Slack chat.postMessage "channel_not_found" error in CI

Slack returns error:"channel_not_found" when the channel argument does not match any conversation visible to the token. A stale ID, a private channel the bot cannot see, or passing a #name where an ID is expected all trigger it.

What this error means

chat.postMessage responds {"ok":false,"error":"channel_not_found"}. The workflow that posted fine yesterday may fail after a channel was renamed or archived.

Slack API

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

Common causes

A private channel the bot is not in

The token cannot resolve a private channel it has not been invited to, so Slack reports the channel as not found rather than forbidden.

A stale or wrong channel identifier

An archived, deleted, or mistyped channel ID (or a #name used where an ID is required) does not resolve.

How to fix it

Resolve the channel ID from the API

List conversations the token can see and copy the exact ID.
Use the ID (starting with C, G, or D) rather than a #name in postMessage.
Invite the bot to private channels so they become visible.

Terminal

curl -sS "https://slack.com/api/conversations.list?types=public_channel,private_channel" \
  -H "Authorization: Bearer $SLACK_BOT_TOKEN" | jq '.channels[] | {id,name}'

Store the ID in a secret or variable

Pin the channel ID as a workflow variable so a rename does not break the notify step.

.github/workflows/ci.yml

env:
  SLACK_CHANNEL_ID: ${{ vars.SLACK_CHANNEL_ID }}

How to prevent it

Address channels by stable ID, not by name.
Invite the bot to private channels it must post to.
Update the stored channel ID when a channel is recreated.