vercel/ai · error · UnsupportedFunctionalityError

'File data URLs in assistant messages are not supported'…

Error message

'File data URLs in assistant messages are not supported' functionality not supported.

What it means

Assistant messages containing a 'file' part whose data is a URL are not convertible to Google's content format, which requires file attachments in assistant turns as inline bytes. The converter throws UnsupportedFunctionalityError when it encounters a file part with `type: 'url'` inside an assistant message.

Solutions

Convert the URL to inline base64 data before sending: fetch the file and pass `{ type: 'data', mediaType, data }`.
Remove or downgrade the file part to plain text (e.g. include the URL as text) in assistant messages.
Persist files as bytes/base64 rather than hosted URLs when targeting Google.

Example fix

// before
{ role: 'assistant', content: [{ type: 'file', data: { type: 'url', url: 'https://cdn/x.png' } }] }
// after
{ role: 'assistant', content: [{ type: 'file', data: { type: 'data', mediaType: 'image/png', data: base64Png } }] }

Defensive patterns

Strategy: validation

Validate before calling

async function assistantFileUrlsResolved(messages) {
  const parts = messages.flatMap(m => m.role === 'assistant' && Array.isArray(m.content) ? m.content : []);
  return parts.every(p => p.type !== 'file' || p.data?.type !== 'url');
}

Type guard

function isDataFilePart(part) {
  return part?.type === 'file' && part?.data?.type === 'data' && typeof part.data.data === 'string' && typeof part.data.mediaType === 'string';
}

Try / catch

try {
  await streamText({ model: googleModel, messages });
} catch (e) {
  if (e?.message?.includes('File data URLs in assistant messages are not supported')) {
    // inline the URLs (fetch + base64) or drop those parts, then retry
  } throw e;
}

Prevention

Convert hosted file URLs to inline base64 at persistence time.
Remember: URL file parts are user-message-only on Google.
Write a converter that normalizes histories per provider before calls.

When it happens

Trigger: An assistant message with a file part like `data: { type: 'url', url: 'https://...' }` in the prompt array passed to a Google Gemini/Vertex model via convertToGoogleMessages.

Common situations: Chat histories recorded from providers that return hosted file URLs (e.g. audio/image generation results); multi-provider agents sharing one persisted message log.

Related errors

AI-assisted analysis of vercel/ai@69428b1f8b (2026-08-30). Data as JSON: /api/errors/8a31c3064b153699. Report an issue: GitHub.

Appendix: source

Thrown at packages/google/src/convert-to-google-messages.ts:405

                    }
                    case 'data': {
                      return {
                        inlineData: {
                          mimeType: part.mediaType,
                          data: convertToBase64(part.data.data),
                        },
                        thought: true,
                        thoughtSignature,
                      };
                    }
                  }
                  break;
                }

                case 'file': {
                  switch (part.data.type) {
                    case 'url': {
                      throw new UnsupportedFunctionalityError({
                        functionality:
                          'File data URLs in assistant messages are not supported',
                      });
                    }
                    case 'reference': {
                      if (isVertexLike) {
                        throw new UnsupportedFunctionalityError({
                          functionality: 'file parts with provider references',
                        });
                      }

                      return {
                        fileData: {
                          mimeType: part.mediaType,
                          fileUri: resolveProviderReference({
                            reference: part.data.reference,
                            provider: 'google',
                          }),

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