diff --git a/core/src/sessions/base_session_service.ts b/core/src/sessions/base_session_service.ts
index 40819b6b3..afec84179 100644
--- a/core/src/sessions/base_session_service.ts
+++ b/core/src/sessions/base_session_service.ts
@@ -193,13 +193,9 @@ export abstract class BaseSessionService {
    * @param request The request to update the session state.
    */
   private updateSessionState({session, event}: AppendEventRequest): void {
-    if (!event.actions || !event.actions.stateDelta) {
-      return;
-    }
-    for (const [key, value] of Object.entries(event.actions.stateDelta)) {
-      if (key.startsWith(State.TEMP_PREFIX)) {
-        continue;
-      }
+    for (const [key, value] of Object.entries(
+      event.actions?.stateDelta ?? {},
+    )) {
       session.state[key] = value;
     }
   }
@@ -216,10 +212,9 @@ export abstract class BaseSessionService {
  * {@link trimTempDeltaState}).
  */
 export function applyTempState(session: Session, event: Event): void {
-  if (!event.actions || !event.actions.stateDelta) {
-    return;
-  }
-  for (const [key, value] of Object.entries(event.actions.stateDelta)) {
+  // `actions` is typed as required but is absent on events deserialized from a
+  // backend; see the VertexAiSessionService `actions: undefined` path.
+  for (const [key, value] of Object.entries(event.actions?.stateDelta ?? {})) {
     if (key.startsWith(State.TEMP_PREFIX)) {
       session.state[key] = value;
     }
diff --git a/core/src/sessions/database_session_service.ts b/core/src/sessions/database_session_service.ts
index 23404eabf..b2be41053 100644
--- a/core/src/sessions/database_session_service.ts
+++ b/core/src/sessions/database_session_service.ts
@@ -10,7 +10,6 @@ import {
   Options as MikroDBOptions,
   MikroORM,
 } from '@mikro-orm/core';
-import {cloneDeep} from 'lodash-es';
 
 import {Event} from '../events/event.js';
 import {randomUUID} from '../utils/env_aware_utils.js';
@@ -379,10 +378,16 @@ export class DatabaseSessionService extends BaseSessionService {
       return event;
     }
 
-    // trimTempDeltaState mutates its argument, so trim a clone for persistence
-    // while keeping the original event (and its temp: keys) intact for
-    // applyTempState after session.state is rebuilt below.
-    const trimmedEvent = trimTempDeltaState(cloneDeep(event));
+    applyTempState(session, event);
+    // Captured before the transaction so it also covers temp: keys written
+    // earlier in this invocation via `State.set`. `session.state` is rebuilt
+    // from persisted rows below, which by construction never hold temp: keys.
+    const tempState = Object.fromEntries(
+      Object.entries(session.state).filter(([key]) =>
+        key.startsWith(State.TEMP_PREFIX),
+      ),
+    );
+    const trimmedEvent = trimTempDeltaState(event);
 
     await em.transactional(async (txEm) => {
       const storageSession = await txEm.findOne(
@@ -505,23 +510,17 @@ export class DatabaseSessionService extends BaseSessionService {
         userStateModel.state,
         storageSession.state,
       );
-      session.state = newMergedState;
-
-      // Apply temp-scoped state to the in-memory session so it is readable for
-      // the remainder of the current invocation. Never persisted:
-      // storageSession.state and the persisted event were built from the
-      // trimmed delta above.
-      applyTempState(session, event);
+      session.state = {...newMergedState, ...tempState};
 
-      const index = session.events.findIndex((e) => e.id === trimmedEvent.id);
+      const index = session.events.findIndex((e) => e.id === event.id);
       if (index >= 0) {
-        session.events[index] = trimmedEvent;
+        session.events[index] = event;
       } else {
-        session.events.push(trimmedEvent);
+        session.events.push(event);
       }
       session.lastUpdateTime = storageSession.updateTime.getTime();
     });
 
-    return trimmedEvent;
+    return event;
   }
 }
diff --git a/core/test/runner/temp_state_visibility_test.ts b/core/test/runner/temp_state_visibility_test.ts
new file mode 100644
index 000000000..efc075ed3
--- /dev/null
+++ b/core/test/runner/temp_state_visibility_test.ts
@@ -0,0 +1,93 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import {
+  BaseLlm,
+  BaseLlmConnection,
+  Event,
+  InMemoryRunner,
+  LlmAgent,
+  LlmRequest,
+  LlmResponse,
+  SequentialAgent,
+  State,
+} from '@google/adk';
+import {describe, expect, it} from 'vitest';
+
+const USER_ID = 'test_user';
+const AGENT_1_OUTPUT = 'result_from_agent_1';
+
+class RecordingLlm extends BaseLlm {
+  readonly requests: LlmRequest[] = [];
+
+  constructor(private readonly text: string) {
+    super({model: 'recording-llm'});
+  }
+
+  async *generateContentAsync(
+    llmRequest: LlmRequest,
+  ): AsyncGenerator<LlmResponse, void> {
+    this.requests.push(llmRequest);
+    yield {content: {role: 'model', parts: [{text: this.text}]}};
+  }
+
+  async connect(_llmRequest: LlmRequest): Promise<BaseLlmConnection> {
+    throw new Error('Method not implemented.');
+  }
+}
+
+describe('temp: state visibility within an invocation', () => {
+  it('makes a temp: outputKey readable by a later sub-agent without persisting it', async () => {
+    const secondModel = new RecordingLlm('rewritten');
+    const agent = new SequentialAgent({
+      name: 'seq',
+      subAgents: [
+        new LlmAgent({
+          name: 'a1',
+          model: new RecordingLlm(AGENT_1_OUTPUT),
+          outputKey: `${State.TEMP_PREFIX}out`,
+        }),
+        new LlmAgent({
+          name: 'a2',
+          model: secondModel,
+          instruction: `Rewrite: {${State.TEMP_PREFIX}out}`,
+        }),
+      ],
+    });
+
+    const runner = new InMemoryRunner({agent});
+    const session = await runner.sessionService.createSession({
+      appName: runner.appName,
+      userId: USER_ID,
+    });
+
+    const events: Event[] = [];
+    for await (const event of runner.runAsync({
+      userId: USER_ID,
+      sessionId: session.id,
+      newMessage: {role: 'user', parts: [{text: 'go'}]},
+    })) {
+      events.push(event);
+    }
+
+    expect(events.map((event) => event.author)).toContain('a2');
+    expect(secondModel.requests).toHaveLength(1);
+    expect(secondModel.requests[0].config?.systemInstruction).toContain(
+      `Rewrite: ${AGENT_1_OUTPUT}`,
+    );
+
+    const storedSession = await runner.sessionService.getSession({
+      appName: runner.appName,
+      userId: USER_ID,
+      sessionId: session.id,
+    });
+    expect(
+      Object.keys(storedSession?.state ?? {}).filter((key) =>
+        key.startsWith(State.TEMP_PREFIX),
+      ),
+    ).toEqual([]);
+  });
+});
diff --git a/core/test/sessions/database_session_service_test.ts b/core/test/sessions/database_session_service_test.ts
index cd051a1cc..9f868a7c2 100644
--- a/core/test/sessions/database_session_service_test.ts
+++ b/core/test/sessions/database_session_service_test.ts
@@ -17,6 +17,11 @@ import {afterEach, beforeEach, describe, expect, it} from 'vitest';
 import {isDatabaseConnectionString} from '../../src/sessions/database_session_service.js';
 import {validateDatabaseSchemaVersion} from '../../src/sessions/db/operations.js';
 
+/** Reaches the ORM the service owns privately, to assert on persisted rows. */
+function ormOf(service: DatabaseSessionService): MikroORM {
+  return (service as unknown as {orm: MikroORM}).orm;
+}
+
 describe('DatabaseSessionService', () => {
   let service: DatabaseSessionService;
 
@@ -31,7 +36,7 @@ describe('DatabaseSessionService', () => {
 
   afterEach(async () => {
     // MikroORM closing
-    const orm = (service as unknown as {orm: MikroORM}).orm;
+    const orm = ormOf(service);
     if (orm) {
       await orm.close();
     }
@@ -371,7 +376,7 @@ describe('DatabaseSessionService', () => {
       allowGlobalContext: true,
     });
     await internalService.init();
-    const orm = (internalService as unknown as {orm: MikroORM}).orm as MikroORM;
+    const orm = ormOf(internalService);
 
     // Manually insert bad version
     const em = orm.em.fork();
@@ -668,7 +673,7 @@ describe('DatabaseSessionService', () => {
 
       await service.appendEvent({session, event});
 
-      const em = (service as unknown as {orm: MikroORM}).orm.em.fork();
+      const em = ormOf(service).em.fork();
       const storedEvents = (await em.find('StorageEvent', {
         sessionId: 's-temp',
       })) as {sessionId: string; eventData: Event}[];
@@ -730,7 +735,7 @@ describe('DatabaseSessionService', () => {
       expect(fetched?.state).not.toHaveProperty(State.TEMP_PREFIX + 'hide');
 
       // The persisted event data is a single, temp-free StorageEvent row.
-      const em = (service as unknown as {orm: MikroORM}).orm.em.fork();
+      const em = ormOf(service).em.fork();
       const storedEvents = (await em.find('StorageEvent', {
         sessionId: 's-temp-apply',
       })) as {sessionId: string; eventData: Event}[];
@@ -742,6 +747,155 @@ describe('DatabaseSessionService', () => {
       ).toBeUndefined();
     });
 
+    it('trims the caller event in place and returns that same event', async () => {
+      const session = await service.createSession({
+        appName: 'test-app',
+        userId: 'test-user',
+        sessionId: 's-temp-in-place',
+      });
+
+      const event = createEvent({
+        timestamp: Date.now(),
+        actions: createEventActions({
+          stateDelta: {
+            [State.TEMP_PREFIX + 'hide']: 'me',
+            'keep': 'me',
+          },
+        }),
+      });
+
+      const returnedEvent = await service.appendEvent({session, event});
+
+      expect(returnedEvent).toBe(event);
+      expect(event.actions?.stateDelta).not.toHaveProperty(
+        State.TEMP_PREFIX + 'hide',
+      );
+      expect(event.actions?.stateDelta).toHaveProperty('keep', 'me');
+    });
+
+    it('keeps temp: state from an earlier event readable when a later event is appended', async () => {
+      const session = await service.createSession({
+        appName: 'test-app',
+        userId: 'test-user',
+        sessionId: 's-temp-across-events',
+      });
+
+      await service.appendEvent({
+        session,
+        event: createEvent({
+          timestamp: Date.now(),
+          actions: createEventActions({
+            stateDelta: {[State.TEMP_PREFIX + 'out']: 'from-agent-1'},
+          }),
+        }),
+      });
+
+      await service.appendEvent({
+        session,
+        event: createEvent({
+          timestamp: Date.now() + 1,
+          actions: createEventActions({stateDelta: {count: 1}}),
+        }),
+      });
+
+      expect(session.state[State.TEMP_PREFIX + 'out']).toBe('from-agent-1');
+      expect(session.state['count']).toBe(1);
+
+      const fetched = await service.getSession({
+        appName: 'test-app',
+        userId: 'test-user',
+        sessionId: 's-temp-across-events',
+      });
+      expect(fetched?.state).toHaveProperty('count', 1);
+      expect(fetched?.state).not.toHaveProperty(State.TEMP_PREFIX + 'out');
+    });
+
+    it('keeps temp: state written straight onto the session state across an append', async () => {
+      const session = await service.createSession({
+        appName: 'test-app',
+        userId: 'test-user',
+        sessionId: 's-temp-state-set',
+      });
+
+      // What `State.set('temp:x', ...)` does during an invocation.
+      session.state[State.TEMP_PREFIX + 'x'] = 'written-by-a-tool';
+
+      await service.appendEvent({
+        session,
+        event: createEvent({
+          timestamp: Date.now(),
+          actions: createEventActions({stateDelta: {count: 1}}),
+        }),
+      });
+
+      expect(session.state[State.TEMP_PREFIX + 'x']).toBe('written-by-a-tool');
+
+      const fetched = await service.getSession({
+        appName: 'test-app',
+        userId: 'test-user',
+        sessionId: 's-temp-state-set',
+      });
+      expect(fetched?.state).not.toHaveProperty(State.TEMP_PREFIX + 'x');
+    });
+
+    it('leaves a partial event untouched: no state applied and no delta trimmed', async () => {
+      const session = await service.createSession({
+        appName: 'test-app',
+        userId: 'test-user',
+        sessionId: 's-temp-partial',
+      });
+
+      const event = createEvent({
+        timestamp: Date.now(),
+        partial: true,
+        actions: createEventActions({
+          stateDelta: {[State.TEMP_PREFIX + 'hide']: 'me', 'keep': 'me'},
+        }),
+      });
+
+      await service.appendEvent({session, event});
+
+      expect(session.state).not.toHaveProperty(State.TEMP_PREFIX + 'hide');
+      expect(session.state).not.toHaveProperty('keep');
+      expect(session.events).toHaveLength(0);
+      expect(event.actions?.stateDelta).toHaveProperty(
+        State.TEMP_PREFIX + 'hide',
+        'me',
+      );
+    });
+
+    it('keeps temp: state when the append reloads a stale session', async () => {
+      const session = await service.createSession({
+        appName: 'test-app',
+        userId: 'test-user',
+        sessionId: 's-temp-stale',
+      });
+
+      await service.appendEvent({
+        session,
+        event: createEvent({
+          timestamp: Date.now(),
+          actions: createEventActions({
+            stateDelta: {[State.TEMP_PREFIX + 'out']: 'from-agent-1'},
+          }),
+        }),
+      });
+
+      // Force the stale-session branch, which reloads state from storage.
+      session.lastUpdateTime = 0;
+
+      await service.appendEvent({
+        session,
+        event: createEvent({
+          timestamp: Date.now() + 1,
+          actions: createEventActions({stateDelta: {count: 1}}),
+        }),
+      });
+
+      expect(session.state[State.TEMP_PREFIX + 'out']).toBe('from-agent-1');
+      expect(session.state['count']).toBe(1);
+    });
+
     it('should align session updateTime with event timestamp', async () => {
       const session = await service.createSession({
         appName: 'test-app',
@@ -756,7 +910,7 @@ describe('DatabaseSessionService', () => {
 
       expect(session.lastUpdateTime).toBe(timestamp);
 
-      const em = (service as unknown as {orm: MikroORM}).orm.em.fork();
+      const em = ormOf(service).em.fork();
       const storedSession = (await em.findOne('StorageSession', {
         id: 's-time',
       })) as {id: string; updateTime: Date};
diff --git a/core/test/sessions/in_memory_session_service_test.ts b/core/test/sessions/in_memory_session_service_test.ts
index cccab1723..efddbbf65 100644
--- a/core/test/sessions/in_memory_session_service_test.ts
+++ b/core/test/sessions/in_memory_session_service_test.ts
@@ -736,6 +736,30 @@ describe('InMemorySessionService', () => {
       expect(fetched?.state).not.toHaveProperty(`${State.TEMP_PREFIX}output`);
       expect(fetched?.state).not.toHaveProperty(`${State.TEMP_PREFIX}output2`);
     });
+
+    it('leaves a partial event untouched: no state applied and no delta trimmed', async () => {
+      const session = await service.createSession({
+        appName: 'app',
+        userId: 'user',
+      });
+      const event = createEvent({
+        timestamp: Date.now(),
+        partial: true,
+        actions: createEventActions({
+          stateDelta: {[`${State.TEMP_PREFIX}k1`]: 'v1', sk: 'v2'},
+        }),
+      });
+
+      await service.appendEvent({session, event});
+
+      expect(session.state).not.toHaveProperty(`${State.TEMP_PREFIX}k1`);
+      expect(session.state).not.toHaveProperty('sk');
+      expect(session.events).toHaveLength(0);
+      expect(event.actions?.stateDelta).toHaveProperty(
+        `${State.TEMP_PREFIX}k1`,
+        'v1',
+      );
+    });
   });
 
   describe('applyTempState (helper)', () => {
@@ -768,16 +792,14 @@ describe('InMemorySessionService', () => {
       expect(session.state).not.toHaveProperty(`${State.APP_PREFIX}a`);
     });
 
-    it('is a no-op (no throw, no mutation) when the event has no actions or no stateDelta', () => {
-      const noActions = createEvent({timestamp: Date.now()});
-      delete (noActions as unknown as {actions?: unknown}).actions;
-      const noStateDelta = createEvent({timestamp: Date.now()});
-      (noStateDelta.actions as unknown as {stateDelta?: unknown}).stateDelta =
-        undefined;
+    it('is a no-op (no throw, no mutation) when the event has an empty stateDelta', () => {
+      const event = createEvent({
+        timestamp: Date.now(),
+        actions: createEventActions({stateDelta: {}}),
+      });
       const session = makeSession({existing: 1});
 
-      expect(() => applyTempState(session, noActions)).not.toThrow();
-      applyTempState(session, noStateDelta);
+      applyTempState(session, event);
 
       expect(session.state).toEqual({existing: 1});
     });
diff --git a/core/test/sessions/vertex_ai_session_service_test.ts b/core/test/sessions/vertex_ai_session_service_test.ts
index 3a8d38659..84dde18c7 100644
--- a/core/test/sessions/vertex_ai_session_service_test.ts
+++ b/core/test/sessions/vertex_ai_session_service_test.ts
@@ -1098,14 +1098,14 @@ describe('VertexAiSessionService', () => {
     });
 
     it('applies temp: state to the in-memory session and does not send temp keys to the backend', async () => {
-      const session = {
+      const session: Session = {
         id: 'append-session',
         appName: '12345',
         userId: 'testUser',
         state: {},
         events: [],
         lastUpdateTime: Date.now(),
-      } as unknown as Session;
+      };
 
       const event = createEvent({
         timestamp: 1620000000000,