fix(auth): forward user-agent to oauth flow requests

src/mcp/client/auth/oauth2.py

@@ -52,6 +52,8 @@
 
 logger = logging.getLogger(__name__)
 
+_FORWARDED_AUTH_FLOW_HEADERS = ("User-Agent",)
+
 
 class PKCEParameters(BaseModel):
     """PKCE (Proof Key for Code Exchange) parameters."""
@@ -477,6 +479,14 @@ def _add_auth_header(self, request: httpx.Request) -> None:
         if self.context.current_tokens and self.context.current_tokens.access_token:  # pragma: no branch
             request.headers["Authorization"] = f"Bearer {self.context.current_tokens.access_token}"
 
+    def _forward_request_headers(self, source_request: httpx.Request, outgoing_request: httpx.Request) -> httpx.Request:
+        """Forward selected caller headers to OAuth flow requests."""
+        for header_name in _FORWARDED_AUTH_FLOW_HEADERS:
+            header_value = source_request.headers.get(header_name)
+            if header_value is not None and header_name not in outgoing_request.headers:
+                outgoing_request.headers[header_name] = header_value
+        return outgoing_request
+
     async def _handle_oauth_metadata_response(self, response: httpx.Response) -> None:
         content = await response.aread()
         metadata = OAuthMetadata.model_validate_json(content)
@@ -508,6 +518,7 @@ async def async_auth_flow(self, request: httpx.Request) -> AsyncGenerator[httpx.
             if not self.context.is_token_valid() and self.context.can_refresh_token():
                 # Try to refresh token
                 refresh_request = await self._refresh_token()  # pragma: no cover
+                refresh_request = self._forward_request_headers(request, refresh_request)  # pragma: no cover
                 refresh_response = yield refresh_request  # pragma: no cover
 
                 if not await self._handle_refresh_response(refresh_response):  # pragma: no cover
@@ -532,6 +543,7 @@ async def async_auth_flow(self, request: httpx.Request) -> AsyncGenerator[httpx.
 
                     for url in prm_discovery_urls:  # pragma: no branch
                         discovery_request = create_oauth_metadata_request(url)
+                        discovery_request = self._forward_request_headers(request, discovery_request)
 
                         discovery_response = yield discovery_request  # sending request
 
@@ -558,6 +570,7 @@ async def async_auth_flow(self, request: httpx.Request) -> AsyncGenerator[httpx.
                     # Step 2: Discover OAuth Authorization Server Metadata (OASM) (with fallback for legacy servers)
                     for url in asm_discovery_urls:  # pragma: no branch
                         oauth_metadata_request = create_oauth_metadata_request(url)
+                        oauth_metadata_request = self._forward_request_headers(request, oauth_metadata_request)
                         oauth_metadata_response = yield oauth_metadata_request
 
                         ok, asm = await handle_auth_metadata_response(oauth_metadata_response)
@@ -596,13 +609,16 @@ async def async_auth_flow(self, request: httpx.Request) -> AsyncGenerator[httpx.
                                 self.context.client_metadata,
                                 self.context.get_authorization_base_url(self.context.server_url),
                             )
+                            registration_request = self._forward_request_headers(request, registration_request)
                             registration_response = yield registration_request
                             client_information = await handle_registration_response(registration_response)
                             self.context.client_info = client_information
                             await self.context.storage.set_client_info(client_information)
 
                     # Step 5: Perform authorization and complete token exchange
-                    token_response = yield await self._perform_authorization()
+                    token_request = await self._perform_authorization()
+                    token_request = self._forward_request_headers(request, token_request)
+                    token_response = yield token_request
                     await self._handle_token_response(token_response)
                 except Exception:  # pragma: no cover
                     logger.exception("OAuth flow error")
@@ -624,7 +640,9 @@ async def async_auth_flow(self, request: httpx.Request) -> AsyncGenerator[httpx.
                         )
 
                         # Step 2b: Perform (re-)authorization and token exchange
-                        token_response = yield await self._perform_authorization()
+                        token_request = await self._perform_authorization()
+                        token_request = self._forward_request_headers(request, token_request)
+                        token_response = yield token_request
                         await self._handle_token_response(token_response)
                     except Exception:  # pragma: no cover
                         logger.exception("OAuth flow error")

tests/client/test_auth.py

@@ -45,7 +45,7 @@ def __init__(self):
         self._client_info: OAuthClientInformationFull | None = None
 
     async def get_tokens(self) -> OAuthToken | None:
-        return self._tokens  # pragma: no cover
+        return self._tokens
 
     async def set_tokens(self, tokens: OAuthToken) -> None:
         self._tokens = tokens
@@ -1167,6 +1167,89 @@ async def test_auth_flow_with_no_tokens(self, oauth_provider: OAuthClientProvide
         assert oauth_provider.context.current_tokens.access_token == "new_access_token"
         assert oauth_provider.context.token_expiry_time is not None
 
+    @pytest.mark.anyio
+    async def test_auth_flow_forwards_user_agent_to_oauth_requests(
+        self, oauth_provider: OAuthClientProvider, mock_storage: MockTokenStorage
+    ):
+        oauth_provider.context.current_tokens = None
+        oauth_provider.context.token_expiry_time = None
+        oauth_provider._initialized = True
+
+        test_request = httpx.Request(
+            "GET", "https://api.example.com/mcp", headers={"User-Agent": "my-custom-client/1.0"}
+        )
+        auth_flow = oauth_provider.async_auth_flow(test_request)
+
+        request = await auth_flow.__anext__()
+        assert request.headers["User-Agent"] == "my-custom-client/1.0"
+
+        response = httpx.Response(
+            401,
+            headers={
+                "WWW-Authenticate": 'Bearer resource_metadata="https://api.example.com/.well-known/oauth-protected-resource"'
+            },
+            request=test_request,
+        )
+
+        discovery_request = await auth_flow.asend(response)
+        assert discovery_request.headers["User-Agent"] == "my-custom-client/1.0"
+
+        discovery_response = httpx.Response(
+            200,
+            content=b'{"resource":"https://api.example.com/v1/mcp","authorization_servers":["https://auth.example.com"]}',
+            request=discovery_request,
+        )
+
+        oauth_metadata_request = await auth_flow.asend(discovery_response)
+        assert oauth_metadata_request.headers["User-Agent"] == "my-custom-client/1.0"
+
+        oauth_metadata_response = httpx.Response(
+            200,
+            content=(
+                b'{"issuer":"https://auth.example.com",'
+                b'"authorization_endpoint":"https://auth.example.com/authorize",'
+                b'"token_endpoint":"https://auth.example.com/token",'
+                b'"registration_endpoint":"https://auth.example.com/register"}'
+            ),
+            request=oauth_metadata_request,
+        )
+
+        registration_request = await auth_flow.asend(oauth_metadata_response)
+        assert registration_request.headers["User-Agent"] == "my-custom-client/1.0"
+
+        registration_response = httpx.Response(
+            201,
+            content=b'{"client_id":"test_client_id","client_secret":"test_client_secret","redirect_uris":["http://localhost:3030/callback"]}',
+            request=registration_request,
+        )
+
+        oauth_provider._perform_authorization_code_grant = mock.AsyncMock(
+            return_value=("test_auth_code", "test_code_verifier")
+        )
+
+        token_request = await auth_flow.asend(registration_response)
+        assert token_request.headers["User-Agent"] == "my-custom-client/1.0"
+
+        token_response = httpx.Response(
+            200,
+            content=(
+                b'{"access_token":"new_access_token","token_type":"Bearer","expires_in":3600,'
+                b'"refresh_token":"new_refresh_token"}'
+            ),
+            request=token_request,
+        )
+
+        final_request = await auth_flow.asend(token_response)
+        assert final_request.headers["Authorization"] == "Bearer new_access_token"
+
+        final_response = httpx.Response(200, request=final_request)
+        with pytest.raises(StopAsyncIteration):
+            await auth_flow.asend(final_response)
+
+        stored_tokens = await mock_storage.get_tokens()
+        assert stored_tokens is not None
+        assert stored_tokens.access_token == "new_access_token"
+
     @pytest.mark.anyio
     async def test_auth_flow_no_unnecessary_retry_after_oauth(
         self, oauth_provider: OAuthClientProvider, mock_storage: MockTokenStorage, valid_tokens: OAuthToken