fix: Add accept json headers for token request

Author: vincent0426

src/mcp/client/auth.py

@@ -424,7 +424,13 @@ async def _exchange_token(self, auth_code: str, code_verifier: str) -> httpx.Req
             token_data["client_secret"] = self.context.client_info.client_secret
 
         return httpx.Request(
-            "POST", token_url, data=token_data, headers={"Content-Type": "application/x-www-form-urlencoded"}
+            "POST",
+            token_url,
+            data=token_data,
+            headers={
+                "Content-Type": "application/x-www-form-urlencoded",
+                "Accept": "application/json",
+            },
         )
 
     async def _handle_token_response(self, response: httpx.Response) -> None:
@@ -478,7 +484,13 @@ async def _refresh_token(self) -> httpx.Request:
             refresh_data["client_secret"] = self.context.client_info.client_secret
 
         return httpx.Request(
-            "POST", token_url, data=refresh_data, headers={"Content-Type": "application/x-www-form-urlencoded"}
+            "POST", 
+            token_url, 
+            data=refresh_data, 
+            headers={
+                "Content-Type": "application/x-www-form-urlencoded", 
+                "Accept": "application/json"
+            },
         )
 
     async def _handle_refresh_response(self, response: httpx.Response) -> bool:

tests/client/test_auth.py

@@ -527,6 +527,7 @@ async def test_token_exchange_request(self, oauth_provider: OAuthClientProvider)
         assert request.method == "POST"
         assert str(request.url) == "https://api.example.com/token"
         assert request.headers["Content-Type"] == "application/x-www-form-urlencoded"
+        assert request.headers["Accept"] == "application/json"
 
         # Check form data
         content = request.content.decode()
@@ -552,6 +553,7 @@ async def test_refresh_token_request(self, oauth_provider: OAuthClientProvider,
         assert request.method == "POST"
         assert str(request.url) == "https://api.example.com/token"
         assert request.headers["Content-Type"] == "application/x-www-form-urlencoded"
+        assert request.headers["Accept"] == "application/json"
 
         # Check form data
         content = request.content.decode()