diff --git a/src/client/src/chatHistoryCache.ts b/src/client/src/chatHistoryCache.ts
new file mode 100644
index 0000000..6868fe3
--- /dev/null
+++ b/src/client/src/chatHistoryCache.ts
@@ -0,0 +1,71 @@
+const CACHE_PREFIX = "pi-web:chat-history:";
+const CACHE_TTL_MS = 30 * 60 * 1000;
+
+export interface RawMessagePage {
+  messages: unknown[];
+  start: number;
+  total: number;
+}
+
+export interface CachedChatHistory extends RawMessagePage {
+  savedAt: number;
+}
+
+export function readChatHistoryCache(sessionId: string): RawMessagePage | undefined {
+  try {
+    const raw = sessionStorage.getItem(cacheKey(sessionId));
+    if (raw === null || raw === "") return undefined;
+    const parsed: unknown = JSON.parse(raw);
+    if (!isCachedHistory(parsed)) return undefined;
+    if (Date.now() - parsed.savedAt > CACHE_TTL_MS) {
+      sessionStorage.removeItem(cacheKey(sessionId));
+      return undefined;
+    }
+    return { messages: parsed.messages, start: parsed.start, total: parsed.total };
+  } catch {
+    return undefined;
+  }
+}
+
+export function writeChatHistoryCache(sessionId: string, page: RawMessagePage): void {
+  try {
+    sessionStorage.setItem(cacheKey(sessionId), JSON.stringify({ ...page, savedAt: Date.now() }));
+  } catch {
+    // Ignore quota/private-mode failures; history paging still works without cache.
+  }
+}
+
+export function mergeChatHistory(existing: RawMessagePage | undefined, incoming: RawMessagePage): RawMessagePage {
+  if (!existing || existing.total !== incoming.total) return incoming;
+
+  const start = Math.min(existing.start, incoming.start);
+  const end = Math.max(existing.start + existing.messages.length, incoming.start + incoming.messages.length);
+  const messages = new Array<unknown>(end - start);
+  copyInto(messages, start, existing);
+  copyInto(messages, start, incoming);
+
+  return { start, total: incoming.total, messages: messages.filter((message) => message !== undefined) };
+}
+
+function copyInto(target: unknown[], targetStart: number, page: RawMessagePage): void {
+  page.messages.forEach((message, index) => {
+    target[page.start - targetStart + index] = message;
+  });
+}
+
+function cacheKey(sessionId: string): string {
+  return `${CACHE_PREFIX}${sessionId}`;
+}
+
+function isCachedHistory(value: unknown): value is CachedChatHistory {
+  return typeof value === "object"
+    && value !== null
+    && "messages" in value
+    && "start" in value
+    && "total" in value
+    && "savedAt" in value
+    && Array.isArray(value.messages)
+    && typeof value.start === "number"
+    && typeof value.total === "number"
+    && typeof value.savedAt === "number";
+}
diff --git a/src/client/src/controllers/sessionController.ts b/src/client/src/controllers/sessionController.ts
index 870d668..068cc64 100644
--- a/src/client/src/controllers/sessionController.ts
+++ b/src/client/src/controllers/sessionController.ts
@@ -2,6 +2,7 @@ import { api, type CommandResult, type SessionActivity, type SessionInfo, type S
 
 const MESSAGE_PAGE_SIZE = 100;
 import { normalizeMessages, textMessage } from "../chatMessages";
+import { readChatHistoryCache, mergeChatHistory, writeChatHistoryCache, type RawMessagePage } from "../chatHistoryCache";
 import { applyTranscriptEvent } from "../chatTranscript";
 import { isShellInput } from "../inputModes";
 import { GlobalSessionSocket, SessionSocket, type SessionUiEvent } from "../sessionSocket";
@@ -48,7 +49,8 @@ export class SessionController {
       const buffered: SessionUiEvent[] = [];
       this.socket.connect(session.id, (event) => buffered.push(event));
       const [page, status] = await Promise.all([api.messages(session.id, { limit: MESSAGE_PAGE_SIZE }), api.status(session.id)]);
-      this.setState({ selectedSession: session, messages: normalizeMessages(page.messages), messagePageStart: page.start, messagePageTotal: page.total, isLoadingEarlierMessages: false, status });
+      const history = this.mergeAndCacheHistory(session.id, page);
+      this.setState({ selectedSession: session, messages: normalizeMessages(history.messages), messagePageStart: history.start, messagePageTotal: history.total, isLoadingEarlierMessages: false, status });
       this.applyStatus(status);
       for (const event of buffered) this.applyEvent(event);
       this.socket.setHandler((event) => { this.applyEvent(event); });
@@ -66,10 +68,11 @@ export class SessionController {
     try {
       const page = await api.messages(session.id, { before: state.messagePageStart, limit: MESSAGE_PAGE_SIZE });
       if (this.getState().selectedSession?.id !== session.id) return;
+      const history = this.mergeAndCacheHistory(session.id, page);
       this.setState({
-        messages: [...normalizeMessages(page.messages), ...this.getState().messages],
-        messagePageStart: page.start,
-        messagePageTotal: page.total,
+        messages: normalizeMessages(history.messages),
+        messagePageStart: history.start,
+        messagePageTotal: history.total,
       });
     } catch (error) {
       this.setState({ error: String(error) });
@@ -142,6 +145,12 @@ export class SessionController {
     }
   }
 
+  private mergeAndCacheHistory(sessionId: string, page: RawMessagePage): RawMessagePage {
+    const history = mergeChatHistory(readChatHistoryCache(sessionId), page);
+    writeChatHistoryCache(sessionId, history);
+    return history;
+  }
+
   private applyCommandResult(result: CommandResult) {
     if (result.type === "select") {
       this.setState({ commandDialog: result });