Slack thread_ts: Reply in a Thread With the API, Tested

To reply in a Slack thread from code, post with chat.postMessage and set thread_ts to the parent message's ts. Add reply_broadcast: true to also show the reply in the channel. Every message has its own ts; thread_ts is the ts of the thread's first message, and every reply carries it. We ran the calls below on 1 October 2026 in a test workspace with a throwaway bot app. Sending a first message from Python is covered in send a Slack message from Python, and editing one in Slack chat.update, tested; this page is only about threads.

ts and thread_ts in a real thread

import os
from slack_sdk import WebClient

client = WebClient(token=os.environ["SLACK_BOT_TOKEN"])
channel = os.environ["CHANNEL"]

parent = client.chat_postMessage(channel=channel, text="Release 3.2 checklist")
root = parent["ts"]
client.chat_postMessage(channel=channel, thread_ts=root, text="Changelog posted")
client.chat_postMessage(channel=channel, thread_ts=root, text="Release 3.2 is live",
                        reply_broadcast=True)

thread = client.conversations_replies(channel=channel, ts=root)
for m in thread["messages"]:
    print(m["ts"], m.get("thread_ts"), m.get("subtype", "-"), m["text"])

Output:

1790818409.508079 1790818409.508079 - Release 3.2 checklist
1790818409.838849 1790818409.508079 - Changelog posted
1790818410.252409 1790818409.508079 thread_broadcast Release 3.2 is live

The first column is each message's own ts. The second is thread_ts, the same for all three. The parent's ts and thread_ts are equal once it has replies, which is how you tell a thread parent from a reply: on a reply they differ. A broadcast reply has the subtype thread_broadcast and shows in the channel as "replied to a thread".

Values that do not start a thread

chat.postMessage does not reject a bad thread_ts. Each call below returned ok: true and posted a plain message in the channel, with no thread_ts in the response:

So if your "reply" shows up as a new message, check that thread_ts is the exact ts string, all six decimals included. Parsing it as a float in your own code is the easy way to lose it. To check from code, read message.thread_ts in the response: it is missing when the post did not thread.

Getting thread_ts from an event

A bot that answers in threads gets the ts from the incoming event. This is the message event our Socket Mode app received for a reply inside a thread:

{
  "type": "message",
  "user": "U0B7L4YK420",
  "ts": "1790818399.158919",
  "thread_ts": "1790818006.406079",
  "parent_user_id": "U0C5UH0S0E6",
  "text": "Reply from a person, to see the event",
  "channel": "C0C5UGC0Q4A",
  "event_ts": "1790818399.158919",
  "channel_type": "channel"
}

A top-level message has no thread_ts. So the rule is: reply to event["thread_ts"] if it exists, otherwise to event["ts"]. This Bolt app does that, and it is the version we ran:

import os
from slack_bolt import App
from slack_bolt.adapter.socket_mode import SocketModeHandler

app = App(token=os.environ["SLACK_BOT_TOKEN"])

@app.event("message")
def reply_in_thread(event, client):
    if event.get("bot_id"):          # skip messages from apps, including our own
        return
    root = event.get("thread_ts") or event["ts"]
    client.chat_postMessage(channel=event["channel"], thread_ts=root,
                            text=f"Got it. thread_ts={root}")

SocketModeHandler(app, os.environ["SLACK_APP_TOKEN"]).start()

The app needs the message.channels bot event, channels:history and chat:write. We typed a question in the channel, then a follow-up inside the thread. Both answers went to the same thread:

Skip messages with bot_id, or the bot answers itself in a loop. That check also skips people's messages posted through an app with a user token: in our test those events carried a bot_id too.

Webhooks thread too

An incoming webhook accepts thread_ts in the JSON body:

curl -X POST -H 'Content-type: application/json' \
  --data '{"text":"Nightly backup finished","thread_ts":"1790818006.406079"}' \
  https://hooks.slack.com/services/T.../B.../XXXX

The response is the plain text ok, and the message arrived in the thread. reply_broadcast: true worked through the webhook as well. A webhook returns no ts, so you cannot start a thread from one and reply later; post the parent with chat.postMessage to get its ts. Setting a webhook up is in how to create a Slack webhook.

Reading a thread back

• • conversations.replies with the parent's ts returned the parent and every reply, oldest first.
• • With a reply's ts, it returned only that one reply, not the thread.
• • Sent as a JSON body, it failed with invalid_arguments and [ERROR] missing required field: channel. It wants form-encoded or query parameters; slack_sdk sends them that way for you.

The parent's reply_count, latest_reply and reply_users fields tell you if a thread has new replies without fetching all of them.

FAQ

Can a scheduled message be a thread reply?

Yes. The chat.scheduleMessage page shows a scheduled reply with thread_ts landing in its thread.

Does reply_broadcast notify the channel?

The reply shows in the channel as "replied to a thread" with a link to the parent, and in the thread pane with "Also sent to the channel" above it. Both are the same message, with one ts.