fix(proxy): preserve and forward OAuth Authorization headers through proxy layer

PR #21039 fixed OAuth token handling at the LLM layer (Authorization: Bearer
instead of x-api-key), but the proxy layer still strips the Authorization
header in clean_headers() before it reaches the Anthropic code. This breaks
OAuth for proxy users (e.g., Claude Code Max through LiteLLM proxy).

Changes:
- Add is_anthropic_oauth_key() helper to detect OAuth tokens (sk-ant-oat*)
- Preserve OAuth Authorization headers in clean_headers() instead of stripping
- Forward OAuth Authorization via ProviderSpecificHeader in
  add_provider_specific_headers_to_request() so tokens only reach
  Anthropic-compatible providers (anthropic, bedrock, vertex_ai)

Fixes #19618

litellm/llms/anthropic/common_utils.py

@@ -22,6 +22,15 @@ from litellm.types.llms.anthropic import (
 from litellm.types.llms.openai import AllMessageValues
 
 
+def is_anthropic_oauth_key(value: Optional[str]) -> 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
+    if value.startswith("Bearer "):
+        value = value[7:]
+    return value.startswith(ANTHROPIC_OAUTH_TOKEN_PREFIX)
+
 def optionally_handle_anthropic_oauth(
     headers: dict, api_key: Optional[str]
 ) -> tuple[dict, Optional[str]]:

litellm/proxy/litellm_pre_call_utils.py

@@ -237,6 +237,8 @@ def clean_headers(
     """
     Removes litellm api key from headers
     """
+    from litellm.llms.anthropic.common_utils import is_anthropic_oauth_key
+
     clean_headers = {}
     litellm_key_lower = (
         litellm_key_header_name.lower() if litellm_key_header_name is not None else None
@@ -244,8 +246,13 @@ def clean_headers(
 
     for header, value in headers.items():
         header_lower = header.lower()
+        # Preserve Authorization header if it contains Anthropic OAuth token (sk-ant-oat*)
+        # This allows OAuth tokens to be forwarded to Anthropic-compatible providers
+        # via add_provider_specific_headers_to_request()
+        if header_lower == "authorization" and is_anthropic_oauth_key(value):
+            clean_headers[header] = value
         # Check if header should be excluded: either in special headers cache or matches custom litellm key
-        if header_lower not in _SPECIAL_HEADERS_CACHE and (
+        elif header_lower not in _SPECIAL_HEADERS_CACHE and (
             litellm_key_lower is None or header_lower != litellm_key_lower
         ):
             clean_headers[header] = value
@@ -1605,6 +1612,8 @@ def add_provider_specific_headers_to_request(
     data: dict,
     headers: dict,
 ):
+    from litellm.llms.anthropic.common_utils import is_anthropic_oauth_key
+
     anthropic_headers = {}
     # boolean to indicate if a header was added
     added_header = False
@@ -1614,6 +1623,14 @@ def add_provider_specific_headers_to_request(
             anthropic_headers[header] = header_value
             added_header = True
 
+    # Check for Authorization header with Anthropic OAuth token (sk-ant-oat*)
+    # This needs to be handled via provider-specific headers to ensure it only
+    # goes to Anthropic-compatible providers, not all providers in the router
+    for header, value in headers.items():
+        if header.lower() == "authorization" and is_anthropic_oauth_key(value):
+            anthropic_headers[header] = value
+            added_header = True
+            break
     if added_header is True:
         # Anthropic headers work across multiple providers
         # Store as comma-separated list so retrieval can match any of them

tests/test_litellm/llms/anthropic/test_anthropic_common_utils.py

@@ -81,4 +81,142 @@ def test_regular_api_keys_still_work():
     # Regular key should be unchanged
     assert extracted_api_key == regular_key
     # OAuth headers should NOT be added
-    assert "anthropic-dangerous-direct-browser-access" not in updated_headers
+    assert "anthropic-dangerous-direct-browser-access" not in updated_headers
+
+class TestIsAnthropicOAuthKey:
+    """Tests for is_anthropic_oauth_key helper function."""
+
+    def test_oauth_token_raw(self):
+        """Raw OAuth token should be detected."""
+        from litellm.llms.anthropic.common_utils import is_anthropic_oauth_key
+
+        assert is_anthropic_oauth_key("sk-ant-oat01-abc123") is True
+        assert is_anthropic_oauth_key("sk-ant-oat02-xyz789") is True
+
+    def test_oauth_token_bearer_format(self):
+        """Bearer-prefixed OAuth token should be detected."""
+        from litellm.llms.anthropic.common_utils import is_anthropic_oauth_key
+
+        assert is_anthropic_oauth_key("Bearer sk-ant-oat01-abc123") is True
+        assert is_anthropic_oauth_key("Bearer sk-ant-oat02-xyz789") is True
+
+    def test_non_oauth_tokens(self):
+        """Non-OAuth values should return False."""
+        from litellm.llms.anthropic.common_utils import is_anthropic_oauth_key
+
+        assert is_anthropic_oauth_key(None) is False
+        assert is_anthropic_oauth_key("") is False
+        assert is_anthropic_oauth_key("sk-ant-api01-abc123") is False
+        assert is_anthropic_oauth_key("Bearer sk-ant-api01-abc123") is False
+
+    def test_case_sensitivity(self):
+        """OAuth prefix matching should be case-sensitive."""
+        from litellm.llms.anthropic.common_utils import is_anthropic_oauth_key
+
+        assert is_anthropic_oauth_key("sk-ant-OAT01-abc123") is False
+        assert is_anthropic_oauth_key("SK-ANT-OAT01-abc123") is False
+
+    def test_just_prefix(self):
+        """Just the prefix with no suffix should still match."""
+        from litellm.llms.anthropic.common_utils import is_anthropic_oauth_key
+
+        assert is_anthropic_oauth_key("sk-ant-oat") is True
+
+
+class TestProxyOAuthHeaderForwarding:
+    """Tests for proxy-layer OAuth header preservation and forwarding."""
+
+    def test_clean_headers_preserves_oauth_authorization(self):
+        """clean_headers should preserve Authorization header with OAuth tokens."""
+        from starlette.datastructures import Headers
+
+        from litellm.proxy.litellm_pre_call_utils import clean_headers
+
+        raw_headers = Headers(
+            raw=[
+                (b"authorization", f"Bearer {FAKE_OAUTH_TOKEN}".encode()),
+                (b"content-type", b"application/json"),
+            ]
+        )
+        cleaned = clean_headers(raw_headers)
+
+        assert "authorization" in cleaned
+        assert cleaned["authorization"] == f"Bearer {FAKE_OAUTH_TOKEN}"
+        assert cleaned["content-type"] == "application/json"
+
+    def test_clean_headers_strips_non_oauth_authorization(self):
+        """clean_headers should strip Authorization header with regular API keys."""
+        from starlette.datastructures import Headers
+
+        from litellm.proxy.litellm_pre_call_utils import clean_headers
+
+        raw_headers = Headers(
+            raw=[
+                (b"authorization", b"Bearer sk-regular-key-123"),
+                (b"content-type", b"application/json"),
+            ]
+        )
+        cleaned = clean_headers(raw_headers)
+
+        assert "authorization" not in cleaned
+        assert cleaned["content-type"] == "application/json"
+
+    def test_add_provider_specific_headers_forwards_oauth(self):
+        """add_provider_specific_headers_to_request should forward OAuth Authorization
+        as a ProviderSpecificHeader scoped to Anthropic-compatible providers."""
+        from litellm.proxy.litellm_pre_call_utils import (
+            add_provider_specific_headers_to_request,
+        )
+
+        data: dict = {}
+        headers = {
+            "authorization": f"Bearer {FAKE_OAUTH_TOKEN}",
+            "content-type": "application/json",
+        }
+
+        add_provider_specific_headers_to_request(data=data, headers=headers)
+
+        assert "provider_specific_header" in data
+        psh = data["provider_specific_header"]
+        assert "anthropic" in psh["custom_llm_provider"]
+        assert "bedrock" in psh["custom_llm_provider"]
+        assert "vertex_ai" in psh["custom_llm_provider"]
+        assert psh["extra_headers"]["authorization"] == f"Bearer {FAKE_OAUTH_TOKEN}"
+
+    def test_add_provider_specific_headers_ignores_non_oauth(self):
+        """add_provider_specific_headers_to_request should not create a
+        ProviderSpecificHeader for non-OAuth Authorization headers."""
+        from litellm.proxy.litellm_pre_call_utils import (
+            add_provider_specific_headers_to_request,
+        )
+
+        data: dict = {}
+        headers = {
+            "authorization": "Bearer sk-regular-key-123",
+            "content-type": "application/json",
+        }
+
+        add_provider_specific_headers_to_request(data=data, headers=headers)
+
+        assert "provider_specific_header" not in data
+
+    def test_add_provider_specific_headers_combines_anthropic_and_oauth(self):
+        """When both anthropic-beta and OAuth Authorization are present, both
+        should be included in the ProviderSpecificHeader."""
+        from litellm.proxy.litellm_pre_call_utils import (
+            add_provider_specific_headers_to_request,
+        )
+
+        data: dict = {}
+        headers = {
+            "authorization": f"Bearer {FAKE_OAUTH_TOKEN}",
+            "anthropic-beta": "oauth-2025-04-20",
+            "content-type": "application/json",
+        }
+
+        add_provider_specific_headers_to_request(data=data, headers=headers)
+
+        assert "provider_specific_header" in data
+        psh = data["provider_specific_header"]
+        assert psh["extra_headers"]["authorization"] == f"Bearer {FAKE_OAUTH_TOKEN}"
+        assert psh["extra_headers"]["anthropic-beta"] == "oauth-2025-04-20"