{
  "python": "3.14.4 (main, Apr  7 2026, 13:13:20) [Clang 21.0.0 (clang-2100.0.123.102)]",
  "sqlite": "3.51.3",
  "platform": "macOS-27.0.1-arm64-arm-64bit-Mach-O",
  "source": "examples/system-design/labs/17/chat.py",
  "sha256": "f68d2bdf228152ed9607d5ae44d0ea07f6bfbdf8c0595651cff87210dab44cdf"
}
$ /opt/homebrew/opt/python@3.14/bin/python3.14 examples/system-design/labs/17/chat.py
{"durable_before_lost_ack": [1, 2], "transport_deliveries": [1, 2, 2, 3, 4, 5], "durable_display": [1, 2, 3, 4, 5], "server_messages": 5, "replayed": 2}
PASS: reconnect fills offline gap; replay and repeated send do not duplicate durable display

EXIT_CODE=0
$ /opt/homebrew/opt/python@3.14/bin/python3.14 examples/system-design/labs/17/chat.py --unsafe
{"durable_before_lost_ack": [1, 2], "transport_deliveries": [1, 2, 2, 3, 4, 5], "durable_display": [1, 2, 3, 4, 5], "server_messages": 5, "replayed": 2}
EXPECTED_REJECTION: append-only rendering duplicates reconnect replay

EXIT_CODE=2