diff --git a/src/mcp/client/auth/oauth2.py b/src/mcp/client/auth/oauth2.py
index 3c546fda2..8fac12a28 100644
--- a/src/mcp/client/auth/oauth2.py
+++ b/src/mcp/client/auth/oauth2.py
@@ -12,7 +12,7 @@
 from collections.abc import AsyncGenerator, Awaitable, Callable
 from dataclasses import dataclass, field
 from typing import Any, Protocol
-from urllib.parse import quote, urlencode, urljoin, urlparse
+from urllib.parse import quote, urljoin, urlparse
 
 import anyio
 import httpx
@@ -353,7 +353,7 @@ 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 = str(httpx.URL(auth_endpoint).copy_merge_params(auth_params))
         await self.context.redirect_handler(authorization_url)
 
         # Wait for callback
diff --git a/tests/client/test_auth.py b/tests/client/test_auth.py
index bb0bce4c9..fc9b3b2e1 100644
--- a/tests/client/test_auth.py
+++ b/tests/client/test_auth.py
@@ -1167,6 +1167,52 @@ 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_authorization_endpoint_preserves_existing_query_params(
+        self, client_metadata: OAuthClientMetadata, mock_storage: MockTokenStorage
+    ):
+        captured_auth_url: str | None = None
+        captured_state: str | None = None
+
+        async def redirect_handler(url: str) -> None:
+            nonlocal captured_auth_url, captured_state
+            captured_auth_url = url
+            captured_state = parse_qs(urlparse(url).query)["state"][0]
+
+        async def callback_handler() -> tuple[str, str | None]:
+            return "test_auth_code", captured_state
+
+        provider = OAuthClientProvider(
+            server_url="https://api.example.com/v1/mcp",
+            client_metadata=client_metadata,
+            storage=mock_storage,
+            redirect_handler=redirect_handler,
+            callback_handler=callback_handler,
+        )
+        provider.context.oauth_metadata = OAuthMetadata(
+            issuer=AnyHttpUrl("https://auth.example.com"),
+            authorization_endpoint=AnyHttpUrl("https://auth.example.com/authorize?prompt=select_account"),
+            token_endpoint=AnyHttpUrl("https://auth.example.com/token"),
+        )
+        provider.context.client_info = OAuthClientInformationFull(
+            client_id="test_client",
+            client_secret="test_secret",
+            redirect_uris=[AnyUrl("http://localhost:3030/callback")],
+        )
+
+        auth_code, code_verifier = await provider._perform_authorization_code_grant()
+
+        assert auth_code == "test_auth_code"
+        assert code_verifier
+        assert captured_auth_url is not None
+        parsed = urlparse(captured_auth_url)
+        params = parse_qs(parsed.query)
+        assert f"{parsed.scheme}://{parsed.netloc}{parsed.path}" == "https://auth.example.com/authorize"
+        assert params["prompt"] == ["select_account"]
+        assert params["response_type"] == ["code"]
+        assert params["client_id"] == ["test_client"]
+        assert params["redirect_uri"] == ["http://localhost:3030/callback"]
+
     @pytest.mark.anyio
     async def test_auth_flow_no_unnecessary_retry_after_oauth(
         self, oauth_provider: OAuthClientProvider, mock_storage: MockTokenStorage, valid_tokens: OAuthToken
@@ -1350,13 +1396,9 @@ async def test_403_insufficient_scope_updates_scope_from_header(
         async def capture_redirect(url: str) -> None:
             nonlocal redirect_captured, captured_state
             redirect_captured = True
-            # Verify the new scope is included in authorization URL
-            assert "scope=admin%3Awrite+admin%3Adelete" in url or "scope=admin:write+admin:delete" in url.replace(
-                "%3A", ":"
-            ).replace("+", " ")
-            # Extract state from redirect URL
             parsed = urlparse(url)
             params = parse_qs(parsed.query)
+            assert params["scope"] == ["admin:write admin:delete"]
             captured_state = params.get("state", [None])[0]
 
         oauth_provider.context.redirect_handler = capture_redirect