vercel/ai · error · UnsupportedFunctionalityError

media type: ${part.mediaType}

Error message

media type: ${part.mediaType}

What it means

convertToAnthropicPrompt throws UnsupportedFunctionalityError when a source message part has a media type that the Anthropic prompt converter does not recognize or support (e.g. an image/* or file media type outside the supported set, or unsupported formats inside document/image blocks). The message is built from the offending part's `mediaType`.

Source

Thrown at packages/anthropic/src/convert-to-anthropic-prompt.ts:402

                                  }
                                : {
                                    type: 'text',
                                    media_type: 'text/plain',
                                    data: convertBytesDataToString(
                                      part.data.data,
                                    ),
                                  },
                            title: metadata.title ?? part.filename,
                            ...(metadata.context && {
                              context: metadata.context,
                            }),
                            ...(enableCitations && {
                              citations: { enabled: true },
                            }),
                            cache_control: cacheControl,
                          });
                        } else {
                          throw new UnsupportedFunctionalityError({
                            functionality: `media type: ${part.mediaType}`,
                          });
                        }
                        break;
                      }
                    }

                    break;
                  }
                }
              }

              break;
            }
            case 'tool': {
              for (let i = 0; i < content.length; i++) {
                const part = content[i];

View on GitHub (pinned to 69428b1f8b)

Solutions

1. Inspect the message parts and remove or convert parts whose mediaType Anthropic does not support (supported: image/jpeg, image/png, image/gif, image/webp; application/pdf documents).
2. Convert unsupported media to a supported format (e.g. transcode audio elsewhere, convert documents to PDF) before sending.
3. Filter the prompt by mediaType before passing it to the Anthropic model.

Example fix

// before
messages: [{ role: 'user', content: [{ type: 'file', data, mediaType: 'audio/wav' }] }]
// after
// transcribe audio separately, then send text or a supported image/PDF part
messages: [{ role: 'user', content: [{ type: 'file', data, mediaType: 'application/pdf' }] }]

const SUPPORTED = new Set(['image/jpeg', 'image/png', 'image/gif', 'image/webp', 'application/pdf']);
for (const msg of messages) {
  for (const part of Array.isArray(msg.content) ? msg.content : []) {
    if ('mediaType' in part && !SUPPORTED.has(part.mediaType)) {
      throw new Error(`Unsupported media type for Anthropic: ${part.mediaType}`);
    }
  }
}

try {
  return await generateText({ model: anthropic(modelId), messages });
} catch (error) {
  if (UnsupportedFunctionalityError.isInstance(error) && error.functionality?.startsWith('media type:')) {
    // sanitize the offending part and retry
    return await generateText({ model: anthropic(modelId), messages: filterUnsupportedParts(messages) });
  }
  throw error;
}

When it happens

Trigger: Sending messages containing file/image parts whose `mediaType` is not one of the types the converter handles (e.g. audio parts, unusual MIME types like 'video/mp4', or unsupported document media types) to a Claude model via generateText/streamText.

Common situations: Piping user-uploaded attachments of arbitrary MIME types straight into Claude; reusing prompt arrays built for a multimodal provider (e.g. Gemini/OpenAI) with Anthropic; citation-enabled document blocks with non-PDF media types.

Related errors

• AI_UnsupportedFunctionalityError
• 'element streams in no-schema mode' functionality not suppor
• 'element streams in object mode' functionality not supported
• 'element streams in enum mode' functionality not supported.
• AI_UnsupportedFunctionalityError

AI-assisted analysis of vercel/ai@69428b1f8b (2026-08-30). Data as JSON: /api/errors/59b57e5d0a1f43b6. Report an issue: GitHub.