diff --git a/litellm/llms/anthropic/common_utils.py b/litellm/llms/anthropic/common_utils.py
index 7199e21014..5b7937208a 100644
--- a/litellm/llms/anthropic/common_utils.py
+++ b/litellm/llms/anthropic/common_utils.py
@@ -452,7 +452,7 @@ class AnthropicModelInfo(BaseLLMModelInfo):
             betas.add(ANTHROPIC_OAUTH_BETA_HEADER)
         elif auth_token and not api_key:
             headers["authorization"] = f"Bearer {auth_token}"
-        else:
+        elif api_key:
             headers["x-api-key"] = api_key
 
         if user_anthropic_beta_headers is not None:
diff --git a/litellm/proxy/pass_through_endpoints/llm_passthrough_endpoints.py b/litellm/proxy/pass_through_endpoints/llm_passthrough_endpoints.py
index f6b83a68ad..96a7b5a27a 100644
--- a/litellm/proxy/pass_through_endpoints/llm_passthrough_endpoints.py
+++ b/litellm/proxy/pass_through_endpoints/llm_passthrough_endpoints.py
@@ -22,6 +22,7 @@ from litellm.constants import (
     ALLOWED_VERTEX_AI_PASSTHROUGH_HEADERS,
     BEDROCK_AGENT_RUNTIME_PASS_THROUGH_ROUTES,
 )
+from litellm.llms.anthropic.common_utils import AnthropicModelInfo
 from litellm.llms.vertex_ai.vertex_llm_base import VertexBase
 from litellm.proxy._types import *
 from litellm.proxy.auth.route_checks import RouteChecks
@@ -606,10 +607,11 @@ async def anthropic_proxy_route(
     is_streaming_request = await is_streaming_request_fn(request)
 
     ## CREATE PASS-THROUGH
+    auth_header = AnthropicModelInfo.get_auth_header(anthropic_api_key or None)
     endpoint_func = create_pass_through_route(
         endpoint=endpoint,
         target=str(updated_url),
-        custom_headers={"x-api-key": "{}".format(anthropic_api_key)} if anthropic_api_key else {},
+        custom_headers=auth_header if auth_header is not None else {},
         _forward_headers=True,
         is_streaming_request=is_streaming_request,
     )  # dynamically construct pass-through endpoint based on incoming path