diff --git a/litellm/llms/anthropic/pass_through/messages/handler.py b/litellm/llms/anthropic/pass_through/messages/handler.py
index 6c4870a91fd..34333e9a65a 100644
--- a/litellm/llms/anthropic/pass_through/messages/handler.py
+++ b/litellm/llms/anthropic/pass_through/messages/handler.py
@@ -679,26 +679,36 @@ def anthropic_messages_handler(
         or kwargs.get("drop_params") is True
     )
 
+    additional_drop_params: Final[Sequence[str] | None] = (
+        cast(  # cast-ok: kwargs are untyped at this compatibility boundary
+            Sequence[str] | None,
+            kwargs.get("additional_drop_params"),
+        )
+    )
+
     filtered_anthropic_messages_params: Final = (
         _drop_unsupported_anthropic_messages_params(
             anthropic_messages_optional_request_params=anthropic_messages_optional_request_params,
             model=model,
             custom_llm_provider=custom_llm_provider,
-            additional_drop_params=kwargs.get("additional_drop_params"),
+            additional_drop_params=additional_drop_params,
         )
         if should_drop_params
         else anthropic_messages_optional_request_params
     )
     thinking_param: Final = filtered_anthropic_messages_params.get("thinking")
-    final_anthropic_messages_params: Final = (
+    final_anthropic_messages_params: Final[Mapping[str, object]] = (
         MappingProxyType(
-            {
-                **filtered_anthropic_messages_params,
-                "thinking": {
-                    **thinking_param,
-                    "display": "summarized",
+            cast(  # cast-ok: both request parameter maps use string keys
+                dict[str, object],
+                {
+                    **filtered_anthropic_messages_params,
+                    "thinking": {
+                        **thinking_param,
+                        "display": "summarized",
+                    },
                 },
-            }
+            )
         )
         if is_reasoning_auto_summary_enabled()
         and isinstance(thinking_param, dict)