parcel-bundler/parcel · error · ThrowableDiagnostic

The inspector module isn't available

Error message

The inspector module isn't available

What it means

Thrown by SamplingProfiler.startProfiling when Node's built-in 'inspector' module cannot be required. Parcel's profiler uses the V8 inspector session API to collect sampling profiles during a build. If require('inspector') throws, the CPU profiling feature is unavailable on the current runtime, so Parcel aborts startup with a ThrowableDiagnostic rather than silently disabling profiling.

Source

Thrown at packages/core/profiler/src/SamplingProfiler.js:48

  lineNumber: string,
  columnNumber: string,
|};

// https://chromedevtools.github.io/devtools-protocol/tot/Profiler#type-PositionTickInfo
type PositionTickInfo = {|
  line: number,
  ticks: number,
|};

export default class SamplingProfiler {
  session: Session;

  startProfiling(): Promise<mixed> {
    let inspector;
    try {
      inspector = require('inspector');
    } catch (err) {
      throw new ThrowableDiagnostic({
        diagnostic: {
          message: `The inspector module isn't available`,
          origin: '@parcel/workers',
          hints: ['Disable build profiling'],
        },
      });
    }

    this.session = new inspector.Session();
    this.session.connect();

    return Promise.all([
      this.sendCommand('Profiler.setSamplingInterval', {
        interval: 100,
      }),
      this.sendCommand('Profiler.enable'),
      this.sendCommand('Profiler.start'),
    ]);

View on GitHub (pinned to 59484858a1)

Solutions

1. Disable build profiling: remove the PARCEL_PROFILE env var / profiling flag and rebuild.
2. Upgrade to a standard Node.js LTS distribution (>= 10) that includes the inspector module — verify with `node -e "require('inspector')"`.
3. If on Alpine/musl, switch to an official Node image or one known to ship inspector.
4. If you need profiling, run Parcel under a full Node.js installation outside the restricted sandbox.

Example fix

// before
PARCEL_PROFILE=true parcel build src/index.html

// after (profiling disabled)
parcel build src/index.html

// Before enabling profiling, verify the inspector module is available.
function canProfile() {
  try { require('inspector'); return true; }
  catch { return false; }
}
if (!canProfile()) {
  console.warn('Profiling disabled: inspector module unavailable');
  delete process.env.PARCEL_PROFILE;
}

// Wrap the profiling-enabled build; degrade gracefully if unsupported.
try {
  await profiler.startProfiling();
} catch (e) {
  if (/inspector module isn't available/.test(e.message)) {
    // proceed without profiling
  } else { throw e; }
}

When it happens

Trigger: Calling the build with profiling enabled (e.g. PARCEL_PROFILE=true or the profiler API) on a runtime that ships without the inspector module — typically a stripped/custom Node build, an older Node version (< 8.0.0), or a non-Node environment bundling @parcel/workers.

Common situations: Running Parcel under a minimal Node runtime (Alpine musl images missing inspector, embedded runtimes like Bun/Den incompatibility), Node.js built with --without-inspector, or attempting to profile inside a sandbox that blocks the inspector. Also seen when a bundler inlines @parcel/workers into a browser bundle where inspector is absent.

Related errors

• Parcel is already profiling
• Parcel is not profiling
• Please provide a worker path!
• No dynamic require possible: ${workerPath}

AI-assisted analysis of parcel-bundler/parcel@59484858a1 (2026-08-13). Data as JSON: /api/errors/12c7ac3996b3bf7d. Report an issue: GitHub.