dotnet/runtime · error · Error

No active JS diagnostic session

Error message

No active JS diagnostic session

What it means

Thrown by collectMetrics() (dotnet-counters) in diagnostics/dotnet-counters.ts when serverSession is undefined. collectMetrics drives a dotnet-counters trace by talking to the in-browser .NET diagnostic server via the JS IPC channel; it needs an established DiagnosticSession. Without serverSession there is no diagnostic server to send the StartTracing/counters command to.

Source

Thrown at src/mono/browser/runtime/diagnostics/dotnet-counters.ts:15

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

export function collectMetrics (options?:DiagnosticCommandOptions):Promise<Uint8Array[]> {
    if (!options) options = {};
    if (!serverSession) {
        throw new Error("No active JS diagnostic session");
    }

    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:() => commandCounters(options),
        onSessionStart,
    });
    return onClosePromise.promise;
}

View on GitHub (pinned to 290d5ab72c)

Solutions

1. Wait for the runtime to be ready and the diagnostic server to advertise before calling collectMetrics.
2. Ensure diagnostics are enabled in the build/config (diagnostic port configured, js-module-diagnostics loaded).
3. Trigger a diagnostic session first (e.g. via the js://counters scenario URL or DOTNET_DiagnosticPorts) so serverSession exists.

Example fix

null

import { serverSession } from "./diagnostics-js";
async function collectMetricsSafe(opts?: DiagnosticCommandOptions) {
  if (!serverSession) {
    throw new Error('No diagnostic session yet; ensure the runtime is ready and diagnostics enabled before calling collectMetrics.');
  }
  return collectMetrics(opts);
}

null

null

When it happens

Trigger: Calling API.collectMetrics() before the .NET diagnostic server has advertised (serverSession set in DiagnosticSession.send), or when diagnostics were not enabled for the runtime.

Common situations: Calling collectMetrics during early startup before the diagnostic server is up. A release build or configuration that did not start the diagnostic server (no diagnostic port configured).

Related errors

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

AI-assisted analysis of dotnet/runtime@290d5ab72c (2026-08-06). Data as JSON: /api/errors/2c59cd9af0f0a8c1. Report an issue: GitHub.