Fix client secret post for client credentials auth

src/mcp/client/auth/extensions/client_credentials.py

@@ -88,8 +88,13 @@ async def _perform_authorization(self) -> httpx.Request:
 
     async def _exchange_token_client_credentials(self) -> httpx.Request:
         """Build token exchange request for client_credentials grant."""
+
+        if not self.context.client_info or not self.context.client_info.client_id:
+            raise OAuthTokenError("No client info available")  # pragma: no cover
+
         token_data: dict[str, Any] = {
             "grant_type": "client_credentials",
+            "client_id": self.context.client_info.client_id,
         }
 
         headers: dict[str, str] = {"Content-Type": "application/x-www-form-urlencoded"}

tests/client/auth/extensions/test_client_credentials.py

@@ -243,6 +243,9 @@ async def test_exchange_token_client_credentials(self, mock_storage: MockTokenSt
         )
         provider.context.protocol_version = "2025-06-18"
 
+        # Initialize the provider first
+        await provider._initialize()
+
         request = await provider._perform_authorization()
 
         assert request.method == "POST"
@@ -269,6 +272,9 @@ async def test_exchange_token_without_scopes(self, mock_storage: MockTokenStorag
         )
         provider.context.protocol_version = "2024-11-05"  # Old version - no resource param
 
+        # Initialize the provider first
+        await provider._initialize()
+
         request = await provider._perform_authorization()
 
         content = urllib.parse.unquote_plus(request.content.decode())