[v1.x] fix(auth): request JSON token responses

src/mcp/client/auth/oauth2.py

@@ -192,6 +192,8 @@ def prepare_token_auth(
         if headers is None:
             headers = {}  # pragma: no cover
 
+        headers.setdefault("Accept", "application/json")
+
         if not self.client_info:
             return data, headers  # pragma: no cover
 

tests/client/test_auth.py

@@ -597,6 +597,7 @@ async def test_token_exchange_request_authorization_code(self, oauth_provider: O
         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()
@@ -623,6 +624,7 @@ 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()
@@ -655,6 +657,7 @@ async def test_basic_auth_token_exchange(self, oauth_provider: OAuthClientProvid
         request = await oauth_provider._exchange_token_authorization_code("test_auth_code", "test_verifier")
 
         # Should use basic auth (registered method)
+        assert request.headers["Accept"] == "application/json"
         assert "Authorization" in request.headers
         assert request.headers["Authorization"].startswith("Basic ")
 
@@ -700,6 +703,7 @@ async def test_basic_auth_refresh_token(self, oauth_provider: OAuthClientProvide
 
         request = await oauth_provider._refresh_token()
 
+        assert request.headers["Accept"] == "application/json"
         assert "Authorization" in request.headers
         assert request.headers["Authorization"].startswith("Basic ")