fix: request identity encoding for streamable HTTP

Author: he-yufeng

src/mcp/client/streamable_http.py

@@ -90,6 +90,7 @@ def _prepare_headers(self) -> dict[str, str]:
         """
         headers: dict[str, str] = {
             "accept": "application/json, text/event-stream",
+            "accept-encoding": "identity",
             "content-type": "application/json",
         }
         # Add session headers if available

tests/shared/test_streamable_http.py

@@ -2283,6 +2283,8 @@ async def test_streamable_http_client_mcp_headers_override_defaults(
                 assert "content-type" in headers_data
                 assert headers_data["content-type"] == "application/json"
 
+                assert headers_data.get("accept-encoding") == "identity"
+
 
 @pytest.mark.anyio
 async def test_streamable_http_client_preserves_custom_with_mcp_headers(
@@ -2318,3 +2320,5 @@ async def test_streamable_http_client_preserves_custom_with_mcp_headers(
 
                 assert "content-type" in headers_data
                 assert headers_data["content-type"] == "application/json"
+
+                assert headers_data.get("accept-encoding") == "identity"