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unsupported_content

HTTP 422. The channel cannot show this content and no fallback was set.

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What it means

Flow never converts content silently. When a channel cannot show what you sent (an effect on Telegram, buttons on iMessage, a template outside WhatsApp, a file above the channel’s size cap), the send is refused unless you said what to send instead.

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Why it happens

  • The content type is not supported on the conversation’s channel.
  • A file is larger than the channel takes, or of a type Flow does not send.
  • A document or archive (PDF, Office files, zip and the like) was uploaded or sent by file_id while malware scanning is not on yet. Send a link to it as text, or an image of it.

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How to fix it

Set fallback: "auto" lets Flow send the nearest thing the channel shows (and report it in delivered_as), or give your own content. Check GET /v1/capabilities?conversation=... before sending rich content.
Every error also carries hint, one sentence specific to your request, and doc_url, this page. Coding agents connected to Flow’s MCP server (https://api.flow.engineer/mcp) can call explain_error with the type to read this page.