vercel/ai · error · UnsupportedFunctionalityError

text file parts

Error message

text file parts

What it means

Mistral chat models don't accept file parts whose content is inline text (data.type === 'text'). convertToMistralChatMessages throws UnsupportedFunctionalityError for these parts.

Source

Thrown at packages/mistral/src/convert-to-mistral-chat-messages.ts:62

      case 'user': {
        messages.push({
          role: 'user',
          content: content.map(part => {
            switch (part.type) {
              case 'text': {
                return { type: 'text', text: part.text };
              }

              case 'file': {
                switch (part.data.type) {
                  case 'reference': {
                    throw new UnsupportedFunctionalityError({
                      functionality: 'file parts with provider references',
                    });
                  }
                  case 'text': {
                    throw new UnsupportedFunctionalityError({
                      functionality: 'text file parts',
                    });
                  }
                  case 'url':
                  case 'data': {
                    const topLevel = getTopLevelMediaType(part.mediaType);

                    if (topLevel === 'image') {
                      return {
                        type: 'image_url',
                        image_url: formatFileUrl({ part }),
                      };
                    } else {
                      if (part.data.type === 'data') {
                        const fullMediaType = resolveFullMediaType({ part });
                        if (fullMediaType !== 'application/pdf') {
                          throw new UnsupportedFunctionalityError({
                            functionality:

View on GitHub (pinned to 69428b1f8b)

Solutions

1. Inline the text content as a regular text part instead of a file part.
2. If a document is required, send it as url or base64 data with a proper mediaType.
3. Convert text file parts to text parts in your prompt-building code.
4. Filter text file parts out for Mistral requests.

Example fix

// before
{ type: 'file', data: { type: 'text', value: 'Hello' } }
// after
{ type: 'text', text: 'Hello' }

function hasTextFilePart(messages: { content: unknown[] }[]): boolean {
  return messages.some(m => (m.content as any[]).some(
    p => p?.type === 'file' && p?.data?.type === 'text'));
}
if (hasTextFilePart(messages)) throw new Error('Convert text file parts to text parts for Mistral');

function isTextFilePart(part: unknown): part is { type: 'file'; data: { type: 'text'; value: string } } {
  return typeof part === 'object' && part !== null &&
    (part as any).type === 'file' &&
    (part as any).data?.type === 'text';
}

try {
  await generateText({ model: mistral(modelId), messages });
} catch (error) {
  if (UnsupportedFunctionalityError.isInstance(error) &&
      error.functionality.includes('text file parts')) {
    // convert file parts with text data into plain text parts and retry
  } else {
    throw error;
  }
}

When it happens

Trigger: Sending a 'file' part with data: { type: 'text', value: '...' } in a prompt to a Mistral model.

Common situations: Trying to pass text documents as file parts generically across providers; converting prompts written for providers that accept text file parts.

Related errors

• file part data type ${part.data.type}
• file parts with provider references
• Only images and PDF file parts are supported
• Unsupported content type in assistant message: ${part.type}
• Unsupported role: ${_exhaustiveCheck}

AI-assisted analysis of vercel/ai@69428b1f8b (2026-08-30). Data as JSON: /api/errors/317f58fd7c914ed9. Report an issue: GitHub.