fix(client): preserve existing query params on OAuth authorization_endpoint

Closes #2776

The authorization code grant built the redirect URL with
`f"{auth_endpoint}?{urlencode(auth_params)}"`, which produces an invalid
URL when the server-advertised authorization_endpoint already carries a
query string. For example Salesforce advertises
`.../services/oauth2/authorize?prompt=select_account`, yielding
`...authorize?prompt=select_account?response_type=code&...` (two `?`
separators), so the client navigates to a malformed URL and the server
rejects the request.

Fix: parse the endpoint, merge its existing query params with the
flow-generated auth_params (flow params win on conflict), and re-encode
into a single well-formed query string. None-valued params are dropped
rather than serialized as the literal "None".

Tests: add TestAuthorizationEndpointWithQuery covering the helper
(no/with/conflicting existing query) plus an end-to-end
_perform_authorization_code_grant assertion that the captured redirect
URL preserves the server param and stays well-formed. 101 passed.

Author: Bartok9

src/mcp/client/auth/oauth2.py

@@ -9,10 +9,10 @@
 import secrets
 import string
 import time
-from collections.abc import AsyncGenerator, Awaitable, Callable
+from collections.abc import AsyncGenerator, Awaitable, Callable, Mapping
 from dataclasses import dataclass, field
 from typing import Any, Protocol
-from urllib.parse import quote, urlencode, urljoin, urlparse
+from urllib.parse import parse_qsl, quote, urlencode, urljoin, urlparse, urlunparse
 
 import anyio
 import httpx
@@ -59,6 +59,22 @@
 logger = logging.getLogger(__name__)
 
 
+def _build_authorization_url(auth_endpoint: str, auth_params: Mapping[str, str | None]) -> str:
+    """Build an authorization URL, preserving any query params already on the endpoint.
+
+    Servers may advertise an ``authorization_endpoint`` that already carries query
+    parameters (e.g. ``https://example.com/authorize?prompt=select_account``).
+    Naively appending ``?<params>`` would produce an invalid URL with two ``?``
+    separators, so the existing query is parsed and merged with ``auth_params``.
+    Flow-generated params take precedence on key conflicts; ``None`` values are
+    dropped rather than serialized as the literal string ``"None"``.
+    """
+    parsed = urlparse(auth_endpoint)
+    merged_params = dict(parse_qsl(parsed.query, keep_blank_values=True))
+    merged_params.update({key: value for key, value in auth_params.items() if value is not None})
+    return urlunparse(parsed._replace(query=urlencode(merged_params)))
+
+
 class PKCEParameters(BaseModel):
     """PKCE (Proof Key for Code Exchange) parameters."""
 
@@ -357,7 +373,7 @@ async def _perform_authorization_code_grant(self) -> tuple[str, str]:
             if "offline_access" in self.context.client_metadata.scope.split():
                 auth_params["prompt"] = "consent"
 
-        authorization_url = f"{auth_endpoint}?{urlencode(auth_params)}"
+        authorization_url = _build_authorization_url(auth_endpoint, auth_params)
         await self.context.redirect_handler(authorization_url)
 
         # Wait for callback

tests/client/test_auth.py

@@ -13,6 +13,7 @@
 
 from mcp.client.auth import OAuthClientProvider, PKCEParameters
 from mcp.client.auth.exceptions import OAuthFlowError, OAuthTokenError
+from mcp.client.auth.oauth2 import _build_authorization_url
 from mcp.client.auth.utils import (
     build_oauth_authorization_server_metadata_discovery_urls,
     build_protected_resource_metadata_discovery_urls,
@@ -3169,3 +3170,86 @@ async def echo_callback() -> AuthorizationCodeResult:
         await auth_flow.asend(httpx.Response(200, request=final_req))
     except StopAsyncIteration:
         pass
+
+
+class TestAuthorizationEndpointWithQuery:
+    """Regression tests for #2776 - authorization_endpoint carrying query params."""
+
+    def test_build_authorization_url_no_existing_query(self):
+        url = _build_authorization_url(
+            "https://auth.example.com/authorize",
+            {"response_type": "code", "client_id": "abc"},
+        )
+        parsed = urlparse(url)
+        params = parse_qs(parsed.query)
+        assert parsed.path == "/authorize"
+        assert params["response_type"] == ["code"]
+        assert params["client_id"] == ["abc"]
+        # No malformed double "?" separator.
+        assert url.count("?") == 1
+
+    def test_build_authorization_url_preserves_existing_query(self):
+        # e.g. Salesforce advertises .../authorize?prompt=select_account
+        url = _build_authorization_url(
+            "https://test.salesforce.com/services/oauth2/authorize?prompt=select_account",
+            {"response_type": "code", "client_id": "abc"},
+        )
+        parsed = urlparse(url)
+        params = parse_qs(parsed.query)
+        assert parsed.path == "/services/oauth2/authorize"
+        # The server-provided param survives...
+        assert params["prompt"] == ["select_account"]
+        # ...alongside the flow-generated params.
+        assert params["response_type"] == ["code"]
+        assert params["client_id"] == ["abc"]
+        # Exactly one "?" - the old f-string produced "...?prompt=...?response_type=...".
+        assert url.count("?") == 1
+
+    def test_build_authorization_url_flow_params_win_on_conflict(self):
+        url = _build_authorization_url(
+            "https://auth.example.com/authorize?response_type=token",
+            {"response_type": "code"},
+        )
+        params = parse_qs(urlparse(url).query)
+        assert params["response_type"] == ["code"]
+
+    @pytest.mark.anyio
+    async def test_perform_authorization_preserves_endpoint_query(self, oauth_provider: OAuthClientProvider):
+        """End-to-end: redirect URL stays valid when the endpoint has a query string."""
+        oauth_provider.context.oauth_metadata = OAuthMetadata(
+            issuer=AnyHttpUrl("https://test.salesforce.com"),
+            authorization_endpoint=AnyHttpUrl(
+                "https://test.salesforce.com/services/oauth2/authorize?prompt=select_account"
+            ),
+            token_endpoint=AnyHttpUrl("https://test.salesforce.com/services/oauth2/token"),
+        )
+        oauth_provider.context.client_info = OAuthClientInformationFull(
+            client_id="test_client_id",
+            client_secret="test_client_secret",
+            redirect_uris=[AnyUrl("http://localhost:3030/callback")],
+        )
+
+        captured_url: str | None = None
+        captured_state: str | None = None
+
+        async def capture_redirect(url: str) -> None:
+            nonlocal captured_url, captured_state
+            captured_url = url
+            captured_state = parse_qs(urlparse(url).query).get("state", [None])[0]
+
+        async def mock_callback() -> AuthorizationCodeResult:
+            return AuthorizationCodeResult(code="test_auth_code", state=captured_state)
+
+        oauth_provider.context.redirect_handler = capture_redirect
+        oauth_provider.context.callback_handler = mock_callback
+
+        await oauth_provider._perform_authorization_code_grant()
+
+        assert captured_url is not None
+        parsed = urlparse(captured_url)
+        params = parse_qs(parsed.query)
+        assert parsed.path == "/services/oauth2/authorize"
+        assert params["prompt"] == ["select_account"]
+        assert params["response_type"] == ["code"]
+        assert params["client_id"] == ["test_client_id"]
+        assert captured_url.count("?") == 1