WebSocket close codes

Close code (uint16)	IANA meaning	Reserved	Custom	Usage
0 - 999			No	Unused
1000	Normal Closure			Successful operation, connection not required anymore
1001	Going Away			Browser tab closing, graceful server shutdown
1002	Protocol error			Endpoint received malformed frame
1003	Unsupported Data			Endpoint received unsupported frame (e.g. binary-only got text frame, ping/pong frames not handled properly)
1004			No	Unused
1005	No Status Rcvd	Yes		Got no close status but transport layer finished normally (e.g. TCP FIN but no previous CLOSE frame)
1006	Abnormal Closure	Yes		Transport layer broke (e.g. couldn't connect, TCP RST)
1007	Invalid frame payload data			Data in endpoint's frame is not consistent (e.g. malformed UTF-8)
1008	Policy Violation			Generic code not applicable to any other (e.g. isn't 1003 nor 1009)
1009	Message Too Big			Endpoint won't process large message
1010	Mandatory Ext.			Client wanted extension(s) that server did not negotiate
1011	Internal Error			Unexpected server problem while operating
1012	Service Restart			Server/service is restarting
1013	Try Again Later			Temporary server condition forced blocking client's application-based request
1014	Bad gateway			Server acting as gateway/proxy got invalid response. Equivalent to HTTP 502
1015	TLS handshake	Yes		Transport layer broke because TLS handshake failed
1016 - 2999		-	Extensions only	Unused. Extensions or future spec can define them
3000 - 3999		Undefined	Yes	Available for applications, registered first come first serve at IANA
3000	Unauthorized	Undefined	Yes	Endpoint must be authorized to perform application-based request. Equivalent to HTTP 401
3003	Forbidden	Undefined	Yes	Endpoint is authorized but has no permissions to perform application-based request. Equivalent to HTTP 403
3008	Timeout	Undefined	Yes	Endpoint took too long to respond to application-based request. Equivalent to HTTP 408
4000 - 4999		Undefined	Yes	Available for applications

• Reserved means WebSocket implementation reports it to application, but per spec cannot be used as actual close code

• Custom means undefined by spec. Application logic and/or WebSocket implementations are allowed to use it however they like, provided common sense that both endpoints will understand them

• Transport layer is the protocol via which WebSocket messages get sent. Usually TCP or QUIC when over HTTP/3

• Extensions are spec that changes how messages are sent. They aren't defined by WebSocket spec, implementations don't have to support them, and per RFC must have their details publicly available. One well used example is permessage-deflate

Relevant links:

• WebSocket spec: https://datatracker.ietf.org/doc/html/rfc6455#section-7.4

• Code registry: https://www.iana.org/assignments/websocket/websocket.xhtml#close-code-number