dotnet/runtime · error · Error

Runtime is not running

Error message

Runtime is not running

What it means

Thrown by setupJsClient when called with startup=false while the .NET runtime is not yet running. Diagnostic clients (metrics, gcdump, cpu profiler) must attach either at startup (startup=true) or after the runtime is ready; attaching at runtime before readiness is invalid.

Source

Thrown at src/native/libs/System.Native.Browser/diagnostics/diagnostic-server-js.ts:150

        if (this.messagesToSend.length === 0) {
            return 0;
        }
        if (this.diagClient && !this.diagClient.skipDownload) {
            downloadBlob(this.messagesToSend);
        }
        this.messagesToSend = [];
        return 0;
    }
}

export function initializeJsClient() {
    nextJsClient = dotnetLoaderExports.createPromiseCompletionSource<IDiagnosticClient>();
    startupJsClient = undefined;
}

export function setupJsClient(client: IDiagnosticClient, startup?: boolean) {
    if (!startup && !dotnetLoaderExports.isRuntimeRunning()) {
        throw new Error("Runtime is not running");
    }
    if (startup) {
        if (startupJsClient) {
            throw new Error("startup diagnostic client already registered");
        }
        startupJsClient = client;
    } else {
        if (nextJsClient.isDone) {
            throw new Error("multiple clients in parallel are not allowed");
        }
        nextJsClient.resolve(client);
    }
}

export function createDiagConnectionJs(socketHandle: number, scenarioName: string): DiagnosticSession {
    if (!fromScenarioNameOnce) {
        fromScenarioNameOnce = true;
        if (scenarioName.startsWith("js://gcdump")) {

View on GitHub (pinned to 290d5ab72c)

Solutions

1. Wait for the runtime-ready signal before calling any runtime diagnostic collect function.
2. If you need data from the very start, register the client at startup with collect*(opts, true).

Example fix

// before: collecting metrics before runtime ready
Module.onRuntimeInitialized = () => collectMetrics();
// after: collect once the runtime reports running
Module.onRuntimeInitialized = () =>
  runtimeReady.then(() => collectMetrics());

// Only attach a runtime diagnostic client once the runtime is running
if (!dotnetLoaderExports.isRuntimeRunning()) {
  throw new Error('Runtime not ready; call collectMetrics after runtime-ready or use the startup hook.');
}
collectMetrics();

When it happens

Trigger: Calling collectMetrics()/collectGcDump()/collectCpuSamples() (without startup=true) before dotnetLoaderExports.isRuntimeRunning() returns true.

Common situations: Invoking diagnostics from an onRuntimeInitialized callback that fires before the runtime reports running; an app-init race where the button handler runs too early.

Related errors

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

AI-assisted analysis of dotnet/runtime@290d5ab72c (2026-08-06). Data as JSON: /api/errors/b727cd6cde73b4c4. Report an issue: GitHub.