Slack chat.scheduleMessage: Limits and Errors We Measured

To schedule a Slack message from code, call chat.scheduleMessage with a channel, the text and post_at, a Unix timestamp in seconds. Slack returns a scheduled_message_id and posts the message at that time. You need a bot or user token with chat:write; the Slack web client's own session token was refused with not_allowed_token_type. We ran every call below with a bot token from Python and slack_sdk on 30 September 2026, against a test channel.

Schedule a message from Python

import os, time
from slack_sdk import WebClient

client = WebClient(token=os.environ["SLACK_BOT_TOKEN"])
resp = client.chat_scheduleMessage(
    channel=os.environ["CHANNEL"],          # channel ID, C...
    post_at=int(time.time()) + 120,         # two minutes from now
    text="Scheduled test: +2 minutes",
)
print(resp["scheduled_message_id"], resp["post_at"])

Output:

Q0C56TJCTLP 1790772151

The message arrived as an ordinary message from the app, with ts 1790772151.59, under a second after post_at. It does not appear in anyone's Drafts & sent > Scheduled tab: with a bot message pending, that tab stayed empty for the workspace owner. Only the API lists it. Scheduling from Slack's own message box is covered in how to delay and schedule Slack messages.

How far ahead, and how soon

We scheduled the same text for different offsets from the current time:

Slack's docs say 120 days; in our run the cut-off sat about an hour past that. Anything under about 12 seconds ahead counts as the past, so a script that computes post_at as "now" gets time_in_past. If you need to send right away, call chat.postMessage instead.

Listing and deleting scheduled messages

msgs = client.chat_scheduledMessages_list(channel=channel)["scheduled_messages"]
for m in msgs:
    client.chat_deleteScheduledMessage(channel=channel, scheduled_message_id=m["id"])

Each listed item has id, channel_id, post_at, date_created and text. Our first run of this loop crashed on the first item with invalid_scheduled_message_id: the list still included the +2 minute message a few seconds after it had posted, and it can no longer be deleted then. Wrap each delete in try/except SlackApiError and skip that error.

A delete that returns ok can still post

We scheduled messages at different distances ahead and deleted each one right away with chat.deleteScheduledMessage, then watched the channel:

for ahead in [45, 60, 75, 90, 120, 180, 300]:
    r = client.chat_scheduleMessage(channel=channel, post_at=int(time.time()) + ahead, text=f"delete window test {ahead}s")
    try:
        client.chat_deleteScheduledMessage(channel=channel, scheduled_message_id=r["scheduled_message_id"])
        print(f"{ahead:>3}s ahead: deleted")
    except SlackApiError as e:
        print(f"{ahead:>3}s ahead: {e.response['error']}")

Within the last minute, Slack refuses the delete. Between about 1 and 5 minutes before the send time, it answers ok and removes the message from chat.scheduledMessages.list, but the message posts anyway. These all arrived in our channel after a successful delete:

So if a scheduled message must not go out, delete it more than 5 minutes before post_at. Inside that window, the reliable fix is chat.delete on the posted message afterwards, which needs its ts from the channel history.

Scheduling many messages at once

We scheduled 35 messages for the same time in one channel, 10 minutes ahead. Slack accepted 31 and refused the 32nd with restricted_too_many. Slack's docs put the limit at 30 messages per channel in any 5-minute window, so treat 30 as the ceiling. For a batch, spread post_at across several minutes or channels. The per-second posting limits are in Slack API rate limits.

FAQ

Can I schedule a message with a webhook?

No. An incoming webhook posts at once. Schedule from code that holds a token, or use a recurring workflow for repeats.

Can a scheduled message be a thread reply?

Yes. We scheduled a reply 15 seconds ahead with the parent's ts as thread_ts, and it arrived inside that thread. How thread_ts works when posting right away is in send a Slack message from Python.

How do I schedule in a user's time zone?

post_at is always UTC seconds. Convert the user's local time first, for example with Python's zoneinfo, and the Slack timezone converter shows the same moment across zones.