reaction/add

Add an emoji reaction to a specific Slack message. Used to signal status, acknowledgement, or intent programmatically without posting a visible message reply.

When to Use

Auto-acknowledge receipt of incoming requests by adding a :eyes: reaction as the workflow begins processing.
Add a :white_check_mark: reaction when the task referenced in a message has been completed successfully.
Signal approval on a proposal message with a :thumbsup: reaction after an automated validation passes.
Mark processed messages with :robot_face: to distinguish them from unprocessed ones in high-volume channels.
Add an :alarm_clock: reaction to messages that require scheduled follow-up so they stand out visually.

Emoji Name Format: Provide the emoji name without colons. Use thumbsup, not :thumbsup:. For custom workspace emojis, use their registered name exactly as it appears in your Slack workspace emoji list.

Configuration

Connection

Field	Required	Description
`botToken`	Required	Slack Bot Token starting with `xoxb-`. Must have the `reactions:write` OAuth scope.

Operation Fields

Field	Required	Description
`channel`	Required	Slack channel ID (e.g. `C08ABCDEFGH`) where the target message was posted.
`messageTs`	Required	The timestamp of the message to react to, in Slack format (e.g. `1748221200.000100`). Acts as the unique message identifier within a channel.
`emojiName`	Required	Emoji name without surrounding colons (e.g. `thumbsup`, `white_check_mark`, `eyes`). Must be a valid standard or workspace custom emoji.

Sample Configuration

{
  "operation": "reaction/add",
  "connection": {
    "botToken": "{{ $credentials.slackBot.token }}"
  },
  "channel": "{{ $trigger.slackEvent.channel }}",
  "messageTs": "{{ $trigger.slackEvent.ts }}",
  "emojiName": "eyes"
}

Validation Errors

Error Code	Cause	Resolution
`already_reacted`	The bot has already added this emoji to the specified message.	Check reaction state first using `reaction/get` if idempotency is required, or ignore this error in the error port handler.
`invalid_name`	The emoji name is not recognized (invalid standard emoji or typo in custom emoji name).	Verify the emoji name in Slack — open emoji picker and copy the exact name shown.
`message_not_found`	No message exists at the given `messageTs` in the specified channel.	Confirm the timestamp and channel ID are correct. Timestamps are channel-scoped.
`too_many_emoji`	The message already has the maximum number of emoji reactions (23 unique emojis per message).	Remove an existing reaction first, or use a different status signaling approach.
`missing_scope`	Bot token lacks `reactions:write` scope.	Add `reactions:write` under OAuth & Permissions and reinstall the Slack App.

Output

Success Port

Field	Type	Description
`channel`	string	Channel ID where the reaction was applied.
`messageTs`	string	Timestamp of the message that received the reaction.
`emojiName`	string	Emoji name that was added (without colons).
`status`	string	`"success"` when the reaction was successfully added.
`errorCode`	string	Empty on success. Contains the Slack API error code on failure.
`payload`	object	Raw Slack API response.

Error Port

Activated on any API error. The already_reacted error is common in at-least-once workflows — consider treating it as a non-fatal condition in your error port handler.

Sample Output

{
  "channel": "C08INBOX001",
  "messageTs": "1748221200.000100",
  "emojiName": "eyes",
  "status": "success",
  "errorCode": "",
  "payload": {
    "ok": true
  }
}

Expression Reference

Expression	Result
`{{ $output.addReaction.channel }}`	Channel ID where the reaction was applied.
`{{ $output.addReaction.messageTs }}`	Message timestamp — pass to `reaction/remove` to undo.
`{{ $output.addReaction.emojiName }}`	Confirmed emoji name that was added.
`{{ $output.addReaction.status }}`	`"success"` or error status string.

Node Policies & GuardRails

Policy Area	Recommendation
Idempotency	The `already_reacted` error indicates the bot already added this emoji. Wire the error port to log and continue rather than failing the workflow — reactions are naturally idempotent in effect.
Emoji Naming	Always strip colons from emoji names before passing to `emojiName`. Colons are a display convention, not part of the API name.
Custom Emoji Consistency	Custom workspace emojis may be deleted or renamed. Build a fallback to a standard emoji if your workflow relies on custom emoji for status signaling.
Rate Limits	Slack's `reactions.add` is Tier 2 (20+ requests/min). For high-volume message processing, batch reaction adds or throttle to stay within limits.
Reaction Lifecycle	Plan a corresponding `reaction/remove` step to clean up status reactions after the workflow completes. Leaving stale reactions misleads users about message processing state.

Examples

Acknowledge Incoming Request with Eyes Emoji

When a new message arrives in #support-requests, immediately add :eyes: to signal the workflow has picked it up.

{
  "operation": "reaction/add",
  "channel": "{{ $trigger.slackEvent.channel }}",
  "messageTs": "{{ $trigger.slackEvent.ts }}",
  "emojiName": "eyes"
}

Mark Completed Task with Check Mark

After completing a task referenced in a Slack message, add a :white_check_mark: to signal completion.

{
  "operation": "reaction/add",
  "channel": "C08TASKS001",
  "messageTs": "{{ $input.taskMessageTs }}",
  "emojiName": "white_check_mark"
}

Flag Message for Follow-Up

When an automated check detects a message requiring manual review, add :alarm_clock: to make it visually distinct for the team.

{
  "operation": "reaction/add",
  "channel": "{{ $input.channelId }}",
  "messageTs": "{{ $input.messageTs }}",
  "emojiName": "alarm_clock"
}