Slack error not_in_channel: Bot is not a member of the channel

The channel resolved — Slack knows exactly which conversation you mean — but the identity behind your token is not a member of it, and the operation requires membership. The chat.postMessage description reads "Cannot post user messages to a channel they are not in." This is the sibling of channel_not_found, and telling them apart matters: channel_not_found is an identifier/visibility problem, not_in_channel is purely a membership problem. A bot that works in one channel and fails in another with this string has almost always just not been invited to the second one.

Membership in Slack is real state, not a permission bit. Bots join channels the same ways humans do: someone runs /invite @botname, or the bot joins a public channel itself via conversations.join (bot tokens with the channels:join scope — public channels only, as the method's reference notes). Private channels can only be joined by invitation from a member; no scope or API call lets a bot force its way in.

There is one deliberate bypass for posting: the chat:write.public scope lets a bot post to public channels it has not joined. Two caveats keep tripping people. First, it applies to public channels only. Second, it grants posting, not reading — conversations.history on that channel still fails without membership, so "the bot can post but can't read replies" is expected behavior under this scope, not a bug. Reading requires joining plus the relevant *:history scopes.

Operationally, this error loves channel migrations: a channel is archived and recreated, or a workflow creates fresh channels per incident/customer, and the bot that inhabited the old channel was never invited to the new one. Automating the invite step in whatever process creates channels eliminates the whole class. When the failure appears in logs, the message content is irrelevant — do not debug your blocks; check membership with conversations.members or just try conversations.join.

{"ok": false, "error": "not_in_channel"}

Decide per feature whether the bot joins channels or posts via chat:write.public, and encode that in onboarding: attempt conversations.join for each configured public destination and prompt for an invite on private ones. Re-verify membership when a destination starts failing — being removed from a channel produces exactly this signature. The post-works-but-read-fails asymmetry is explained in Slack bot not responding.