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 called at runtime (startup=false) but serverSession is undefined: no JS diagnostic session exists to attach the CPU-sampling client to. Identical precondition to the counters variant, scoped to the CPU profiler.

Source

Thrown at src/native/libs/System.Native.Browser/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 { commandResumeRuntime, commandStopTracing, commandSampleProfiler } from "./client-commands";
import { dotnetApi, dotnetLoaderExports, Module } from "./cross-module";
import { serverSession, setupJsClient } from "./diagnostic-server-js";
import { IDiagnosticSession } from "./types";

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

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

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

View on GitHub (pinned to 290d5ab72c)

Solutions

1. Ensure the runtime is running and DOTNET_DiagnosticPorts is configured so a JS session exists.
2. Use collectCpuSamples(opts, true) to sample from startup.
3. Call collectCpuSamples only after a diagnostic session is established.

Example fix

// before: runtime CPU profiling with no session
collectCpuSamples(); // throws
// after: register at startup
collectCpuSamples({ durationSeconds: 30 }, true);

// Verify a session exists (or use startup) before CPU sampling
import { serverSession } from './diagnostic-server-js';
import { dotnetApi } from './cross-module';
if (!dotnetApi.getConfig().environmentVariables?.DOTNET_WasmPerformanceInstrumentation) {
  throw new Error('Enable WasmPerformanceInstrumentation before profiling.');
}
if (!serverSession) collectCpuSamples({ durationSeconds: 30 }, true);
else collectCpuSamples({ durationSeconds: 30 });

When it happens

Trigger: collectCpuSamples() invoked after runtime start but before the diagnostic server completed a session handshake (serverSession undefined).

Common situations: Calling the profiler before the runtime/session is ready; diagnostics disabled; missing DOTNET_DiagnosticPorts.

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/830d58b060637d62. Report an issue: GitHub.