessions: {
+        sessions: evictStale({
           ...state.sessions,
-          [sessionKey]: {
-            messages,
-            streamText: existing?.streamText ?? null,
-            thinkingText: existing?.thinkingText ?? null,
-            isRunning: existing?.isRunning ?? false,
-          },
-        },
+          [sessionKey]: touchSession(state.sessions[sessionKey], { messages: updater(current) }),
+        }),
       };
     });
   },
 
-  updateSessionMessages: (sessionKey: string, updater: (prev: ChatMessage[]) => ChatMessage[]) => {
-    set((state) => {
-      const existing = state.sessions[sessionKey];
-      const current = existing?.messages ?? [];
-      return {
-        sessions: {
-          ...state.sessions,
-          [sessionKey]: {
-            messages: updater(current),
-            streamText: existing?.streamText ?? null,
-            thinkingText: existing?.thinkingText ?? null,
-            isRunning: existing?.isRunning ?? false,
-          },
-        },
-      };
-    });
+  setSessionStream: (sessionKey, streamText) => {
+    set((state) => ({
+      sessions: {
+        ...state.sessions,
+        [sessionKey]: touchSession(state.sessions[sessionKey], { streamText }),
+      },
+    }));
   },
 
-  setSessionStream: (sessionKey: string, streamText: string | null) => {
-    set((state) => {
-      const existing = state.sessions[sessionKey];
-      return {
-        sessions: {
-          ...state.sessions,
-          [sessionKey]: {
-            messages: existing?.messages ?? [],
-            streamText,
-            thinkingText: existing?.thinkingText ?? null,
-            isRunning: existing?.isRunning ?? false,
-          },
-        },
-      };
-    });
+  setSessionThinking: (sessionKey, thinkingText) => {
+    set((state) => ({
+      sessions: {
+        ...state.sessions,
+        [sessionKey]: touchSession(state.sessions[sessionKey], { thinkingText }),
+      },
+    }));
   },
 
-  setSessionThinking: (sessionKey: string, thinkingText: string | null) => {
-    set((state) => {
-      const existing = state.sessions[sessionKey];
-      return {
-        sessions: {
-          ...state.sessions,
-          [sessionKey]: {
-            messages: existing?.messages ?? [],
-            streamText: existing?.streamText ?? null,
-            thinkingText,
-            isRunning: existing?.isRunning ?? false,
-          },
-        },
-      };
-    });
-  },
-
-  setSessionRunning: (sessionKey: string, isRunning: boolean) => {
-    set((state) => {
-      const existing = state.sessions[sessionKey];
-      return {
-        sessions: {
-          ...state.sessions,
-          [sessionKey]: {
-            messages: existing?.messages ?? [],
-            streamText: existing?.streamText ?? null,
-            thinkingText: existing?.thinkingText ?? null,
-            isRunning,
-          },
-        },
-      };
-    });
+  setSessionRunning: (sessionKey, isRunning) => {
+    set((state) => ({
+      sessions: {
+        ...state.sessions,
+        [sessionKey]: touchSession(state.sessions[sessionKey], { isRunning }),
+      },
+    }));
   },
 }));