mozilla/pdf.js · error · Error

No valid response from the AI service.

Error message

No valid response from the AI service.

What it means

Thrown by StampEditor.mlGuessAltText when the guess response is present, error-free, and not cancelled, but lacks an `output` string. It guards the final step that uses response.output as the alt text, so an empty/malformed success response is rejected rather than silently producing empty alt text.

Source

Thrown at src/display/editor/stamp.js:192

      name: "altText",
      request: {
        data,
        width,
        height,
        channels: data.length / (width * height),
      },
    });
    if (!response) {
      throw new Error("No response from the AI service.");
    }
    if (response.error) {
      throw new Error("Error from the AI service.");
    }
    if (response.cancel) {
      return null;
    }
    if (!response.output) {
      throw new Error("No valid response from the AI service.");
    }
    const altText = response.output;
    await this.setGuessedAltText(altText);
    if (updateAltTextData && !this.hasAltTextData()) {
      this.altTextData = { alt: altText, decorative: false };
    }
    return altText;
  }

  #getBitmap() {
    if (this.#bitmapId) {
      this._uiManager.enableWaiting(true);
      this._uiManager.imageManager
        .getFromId(this.#bitmapId)
        .then(data => this.#getBitmapFetched(data, /* fromId = */ true))
        .finally(() => this.#getBitmapDone());
      return;
    }

View on GitHub (pinned to 5903d58d58)

Solutions

1. Verify the ML backend returns { output: <non-empty string> } on success.
2. Ensure the alt-text model is the version expecting the {data,width,height,channels} request shape.
3. Catch the error and degrade to manual alt-text entry.
4. Log the full response object upstream of the throw to confirm the missing key.

Example fix

// before
const alt = await editor.mlGuessAltText();
// after
try {
  const alt = await editor.mlGuessAltText();
} catch (e) {
  console.warn('Alt-text guess returned no usable output');
}

// Cannot fully validate server output client-side; at least confirm the backend contract.
// Ensure your mlManager normalizes responses to { output: string } on success.

function isValidAltTextResponse(r) {
  return !!r && !r.error && !r.cancel && typeof r.output === 'string' && r.output.length > 0;
}

try {
  const alt = await editor.mlGuessAltText();
} catch (e) {
  if (e.message === 'No valid response from the AI service.') {
    // schema mismatch; fall back to manual entry
  }
}

When it happens

Trigger: mlManager.guess resolves to an object with no error, no cancel, and a falsy `output` (e.g. { output: '' } or { } ).

Common situations: The model returned an empty string for an unrecognizable image; the backend schema changed and the text moved off the `output` key; a proxy trimmed the response body.

Related errors

• No response from the AI service.
• Error from the AI service.
• No ML.
• ML isn't enabled for alt text.

AI-assisted analysis of mozilla/pdf.js@5903d58d58 (2026-08-13). Data as JSON: /api/errors/5678be58c8c1314e. Report an issue: GitHub.