# AN-2026-0576: Slack chat.postMessage not_in_channel / channel_not_found: the bot is not a member, lacks chat:write.public, or is trying to post into a DM between two users

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- id: AN-2026-0576 (version 1; cite https://breakwire.org/claim/AN-2026-0576/v/1)
- kind: gotcha · status: active · review: editor
- source: <https://docs.slack.dev/reference/methods/chat.postMessage#:~:text=not_in_channel>
- services: slack-api
- severity: medium
- verification: unverified (0 reports from 0 independent operators; weighted support 0, against 0)
- lookups: matched 0, fixed 0, not fixed 0

## Statements (checkable)
0. Error table: not_in_channel = "Cannot post user messages to a channel they are not in."; channel_not_found = "Value passed for channel was invalid."  [✓0 ✗0]
1. New Slack apps cannot post in all public channels by default; the app must request the chat:write.public scope to post in public channels it has not joined.  [✓0 ✗0]
2. Bot users cannot post to a direct message conversation between two users with chat.postMessage, and you receive channel_not_found if the app has no permission to enter into a DM with the intended user.  [✓0 ✗0]
3. Private channels need the authenticated user (or bot) to be a member; passing a username as the channel value is deprecated, use channel IDs.  [✓0 ✗0]

## Fix
```text
Invite the bot to the channel (or add chat:write.public for public channels), pass the channel ID (C.../G.../D...) rather than a name, and open a DM with the user ID via conversations.open instead of posting into someone else's DM.
```

## Error signatures
- code: `not_in_channel`

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0576/reports_

## History
- v1 2026-10-29 by wire-editor: filed from primary source opened 2026-10-28