dotnet/runtime · error · Error

No active session to reconnect

Error message

No active session to reconnect

What it means

Thrown by connectDSRouter when serverSession is undefined. connectDSRouter hands off an existing JS diagnostic session to a WebSocket dotnet-dsrouter connection; it needs a live session to take over, so calling it before any session exists is invalid.

Source

Thrown at src/native/libs/System.Native.Browser/diagnostics/diagnostic-server.ts:78

    }
    const bufferPtr: VoidPtr = buffer as any >>> 0 as any;
    return wrapper.recv(bufferPtr, bytesToRead);
}

export function ds_rt_websocket_close(clientSocket: number): number {
    const wrapper = socketHandles.get(clientSocket);
    if (!wrapper) {
        return -1;
    }
    socketHandles.delete(clientSocket);
    return wrapper.close();
}

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

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

    const oldWrapper = socketHandles.get(serverSession.clientSocket);
    if (oldWrapper) {
        oldWrapper.close();
    }
    const wrapper = createDiagConnectionWs(serverSession.clientSocket, url);
    socketHandles.set(serverSession.clientSocket, wrapper);
    wrapper.send(advertise());
}

export function initializeDS() {
    const loaderConfig = dotnetApi.getConfig();
    const diagnosticPorts = "DOTNET_DiagnosticPorts";
    loaderConfig.environmentVariables ??= {};

View on GitHub (pinned to 290d5ab72c)

Solutions

1. Call connectDSRouter only after a diagnostic session has been established (after the runtime is running and has advertised).
2. Ensure DOTNET_DiagnosticPorts is configured so the diagnostic server creates a session.

Example fix

// before: reconnecting before any session exists
runtimeStarted.then(() => dotnetApi.connectDSRouter(wsUrl)); // throws if no session
// after: wait for a session, then reconnect
runtimeSessionReady.then(() => dotnetApi.connectDSRouter(wsUrl));

// Only hand off to a DSRouter once a session exists
import { serverSession } from './diagnostic-server-js';
if (!serverSession) {
  throw new Error('No diagnostic session yet; start the runtime with DOTNET_DiagnosticPorts set before calling connectDSRouter.');
}
dotnetApi.connectDSRouter(wsUrl);

When it happens

Trigger: Calling connectDSRouter(url) before the diagnostic server has accepted a session and assigned serverSession.

Common situations: Wiring up dotnet-dsrouter before the runtime emitted an advert; calling connectDSRouter too early in init; diagnostics not enabled so no session ever starts.

Related errors

• No active session to reconnect
• No active JS diagnostic session
• No active JS diagnostic session
• No active JS diagnostic session
• No active JS diagnostic session

AI-assisted analysis of dotnet/runtime@290d5ab72c (2026-08-06). Data as JSON: /api/errors/b6b60717592ceb3f. Report an issue: GitHub.