vercel/ai · error · UnsupportedFunctionalityError

'text file parts' functionality not supported.

Error message

'text file parts' functionality not supported.

What it means

xAI does not accept file parts whose data payload is text (i.e. a file part whose content is inline text rather than binary/reference). When converting file blocks for the Responses API, a 'text' data case throws UnsupportedFunctionalityError because there is no valid xAI representation.

Source

Thrown at packages/xai/src/responses/convert-to-xai-responses-input.ts:65

            case 'text': {
              contentParts.push({ type: 'input_text', text: block.text });
              break;
            }

            case 'file': {
              switch (block.data.type) {
                case 'reference': {
                  contentParts.push({
                    type: 'input_file',
                    file_id: resolveProviderReference({
                      reference: block.data.reference,
                      provider: 'xai',
                    }),
                  });
                  break;
                }
                case 'text': {
                  throw new UnsupportedFunctionalityError({
                    functionality: 'text file parts',
                  });
                }
                case 'url':
                case 'data': {
                  if (getTopLevelMediaType(block.mediaType) === 'image') {
                    const imageUrl =
                      block.data.type === 'url'
                        ? block.data.url.toString()
                        : `data:${resolveFullMediaType({ part: block })};base64,${convertToBase64(block.data.data)}`;

                    const filePartOptions = await parseProviderOptions({
                      provider: 'xai',
                      providerOptions: block.providerOptions,
                      schema: xaiFilePartProviderOptions,
                    });

                    contentParts.push({

View on GitHub (pinned to 69428b1f8b)

Solutions

1. Send the content as a plain text part instead of a file part
2. Upload the document to xAI's Files API and pass a provider reference (file_id) in the file part
3. Use a data/url file part with an image media type if the content is an image

Example fix

// before
content: [{ type: 'file', data: { type: 'text', text: doc }, mediaType: 'text/plain' }]
// after
content: [{ type: 'text', text: doc }]

const hasTextFilePart = prompt.some(m => m.content.some(p => p.type === 'file' && p.data.type === 'text'));
if (hasTextFilePart) throw new Error('convert text file parts to plain text parts for xai');

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

try {
  await generateText({ model, prompt });
} catch (e) {
  if ((e as any).name === 'AI_UnsupportedFunctionalityError' && (e as Error).message.includes('text file parts')) {
    // fall back to text-part rendering of the file content
  }
}

When it happens

Trigger: Building a prompt with a file part whose data is of type 'text' (content: [{ type: 'file', data: { type: 'text', ... } }]) and calling generateText/streamText with an xai responses model.

Common situations: Piping documents retrieved as raw text into file parts; porting prompts from another provider that supports text file parts; dynamically generated file content typed as text.

Related errors

• 'file part media type ${block.mediaType} as inline data (xAI
• AI_UnsupportedFunctionalityError
• file parts with provider references
• File URL data
• allowedTools with only tools that cannot be allow-listed (${

AI-assisted analysis of vercel/ai@69428b1f8b (2026-08-30). Data as JSON: /api/errors/c34f272e3e0a9c93. Report an issue: GitHub.