Slack Fast

CI Release Go Report Card Go Version License: MIT

The fastest Slack MCP server. Written in Go, starts in ~10ms. No runtime, no dependencies — just a single binary.

Post messages, read history, and reply to threads from AI editors like Cursor, Windsurf, Claude Desktop, or your terminal.

Japanese / 日本語版はこちら

Table of Contents

• Why slack-fast-mcp?
• Design Philosophy
• What Can You Do?
• Quick Start
• MCP Tools
• CLI Usage
• Configuration
• Security
• Troubleshooting
• Roadmap
• Contributing
• Acknowledgments
• License

Why slack-fast-mcp?

MCP servers start a new process for every request. Startup speed directly impacts your experience.

Benchmark: startup time measured on Apple M1 (macOS). Actual values vary by hardware. Reproduce: make bench (scripts/benchmark.sh)

Features

• 5 MCP Tools — slack_post_message, slack_get_history, slack_post_thread, slack_add_reaction, slack_remove_reaction
• CLI Mode — Use from the terminal: post, history, reply
• Setup Wizard — Interactive slack-fast-mcp setup for easy configuration
• Per-project Config — .slack-mcp.json for project-specific Slack settings
• Cross-platform — macOS, Linux, Windows binaries available
• Secure — Environment variable references for tokens, hardcoded-token warnings

Design Philosophy

Do a few things exceptionally well, rather than everything adequately.

There are many Slack MCP servers available. slack-fast-mcp takes a deliberately minimalist approach:

Choose slack-fast-mcp if you value:

• Speed — Sub-10ms cold starts on every request
• Simplicity — One binary, zero runtime dependencies, 3-minute setup
• Developer experience — CLI mode, setup wizard, project-scoped config
• Clean auth — Standard Slack Bot Token, no workarounds needed

Consider alternatives if you need:

Message search, DM/Group DM support, browser-token authentication, or SSE/HTTP transports.

See also: korotovsky/slack-mcp-server (1k+ stars, Go, feature-rich) is a great option if you need advanced capabilities.

What Can You Do?

Here are some real-world scenarios where slack-fast-mcp shines:

• Daily standup from your editor — Ask AI to post a progress update to #daily-standup without leaving your code
• Pull request notifications — Let AI post a summary to Slack when you finish a PR
• Thread-based collaboration — Read and reply to Slack threads directly from Cursor or Claude Desktop
• CI/CD status reporting — Pipe build results to a Slack channel via CLI
• Emoji reactions — Let AI react to messages with emoji (e.g. :thumbsup: for acknowledgment, :eyes: for "looking into it")
• Team log / journal — Auto-post session summaries to your personal #times-* channel

Quick Start

Prerequisites

• A Slack workspace you can add apps to
• One of: macOS, Linux, or Windows

1. Install

Option A: Download binary (recommended)

Download the latest binary from GitHub Releases:

# macOS (Apple Silicon)
curl -LO https://github.com/kai-kou/slack-fast-mcp/releases/latest/download/slack-fast-mcp_Darwin_arm64.tar.gz
tar xzf slack-fast-mcp_Darwin_arm64.tar.gz
sudo mv slack-fast-mcp /usr/local/bin/

# macOS (Intel)
curl -LO https://github.com/kai-kou/slack-fast-mcp/releases/latest/download/slack-fast-mcp_Darwin_amd64.tar.gz
tar xzf slack-fast-mcp_Darwin_amd64.tar.gz
sudo mv slack-fast-mcp /usr/local/bin/

# Linux (x86_64)
curl -LO https://github.com/kai-kou/slack-fast-mcp/releases/latest/download/slack-fast-mcp_Linux_amd64.tar.gz
tar xzf slack-fast-mcp_Linux_amd64.tar.gz
sudo mv slack-fast-mcp /usr/local/bin/

# Windows (PowerShell)
Invoke-WebRequest -Uri "https://github.com/kai-kou/slack-fast-mcp/releases/latest/download/slack-fast-mcp_Windows_amd64.zip" -OutFile slack-fast-mcp.zip
Expand-Archive slack-fast-mcp.zip -DestinationPath "$env:USERPROFILE\bin"

macOS Gatekeeper: If you see a warning, run: xattr -d com.apple.quarantine /usr/local/bin/slack-fast-mcp

Option B: Go install

go install github.com/kai-kou/slack-fast-mcp/cmd/slack-fast-mcp@latest

Option C: Build from source

git clone https://github.com/kai-kou/slack-fast-mcp.git
cd slack-fast-mcp && make build

Verify:

slack-fast-mcp version

2. Create a Slack App

For a detailed walkthrough with screenshots, see the Slack App Setup Guide.

1. Go to api.slack.com/apps → Create New App → From scratch

2. Add Bot Token Scopes under OAuth & Permissions:

   Scope	Purpose	Required?
   chat:write	Post messages	Yes
   channels:history	Read public channel history	Yes
   channels:read	Resolve channel names	Yes
   reactions:write	Add/remove emoji reactions	Recommended
   users:read	Display usernames in history	Recommended
   groups:history	Read private channel history	Optional
   groups:read	Resolve private channel names	Optional

3. Install the app to your workspace

4. Copy the Bot User OAuth Token (xoxb-...)

3. Configure

The easiest way — run the setup wizard:

slack-fast-mcp setup

Or configure manually:

export SLACK_BOT_TOKEN='xoxb-your-token-here'

# Example: Add to ~/.zprofile (macOS with zsh)
echo "export SLACK_BOT_TOKEN='xoxb-your-token-here'" >> ~/.zprofile
source ~/.zprofile

4. Add to Your AI Editor

Cursor / Windsurf

Add to .cursor/mcp.json (or .windsurf/mcp.json):

{
  "mcpServers": {
    "slack-fast-mcp": {
      "command": "/usr/local/bin/slack-fast-mcp",
      "args": [],
      "env": {
        "SLACK_BOT_TOKEN": "${SLACK_BOT_TOKEN}"
      }
    }
  }
}

Claude Desktop

Add to Claude Desktop's MCP config (Settings → Developer → MCP Servers):

{
  "slack-fast-mcp": {
    "command": "/usr/local/bin/slack-fast-mcp",
    "args": [],
    "env": {
      "SLACK_BOT_TOKEN": "your-token-here"
    }
  }
}

Note: Claude Desktop may not support ${VAR} environment variable expansion. If you set the token directly, ensure this config file is not committed to Git. It is typically stored in your user directory, so this is usually safe.

slack-fast-mcp works with any MCP-compatible tool via stdio transport.

5. Invite the Bot

This step is required. The bot cannot post to or read from a channel unless it has been invited.

In Slack, open the target channel and type:

/invite @your-bot-name

MCP Tools

Five tools are available when connected as an MCP server:

slack_post_message

Post a message to a Slack channel.

Example:

slack_post_message(channel: "general", message: "Hello from MCP!")

slack_get_history

Get message history from a channel.

Example:

slack_get_history(channel: "general", limit: 5)

slack_post_thread

Reply to a message thread.

Example:

slack_post_thread(channel: "general", thread_ts: "1234567890.123456", message: "Got it!")

slack_add_reaction

Add an emoji reaction to a message.

Example:

slack_add_reaction(channel: "general", timestamp: "1234567890.123456", reaction: "thumbsup")

slack_remove_reaction

Remove an emoji reaction from a message.

Example:

slack_remove_reaction(channel: "general", timestamp: "1234567890.123456", reaction: "thumbsup")

CLI Usage

slack-fast-mcp also works as a standalone CLI tool:

# Post a message
slack-fast-mcp post --channel general --message "Hello from CLI!"

# Get channel history
slack-fast-mcp history --channel general --limit 20

# Reply to a thread
slack-fast-mcp reply --channel general --thread-ts 1234567890.123456 --message "Reply here"

# Add a reaction
slack-fast-mcp react --channel general --timestamp 1234567890.123456 --reaction thumbsup

# Remove a reaction
slack-fast-mcp unreact --channel general --timestamp 1234567890.123456 --reaction thumbsup

# JSON output (pipe to jq for pretty printing)
slack-fast-mcp history --channel general --json | jq '.messages[].text'

# Start as MCP server (default when no subcommand is given)
slack-fast-mcp serve

# Show version
slack-fast-mcp version

# Run setup wizard
slack-fast-mcp setup

Configuration

Configuration is resolved in the following priority (highest first):

Project Config: .slack-mcp.json

Create a .slack-mcp.json in your project root to set defaults:

{
  "token": "${SLACK_BOT_TOKEN}",
  "default_channel": "general",
  "display_name": "my-bot"
}

Environment Variables

Security

Token Management

• Never hardcode tokens in files committed to Git
• Use ${SLACK_BOT_TOKEN} environment variable references in config files
• The tool detects and warns if it finds hardcoded tokens (starting with xoxb-, xoxp-, xoxs-)

Recommended .gitignore

.slack-mcp.json

What This Tool Does NOT Do

• Does not store any data locally (messages, tokens, or credentials)
• Does not have admin/management permissions — only reads and posts messages
• All communication with Slack is over HTTPS

If a Token Is Leaked

1. Go to api.slack.com/apps
2. Select your app → OAuth & Permissions
3. Click Revoke Token to invalidate the compromised token
4. Reinstall the app to generate a new token

Troubleshooting

Binary fails to start

Symptoms: MCP client (Cursor, etc.) shows a connection error, and logs contain repeated messages like:

[info] Starting new stdio process with command: .../slack-fast-mcp
[info] Server creation in progress for getInstructions, waiting
[error] No server info found

Cause: On macOS, the binary may hang at the dynamic linker (dyld) level and never reach the Go runtime. This can happen when:

• Using a stale binary after updating the Go toolchain
• Using a cross-compiled or GoReleaser-built binary that is incompatible with the current macOS version
• After a major macOS update

Diagnosis:

# Check if the binary responds normally
slack-fast-mcp version

# If the version command does not respond within a few seconds, the binary is broken

Fix:

# Rebuild the binary from source
cd /path/to/slack-fast-mcp
make build
# The binary will be generated at ./build/slack-fast-mcp

# Or rebuild directly with go build
go build -o slack-fast-mcp ./cmd/slack-fast-mcp/

# Copy the rebuilt binary to the path configured in your MCP settings
sudo cp ./build/slack-fast-mcp /usr/local/bin/

After rebuilding, reload the MCP server in your editor's settings.

For more details, see the Slack App Setup Guide.

Roadmap

Contributing

Contributions are welcome! Please feel free to submit a Pull Request.

• Report a bug
• Request a feature
• Improve documentation

See CONTRIBUTING.md for guidelines.

Development

git clone https://github.com/kai-kou/slack-fast-mcp.git
cd slack-fast-mcp

make build         # Build the binary
make test          # Run tests
make test-race     # Run tests with race detector
make quality       # Full quality gate (vet, build, test, coverage, smoke)
make smoke         # Smoke test the binary
make help          # Show all available targets

Acknowledgments

Built with these excellent libraries:

• mcp-go — Go MCP SDK
• slack-go/slack — Slack API client for Go
• cobra — CLI framework for Go

License

MIT License