Dispatch Handoff Attachments

When calling start-workspace-app-creation, pass source files in the attachments argument rather than embedding their bytes in prompt.

Uploaded files

Images and PDFs use an upload attachment with a matching base64 data URL:

{
  type: "upload",
  contentType: "image/webp" | "image/png" | "image/jpeg" | "image/gif" | "application/pdf",
  name: string,
  dataUrl: "data:<contentType>;base64,<BASE64_CONTENT>",
  id: string
}

Text and JSON files provide decoded contents instead of a data URL:

{
  type: "upload",
  contentType: "text/plain" | "application/json",
  name: string,
  dataUrl: "",
  text: string,
  id: string
}

Send any other text-based file (Markdown, CSV, source code, plain text) as text/plain with its real file name, for example notes.md. Use a stable or unique id for each attachment. A malformed attachment is rejected with invalid_attachment and a message naming the problem; fix that attachment and retry, because an unchanged retry fails the same way.

Public URLs

For an existing public file that Builder can fetch, use:

{
  type: "url",
  value: "https://..."
}

Do not pass private, localhost, or internal-network URLs.

Handoff behavior

Keep the original task in prompt and tell the Builder agent how each attachment should be used. Put multiple attachments in the same attachments array. Attachments are model context only and are not copied into the generated app's workspace.