diff --git a/core/src/sessions/in_memory_session_service.ts b/core/src/sessions/in_memory_session_service.ts
index e521f2fb3..fa19f8bd4 100644
--- a/core/src/sessions/in_memory_session_service.ts
+++ b/core/src/sessions/in_memory_session_service.ts
@@ -18,10 +18,9 @@ import {
   ListSessionsRequest,
   ListSessionsResponse,
   mergeStates,
-  trimTempState,
 } from './base_session_service.js';
 import {createSession, Session} from './session.js';
-import {extractStateDelta} from './state_utils.js';
+import {extractStateDelta, StateDeltas} from './state_utils.js';
 
 /**
  * Checks if the given URI is an in-memory memory service URI.
@@ -52,18 +51,45 @@ export class InMemorySessionService extends BaseSessionService {
    */
   private appState: Record<string, Record<string, unknown>> = {};
 
+  /**
+   * Merges app- and user-scoped deltas into the stores shared across sessions.
+   *
+   * Empty deltas are skipped so that apps and users which never set scoped
+   * state gain no entry at all.
+   */
+  private applyScopedDeltas(
+    appName: string,
+    userId: string,
+    {app, user}: StateDeltas,
+  ): void {
+    if (Object.keys(app).length > 0) {
+      this.appState[appName] = {...this.appState[appName], ...app};
+    }
+
+    if (Object.keys(user).length > 0) {
+      this.userState[appName] = this.userState[appName] || {};
+      this.userState[appName][userId] = {
+        ...this.userState[appName][userId],
+        ...user,
+      };
+    }
+  }
+
   async createSession({
     appName,
     userId,
     state,
     sessionId,
   }: CreateSessionRequest): Promise<Session> {
-    const filteredState = state ? trimTempState(state) : undefined;
+    const deltas = extractStateDelta(state);
+    // Must precede the mergeStates() below, which re-reads the scope stores.
+    this.applyScopedDeltas(appName, userId, deltas);
+
     const session = createSession({
       id: sessionId || randomUUID(),
       appName,
       userId,
-      state: filteredState,
+      state: deltas.session,
       events: [],
       lastUpdateTime: Date.now(),
     });
@@ -275,21 +301,11 @@ export class InMemorySessionService extends BaseSessionService {
     if (event.actions && event.actions.stateDelta) {
       // The session bucket is deliberately ignored: session state is applied by
       // super.appendEvent(), which keeps the `app:`/`user:` prefixes on the key.
-      const {app: appDelta, user: userDelta} = extractStateDelta(
-        event.actions.stateDelta,
+      this.applyScopedDeltas(
+        appName,
+        userId,
+        extractStateDelta(event.actions.stateDelta),
       );
-
-      if (Object.keys(appDelta).length > 0) {
-        this.appState[appName] = {...this.appState[appName], ...appDelta};
-      }
-
-      if (Object.keys(userDelta).length > 0) {
-        this.userState[appName] = this.userState[appName] || {};
-        this.userState[appName][userId] = {
-          ...this.userState[appName][userId],
-          ...userDelta,
-        };
-      }
     }
 
     const storageSession: Session = this.sessions[appName][userId][sessionId];
diff --git a/core/test/sessions/in_memory_session_service_test.ts b/core/test/sessions/in_memory_session_service_test.ts
index 64f54119c..56316e6c7 100644
--- a/core/test/sessions/in_memory_session_service_test.ts
+++ b/core/test/sessions/in_memory_session_service_test.ts
@@ -100,6 +100,141 @@ describe('InMemorySessionService', () => {
       expect(session.state).toHaveProperty('normalKey', 'value');
       expect(session.state).not.toHaveProperty(`${State.TEMP_PREFIX}tempKey`);
     });
+
+    it('shares app: state from initial state with other sessions of the same app', async () => {
+      const appName = 'app';
+      await service.createSession({
+        appName,
+        userId: 'user1',
+        state: {[`${State.APP_PREFIX}config`]: 'dark-mode'},
+      });
+
+      const session2 = await service.createSession({appName, userId: 'user2'});
+
+      expect(session2.state).toHaveProperty(
+        `${State.APP_PREFIX}config`,
+        'dark-mode',
+      );
+    });
+
+    it('shares user: state from initial state with other sessions of the same user', async () => {
+      const appName = 'app';
+      const userId = 'user1';
+      await service.createSession({
+        appName,
+        userId,
+        state: {[`${State.USER_PREFIX}pref`]: 'A'},
+      });
+
+      const session2 = await service.createSession({appName, userId});
+
+      expect(session2.state).toHaveProperty(`${State.USER_PREFIX}pref`, 'A');
+    });
+
+    it('does not share user: state from initial state with a different user', async () => {
+      const appName = 'app';
+      await service.createSession({
+        appName,
+        userId: 'user1',
+        state: {[`${State.USER_PREFIX}pref`]: 'A'},
+      });
+
+      const session2 = await service.createSession({appName, userId: 'user2'});
+
+      expect(session2.state).not.toHaveProperty(`${State.USER_PREFIX}pref`);
+    });
+
+    it('keeps unprefixed initial state scoped to the session that created it', async () => {
+      const appName = 'app';
+      const userId = 'user';
+      const session1 = await service.createSession({
+        appName,
+        userId,
+        state: {sessionKey: 'value'},
+      });
+
+      const session2 = await service.createSession({appName, userId});
+      const retrieved = await service.getSession({
+        appName,
+        userId,
+        sessionId: session1.id,
+      });
+
+      expect(retrieved?.state).toEqual({sessionKey: 'value'});
+      expect(session2.state).not.toHaveProperty('sessionKey');
+    });
+
+    it('drops temp: keys from initial state instead of promoting them', async () => {
+      const appName = 'app';
+      const session1 = await service.createSession({
+        appName,
+        userId: 'user1',
+        state: {
+          [`${State.TEMP_PREFIX}scratch`]: 'dropped',
+          [`${State.APP_PREFIX}kept`]: 1,
+        },
+      });
+
+      const session2 = await service.createSession({appName, userId: 'user2'});
+
+      expect(session1.state).toEqual({[`${State.APP_PREFIX}kept`]: 1});
+      expect(session2.state).toEqual({[`${State.APP_PREFIX}kept`]: 1});
+    });
+
+    it('lets app: initial state overwrite an existing app value', async () => {
+      const appName = 'app';
+      const userId = 'user';
+      const session1 = await service.createSession({
+        appName,
+        userId,
+        state: {[`${State.APP_PREFIX}config`]: 'dark-mode'},
+      });
+
+      const session2 = await service.createSession({
+        appName,
+        userId,
+        state: {[`${State.APP_PREFIX}config`]: 'light-mode'},
+      });
+      const retrieved = await service.getSession({
+        appName,
+        userId,
+        sessionId: session1.id,
+      });
+
+      expect(session2.state).toHaveProperty(
+        `${State.APP_PREFIX}config`,
+        'light-mode',
+      );
+      expect(retrieved?.state).toHaveProperty(
+        `${State.APP_PREFIX}config`,
+        'light-mode',
+      );
+    });
+
+    it('merges initial app: state with app state set by an event', async () => {
+      const appName = 'app';
+      const session1 = await service.createSession({
+        appName,
+        userId: 'user1',
+        state: {[`${State.APP_PREFIX}k1`]: 'v1'},
+      });
+      await service.appendEvent({
+        session: session1,
+        event: createEvent({
+          timestamp: Date.now(),
+          actions: createEventActions({
+            stateDelta: {[`${State.APP_PREFIX}k2`]: 'v2'},
+          }),
+        }),
+      });
+
+      const session2 = await service.createSession({appName, userId: 'user2'});
+
+      expect(session2.state).toEqual({
+        [`${State.APP_PREFIX}k1`]: 'v1',
+        [`${State.APP_PREFIX}k2`]: 'v2',
+      });
+    });
   });
 
   describe('getSession', () => {