Discord error 4002: Decode error

Discord could not decode a frame you sent. Where 4001 means the payload parsed but the opcode was wrong, 4002 means it did not parse at all: invalid JSON, an ETF payload when you connected with encoding=json (or vice versa), a compressed frame sent when you did not negotiate compression, or a binary frame where text was expected. Payload size is also a factor; the gateway enforces a maximum size for inbound frames and an oversized payload is rejected.

Again this is a custom-client problem in almost every case. With a library, the only realistic trigger is mismatched connection options (for example enabling zlib-stream transport compression without the library actually compressing) or an outdated library sending a payload format the gateway version no longer accepts.

// WebSocket close frame
code: 4002
reason: "Decode error"

Negotiate encoding and compression in exactly one place and derive both the URL and the serializer from the same setting. Add a size guard on any payload built from variable-length data before it is queued for the socket.

Encoding bugs are deterministic, which makes them the easiest close code to reproduce and unit test: capture the exact frame, replay it through your serializer, and assert round-tripping. The gateway connection parameters (version, encoding, compression) sit with the connection limits on the Discord limits page; the REST twin of a body Discord cannot parse is 50109.