Can ngrok forward client cancellation to the backend?

Direct Answer

Yes. ngrok propagates client-initiated cancellation to the upstream service, but it does so by closing the backend connection rather than sending a distinct HTTP abort message. For WebSocket tunnels, close frames are forwarded with their code and reason intact.

HTTP Request Cancellation

ngrok operates as a reverse proxy. When a client disconnects mid-request, ngrok detects the TCP connection close and tears down the corresponding upstream connection to your backend. The backend does not receive a special HTTP "Abort" signal; instead, it observes a connection reset or an unexpected EOF on its socket. This is the standard cancellation mechanism for HTTP traffic through a proxy.

WebSocket Close Frames

For WebSocket endpoints, ngrok forwards the WebSocket close frame from the client to the backend server. The close code and reason string are preserved as part of the protocol exchange. This allows the backend to distinguish a normal closure from an abnormal disconnect, provided the client itself sends a well-formed close frame.

Configuration and Timeout Settings

There is no ngrok configuration option that enables or disables cancellation propagation; the behavior is inherent to the proxy architecture and applies by default. Settings such as the agent's request timeout or WebSocket idle limits control how long ngrok waits for backend activity, but they do not toggle abort forwarding. The control API can terminate an entire tunnel or session, which closes all in-flight connections, but that is an administrative action distinct from relaying an individual client cancellation.

Verification Steps

1. HTTP: Run a local backend that logs socket close events and connection errors. Expose it through an ngrok HTTP tunnel, start a long-running request through the public URL, cancel it from the client, and confirm the backend logs an immediate connection close or EOF.
2. WebSocket: Establish a tunnel, open a WebSocket client through the ngrok URL, send a close frame with a specific code, and verify the backend receives the close event with the expected code and reason.

Assumptions and Uncertainty

Confirmed by proxy architecture: HTTP cancellation manifests as an upstream TCP close. Likely but version-dependent: WebSocket close-frame forwarding preserves code and reason across all agent versions and plans. Because no live version was tested here, verify WebSocket behavior against your specific ngrok agent version. If your backend is not observing the close, share the protocol and agent version in use, since version-specific handling may affect the outcome.