dotnet/runtime · error · Error

No active JS diagnostic session

Error message

No active JS diagnostic session

What it means

collectMetrics() throws at its guard when called (startup=false) and serverSession is undefined. The diagnostic server inside the dotnet VM must have already advertised a session (setting serverSession) before counters can be collected. Same precondition as error 113 but checked locally in dotnet-counters.ts.

Solutions

1. Await dotnet.create() then verify the diagnostics JS module loaded (resources.jsModuleDiagnostics non-empty).
2. Pass startup=true to register the client at boot: collectMetrics({}, true) — this bypasses the serverSession check.
3. Set DOTNET_DiagnosticPorts=js://ready so the runtime opens a JS session.
4. Re-check after runtime exit; do not call collectors on a dead runtime.

Example fix

// before
collectMetrics(); // serverSession undefined

// after: register as startup client or wait
const runtime = await dotnet.create();
await collectMetrics(); // session exists after first advert

function canCollectMetrics(): boolean {
  // serverSession exported from diagnostic-server-js
  return !!serverSession;
}

function isSessionReady(): boolean { return !!serverSession; }

try { await collectMetrics(); } catch (e) {
  if (/No active JS diagnostic session/.test((e as Error).message)) {
    collectMetrics({}, true); // register at startup slot, then create runtime
    await dotnet.create();
  } else throw e;
}

When it happens

Trigger: Calling collectMetrics() before any advert arrived from the runtime. Calling it after the session closed and serverSession was cleared. Diagnostics module not loaded (jsModuleDiagnostics absent) so no session ever starts.

Common situations: Forgetting to enable diagnostics at build (diagnostics module not linked). Calling collectMetrics() too early in startup. Runtime exited before the call.

Related errors

• No active JS diagnostic session
• Diag module already loaded
• No active JS diagnostic session
• No active session to reconnect
• Runtime is not running

AI-assisted analysis of dotnet/runtime@60108ba66e (2026-08-10). Data as JSON: /api/errors/5d4703442ac1e2e4. Report an issue: GitHub.

Appendix: source

Thrown at src/native/libs/System.Native.Browser/diagnostics/dotnet-counters.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, commandCounters } from "./client-commands";
import { dotnetLoaderExports, Module } from "./cross-module";
import { serverSession, setupJsClient } from "./diagnostic-server-js";
import { IDiagnosticSession } from "./types";

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

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

View on GitHub (pinned to 60108ba66e)