diff --git a/litellm/llms/anthropic/experimental_pass_through/messages/transformation.py b/litellm/llms/anthropic/experimental_pass_through/messages/transformation.py
index 032bf0130ce..75146922a39 100644
--- a/litellm/llms/anthropic/experimental_pass_through/messages/transformation.py
+++ b/litellm/llms/anthropic/experimental_pass_through/messages/transformation.py
@@ -8,6 +8,7 @@ from litellm.constants import (
     DEFAULT_REASONING_EFFORT_MEDIUM_THINKING_BUDGET,
     DEFAULT_REASONING_EFFORT_XHIGH_THINKING_BUDGET,
 )
+from litellm.exceptions import AuthenticationError
 from litellm.litellm_core_utils.litellm_logging import Logging as LiteLLMLoggingObj
 from litellm.litellm_core_utils.litellm_logging import verbose_logger
 from litellm.llms.base_llm.anthropic_messages.transformation import (
@@ -309,8 +310,17 @@ class AnthropicMessagesConfig(BaseAnthropicMessagesConfig):
 
         if "x-api-key" not in headers and "authorization" not in headers:
             auth_header: Final = AnthropicModelInfo.get_auth_header(api_key)
-            if auth_header is not None:
-                headers.update(auth_header)
+            if auth_header is None:
+                raise AuthenticationError(
+                    message=(
+                        "Missing Anthropic API Key - A call is being made to anthropic but no key is set "
+                        "either in the environment variables or via params. Please set `ANTHROPIC_API_KEY` "
+                        "or `ANTHROPIC_AUTH_TOKEN` in your environment vars"
+                    ),
+                    llm_provider=self._resolved_provider,
+                    model=model,
+                )
+            headers.update(auth_header)
         if "anthropic-version" not in headers:
             headers["anthropic-version"] = DEFAULT_ANTHROPIC_API_VERSION
         if "content-type" not in headers:
diff --git a/tests/test_litellm/llms/anthropic/test_anthropic_common_utils.py b/tests/test_litellm/llms/anthropic/test_anthropic_common_utils.py
index c27362bf49f..a96c44ac5c0 100644
--- a/tests/test_litellm/llms/anthropic/test_anthropic_common_utils.py
+++ b/tests/test_litellm/llms/anthropic/test_anthropic_common_utils.py
@@ -1227,6 +1227,50 @@ class TestPassthroughAuthToken:
         assert updated_headers["x-api-key"] == FAKE_REGULAR_KEY
         assert "authorization" not in updated_headers
 
+    def test_passthrough_missing_credentials_raises_authentication_error(self):
+        """Passthrough endpoint should raise locally instead of forwarding an unauthenticated request."""
+        from unittest.mock import patch as mock_patch
+
+        import litellm
+        from litellm.llms.anthropic.experimental_pass_through.messages.transformation import (
+            AnthropicMessagesConfig,
+        )
+
+        config = AnthropicMessagesConfig()
+        with mock_patch.dict("os.environ", {}, clear=True):
+            with pytest.raises(litellm.AuthenticationError, match="Missing Anthropic API Key"):
+                config.validate_anthropic_messages_environment(
+                    headers={},
+                    model="claude-sonnet-4-5-20250929",
+                    messages=[{"role": "user", "content": "Hello"}],
+                    optional_params={},
+                    litellm_params={},
+                    api_key=None,
+                    api_base=None,
+                )
+
+    def test_passthrough_client_x_api_key_header_is_kept(self):
+        """A client-forwarded x-api-key header should satisfy validation without env credentials."""
+        from unittest.mock import patch as mock_patch
+
+        from litellm.llms.anthropic.experimental_pass_through.messages.transformation import (
+            AnthropicMessagesConfig,
+        )
+
+        config = AnthropicMessagesConfig()
+        with mock_patch.dict("os.environ", {}, clear=True):
+            updated_headers, _ = config.validate_anthropic_messages_environment(
+                headers={"x-api-key": FAKE_REGULAR_KEY},
+                model="claude-sonnet-4-5-20250929",
+                messages=[{"role": "user", "content": "Hello"}],
+                optional_params={},
+                litellm_params={},
+                api_key=None,
+                api_base=None,
+            )
+
+        assert updated_headers["x-api-key"] == FAKE_REGULAR_KEY
+
     def test_passthrough_get_complete_url_honours_base_url_env(self):
         """get_complete_url should use ANTHROPIC_BASE_URL when api_base is None."""
         from unittest.mock import patch as mock_patch