linshenkx/prompt-optimizer · error · EvaluationValidationError

${label} #${index + 1} label must not be empty.

Error message

${label} #${index + 1} label must not be empty.

What it means

Part of evaluation input validation: every media item in an evaluation's content block must carry a non-empty label. The service iterates mediaItems and throws EvaluationValidationError when item.label is missing or whitespace, including the 1-based index of the offending item in the message.

Source

Thrown at packages/core/src/services/evaluation/service.ts:1675

  private hasSnapshotOutputMedia(snapshot: EvaluationSnapshot | null | undefined): boolean {
    return this.hasBlockMedia(snapshot?.outputBlock);
  }

  private hasMediaPayload(mediaItem: EvaluationMediaItem | null | undefined): boolean {
    const assetId = mediaItem?.assetId?.trim() || '';
    const b64 = mediaItem?.b64?.trim() || '';
    return !!assetId || !!b64;
  }

  private validateMediaItems(mediaItems: EvaluationMediaItem[], label: string): void {
    mediaItems.forEach((item, index) => {
      const itemLabel = item?.label?.trim() || '';
      const assetId = item?.assetId?.trim() || '';
      const b64 = item?.b64?.trim() || '';

      if (!itemLabel) {
        throw new EvaluationValidationError(`${label} #${index + 1} label must not be empty.`);
      }

      if (!assetId && !b64) {
        throw new EvaluationValidationError(
          `${label} #${index + 1} must provide either assetId or b64.`
        );
      }

      if (assetId && b64) {
        throw new EvaluationValidationError(
          `${label} #${index + 1} must not provide both assetId and b64.`
        );
      }
    });
  }

  private validateContentBlock(
    block: EvaluationContentBlock | undefined,

View on GitHub (pinned to 3e677b1d9f)

Solutions

1. Set a unique non-empty label on every media item in the media array
2. Inspect the error message: it names the offending item by index — fix that exact entry
3. Add a client-side check over media arrays before submit (see validation snippet)

Example fix

// before
media: [{ assetId: 'img-1' }]

// after
media: [{ label: 'expected output screenshot', assetId: 'img-1' }]

function validateMediaLabels(media: { label?: string }[]): void {
  media.forEach((item, i) => {
    if (!item?.label?.trim()) throw new Error(`media[${i}] is missing a label`);
  });
}

const hasLabel = (m: { label?: string } | undefined): boolean => !!m?.label?.trim();

try {
  await evaluationService.create(request);
} catch (err) {
  if (err instanceof EvaluationValidationError && /label must not be empty/.test(err.message)) {
    showFieldError(err.message); // surface index from message to the user
  } else throw err;
}

When it happens

Trigger: Submitting an evaluation (or snapshot/test case) whose input content block includes a media array where one of the items has label: '', label omitted, or a whitespace-only string. The message embeds the block label (e.g. 'Test case #2 input media') and the item index.

Common situations: Generating media items programmatically and forgetting to set label; parsing media from CSV/JSON where the label column is empty; trimming/copying sample payloads and dropping the label field.

Understand the failure class

Background: Schema validation failed / invalid input schema: payload rejected because its shape doesn't match the expected schema — this error's family across 28 libraries.

Related errors

• ${label} #${index + 1} must provide either assetId or b64.
• ${label} #${index + 1} must not provide both assetId and b64
• ${label} label must not be empty.
• Result evaluation snapshot testCaseId must match testCase.id
• Image result evaluation requires at least one output image e

AI-assisted analysis of linshenkx/prompt-optimizer@3e677b1d9f (2026-08-27). Data as JSON: /api/errors/1c927b7b1646e751. Report an issue: GitHub.