Use Slack publishing for messages you want to prepare and review in advance. For example, schedule a launch announcement with an image and a document for your team's project channel.
Connect the intended workspace
- In Postly, open Channels, choose Add account, and select Slack.
- Confirm the Slack workspace shown on the authorization screen. Ask its administrator for approval if your workspace requires it.
- Authorize Postly, then invite the Postly app to each intended channel in Slack.
- Return to Postly and select the channels the app has joined. Invite it before expecting a private channel to appear.
The requested bot permissions are chat:write, files:write, channels:read, and groups:read. They support sending content and discovering channels. Postly does not request message-history access.
Prepare and review a message
Write the message in the social editor, select a Slack channel, and inspect its Preview settings. Choose Slack formatting, link previews and media-link previews. Mentions are off by default; enable them deliberately before using a person or group mention.
You can attach up to ten supported files, with a Postly limit of 100 MiB per file. Slack can apply additional workspace or account restrictions. When a message includes text and files, Postly sends the text first and places the files in its thread. A file-only publication goes directly to the selected channel.
To reply to an existing message, enter its Slack thread timestamp in Preview. Broadcast reply applies to a text reply in a thread; it is not a separate broadcast setting for file uploads.
Save reusable formatting and preview choices in Workspace Settings → Publishing presets. Thread destinations are chosen for each publication rather than saved as a general preset.
Publish through an import or integration
Select slack as the platform in a social CSV or Google Sheets row. The supported columns include slack_mrkdwn, slack_unfurl_links, slack_unfurl_media, slack_allow_mentions, slack_thread_ts, and slack_reply_broadcast. Use yes/no for boolean choices and leave the thread timestamp blank for a new channel message.
API and MCP publishing use the same Slack settings. Confirm the selected Slack channel and inspect the eventual publishing result; accepting a scheduled request does not mean the message has already been sent.
Resolve a connection or publishing problem
- A channel is missing: confirm the workspace, invite Postly to that channel, then refresh the channel selection.
- Posting is denied: check that the app is still installed and allowed to post there. Private channels require an invitation.
- A request timed out: check Slack and Postly's saved result before retrying. A timeout alone does not prove that nothing was posted.
- Part of the content is already live: retain the existing message link and inspect which files were accepted. Do not resend the complete message just to resolve an uncertain file upload.
Publishing prepared messages is separate from sending Postly alerts to Slack. Configure publishing results, approval and connection alerts under Product notifications → Slack.