Slack reactions.add API: Names, Errors and the 23-Reaction Limit

To add a reaction with the Slack API, call reactions.add with the message's channel, its timestamp (the message ts), and the emoji name without colons, using a token with the reactions:write scope. We tested it on 1 October 2026 in our own test workspace with a bot token and a user token. Each app or person can put 23 different reactions on one message; the 24th returns too_many_reactions.

Add a reaction

This is the script we ran against a message our bot had posted:

import os
from slack_sdk import WebClient
from slack_sdk.errors import SlackApiError

client = WebClient(token=os.environ["SLACK_BOT_TOKEN"])
CHANNEL, TS = "C0C5QG3TQKV", "1790838915.131049"

for name in ["thumbsup", "thumbsup", "+1", ":eyes:", "wave::skin-tone-3", "squirrel", "shipit"]:
    try:
        client.reactions_add(channel=CHANNEL, timestamp=TS, name=name)
        print(f"{name:20} ok")
    except SlackApiError as e:
        print(f"{name:20} {e.response['error']}")

msg = client.reactions_get(channel=CHANNEL, timestamp=TS)["message"]
print([r["name"] for r in msg["reactions"]])

Output:

thumbsup             ok
thumbsup             already_reacted
+1                   already_reacted
:eyes:               invalid_name
wave::skin-tone-3    ok
squirrel             ok
shipit               already_reacted
['umbrella', '+1', 'squirrel', 'wave::skin-tone-3']

What the output shows:

• • thumbsup and +1 are the same emoji. After thumbsup, adding +1 returned already_reacted, and reactions.get lists the reaction as +1.
• • A name in colons, :eyes:, is invalid_name. Strip the colons first.
• • A skin tone goes after a double colon: wave::skin-tone-3. The single-colon form wave:skin-tone-3 returned invalid_name. Our Slack skin tone page lists the six tone numbers.
• • Custom emoji work by name. shipit is an alias of squirrel in this workspace (emoji.list returns alias:squirrel for it), so it was already there. Before you react with a custom emoji, upload it; the Slack emoji maker makes one at the right size.

Names are case-sensitive in practice: Eyes returned invalid_name, eyes worked.

Errors we got

The not_in_channel row only applies to the bot. In the same channel, the user token's reactions.add returned ok: true, because the user was a member. The fix for the bot is conversations.join or an invite; not_in_channel covers both.

The 23-reaction limit is per person

Our bot added different emoji one at a time. The 24th, umbrella, returned too_many_reactions. Then our user token added umbrella to the same message, and it worked. The user went on until its own 24th different emoji was refused. The message ended with 44 different reactions: 23 from the user and 22 from the bot, with umbrella from both. That count is after we removed two of the bot's reactions:

So the limit counts different emoji per reacting user or bot, not per message. It matches what a person sees in the Slack app, where the emoji picker stops at 23 too, as in our eyes emoji test. After reactions.remove took one of the bot's reactions off, the bot could add a new one again. Removing a reaction that both the bot and the user had left took off only the bot's; the user's stayed with count 1.

The reaction_added event

With reaction_added in the app's bot events and reactions:read in its scopes, Slack sent one event per reaction, including the ones our own bot added. This is the event for the bot's thumbsup:

{
  "type": "reaction_added",
  "user": "U0C5WG3PXHQ",
  "reaction": "+1",
  "item": {
    "type": "message",
    "channel": "C0C5QG3TQKV",
    "ts": "1790838871.694209",
    "channel_type": "channel"
  },
  "item_user": "U0C5WG3PXHQ",
  "event_ts": "1790838872.000300"
}

The reaction field holds the stored name, +1, not the thumbsup we sent. If your code matches reaction names, compare against the stored names. Skin tones arrive as wave::skin-tone-3. Because your own bot's reactions come back as events, skip events whose user is your bot's user ID, or a bot that answers every reaction with a reaction will trigger itself.

FAQ

Can a bot react to a message in a DM?

Yes. Our bot added eyes to a message the user had sent in the bot's own DM, with reactions:write and the DM's D channel ID, and got ok: true.

How do I find which reactions a message has?

Call reactions.get with the channel and timestamp. It returns each name with its count and the users who added it. To find messages by reaction in the Slack app, use has:: search.