feat-add-reactions

README.md

@@ -84,6 +84,16 @@ Get list of channels
   - `limit` (number, default: 100): The maximum number of items to return. Must be an integer between 1 and 1000 (maximum 999).
   - `cursor` (string, optional): Cursor for pagination. Use the value of the last row and column in the response as next_cursor field returned from the previous request.
 
+### 6. reactions_add:
+Add an emoji reaction to a message in a public channel, private channel, or direct message (DM, or IM) conversation.
+
+> **Note:** Adding reactions is disabled by default for safety. To enable, set the `SLACK_MCP_ADD_MESSAGE_TOOL` environment variable. If set to a comma-separated list of channel IDs, reactions are enabled only for those specific channels. See the Environment Variables section below for details.
+
+- **Parameters:**
+  - `channel_id` (string, required): ID of the channel in format `Cxxxxxxxxxx` or its name starting with `#...` or `@...` aka `#general` or `@username_dm`.
+  - `timestamp` (string, required): Timestamp of the message to add reaction to, in format `1234567890.123456`.
+  - `emoji` (string, required): The name of the emoji to add as a reaction (without colons). Example: `thumbsup`, `heart`, `rocket`.
+
 ## Resources
 
 The Slack MCP Server exposes two special directory resources for easy access to workspace metadata:
@@ -135,7 +145,7 @@ Fetches a CSV directory of all users in the workspace.
 | `SLACK_MCP_SERVER_CA`             | No        | `nil`                     | Path to CA certificate                                                                                                                                                                                                                                                                    |
 | `SLACK_MCP_SERVER_CA_TOOLKIT`     | No        | `nil`                     | Inject HTTPToolkit CA certificate to root trust-store for MitM debugging                                                                                                                                                                                                                  |
 | `SLACK_MCP_SERVER_CA_INSECURE`    | No        | `false`                   | Trust all insecure requests (NOT RECOMMENDED)                                                                                                                                                                                                                                             |
-| `SLACK_MCP_ADD_MESSAGE_TOOL`      | No        | `nil`                     | Enable message posting via `conversations_add_message` by setting it to true for all channels, a comma-separated list of channel IDs to whitelist specific channels, or use `!` before a channel ID to allow all except specified ones, while an empty value disables posting by default. |
+| `SLACK_MCP_ADD_MESSAGE_TOOL`      | No        | `nil`                     | Enable message posting via `conversations_add_message` and emoji reactions via `reactions_add` by setting it to true for all channels, a comma-separated list of channel IDs to whitelist specific channels, or use `!` before a channel ID to allow all except specified ones, while an empty value disables these tools by default. |
 | `SLACK_MCP_ADD_MESSAGE_MARK`      | No        | `nil`                     | When the `conversations_add_message` tool is enabled, any new message sent will automatically be marked as read.                                                                                                                                                                          |
 | `SLACK_MCP_ADD_MESSAGE_UNFURLING` | No        | `nil`                     | Enable to let Slack unfurl posted links or set comma-separated list of domains e.g. `github.com,slack.com` to whitelist unfurling only for them. If text contains whitelisted and unknown domain unfurling will be disabled for security reasons.                                         |
 | `SLACK_MCP_USERS_CACHE`           | No        | `~/Library/Caches/slack-mcp-server/users_cache.json` (macOS)<br>`~/.cache/slack-mcp-server/users_cache.json` (Linux)<br>`%LocalAppData%/slack-mcp-server/users_cache.json` (Windows) | Path to the users cache file. Used to cache Slack user information to avoid repeated API calls on startup. |

pkg/handler/conversations.go

@@ -79,6 +79,12 @@ type addMessageParams struct {
 	contentType string
 }
 
+type addReactionParams struct {
+	channel   string
+	timestamp string
+	emoji     string
+}
+
 type ConversationsHandler struct {
 	apiProvider *provider.ApiProvider
 	logger      *zap.Logger
@@ -155,6 +161,12 @@ func (ch *ConversationsHandler) UsersResource(ctx context.Context, request mcp.R
 func (ch *ConversationsHandler) ConversationsAddMessageHandler(ctx context.Context, request mcp.CallToolRequest) (*mcp.CallToolResult, error) {
 	ch.logger.Debug("ConversationsAddMessageHandler called", zap.Any("params", request.Params))
 
+	// provider readiness
+	if ready, err := ch.apiProvider.IsReady(); !ready {
+		ch.logger.Error("API provider not ready", zap.Error(err))
+		return nil, err
+	}
+
 	params, err := ch.parseParamsToolAddMessage(request)
 	if err != nil {
 		ch.logger.Error("Failed to parse add-message params", zap.Error(err))
@@ -230,6 +242,42 @@ func (ch *ConversationsHandler) ConversationsAddMessageHandler(ctx context.Conte
 	return marshalMessagesToCSV(messages)
 }
 
+// ReactionsAddHandler adds an emoji reaction to a message
+func (ch *ConversationsHandler) ReactionsAddHandler(ctx context.Context, request mcp.CallToolRequest) (*mcp.CallToolResult, error) {
+	ch.logger.Debug("ReactionsAddHandler called", zap.Any("params", request.Params))
+
+	// provider readiness
+	if ready, err := ch.apiProvider.IsReady(); !ready {
+		ch.logger.Error("API provider not ready", zap.Error(err))
+		return nil, err
+	}
+
+	params, err := ch.parseParamsToolAddReaction(request)
+	if err != nil {
+		ch.logger.Error("Failed to parse add-reaction params", zap.Error(err))
+		return nil, err
+	}
+
+	itemRef := slack.ItemRef{
+		Channel:   params.channel,
+		Timestamp: params.timestamp,
+	}
+
+	ch.logger.Debug("Adding reaction to Slack message",
+		zap.String("channel", params.channel),
+		zap.String("timestamp", params.timestamp),
+		zap.String("emoji", params.emoji),
+	)
+
+	err = ch.apiProvider.Slack().AddReactionContext(ctx, params.emoji, itemRef)
+	if err != nil {
+		ch.logger.Error("Slack AddReactionContext failed", zap.Error(err))
+		return nil, err
+	}
+
+	return mcp.NewToolResultText(fmt.Sprintf("Successfully added :%s: reaction to message %s in channel %s", params.emoji, params.timestamp, params.channel)), nil
+}
+
 // ConversationsHistoryHandler streams conversation history as CSV
 func (ch *ConversationsHandler) ConversationsHistoryHandler(ctx context.Context, request mcp.CallToolRequest) (*mcp.CallToolResult, error) {
 	ch.logger.Debug("ConversationsHistoryHandler called", zap.Any("params", request.Params))
@@ -363,6 +411,18 @@ func isChannelAllowed(channel string) bool {
 	return !isNegated
 }
 
+func (ch *ConversationsHandler) resolveChannelID(channel string) (string, error) {
+	if !strings.HasPrefix(channel, "#") && !strings.HasPrefix(channel, "@") {
+		return channel, nil
+	}
+	channelsMaps := ch.apiProvider.ProvideChannelsMaps()
+	chn, ok := channelsMaps.ChannelsInv[channel]
+	if !ok {
+		return "", fmt.Errorf("channel %q not found", channel)
+	}
+	return channelsMaps.Channels[chn].ID, nil
+}
+
 func (ch *ConversationsHandler) convertMessagesFromHistory(slackMessages []slack.Message, channel string, includeActivity bool) []Message {
 	usersMap := ch.apiProvider.ProvideUsersMap()
 	var messages []Message
@@ -552,14 +612,10 @@ func (ch *ConversationsHandler) parseParamsToolAddMessage(request mcp.CallToolRe
 		ch.logger.Error("channel_id missing in add-message params")
 		return nil, errors.New("channel_id must be a string")
 	}
-	if strings.HasPrefix(channel, "#") || strings.HasPrefix(channel, "@") {
-		channelsMaps := ch.apiProvider.ProvideChannelsMaps()
-		chn, ok := channelsMaps.ChannelsInv[channel]
-		if !ok {
-			ch.logger.Error("Channel not found", zap.String("channel", channel))
-			return nil, fmt.Errorf("channel %q not found", channel)
-		}
-		channel = channelsMaps.Channels[chn].ID
+	channel, err := ch.resolveChannelID(channel)
+	if err != nil {
+		ch.logger.Error("Channel not found", zap.String("channel", channel), zap.Error(err))
+		return nil, err
 	}
 	if !isChannelAllowed(channel) {
 		ch.logger.Warn("Add-message tool not allowed for channel", zap.String("channel", channel), zap.String("policy", toolConfig))
@@ -592,6 +648,49 @@ func (ch *ConversationsHandler) parseParamsToolAddMessage(request mcp.CallToolRe
 	}, nil
 }
 
+func (ch *ConversationsHandler) parseParamsToolAddReaction(request mcp.CallToolRequest) (*addReactionParams, error) {
+	toolConfig := os.Getenv("SLACK_MCP_ADD_MESSAGE_TOOL")
+	if toolConfig == "" {
+		ch.logger.Error("Reactions tool disabled by default")
+		return nil, errors.New(
+			"by default, the reactions_add tool is disabled to guard Slack workspaces against accidental spamming. " +
+				"To enable it, set the SLACK_MCP_ADD_MESSAGE_TOOL environment variable to true, 1, or comma separated list of channels " +
+				"to limit where the MCP can add reactions, e.g. 'SLACK_MCP_ADD_MESSAGE_TOOL=C1234567890,D0987654321', 'SLACK_MCP_ADD_MESSAGE_TOOL=!C1234567890' " +
+				"to enable all except one or 'SLACK_MCP_ADD_MESSAGE_TOOL=true' for all channels and DMs",
+		)
+	}
+
+	channel := request.GetString("channel_id", "")
+	if channel == "" {
+		return nil, errors.New("channel_id is required")
+	}
+	channel, err := ch.resolveChannelID(channel)
+	if err != nil {
+		ch.logger.Error("Channel not found", zap.String("channel", channel), zap.Error(err))
+		return nil, err
+	}
+	if !isChannelAllowed(channel) {
+		ch.logger.Warn("Reactions tool not allowed for channel", zap.String("channel", channel), zap.String("policy", toolConfig))
+		return nil, fmt.Errorf("reactions_add tool is not allowed for channel %q, applied policy: %s", channel, toolConfig)
+	}
+
+	timestamp := request.GetString("timestamp", "")
+	if timestamp == "" {
+		return nil, errors.New("timestamp is required")
+	}
+
+	emoji := strings.Trim(request.GetString("emoji", ""), ":")
+	if emoji == "" {
+		return nil, errors.New("emoji is required")
+	}
+
+	return &addReactionParams{
+		channel:   channel,
+		timestamp: timestamp,
+		emoji:     emoji,
+	}, nil
+}
+
 func (ch *ConversationsHandler) parseParamsToolSearch(req mcp.CallToolRequest) (*searchParams, error) {
 	rawQuery := strings.TrimSpace(req.GetString("search_query", ""))
 	freeText, filters := splitQuery(rawQuery)

pkg/provider/api.go

@@ -76,6 +76,7 @@ type SlackAPI interface {
 	GetUsersInfo(users ...string) (*[]slack.User, error)
 	PostMessageContext(ctx context.Context, channel string, options ...slack.MsgOption) (string, string, error)
 	MarkConversationContext(ctx context.Context, channel, ts string) error
+	AddReactionContext(ctx context.Context, name string, item slack.ItemRef) error
 
 	// Used to get messages
 	GetConversationHistoryContext(ctx context.Context, params *slack.GetConversationHistoryParameters) (*slack.GetConversationHistoryResponse, error)
@@ -284,6 +285,10 @@ func (c *MCPSlackClient) PostMessageContext(ctx context.Context, channelID strin
 	return c.slackClient.PostMessageContext(ctx, channelID, options...)
 }
 
+func (c *MCPSlackClient) AddReactionContext(ctx context.Context, name string, item slack.ItemRef) error {
+	return c.slackClient.AddReactionContext(ctx, name, item)
+}
+
 func (c *MCPSlackClient) ClientUserBoot(ctx context.Context) (*edge.ClientUserBootResponse, error) {
 	return c.edgeClient.ClientUserBoot(ctx)
 }

pkg/server/server.go

@@ -93,6 +93,22 @@ func NewMCPServer(provider *provider.ApiProvider, logger *zap.Logger) *MCPServer
 		),
 	), conversationsHandler.ConversationsAddMessageHandler)
 
+	s.AddTool(mcp.NewTool("reactions_add",
+		mcp.WithDescription("Add an emoji reaction to a message in a public channel, private channel, or direct message (DM, or IM) conversation."),
+		mcp.WithString("channel_id",
+			mcp.Required(),
+			mcp.Description("ID of the channel in format Cxxxxxxxxxx or its name starting with #... or @... aka #general or @username_dm."),
+		),
+		mcp.WithString("timestamp",
+			mcp.Required(),
+			mcp.Description("Timestamp of the message to add reaction to, in format 1234567890.123456."),
+		),
+		mcp.WithString("emoji",
+			mcp.Required(),
+			mcp.Description("The name of the emoji to add as a reaction (without colons). Example: 'thumbsup', 'heart', 'rocket'."),
+		),
+	), conversationsHandler.ReactionsAddHandler)
+
 	conversationsSearchTool := mcp.NewTool("conversations_search_messages",
 		mcp.WithDescription("Search messages in a public channel, private channel, or direct message (DM, or IM) conversation using filters. All filters are optional, if not provided then search_query is required."),
 		mcp.WithString("search_query",