NEVER Do (Expert Networking Rules)

NEVER use Reliable for high-frequency data (Position/Movement) — Reliable packets block all following packets until acknowledged (Head-of-Line blocking). Use UnreliableOrdered.
NEVER trust the client for shared state (Health/Money/Inventory) — Clients should only suggest actions. The Server MUST validate and broadcast the result.
NEVER hardcode the Server IP — Always allow for discovery (net_lan_discovery.gd) or pass the IP via CLI/UI.

NEVER send the same data every frame — Use net_adaptive_sync_throttle.gd to only send updates when state changes significantly or at fixed Hz intervals (e.g., 20Hz).
NEVER use JSON for high-speed synchronization — String serialization is massive over the wire. Use bit-packing (net_packet_bit_packer.gd) to keep packets under 100 bytes.
NEVER broadcast to all peers if it's not needed — In large worlds, use Interest Management (net_visibility_grid_culling.gd). A player in the forest doesn't need the position of a player in the city.

NEVER allow unlimited RPC calls per second — An attacker can flood the server with 1,000 "Attack" RPCs to crash the game. Use net_packet_rate_limiter.gd.
NEVER expose PeerIDs to the end-user — PeerIDs are internal. Always map them to persistent UserIDs (net_custom_id_mapper.gd) from a database.
NEVER run a dedicated server with a GUI enabled — Use --headless (net_headless_server_auto_start.gd) to save resources and ensure stability.

godot-multiplayer-networking