parcel-bundler/parcel · error · Error

Cannot change an asset's uniqueKey after it has been set.

Error message

Cannot change an asset's uniqueKey after it has been set.

What it means

`uniqueKey` is the asset's stable cache identity. It is write-once: once assigned (non-null), subsequent assignments are rejected because changing it would invalidate cached outputs and corrupt dependency resolution. The setter checks the current value and throws if it is already set.

Solutions

1. Check `asset.uniqueKey == null` before assigning.
2. Compute the key once (e.g. at asset creation) and reuse it.
3. If you need a different identity, create a new asset instead of mutating the existing one.

Example fix

// before
asset.uniqueKey = computeKey(); // throws on second pass

// after
if (asset.uniqueKey == null) asset.uniqueKey = computeKey();

// Only assign uniqueKey when it is unset.
if (asset.value.uniqueKey == null) {
  asset.value.uniqueKey = computeKey();
}

function canSetUniqueKey(asset: {value: {uniqueKey?: ?string}}): boolean {
  return asset.value.uniqueKey == null;
}

When it happens

Trigger: A transformer calls `asset.uniqueKey = '...'` on an asset that already has a non-null `uniqueKey`.

Common situations: A transformer that sets uniqueKey on every pass; code that copies/merges assets and reassigns the key; running a transformer twice over the same asset.

Related errors

• Cannot call getCode() on an asset with a dirty AST. For…
• Asset has an AST but no generate method is available on the…
• Config result is not hashable because it contains…
• failed to locate file
• IDBCache is only supported in the browser

AI-assisted analysis of parcel-bundler/parcel@59484858a1 (2026-08-13). Data as JSON: /api/errors/857ab1f05c38e765. Report an issue: GitHub.

Appendix: source

Thrown at packages/core/core/src/public/Asset.js:286

  set isBundleSplittable(isBundleSplittable: boolean): void {
    this.#asset.value.isBundleSplittable = isBundleSplittable;
  }

  get sideEffects(): boolean {
    return this.#asset.value.sideEffects;
  }

  set sideEffects(sideEffects: boolean): void {
    this.#asset.value.sideEffects = sideEffects;
  }

  get uniqueKey(): ?string {
    return this.#asset.value.uniqueKey;
  }

  set uniqueKey(uniqueKey: ?string): void {
    if (this.#asset.value.uniqueKey != null) {
      throw new Error(
        "Cannot change an asset's uniqueKey after it has been set.",
      );
    }
    this.#asset.value.uniqueKey = uniqueKey;
  }

  get symbols(): IMutableAssetSymbols {
    return new MutableAssetSymbols(this.#asset.options, this.#asset.value);
  }

  addDependency(dep: DependencyOptions): string {
    return this.#asset.addDependency(dep);
  }

  invalidateOnFileChange(filePath: FilePath): void {
    this.#asset.invalidateOnFileChange(
      toProjectPath(this.#asset.options.projectRoot, filePath),
    );

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