Discord error 4001: Unknown opcode

Your bot sent a gateway payload whose op value is not one Discord accepts from clients, or whose d data did not match the expected shape for that opcode. Clients are only allowed to send a small set: 1 Heartbeat, 2 Identify, 3 Presence Update, 4 Voice State Update, 6 Resume, 8 Request Guild Members, and 31 Request Soundboard Sounds. Anything else, including the server-only opcodes (0 Dispatch, 7 Reconnect, 9 Invalid Session, 10 Hello, 11 Heartbeat ACK), is rejected with 4001.

If you use a mainstream library you will essentially never see this. It appears in custom gateway implementations, in bots that forward raw payloads (for example a voice library that sends voice-gateway opcodes to the main gateway), or when an op is accidentally sent as a string. It is not reconnect-safe in the sense that reconnecting without fixing the sender will produce it again, although Discord marks it as reconnectable.

// WebSocket close frame
code: 4001
reason: "Unknown opcode"

// a payload that would cause it
{ "op": 12, "d": {} }   // 12 is not a client-sendable opcode

Unless you are writing a library, do not hand-send gateway frames. If you are, build a typed encoder for the client-sendable opcodes and unit-test each payload shape against the documented schema.

The client-sendable opcode list is short by design; anything else belongs to Discord's side of the protocol. The gateway send budget those opcodes share is on the Discord limits page, and malformed-but-parseable frames fail one step earlier with 4002.