diff --git a/homeassistant/components/websocket_api/messages.py b/homeassistant/components/websocket_api/messages.py
index ec1ab267a37..341aebdf9e4 100644
--- a/homeassistant/components/websocket_api/messages.py
+++ b/homeassistant/components/websocket_api/messages.py
@@ -3,7 +3,7 @@ from __future__ import annotations
 
 from functools import lru_cache
 import logging
-from typing import Any, Final
+from typing import TYPE_CHECKING, Any, Final, cast
 
 import voluptuous as vol
 
@@ -121,14 +121,16 @@ def _state_diff_event(event: Event) -> dict:
     """
     if (event_new_state := event.data["new_state"]) is None:
         return {ENTITY_EVENT_REMOVE: [event.data["entity_id"]]}
-    assert isinstance(event_new_state, State)
+    if TYPE_CHECKING:
+        event_new_state = cast(State, event_new_state)
     if (event_old_state := event.data["old_state"]) is None:
         return {
             ENTITY_EVENT_ADD: {
                 event_new_state.entity_id: event_new_state.as_compressed_state()
             }
         }
-    assert isinstance(event_old_state, State)
+    if TYPE_CHECKING:
+        event_old_state = cast(State, event_old_state)
     return _state_diff(event_old_state, event_new_state)
 
 
@@ -136,27 +138,28 @@ def _state_diff(
     old_state: State, new_state: State
 ) -> dict[str, dict[str, dict[str, dict[str, str | list[str]]]]]:
     """Create a diff dict that can be used to overlay changes."""
-    diff: dict = {STATE_DIFF_ADDITIONS: {}}
-    additions = diff[STATE_DIFF_ADDITIONS]
+    additions: dict[str, Any] = {}
+    diff: dict[str, dict[str, Any]] = {STATE_DIFF_ADDITIONS: additions}
+    new_state_context = new_state.context
+    old_state_context = old_state.context
     if old_state.state != new_state.state:
         additions[COMPRESSED_STATE_STATE] = new_state.state
     if old_state.last_changed != new_state.last_changed:
         additions[COMPRESSED_STATE_LAST_CHANGED] = new_state.last_changed.timestamp()
     elif old_state.last_updated != new_state.last_updated:
         additions[COMPRESSED_STATE_LAST_UPDATED] = new_state.last_updated.timestamp()
-    if old_state.context.parent_id != new_state.context.parent_id:
-        additions.setdefault(COMPRESSED_STATE_CONTEXT, {})[
-            "parent_id"
-        ] = new_state.context.parent_id
-    if old_state.context.user_id != new_state.context.user_id:
-        additions.setdefault(COMPRESSED_STATE_CONTEXT, {})[
-            "user_id"
-        ] = new_state.context.user_id
-    if old_state.context.id != new_state.context.id:
+    if old_state_context.parent_id != new_state_context.parent_id:
+        additions[COMPRESSED_STATE_CONTEXT] = {"parent_id": new_state_context.parent_id}
+    if old_state_context.user_id != new_state_context.user_id:
         if COMPRESSED_STATE_CONTEXT in additions:
-            additions[COMPRESSED_STATE_CONTEXT]["id"] = new_state.context.id
+            additions[COMPRESSED_STATE_CONTEXT]["user_id"] = new_state_context.user_id
         else:
-            additions[COMPRESSED_STATE_CONTEXT] = new_state.context.id
+            additions[COMPRESSED_STATE_CONTEXT] = {"user_id": new_state_context.user_id}
+    if old_state_context.id != new_state_context.id:
+        if COMPRESSED_STATE_CONTEXT in additions:
+            additions[COMPRESSED_STATE_CONTEXT]["id"] = new_state_context.id
+        else:
+            additions[COMPRESSED_STATE_CONTEXT] = new_state_context.id
     if (old_attributes := old_state.attributes) != (
         new_attributes := new_state.attributes
     ):
diff --git a/tests/components/websocket_api/test_messages.py b/tests/components/websocket_api/test_messages.py
index ec7523a02e5..d2102b651b7 100644
--- a/tests/components/websocket_api/test_messages.py
+++ b/tests/components/websocket_api/test_messages.py
@@ -3,11 +3,14 @@ import pytest
 
 from homeassistant.components.websocket_api.messages import (
     _cached_event_message as lru_event_cache,
+    _state_diff_event,
     cached_event_message,
     message_to_json,
 )
 from homeassistant.const import EVENT_STATE_CHANGED
-from homeassistant.core import HomeAssistant, callback
+from homeassistant.core import Context, Event, HomeAssistant, State, callback
+
+from tests.common import async_capture_events
 
 
 async def test_cached_event_message(hass: HomeAssistant) -> None:
@@ -79,6 +82,147 @@ async def test_cached_event_message_with_different_idens(hass: HomeAssistant) ->
     assert cache_info.currsize == 1
 
 
+async def test_state_diff_event(hass: HomeAssistant) -> None:
+    """Test building state_diff_message."""
+    state_change_events = async_capture_events(hass, EVENT_STATE_CHANGED)
+    context = Context(user_id="user-id", parent_id="parent-id", id="id")
+    hass.states.async_set("light.window", "on", context=context)
+    hass.states.async_set("light.window", "off", context=context)
+    await hass.async_block_till_done()
+
+    last_state_event: Event = state_change_events[-1]
+    new_state: State = last_state_event.data["new_state"]
+    message = _state_diff_event(last_state_event)
+    assert message == {
+        "c": {
+            "light.window": {
+                "+": {"lc": new_state.last_changed.timestamp(), "s": "off"}
+            }
+        }
+    }
+
+    hass.states.async_set(
+        "light.window",
+        "red",
+        context=Context(user_id="user-id", parent_id="new-parent-id", id="id"),
+    )
+    await hass.async_block_till_done()
+    last_state_event: Event = state_change_events[-1]
+    new_state: State = last_state_event.data["new_state"]
+    message = _state_diff_event(last_state_event)
+
+    assert message == {
+        "c": {
+            "light.window": {
+                "+": {
+                    "c": {"parent_id": "new-parent-id"},
+                    "lc": new_state.last_changed.timestamp(),
+                    "s": "red",
+                }
+            }
+        }
+    }
+
+    hass.states.async_set(
+        "light.window",
+        "green",
+        context=Context(
+            user_id="new-user-id", parent_id="another-new-parent-id", id="id"
+        ),
+    )
+    await hass.async_block_till_done()
+    last_state_event: Event = state_change_events[-1]
+    new_state: State = last_state_event.data["new_state"]
+    message = _state_diff_event(last_state_event)
+
+    assert message == {
+        "c": {
+            "light.window": {
+                "+": {
+                    "c": {
+                        "parent_id": "another-new-parent-id",
+                        "user_id": "new-user-id",
+                    },
+                    "lc": new_state.last_changed.timestamp(),
+                    "s": "green",
+                }
+            }
+        }
+    }
+
+    hass.states.async_set(
+        "light.window",
+        "blue",
+        context=Context(
+            user_id="another-new-user-id", parent_id="another-new-parent-id", id="id"
+        ),
+    )
+    await hass.async_block_till_done()
+    last_state_event: Event = state_change_events[-1]
+    new_state: State = last_state_event.data["new_state"]
+    message = _state_diff_event(last_state_event)
+
+    assert message == {
+        "c": {
+            "light.window": {
+                "+": {
+                    "c": {"user_id": "another-new-user-id"},
+                    "lc": new_state.last_changed.timestamp(),
+                    "s": "blue",
+                }
+            }
+        }
+    }
+
+    hass.states.async_set(
+        "light.window",
+        "yellow",
+        context=Context(
+            user_id="another-new-user-id",
+            parent_id="another-new-parent-id",
+            id="id-new",
+        ),
+    )
+    await hass.async_block_till_done()
+    last_state_event: Event = state_change_events[-1]
+    new_state: State = last_state_event.data["new_state"]
+    message = _state_diff_event(last_state_event)
+
+    assert message == {
+        "c": {
+            "light.window": {
+                "+": {
+                    "c": "id-new",
+                    "lc": new_state.last_changed.timestamp(),
+                    "s": "yellow",
+                }
+            }
+        }
+    }
+
+    new_context = Context()
+    hass.states.async_set(
+        "light.window", "purple", {"new": "attr"}, context=new_context
+    )
+    await hass.async_block_till_done()
+    last_state_event: Event = state_change_events[-1]
+    new_state: State = last_state_event.data["new_state"]
+    message = _state_diff_event(last_state_event)
+
+    assert message == {
+        "c": {
+            "light.window": {
+                "+": {
+                    "a": {"new": "attr"},
+                    "c": {"id": new_context.id, "parent_id": None, "user_id": None},
+                    "lc": new_state.last_changed.timestamp(),
+                    "s": "purple",
+                }
+            }
+        }
+    }
+
+
 async def test_message_to_json(caplog: pytest.LogCaptureFixture) -> None:
     """Test we can serialize websocket messages."""