not_implemented

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

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.

Why it happens

Using WhatsApp before it is live (it waits on Meta’s approval; Telegram and iMessage are live).
Asking for an iMessage line with POST /v1/senders: iMessage lines are connected by the Flow team, not by API.
An endpoint that arrives in a later release.

How to fix it

Build on what is live now (Telegram, iMessage replies, and the endpoints that answer). For an iMessage line, ask the Flow team; it then appears in GET /v1/senders with your live key.

curl https://api.flow.engineer/v1/senders -H "Authorization: Bearer $FLOW_MESSAGING_KEY"   # what you can send from today

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.