diff --git a/litellm/llms/anthropic/common_utils.py b/litellm/llms/anthropic/common_utils.py
index c73376ba498..af1127839aa 100644
--- a/litellm/llms/anthropic/common_utils.py
+++ b/litellm/llms/anthropic/common_utils.py
@@ -75,8 +75,10 @@ def is_anthropic_oauth_key(value: str | None) -> bool:
     """Check if a value contains an Anthropic OAuth token (sk-ant-oat*)."""
     if value is None:
         return False
-    # Handle both raw token and "Bearer <token>" format
-    value = value.removeprefix("Bearer ")
+    # Handle both raw token and "Bearer <token>" format, case-insensitive scheme
+    scheme, _, token = value.partition(" ")
+    if scheme.lower() == "bearer":
+        value = token
     return value.startswith(ANTHROPIC_OAUTH_TOKEN_PREFIX)
 
 
diff --git a/litellm/proxy/pass_through_endpoints/llm_passthrough_endpoints.py b/litellm/proxy/pass_through_endpoints/llm_passthrough_endpoints.py
index 9a3bc82c6fa..4cd466ab6bb 100644
--- a/litellm/proxy/pass_through_endpoints/llm_passthrough_endpoints.py
+++ b/litellm/proxy/pass_through_endpoints/llm_passthrough_endpoints.py
@@ -26,7 +26,7 @@ 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.anthropic.common_utils import AnthropicModelInfo, is_anthropic_oauth_key
 from litellm.llms.vertex_ai.vertex_llm_base import VertexBase
 from litellm.proxy._types import *
 from litellm.proxy.auth.handle_jwt import JWTHandler
@@ -643,7 +643,10 @@ async def anthropic_proxy_route(
     is_streaming_request: Final = await is_streaming_request_fn(request)
 
     ## CREATE PASS-THROUGH
-    auth_header: Final = AnthropicModelInfo.get_auth_header(anthropic_api_key or None)
+    client_forwards_own_oauth: Final = is_anthropic_oauth_key(request.headers.get("authorization"))
+    auth_header: Final = (
+        None if client_forwards_own_oauth else AnthropicModelInfo.get_auth_header(anthropic_api_key or None)
+    )
     endpoint_func: Final = create_pass_through_route(
         endpoint=endpoint,
         target=str(updated_url),
diff --git a/tests/test_litellm/proxy/pass_through_endpoints/test_llm_pass_through_endpoints.py b/tests/test_litellm/proxy/pass_through_endpoints/test_llm_pass_through_endpoints.py
index 3b506324ad7..c9dcecd60d0 100644
--- a/tests/test_litellm/proxy/pass_through_endpoints/test_llm_pass_through_endpoints.py
+++ b/tests/test_litellm/proxy/pass_through_endpoints/test_llm_pass_through_endpoints.py
@@ -22,6 +22,7 @@ from litellm.proxy.pass_through_endpoints.llm_passthrough_endpoints import (
     BaseOpenAIPassThroughHandler,
     RouteChecks,
     _join_url_paths,
+    anthropic_proxy_route,
     azure_proxy_route,
     bedrock_llm_proxy_route,
     bedrock_proxy_route,
@@ -3010,6 +3011,103 @@ class TestOpenAIPassthroughRoute:
             assert result == {"id": "asst_123", "object": "assistant"}
 
 
+class TestAnthropicProxyRoute:
+    """Regression test for issue #37344."""
+
+    @pytest.mark.asyncio
+    async def test_anthropic_passthrough_omits_server_api_key_when_client_forwards_oauth(self):
+        mock_request = MagicMock(spec=Request)
+        mock_request.method = "POST"
+        mock_request.headers = {"authorization": "Bearer sk-ant-oat01-canary"}
+        mock_request.query_params = {}
+        mock_response = MagicMock(spec=Response)
+        mock_user_api_key_dict = MagicMock()
+
+        with (
+            patch(
+                "litellm.proxy.pass_through_endpoints.llm_passthrough_endpoints.passthrough_endpoint_router.get_credentials",
+                return_value="sk-ant-server-configured-key",
+            ),
+            patch(
+                "litellm.proxy.pass_through_endpoints.llm_passthrough_endpoints.create_pass_through_route"
+            ) as mock_create_route,
+        ):
+            mock_endpoint_func = AsyncMock(return_value={"id": "msg_123"})
+            mock_create_route.return_value = mock_endpoint_func
+
+            await anthropic_proxy_route(
+                endpoint="v1/messages",
+                request=mock_request,
+                fastapi_response=mock_response,
+                user_api_key_dict=mock_user_api_key_dict,
+            )
+
+            call_args = mock_create_route.call_args[1]
+            assert call_args["custom_headers"] == {}
+
+    @pytest.mark.asyncio
+    async def test_anthropic_passthrough_uses_server_api_key_without_client_oauth(self):
+        mock_request = MagicMock(spec=Request)
+        mock_request.method = "POST"
+        mock_request.headers = {}
+        mock_request.query_params = {}
+        mock_response = MagicMock(spec=Response)
+        mock_user_api_key_dict = MagicMock()
+
+        with (
+            patch(
+                "litellm.proxy.pass_through_endpoints.llm_passthrough_endpoints.passthrough_endpoint_router.get_credentials",
+                return_value="sk-ant-server-configured-key",
+            ),
+            patch(
+                "litellm.proxy.pass_through_endpoints.llm_passthrough_endpoints.create_pass_through_route"
+            ) as mock_create_route,
+        ):
+            mock_endpoint_func = AsyncMock(return_value={"id": "msg_123"})
+            mock_create_route.return_value = mock_endpoint_func
+
+            await anthropic_proxy_route(
+                endpoint="v1/messages",
+                request=mock_request,
+                fastapi_response=mock_response,
+                user_api_key_dict=mock_user_api_key_dict,
+            )
+
+            call_args = mock_create_route.call_args[1]
+            assert call_args["custom_headers"] == {"x-api-key": "sk-ant-server-configured-key"}
+
+    @pytest.mark.asyncio
+    async def test_anthropic_passthrough_omits_server_api_key_for_lowercase_bearer_scheme(self):
+        mock_request = MagicMock(spec=Request)
+        mock_request.method = "POST"
+        mock_request.headers = {"authorization": "bearer sk-ant-oat01-canary"}
+        mock_request.query_params = {}
+        mock_response = MagicMock(spec=Response)
+        mock_user_api_key_dict = MagicMock()
+
+        with (
+            patch(
+                "litellm.proxy.pass_through_endpoints.llm_passthrough_endpoints.passthrough_endpoint_router.get_credentials",
+                return_value="sk-ant-server-configured-key",
+            ),
+            patch(
+                "litellm.proxy.pass_through_endpoints.llm_passthrough_endpoints.create_pass_through_route"
+            ) as mock_create_route,
+        ):
+            mock_endpoint_func = AsyncMock(return_value={"id": "msg_123"})
+            mock_create_route.return_value = mock_endpoint_func
+
+            await anthropic_proxy_route(
+                endpoint="v1/messages",
+                request=mock_request,
+                fastapi_response=mock_response,
+                user_api_key_dict=mock_user_api_key_dict,
+            )
+
+            call_args = mock_create_route.call_args[1]
+            assert call_args["custom_headers"] == {}
+
+
 def _resolve_route_name(method: str, path: str) -> str | None:
     from starlette.routing import Match