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HTTP 501. This endpoint or channel is not live yet during the beta.

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What it means

Flow Messaging is in beta. Some endpoints and channels are in the spec before they are live, and answer not_implemented until then.

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Why it happens

  • Asking for an iMessage line, a WhatsApp number or a US SMS number with POST /v0/senders: these are connected by the Flow team, not by API. This answer comes first, with a test or a live key.
  • An endpoint that arrives in a later release.

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How to fix it

Build on what is live now (Telegram, iMessage replies, and the endpoints that answer). For an iMessage line, a WhatsApp number or a US SMS number, write to the Flow team at samyak@flow.engineer or prabal@flow.engineer with your app ID (GET /v0/app) and the number’s details, never secrets or tokens; it then appears in GET /v0/senders with your live key. Meanwhile, build and test on simulated conversations of that channel with a test key (POST /v0/test/conversations with "channel": "imessage", "sms" or "whatsapp"): they keep the channel’s rules and need no number.
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.