fix(anthropic): support Bearer auth for custom api_base endpoints (Fixes #30926)

litellm/llms/anthropic/batches/transformation.py

@@ -43,7 +43,7 @@ class AnthropicBatchesConfig(BaseBatchesConfig):
         api_base: Optional[str] = None,
     ) -> dict:
         """Validate and prepare environment-specific headers and parameters."""
-        auth_header = self.anthropic_model_info.get_auth_header(api_key)
+        auth_header = self.anthropic_model_info.get_auth_header(api_key, api_base)
         if auth_header is None:
             raise ValueError(
                 "Missing Anthropic API Key - A call is being made to anthropic but no key is set either in the environment variables or via params"

litellm/llms/anthropic/chat/handler.py

@@ -364,6 +364,7 @@ class AnthropicChatCompletion(BaseLLM):
             messages=messages,
             optional_params={**optional_params, "is_vertex_request": is_vertex_request},
             litellm_params=litellm_params,
+            api_base=api_base,
         )
 
         config = ProviderConfigManager.get_provider_chat_config(

litellm/llms/anthropic/common_utils.py

@@ -552,6 +552,7 @@ class AnthropicModelInfo(BaseLLMModelInfo):
         user_anthropic_beta_headers: Optional[List[str]] = None,
         code_execution_tool_used: bool = False,
         container_with_skills_used: bool = False,
+        api_base: Optional[str] = None,
     ) -> dict:
         betas = set()
         # Anthropic no longer requires the prompt-caching beta header
@@ -600,7 +601,12 @@ class AnthropicModelInfo(BaseLLMModelInfo):
         elif auth_token and not api_key:
             headers["authorization"] = f"Bearer {auth_token}"
         elif api_key:
-            headers["x-api-key"] = api_key
+            if api_key.startswith("Bearer "):
+                headers["authorization"] = api_key
+            elif api_base and "api.anthropic.com" not in api_base and not api_key.startswith("sk-ant-"):
+                headers["authorization"] = f"Bearer {api_key}"
+            else:
+                headers["x-api-key"] = api_key
 
         if user_anthropic_beta_headers is not None:
             betas.update(user_anthropic_beta_headers)
@@ -629,6 +635,8 @@ class AnthropicModelInfo(BaseLLMModelInfo):
         api_key: Optional[str] = None,
         api_base: Optional[str] = None,
     ) -> Dict:
+        if api_base is None and isinstance(litellm_params, dict):
+            api_base = litellm_params.get("api_base")
         # Check for Anthropic OAuth token in headers
         headers, api_key = optionally_handle_anthropic_oauth(
             headers=headers, api_key=api_key
@@ -684,6 +692,7 @@ class AnthropicModelInfo(BaseLLMModelInfo):
             effort_used=effort_used,
             code_execution_tool_used=code_execution_tool_used,
             container_with_skills_used=container_with_skills_used,
+            api_base=api_base,
         )
 
         headers = {**headers, **anthropic_headers}
@@ -719,7 +728,7 @@ class AnthropicModelInfo(BaseLLMModelInfo):
         return auth_token or get_secret_str("ANTHROPIC_AUTH_TOKEN")
 
     @staticmethod
-    def get_auth_header(api_key: Optional[str] = None) -> Optional[dict]:
+    def get_auth_header(api_key: Optional[str] = None, api_base: Optional[str] = None) -> Optional[dict]:
         """Resolve Anthropic credentials and return the appropriate auth header dict.
 
         Checks ANTHROPIC_API_KEY first (-> x-api-key), then
@@ -730,6 +739,10 @@ class AnthropicModelInfo(BaseLLMModelInfo):
         if resolved_key is not None:
             if is_anthropic_oauth_key(resolved_key):
                 return {"authorization": f"Bearer {resolved_key}"}
+            if resolved_key.startswith("Bearer "):
+                return {"authorization": resolved_key}
+            if api_base and "api.anthropic.com" not in api_base and not resolved_key.startswith("sk-ant-"):
+                return {"authorization": f"Bearer {resolved_key}"}
             return {"x-api-key": resolved_key}
         auth_token = AnthropicModelInfo.get_auth_token()
         if auth_token is not None:
@@ -744,7 +757,7 @@ class AnthropicModelInfo(BaseLLMModelInfo):
         self, api_key: Optional[str] = None, api_base: Optional[str] = None
     ) -> List[str]:
         api_base = AnthropicModelInfo.get_api_base(api_base)
-        auth_header = AnthropicModelInfo.get_auth_header(api_key)
+        auth_header = AnthropicModelInfo.get_auth_header(api_key, api_base)
         if api_base is None or auth_header is None:
             raise ValueError(
                 "ANTHROPIC_API_BASE/ANTHROPIC_BASE_URL or ANTHROPIC_API_KEY/ANTHROPIC_AUTH_TOKEN is not set. Please set the environment variable, to query Anthropic's `/models` endpoint."

litellm/llms/anthropic/files/handler.py

@@ -84,7 +84,7 @@ class AnthropicFilesHandler:
 
         # Get Anthropic API credentials
         api_base = self.anthropic_model_info.get_api_base(api_base)
-        auth_header = self.anthropic_model_info.get_auth_header(api_key)
+        auth_header = self.anthropic_model_info.get_auth_header(api_key, api_base)
 
         if auth_header is None:
             raise ValueError("Missing Anthropic API Key")

litellm/llms/anthropic/files/transformation.py

@@ -95,7 +95,7 @@ class AnthropicFilesConfig(BaseFilesConfig):
         api_key: Optional[str] = None,
         api_base: Optional[str] = None,
     ) -> dict:
-        auth_header = AnthropicModelInfo.get_auth_header(api_key)
+        auth_header = AnthropicModelInfo.get_auth_header(api_key, api_base)
         if auth_header is None:
             raise ValueError(
                 "Anthropic API key is required. Set ANTHROPIC_API_KEY or ANTHROPIC_AUTH_TOKEN environment variable or pass api_key parameter."

litellm/llms/anthropic/skills/transformation.py

@@ -38,10 +38,12 @@ class AnthropicSkillsConfig(BaseSkillsAPIConfig):
 
         # Get API key from litellm_params if available
         api_key = None
+        api_base = None
         if litellm_params is not None:
             api_key = litellm_params.api_key
+            api_base = litellm_params.api_base
 
-        auth_header = AnthropicModelInfo.get_auth_header(api_key)
+        auth_header = AnthropicModelInfo.get_auth_header(api_key, api_base)
         if auth_header is None:
             raise ValueError(
                 "ANTHROPIC_API_KEY or ANTHROPIC_AUTH_TOKEN is required for Skills API"

tests/test_litellm/llms/anthropic/test_anthropic_common_utils.py

@@ -159,6 +159,57 @@ class TestGetAnthropicHeaders:
         assert "authorization" not in headers
         assert "anthropic-dangerous-direct-browser-access" not in headers
 
+    def test_custom_api_base_uses_bearer_header(self):
+        """Custom api_base and non-standard API key should produce Authorization: Bearer header."""
+        from litellm.llms.anthropic.common_utils import AnthropicModelInfo
+
+        config = AnthropicModelInfo()
+        headers = config.get_anthropic_headers(
+            api_key="my-custom-ollama-token",
+            computer_tool_used=False,
+            prompt_caching_set=False,
+            pdf_used=False,
+            is_vertex_request=False,
+            api_base="https://ollama.com/",
+        )
+
+        assert headers["authorization"] == "Bearer my-custom-ollama-token"
+        assert "x-api-key" not in headers
+
+    def test_custom_api_base_uses_bearer_header_already_starts_with_bearer(self):
+        """If the key already starts with Bearer, use it directly."""
+        from litellm.llms.anthropic.common_utils import AnthropicModelInfo
+
+        config = AnthropicModelInfo()
+        headers = config.get_anthropic_headers(
+            api_key="Bearer my-custom-ollama-token",
+            computer_tool_used=False,
+            prompt_caching_set=False,
+            pdf_used=False,
+            is_vertex_request=False,
+            api_base="https://ollama.com/",
+        )
+
+        assert headers["authorization"] == "Bearer my-custom-ollama-token"
+        assert "x-api-key" not in headers
+
+    def test_custom_api_base_uses_x_api_key_when_standard_key(self):
+        """If the key is standard sk-ant- key, use x-api-key even with custom api_base."""
+        from litellm.llms.anthropic.common_utils import AnthropicModelInfo
+
+        config = AnthropicModelInfo()
+        headers = config.get_anthropic_headers(
+            api_key=FAKE_REGULAR_KEY,
+            computer_tool_used=False,
+            prompt_caching_set=False,
+            pdf_used=False,
+            is_vertex_request=False,
+            api_base="https://ollama.com/",
+        )
+
+        assert headers["x-api-key"] == FAKE_REGULAR_KEY
+        assert "authorization" not in headers
+
     def test_oauth_includes_standard_headers(self):
         """OAuth path should still include standard Anthropic headers."""
         from litellm.llms.anthropic.common_utils import AnthropicModelInfo
@@ -242,6 +293,46 @@ class TestValidateEnvironmentOAuth:
 
         assert updated_headers["x-api-key"] == FAKE_REGULAR_KEY
         assert "authorization" not in updated_headers
+
+    def test_custom_api_base_via_param(self):
+        """validate_environment with custom api_base via parameter and custom key should use Bearer."""
+        from litellm.llms.anthropic.common_utils import AnthropicModelInfo
+
+        config = AnthropicModelInfo()
+        headers = {}
+
+        updated_headers = config.validate_environment(
+            headers=headers,
+            model="claude-sonnet-4-5-20250929",
+            messages=[{"role": "user", "content": "Hello"}],
+            optional_params={},
+            litellm_params={},
+            api_key="custom-api-key",
+            api_base="https://custom-gateway.com",
+        )
+
+        assert updated_headers["authorization"] == "Bearer custom-api-key"
+        assert "x-api-key" not in updated_headers
+
+    def test_custom_api_base_via_litellm_params(self):
+        """validate_environment with custom api_base inside litellm_params should use Bearer."""
+        from litellm.llms.anthropic.common_utils import AnthropicModelInfo
+
+        config = AnthropicModelInfo()
+        headers = {}
+
+        updated_headers = config.validate_environment(
+            headers=headers,
+            model="claude-sonnet-4-5-20250929",
+            messages=[{"role": "user", "content": "Hello"}],
+            optional_params={},
+            litellm_params={"api_base": "https://custom-gateway.com"},
+            api_key="custom-api-key",
+            api_base=None,
+        )
+
+        assert updated_headers["authorization"] == "Bearer custom-api-key"
+        assert "x-api-key" not in updated_headers
         assert "anthropic-dangerous-direct-browser-access" not in updated_headers
 
 
@@ -1004,6 +1095,20 @@ class TestGetAuthHeader:
             result = AnthropicModelInfo.get_auth_header()
             assert result == {"authorization": f"Bearer {FAKE_OAUTH_TOKEN}"}
 
+    def test_custom_api_base_get_auth_header_uses_bearer(self):
+        """Non-standard API key and custom api_base should return Authorization: Bearer."""
+        from litellm.llms.anthropic.common_utils import AnthropicModelInfo
+
+        result = AnthropicModelInfo.get_auth_header(api_key="my-custom-key", api_base="https://custom-gateway.com")
+        assert result == {"authorization": "Bearer my-custom-key"}
+
+    def test_custom_api_base_get_auth_header_uses_x_api_key_when_standard(self):
+        """Standard sk-ant- key with custom api_base should still return x-api-key."""
+        from litellm.llms.anthropic.common_utils import AnthropicModelInfo
+
+        result = AnthropicModelInfo.get_auth_header(api_key=FAKE_REGULAR_KEY, api_base="https://custom-gateway.com")
+        assert result == {"x-api-key": FAKE_REGULAR_KEY}
+
 
 class TestGetApiBaseFallbackChain:
     """Tests for AnthropicModelInfo.get_api_base() fallback to ANTHROPIC_BASE_URL."""