channel_error

HTTP 502, or in message.failed. The channel refused or failed the message.

What it means

Telegram, WhatsApp or iMessage did not take the message. error.channel_code carries the channel’s own code and error.message its text. Errors after the message was queued arrive as a message.failed event.

Why it happens

The contact blocked the bot, or never pressed Start on a Telegram bot.
Malformed markdown, or text over the channel’s limit.
The bot token was revoked.
The channel failed or timed out on a typing indicator or a read receipt. These call the channel at once (POST /v1/conversations/{id}/typing and /read), so they answer 502 directly.

How to fix it

Read error.hint: it maps the channel’s reason to the change to make. Fix that and send again; retrying unchanged only helps when the hint says the channel failed for now. From typing or a read receipt it is safe to ignore: never hold back a reply because the indicator failed.

if (event.type === "message.failed") console.log(event.data.message.error.hint);

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.