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 is not supported.

What it means

Assistant ('reasoning-file') parts whose data is a URL cannot be converted for Google: the Gemini/Vertex content format requires reasoning/thinking file attachments as inline bytes, not remote URLs. The converter throws UnsupportedFunctionalityError when it hits a reasoning-file part with `type: 'url'`.

Solutions

Download the file and resend it as inline data: `data: { type: 'data', mediaType, data: base64 }`.
Strip reasoning-file parts from the history when switching to Google models.
Store base64/raw bytes instead of URLs in your message persistence layer.

Example fix

// before
{ type: 'reasoning-file', data: { type: 'url', url: 'https://.../thinking.json' } }
// after
{ type: 'reasoning-file', data: { type: 'data', mediaType: 'application/json', data: base64Content } }

Defensive patterns

Strategy: try-catch

Validate before calling

function reasoningFilesHaveNoUrls(messages) {
  return messages.flatMap(m => Array.isArray(m.content) ? m.content : [])
    .every(p => p.type !== 'reasoning-file' || p.data?.type !== 'url');
}

Type guard

function isDataReasoningFile(part) {
  return part?.type === 'reasoning-file' && part?.data?.type === 'data';
}

Try / catch

try {
  await streamText({ model: googleModel, messages });
} catch (e) {
  if (e?.message?.includes('File data URLs in assistant messages are not supported')) {
    const sanitized = messages.map(m => m.role === 'assistant'
      ? { ...m, content: m.content.filter(p => p.type !== 'reasoning-file') } : m);
    return streamText({ model: googleModel, messages: sanitized });
  } throw e;
}

Prevention

Persist reasoning files as base64 bytes, not hosted URLs.
Strip reasoning-file parts when switching providers mid-conversation.
Normalize message history to a provider-neutral, data-inline format.

When it happens

Trigger: A message array containing an assistant message with a `reasoning-file` part whose `data.type` is 'url' (e.g. from a previous provider response that returned a hosted file URL), sent to a Google model.

Common situations: Carrying multi-provider conversation history where an earlier provider returned hosted thinking-file URLs; persisting assistant outputs as URLs in a database and replaying them against Google.

Related errors

AI-assisted analysis of vercel/ai@69428b1f8b (2026-08-30). Data as JSON: /api/errors/6fcf82ae53328cb9. Report an issue: GitHub.

Appendix: source

Thrown at packages/google/src/convert-to-google-messages.ts:383

                        text: part.text,
                        thoughtSignature,
                      };
                }

                case 'reasoning': {
                  return part.text.length === 0
                    ? undefined
                    : {
                        text: part.text,
                        thought: true,
                        thoughtSignature,
                      };
                }

                case 'reasoning-file': {
                  switch (part.data.type) {
                    case 'url': {
                      throw new UnsupportedFunctionalityError({
                        functionality:
                          'File data URLs in assistant messages are not supported',
                      });
                    }
                    case 'data': {
                      return {
                        inlineData: {
                          mimeType: part.mediaType,
                          data: convertToBase64(part.data.data),
                        },
                        thought: true,
                        thoughtSignature,
                      };
                    }
                  }
                  break;
                }

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