diff --git a/src/anthropic/lib/streaming/_beta_messages.py b/src/anthropic/lib/streaming/_beta_messages.py
index b6241d2e..e9325f55 100644
--- a/src/anthropic/lib/streaming/_beta_messages.py
+++ b/src/anthropic/lib/streaming/_beta_messages.py
@@ -1,14 +1,14 @@
 from __future__ import annotations
 
 from types import TracebackType
-from typing import TYPE_CHECKING, Any, Callable, cast
+from typing import TYPE_CHECKING, Any, Type, Callable, cast
 from typing_extensions import Self, Iterator, Awaitable, AsyncIterator, assert_never
 
 import httpx
 from pydantic import BaseModel
 
 from ..._utils import consume_sync_iterator, consume_async_iterator
-from ..._models import build, construct_type
+from ..._models import build, construct_type, construct_type_unchecked
 from ._beta_types import (
     BetaTextEvent,
     BetaCitationEvent,
@@ -372,8 +372,16 @@ def accumulate_event(
     event: BetaRawMessageStreamEvent,
     current_snapshot: BetaMessage | None,
 ) -> BetaMessage:
-    if not isinstance(event, BaseModel):  # pyright: ignore[reportUnnecessaryIsInstance]
-        raise TypeError(f"Unexpected event runtime type - {event}")
+    if not isinstance(cast(Any, event), BaseModel):
+        event = cast(  # pyright: ignore[reportUnnecessaryCast]
+            BetaRawMessageStreamEvent,
+            construct_type_unchecked(
+                type_=cast(Type[BetaRawMessageStreamEvent], BetaRawMessageStreamEvent),
+                value=event,
+            ),
+        )
+        if not isinstance(cast(Any, event), BaseModel):
+            raise TypeError(f"Unexpected event runtime type, after deserialising twice - {event} - {type(event)}")
 
     if current_snapshot is None:
         if event.type == "message_start":
diff --git a/src/anthropic/lib/streaming/_messages.py b/src/anthropic/lib/streaming/_messages.py
index 146a1bab..082ed383 100644
--- a/src/anthropic/lib/streaming/_messages.py
+++ b/src/anthropic/lib/streaming/_messages.py
@@ -1,7 +1,7 @@
 from __future__ import annotations
 
 from types import TracebackType
-from typing import TYPE_CHECKING, Any, Callable, cast
+from typing import TYPE_CHECKING, Any, Type, Callable, cast
 from typing_extensions import Self, Iterator, Awaitable, AsyncIterator, assert_never
 
 import httpx
@@ -17,7 +17,7 @@
 )
 from ...types import Message, ContentBlock, RawMessageStreamEvent
 from ..._utils import consume_sync_iterator, consume_async_iterator
-from ..._models import build, construct_type
+from ..._models import build, construct_type, construct_type_unchecked
 from ..._streaming import Stream, AsyncStream
 
 
@@ -372,8 +372,16 @@ def accumulate_event(
     event: RawMessageStreamEvent,
     current_snapshot: Message | None,
 ) -> Message:
-    if not isinstance(event, BaseModel):  # pyright: ignore[reportUnnecessaryIsInstance]
-        raise TypeError(f"Unexpected event runtime type - {event}")
+    if not isinstance(cast(Any, event), BaseModel):
+        event = cast(  # pyright: ignore[reportUnnecessaryCast]
+            RawMessageStreamEvent,
+            construct_type_unchecked(
+                type_=cast(Type[RawMessageStreamEvent], RawMessageStreamEvent),
+                value=event,
+            ),
+        )
+        if not isinstance(cast(Any, event), BaseModel):
+            raise TypeError(f"Unexpected event runtime type, after deserialising twice - {event} - {type(event)}")
 
     if current_snapshot is None:
         if event.type == "message_start":