claude-slack-event-channel-mcp

A one-way (listen-only) Claude Code channel that forwards Slack events into a Claude Code session. It connects to Slack via Socket Mode, filters messages and reactions, and pushes them to Claude as <channel> notifications so Claude can react to things happening in Slack without leaving the terminal.

What is a channel?

A channel is an MCP server that Claude Code spawns as a subprocess and talks to over stdio. Instead of waiting for the user to type, the channel pushes events into the session. One-way channels (like this one) forward alerts, webhooks, or chat messages for Claude to act on; two-way channels also expose a reply tool so Claude can send messages back. This server is one-way: Slack → Claude only.

The channel contract is just three things, all implemented in main.ts:

1. Declare the capability — the experimental: { 'claude/channel': {} } key in the McpServer constructor is what registers the notification listener and makes this an MCP server a channel.
2. Emit notifications — for each Slack event, the server calls mcp.server.notification({ method: 'notifications/claude/channel', params: { content, meta } }).
3. Connect over stdio — mcp.connect(new StdioServerTransport()); Claude Code owns the process.

The instructions string in the constructor is added to Claude's system prompt so Claude knows what these events mean and that the channel is one-way.

How an event reaches Claude

When a Slack event passes the filters, content becomes the body of a <channel> tag and each meta key becomes a tag attribute (the source attribute is filled in automatically from the server's configured name, slack). For example a thread reply arrives in Claude's context as:

<channel source="slack" kind="message" channel="C0123ABC" user="U0456DEF" ts="1718000000.000100" thread_ts="1718000000.000001">
deploy looks stuck, can you check?
</channel>

kind="reaction_added" / kind="reaction_removed" events carry a reaction emoji name and the target ts instead.

Note on meta keys: keys must be identifiers (letters, digits, underscores only). Keys with hyphens or other characters are silently dropped, which is why this server uses thread_ts, not thread-ts.

Delivery is fire-and-forget: notifications are not acknowledged. If the session hasn't loaded this server as a channel, or org policy blocks it, events are dropped silently with no error. Events that arrive while Claude is busy are queued and delivered together on the next turn.

Requirements

• Bun
• Claude Code v2.1.80 or later (channels are in research preview). On Team/Enterprise plans an admin must enable channels first.
• A Slack app with Socket Mode enabled and an App-Level Token (xapp-...) carrying the connections:write scope. Subscribe the app to the events you want forwarded (message.channels, reaction_added, reaction_removed, …) and invite it to the target channels.

Install

bun install

Configure

Copy .env.example to .env and fill in the values (loaded via dotenv):

Variable	Required	Description
SLACK_APP_TOKEN	yes	App-Level Token (xapp-...), needs connections:write.
SLACK_WATCH_CHANNELS	no	Comma-separated channel IDs to forward. Empty = all channels.
SLACK_ALLOWED_SENDERS	no	Comma-separated user IDs / bot_ids allowed. Empty = allow all.

Security — this is a prompt injection vector. Every forwarded message becomes Claude's input. Anyone who can post in a watched channel can put text in front of Claude. Leaving SLACK_ALLOWED_SENDERS empty is only safe for channels that solely trust an alert bot; always set an allowlist when listening to human messages. The server gates on the sender's identity (user / bot_id), not the channel, so membership in a watched channel alone is not enough.

Register with Claude Code

Claude Code launches the server itself — you do not run it standalone. Add it to your MCP config so Claude Code spawns it as a subprocess. Project-level .mcp.json (relative path):

{
  "mcpServers": {
    "slack": { "command": "bun", "args": ["./main.ts"] }
  }
}

For user-level config in ~/.claude.json, use the absolute path to main.ts so it resolves from any project.

Run during the research preview

Custom channels aren't on the approved allowlist yet, so start Claude Code with the development flag (this bypasses the allowlist for this one entry after a confirmation prompt; org policy still applies):

claude --dangerously-load-development-channels server:slack

The first time, Claude Code asks for consent to use the new server from .mcp.json — select Use this MCP server. A dim notice under the startup banner confirms the channel is registered. Once running, posting in a watched Slack channel pushes the event straight into the session.

If events don't arrive, run /mcp in the session to check the server's status; "Failed to connect" usually means an import/dependency error — check ~/.claude/debug/<session-id>.txt.

Local development

You can run the server directly to verify it connects to Slack (it will throw if SLACK_APP_TOKEN is missing). Note that outside Claude Code there's no channel listener, so notifications go nowhere — this is only useful for checking the Socket Mode connection and your filters.

bun run start   # or: bun run dev  (watch mode)

Scripts

Script	Description
bun run start	Run the server.
bun run dev	Run with file watching.
bun run check	Biome lint + format (auto-fix).
bun run lint	Biome lint only.
bun run format	Biome format (write).
bun run typecheck	TypeScript type check.

See also

• Channels reference — the full channel contract.
• Channels — installing and enabling channels.
• Working channel implementations (Telegram, Discord, iMessage, fakechat) — including two-way reply tools and sender-pairing flows.