WebSockets now automatically reply to Close frames

The Workers runtime now automatically sends a reciprocal Close frame when it receives a Close frame from the peer. The readyState transitions to CLOSED before the close event fires. This matches the WebSocket specification and standard browser behavior.

This change is enabled by default for Workers using compatibility dates on or after 2026-04-07 (via the web_socket_auto_reply_to_close compatibility flag). Existing code that manually calls close() inside the close event handler will continue to work — the call is silently ignored when the WebSocket is already closed.

const [client, server] = Object.values(new WebSocketPair()); server.accept(); server.addEventListener("close", (event) => { // readyState is already CLOSED — no need to call server.close(). console.log(server.readyState); // WebSocket.CLOSED console.log(event.code); // 1000 console.log(event.wasClean); // true });

Half-open mode for WebSocket proxying

The automatic close behavior can interfere with WebSocket proxying, where a Worker sits between a client and a backend and needs to coordinate the close on both sides independently. To support this use case, pass { allowHalfOpen: true } to accept():

const [client, server] = Object.values(new WebSocketPair()); server.accept({ allowHalfOpen: true }); server.addEventListener("close", (event) => { // readyState is still CLOSING here, giving you time // to coordinate the close on the other side. console.log(server.readyState); // WebSocket.CLOSING // Manually close when ready. server.close(event.code, "done"); });

For more information, refer to WebSockets Close behavior.