diff --git a/src/google/adk/integrations/redis/_redis_session_service.py b/src/google/adk/integrations/redis/_redis_session_service.py
index 2717b76720..041940db1f 100644
--- a/src/google/adk/integrations/redis/_redis_session_service.py
+++ b/src/google/adk/integrations/redis/_redis_session_service.py
@@ -382,9 +382,23 @@ async def append_event(self, session: Session, event: Event) -> Event:
     )
 
     key = self._session_key(session.app_name, session.user_id, session.id)
-    await client.set(
-        key,
-        storage_session.model_dump_json(),
-        ex=self.config.ttl_seconds if self.config.ttl_seconds > 0 else None,
-    )
+
+    async def append_to_storage(pipe: Any) -> None:
+      raw = await pipe.get(key)
+      if raw:
+        # The caller may hold a filtered view. Reload on every transaction
+        # attempt so concurrent appends are preserved as well.
+        storage_session.events = Session.model_validate_json(raw).events
+        if not event.partial:
+          storage_session.events.append(event)
+      else:
+        storage_session.events = session.events
+      pipe.multi()
+      pipe.set(
+          key,
+          storage_session.model_dump_json(),
+          ex=self.config.ttl_seconds if self.config.ttl_seconds > 0 else None,
+      )
+
+    await client.transaction(append_to_storage, key)
     return event
diff --git a/tests/unittests/integrations/redis/_fake_redis.py b/tests/unittests/integrations/redis/_fake_redis.py
index 68b30befd3..3047c4fb6a 100644
--- a/tests/unittests/integrations/redis/_fake_redis.py
+++ b/tests/unittests/integrations/redis/_fake_redis.py
@@ -17,7 +17,11 @@
 from __future__ import annotations
 
 from collections.abc import AsyncIterator
+from collections.abc import Awaitable
+from collections.abc import Callable
 import re
+from typing import Any
+from unittest.mock import Mock
 
 
 class FakeRedisAsync:
@@ -29,6 +33,7 @@ def __init__(self) -> None:
     self._created_at: dict[str, float] = {}
     self._current_time: float = 0.0
     self.scan_patterns: list[str] = []
+    self._versions: dict[str, int] = {}
 
   def advance_time(self, seconds: float) -> None:
     self._current_time += seconds
@@ -43,6 +48,7 @@ def _is_expired(self, key: str) -> bool:
         self._store.pop(key, None)
         self._ex_store.pop(key, None)
         self._created_at.pop(key, None)
+        self._versions[key] = self._versions.get(key, 0) + 1
         return True
     return False
 
@@ -63,6 +69,7 @@ async def set(
     self._store[key] = value
     self._ex_store[key] = ex
     self._created_at[key] = self._current_time
+    self._versions[key] = self._versions.get(key, 0) + 1
     return True
 
   async def delete(self, key: str) -> int:
@@ -70,9 +77,27 @@ async def delete(self, key: str) -> int:
     self._created_at.pop(key, None)
     if key in self._store:
       del self._store[key]
+      self._versions[key] = self._versions.get(key, 0) + 1
       return 1
     return 0
 
+  async def transaction(
+      self, func: Callable[[Any], Awaitable[None]], key: str
+  ) -> list[bool | None]:
+    """Retries a queued write if the watched key changed or expired."""
+    while True:
+      self._is_expired(key)
+      version = self._versions.get(key, 0)
+      pipe = Mock(get=self.get)
+      await func(pipe)
+      self._is_expired(key)
+      if self._versions.get(key, 0) != version:
+        continue
+      return [
+          await self.set(*call.args, **call.kwargs)
+          for call in pipe.set.call_args_list
+      ]
+
   @staticmethod
   def _glob_match(pattern: str, key: str) -> bool:
     """Matches a key the way Redis glob-style patterns do."""
diff --git a/tests/unittests/integrations/redis/test_redis_session_service.py b/tests/unittests/integrations/redis/test_redis_session_service.py
index 8197dcd91b..cf8a03cee9 100644
--- a/tests/unittests/integrations/redis/test_redis_session_service.py
+++ b/tests/unittests/integrations/redis/test_redis_session_service.py
@@ -24,6 +24,7 @@
 from google.adk.integrations.redis._config import RedisSessionServiceConfig
 from google.adk.integrations.redis._redis_session_service import RedisSessionService
 from google.adk.sessions.base_session_service import GetSessionConfig
+from google.genai import types
 import pytest
 
 from ._fake_redis import FakeRedisAsync
@@ -176,6 +177,137 @@ async def test_get_session_with_after_timestamp(session_service):
   assert [e.author for e in fetched.events] == ["user_3", "user_4"]
 
 
+@pytest.mark.asyncio
+@pytest.mark.parametrize(
+    "config, start",
+    [
+        (None, 0),
+        (GetSessionConfig(num_recent_events=2), 2),
+        (GetSessionConfig(num_recent_events=0), 4),
+        (GetSessionConfig(after_timestamp=102), 2),
+        (GetSessionConfig(num_recent_events=3, after_timestamp=102), 2),
+    ],
+)
+async def test_append_event_preserves_filtered_history(
+    session_service, config, start
+):
+  """A filtered view can grow without removing stored events."""
+  session = await session_service.create_session(
+      app_name="app1", user_id="u1", state={"original": "value"}
+  )
+  events = [
+      Event(
+          author="user",
+          invocation_id=f"invocation-{i}",
+          timestamp=100.0 + i,
+          content=types.Content(parts=[types.Part(text=f"event-{i}")]),
+      )
+      for i in range(6)
+  ]
+  for event in events[:4]:
+    await session_service.append_event(session, event)
+  view = await session_service.get_session(
+      app_name="app1", user_id="u1", session_id=session.id, config=config
+  )
+  assert view.events == events[start:4]
+  events[4].actions.state_delta = {
+      "added": 1,
+      "app:mode": "test",
+      "user:theme": "dark",
+      "temp:scratch": "local",
+  }
+
+  for event in events[4:]:
+    assert await session_service.append_event(view, event) is event
+
+  stored = await session_service.get_session(
+      app_name="app1", user_id="u1", session_id=session.id
+  )
+  assert stored.events == events
+  assert view.events == events[start:]
+  assert stored.state == {
+      "original": "value",
+      "added": 1,
+      "app:mode": "test",
+      "user:theme": "dark",
+  }
+  assert view.state["temp:scratch"] == "local"
+  assert "temp:scratch" not in stored.events[4].actions.state_delta
+
+
+@pytest.mark.asyncio
+async def test_partial_event_preserves_filtered_history(session_service):
+  session = await session_service.create_session(app_name="app1", user_id="u1")
+  old_event = Event(author="user", invocation_id="old")
+  await session_service.append_event(session, old_event)
+  view = await session_service.get_session(
+      app_name="app1",
+      user_id="u1",
+      session_id=session.id,
+      config=GetSessionConfig(num_recent_events=0),
+  )
+  partial = Event(author="agent", invocation_id="partial", partial=True)
+
+  assert await session_service.append_event(view, partial) is partial
+
+  stored = await session_service.get_session(
+      app_name="app1", user_id="u1", session_id=session.id
+  )
+  assert stored.events == [old_event]
+  assert view.events == []
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize("change", ["append", "delete", "expire"])
+async def test_append_event_retries_storage_changes(
+    session_service, fake_redis, monkeypatch, change
+):
+  """A concurrent append is retained; missing storage keeps recreation behavior."""
+  session = await session_service.create_session(app_name="app1", user_id="u1")
+  old_event = Event(author="user", invocation_id="old")
+  await session_service.append_event(session, old_event)
+  view = await session_service.get_session(
+      app_name="app1",
+      user_id="u1",
+      session_id=session.id,
+      config=GetSessionConfig(num_recent_events=0),
+  )
+  other_event = Event(author="agent", invocation_id="other")
+  new_event = Event(author="user", invocation_id="new")
+  key = session_service._session_key("app1", "u1", session.id)
+  original_get = fake_redis.get
+  changed = False
+
+  async def get_with_concurrent_change(read_key):
+    nonlocal changed
+    raw = await original_get(read_key)
+    if read_key == key and not changed:
+      changed = True
+      if change == "append":
+        await session_service.append_event(session, other_event)
+      elif change == "delete":
+        await session_service.delete_session(
+            app_name="app1", user_id="u1", session_id=session.id
+        )
+      else:
+        fake_redis.advance_time(3601)
+    return raw
+
+  monkeypatch.setattr(fake_redis, "get", get_with_concurrent_change)
+  await session_service.append_event(view, new_event)
+
+  stored = await session_service.get_session(
+      app_name="app1", user_id="u1", session_id=session.id
+  )
+  expected = [old_event, other_event] if change == "append" else []
+  assert stored.events == expected + [new_event]
+  assert view.events == [new_event]
+  fake_redis.advance_time(3599)
+  assert await original_get(key) is not None
+  fake_redis.advance_time(2)
+  assert await original_get(key) is None
+
+
 @pytest.mark.asyncio
 async def test_list_sessions(session_service):
   await session_service.create_session(