vercel/ai · error · UnsupportedFunctionalityError

file part data type ${part.data.type}

Error message

file part data type ${part.data.type}

What it means

Mistral's chat API has no representation for this file part's data type, so formatFileUrl throws UnsupportedFunctionalityError listing the unsupported type. Only 'url' and 'data' (base64) file parts can be converted to Mistral message format.

Source

Thrown at packages/mistral/src/convert-to-mistral-chat-messages.ts:25

  MistralAssistantMessageContent,
  MistralPrompt,
} from './mistral-chat-prompt';
import {
  convertToBase64,
  getTopLevelMediaType,
  resolveFullMediaType,
} from '@ai-sdk/provider-utils';

function formatFileUrl({ part }: { part: LanguageModelV4FilePart }): string {
  if (part.data.type === 'url') {
    return part.data.url.toString();
  }

  if (part.data.type === 'data') {
    return `data:${resolveFullMediaType({ part })};base64,${convertToBase64(part.data.data)}`;
  }

  throw new UnsupportedFunctionalityError({
    functionality: `file part data type ${part.data.type}`,
  });
}

export function convertToMistralChatMessages(
  prompt: LanguageModelV4Prompt,
): MistralPrompt {
  const messages: MistralPrompt = [];

  for (let i = 0; i < prompt.length; i++) {
    const { role, content } = prompt[i];
    const isLastMessage = i === prompt.length - 1;

    switch (role) {
      case 'system': {
        messages.push({ role: 'system', content });
        break;
      }

View on GitHub (pinned to 69428b1f8b)

Solutions

1. Send file parts with url or binary data instead of the unsupported data type.
2. Download referenced files and resend them as url or base64 data parts.
3. Filter out unsupported file parts before constructing the prompt.
4. Use a provider that supports the part type for that request.

Example fix

// before: provider reference
{ type: 'file', data: { type: 'reference', ... } }
// after: send binary data
{ type: 'file', mediaType: 'application/pdf', data: { type: 'data', data: pdfBytes } }

function isMistralCompatibleFilePart(part: { type: string; data: { type: string } }): boolean {
  return part.type !== 'file' || ['url', 'data'].includes(part.data.type);
}
// filter before building the prompt:
const safeContent = content.filter(isMistralCompatibleFilePart);

function isConvertibleFileData(data: unknown): data is { type: 'url'; url: string } | { type: 'data'; data: Uint8Array } {
  return (
    typeof data === 'object' && data !== null &&
    ('url' in (data as any) || 'data' in (data as any)) &&
    ['url', 'data'].includes((data as { type: string }).type)
  );
}

try {
  await generateText({ model: mistral(modelId), prompt });
} catch (error) {
  if (UnsupportedFunctionalityError.isInstance(error)) {
    console.warn('Unsupported part for Mistral:', error.functionality);
  } else {
    throw error;
  }
}

When it happens

Trigger: Passing a file part whose data.type is neither 'url' nor 'data' (e.g. 'reference' or 'text') into a message sent to a Mistral chat model via streamText/generateText.

Common situations: Building prompts with provider-specific file references (from another provider's output) and sending them to Mistral; generic prompt-building code that attaches file parts of every kind.

Related errors

• text file parts
• 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/970ad14868cb7641. Report an issue: GitHub.