dotnet/runtime · error · Error

No active session to reconnect

Error message

No active session to reconnect

What it means

Thrown by `connectDSRouter` when there is no existing `serverSession` to migrate. The function's purpose is to take over an already established diagnostic session and redirect it to a new WebSocket URL (e.g. switching to dotnet-dsrouter); without a live session there is nothing to reconnect.

Solutions

1. Wait until `serverSession` exists (the runtime has advertised) before calling `connectDSRouter`.
2. Do not call `connectDSRouter` after the diagnostic session was closed; re-establish one first.
3. Ensure diagnostics are enabled in the runtime so an advert is produced.

Example fix

// before: no session yet
connectDSRouter('ws://localhost:8080'); // throws

// after: wait for the session, then switch the router
await waitForDiagSession();
connectDSRouter('ws://localhost:8080');

function canConnectDSRouter () {
  return !!serverSession;
}
if (canConnectDSRouter()) connectDSRouter(url);

function hasSessionToReconnect (): boolean {
  return typeof serverSession !== 'undefined' && serverSession !== null;
}

When it happens

Trigger: Calling `connectDSRouter(url)` (index.ts:82) before the runtime has advertised a diagnostic session, or after the session was closed. The guard at index.ts:83-85 fires because `serverSession` is undefined.

Common situations: Calling `connectDSRouter` at startup before the advert arrives; calling it after the prior session ended; runtime published without diagnostics.

Related errors

• No active session to reconnect
• multiple clients in parallel are not allowed
• No active JS diagnostic session
• No active JS diagnostic session
• No active JS diagnostic session

AI-assisted analysis of dotnet/runtime@60108ba66e (2026-08-10). Data as JSON: /api/errors/443f26f7eda54bd9. Report an issue: GitHub.

Appendix: source

Thrown at src/mono/browser/runtime/diagnostics/index.ts:84

            return -1;
        }
        socket_handles.delete(client_socket);
        return wrapper.close();
    };

    globalObjects.api.collectCpuSamples = collectCpuSamples;
    globalObjects.api.collectMetrics = collectMetrics;
    globalObjects.api.collectGcDump = collectGcDump;
    globalObjects.api.connectDSRouter = connectDSRouter;

    cleanup_js_client();
}

// this will take over the existing connection to JS and send new advert message to WS client
// use dotnet-dsrouter server-websocket -v trace
function connectDSRouter (url: string): void {
    if (!serverSession) {
        throw new Error("No active session to reconnect");
    }

    // make sure new sessions hit the new URL
    url_override = url;

    const wrapper = createDiagConnectionWs(serverSession.client_socket, url);
    socket_handles.set(serverSession.client_socket, wrapper);
    wrapper.send(advertise());
}

View on GitHub (pinned to 60108ba66e)