Unclear What the new check rejects, and so which responses are now blocked, is not known.
The sandbox is a restricted environment Claude Code can run commands in, and its network proxy is the go-between that handles their web traffic. When a response comes back from the outside server, the proxy used to forward its status and headers to the command and stream the rest straight through. It now runs the status and headers through a check first and forwards the response only if that check passes. This applies both to the normal proxy path and to the path that decrypts secure (TLS) traffic for inspection.
Errors from the outside server are still logged as before.
This looks like a tightening of how the sandbox proxy handles responses, which matters if you rely on the sandbox to filter network access. Some responses that used to get through may now be stopped.
What the new check rejects, and so which responses are now blocked, is not known.