What causes "Named pipeline '${k.slice(0, i + 1)}' is reserved." in parcel-bundler/parcel?

Rename the pipeline key to something that doesn't collide with a reserved scheme — use a plain alphanumeric name without a reserved prefix.

parcel-bundler/parcel · error · ThrowableDiagnostic

Named pipeline ' ' is reserved.

Error message

Named pipeline '${k.slice(0, i + 1)}' is reserved.

What it means

Thrown by processMap() when a key in a config map (e.g., transformers, bundlers) uses a reserved URL scheme prefix. The reserved set is node:, npm:, http:, https:, data:, tel:, mailto:. These schemes have special meaning in Parcel's dependency resolution and cannot be used as named pipeline identifiers. The diagnostic links to the URL schemes documentation.

Solutions

Rename the pipeline key to something that doesn't collide with a reserved scheme — use a plain alphanumeric name without a reserved prefix.
If you need protocol-specific behavior, use the URL scheme in import specifiers (e.g., import 'npm:react') rather than as a pipeline name.
Read the linked documentation: https://parceljs.org/features/dependency-resolution/#url-schemes.

Example fix

// before — .parcelrc
{
  "extends": "@parcel/config-default",
  "transformers": {
    "http:*": ["my-http-transformer"]
  }
}

// after — use a non-reserved name
{
  "extends": "@parcel/config-default",
  "transformers": {
    "web-resource:*": ["my-http-transformer"]
  }
}

Defensive patterns

Strategy: validation

Validate before calling

const RESERVED = new Set(['node:', 'npm:', 'http:', 'https:', 'data:', 'tel:', 'mailto:']);
function validateNoReservedPipelineKeys(parcelrc) {
  for (const section of ['transformers', 'bundlers', 'namers', 'runners']) {
    const map = parcelrc[section];
    if (!map) continue;
    for (const key of Object.keys(map)) {
      const i = key.indexOf(':');
      if (i > 0 && RESERVED.has(key.slice(0, i + 1))) {
        throw new Error(`Pipeline key "${key}" uses reserved scheme "${key.slice(0, i + 1)}".`);
      }
    }
  }
}

Prevention

Familiarize yourself with Parcel's reserved URL schemes before writing custom pipeline keys.
Use descriptive alphanumeric names for named pipelines.
Review the URL schemes documentation: https://parceljs.org/features/dependency-resolution/#url-schemes.

When it happens

Trigger: Iterating keys in a config map within .parcelrc. For each key k, if it contains ':' and the prefix (including the colon) is in RESERVED_PIPELINES, the error fires. E.g., a transformers entry keyed "npm:foo" or "http:bar".

Common situations: Trying to create a named pipeline for a specific protocol like "http:" or "npm:"; misunderstanding that these prefixes are Parcel's built-in URL schemes for resolution; copy-pasting a config that used a scheme-like name.

Related errors

AI-assisted analysis of parcel-bundler/parcel@59484858a1 (2026-08-13). Data as JSON: /api/errors/f1aa87dc980c11f7. Report an issue: GitHub.

Appendix: source

Thrown at packages/core/core/src/requests/ParcelConfigRequest.js:297

]);

async function processMap(
  // $FlowFixMe
  map: ?ConfigMap<any, any>,
  keyPath: string,
  filePath: FilePath,
  options: ParcelOptions,
  // $FlowFixMe
): Promise<ConfigMap<any, any> | typeof undefined> {
  if (!map) return undefined;

  // $FlowFixMe
  let res: ConfigMap<any, any> = {};
  for (let k in map) {
    let i = k.indexOf(':');
    if (i > 0 && RESERVED_PIPELINES.has(k.slice(0, i + 1))) {
      let code = await options.inputFS.readFile(filePath, 'utf8');
      throw new ThrowableDiagnostic({
        diagnostic: {
          message: `Named pipeline '${k.slice(0, i + 1)}' is reserved.`,
          origin: '@parcel/core',
          codeFrames: [
            {
              filePath: filePath,
              language: 'json5',
              code,
              codeHighlights: generateJSONCodeHighlights(code, [
                {
                  key: `${keyPath}/${k}`,
                  type: 'key',
                },
              ]),
            },
          ],
          documentationURL:
            'https://parceljs.org/features/dependency-resolution/#url-schemes',

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