I used to call API https://ipnpb.paypal.com/cgi-bin/webscr to get the transaction detail.
But today, the api starts to return 400 bad request: Your browser sent a request that this server could not understand.
The body: {"cmd": "_notify-synch", "tx": transaction_id, "at": identity_token} worked well in the past and just return 400 bad request today.