diff --git a/litellm/llms/vertex_ai/vertex_ai_partner_models/anthropic/experimental_pass_through/transformation.py b/litellm/llms/vertex_ai/vertex_ai_partner_models/anthropic/experimental_pass_through/transformation.py
index 6f8578259db..0bedef3276b 100644
--- a/litellm/llms/vertex_ai/vertex_ai_partner_models/anthropic/experimental_pass_through/transformation.py
+++ b/litellm/llms/vertex_ai/vertex_ai_partner_models/anthropic/experimental_pass_through/transformation.py
@@ -58,7 +58,7 @@ class VertexAIPartnerModelsAnthropicMessagesConfig(AnthropicMessagesConfig, Vert
         
         # Add beta headers for Vertex AI
         tools = optional_params.get("tools", [])
-        beta_values = set()
+        beta_values: set[str] = set()
         
         # Get existing beta headers if any
         existing_beta = headers.get("anthropic-beta")