fix(tools): fix type error in append_messages lambda return type

- Add cast() to fix pyright type error where lambda returns dict instead of ParseMessageCreateParamsBase
- Fix bug: use params["messages"] (existing conversation) instead of messages (input variable)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: fede-kamel

src/anthropic/lib/tools/_beta_runner.py

@@ -15,6 +15,7 @@
     Iterator,
     Coroutine,
     AsyncIterator,
+    cast,
 )
 from contextlib import contextmanager, asynccontextmanager
 from typing_extensions import TypedDict, override
@@ -108,7 +109,12 @@ def append_messages(self, *messages: BetaMessageParam | ParsedBetaMessage[Respon
             for message in messages
         ]
         self._messages_modified = True
-        self.set_messages_params(lambda params: {**params, "messages": [*messages, *message_params]})
+        self.set_messages_params(
+            lambda params: cast(
+                ParseMessageCreateParamsBase[ResponseFormatT],
+                {**params, "messages": [*params["messages"], *message_params]},
+            )
+        )
         self._cached_tool_call_response = None
 
     def _should_stop(self) -> bool: