dotnet/runtime · error · Error

No active JS diagnostic session

Error message

No active JS diagnostic session

What it means

Thrown by `collectCpuSamples` (dotnet-cpu-profiler) when no JS diagnostic session exists. Same precondition as the counters collector: the runtime must have advertised a session so the profiler can send its StartSampling command over the established transport.

Solutions

1. Defer `collectCpuSamples` until the runtime is ready and `serverSession` exists.
2. Publish the runtime with diagnostics enabled.
3. Confirm `DOTNET_DiagnosticPorts` is configured to expose the JS transport.

Example fix

// before: called before session
const s = collectCpuSamples({}); // throws

// after: wait for the diagnostic session
await waitForDiagSession();
const s = collectCpuSamples({ durationSeconds: 5 });

function canCollectCpuSamples () {
  return !!serverSession;
}
if (canCollectCpuSamples()) collectCpuSamples({ durationSeconds: 5 });

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

When it happens

Trigger: Calling `collectCpuSamples(options)` (dotnet-cpu-profiler.ts:11) before `serverSession` is set. Occurs when the profiler is invoked at startup, when the runtime has no diagnostics enabled, or before the advert handshake completes.

Common situations: Profiling before runtime ready; runtime published without the diagnostics module; misconfigured diagnostic ports.

Related errors

• No active JS diagnostic session
• No active JS diagnostic session
• Failed on mono_wasm_get_dbg_command_info
• method instrumentation is not enabled, please enable it…
• multiple clients in parallel are not allowed

AI-assisted analysis of dotnet/runtime@60108ba66e (2026-08-10). Data as JSON: /api/errors/822d9aca775761fa. Report an issue: GitHub.

Appendix: source

Thrown at src/mono/browser/runtime/diagnostics/dotnet-cpu-profiler.ts:14

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

import type { DiagnosticCommandOptions } from "../types";

import { commandStopTracing, commandSampleProfiler } from "./client-commands";
import { loaderHelpers, Module, runtimeHelpers } from "./globals";
import { serverSession, setupJsClient } from "./diagnostics-js";
import { IDiagnosticSession } from "./common";

export function collectCpuSamples (options?:DiagnosticCommandOptions):Promise<Uint8Array[]> {
    if (!options) options = {};
    if (!serverSession) {
        throw new Error("No active JS diagnostic session");
    }
    if (!runtimeHelpers.config.environmentVariables!["DOTNET_WasmPerformanceInstrumentation"]) {
        throw new Error("method instrumentation is not enabled, please enable it with WasmPerformanceInstrumentation MSBuild property");
    }

    const onClosePromise = loaderHelpers.createPromiseController<Uint8Array[]>();
    function onSessionStart (session: IDiagnosticSession): void {
        // stop tracing after period of monitoring
        Module.safeSetTimeout(() => {
            session.sendCommand(commandStopTracing(session.session_id));
        }, 1000 * (options?.durationSeconds ?? 60));
    }

    setupJsClient({
        onClosePromise:onClosePromise.promise_control,
        skipDownload:options.skipDownload,
        commandOnAdvertise: () => commandSampleProfiler(options),
        onSessionStart,

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