yt-slack-mcp

A host-agnostic Model Context Protocol server for Slack. It communicates over stdio and can be used with any MCP client that launches local servers.

Features

• Read Slack channel history and thread replies.

• Read message permalinks from channels, DMs, and group DMs.

• Return image attachments inline, extract text from PDFs, and return text/* attachment contents inline.

• Retrieve private attachment contents through the authenticated Slack browser session.

• Reply to a thread, add or remove a reaction, edit an authored message or thread reply, or upload local files only in explicitly allowed channel IDs.

• Have no plain top-level text post: a reply needs an existing thread, and a file upload with an initial_comment is the only way to start a top-level message.

• Prefix every message the agent posts with 🤖: — or 🤖 <model>: when the call names the model — so agent replies stand apart from the account's own messages; an edit can opt out with agent_authored: false.

Attachment limits: images and text files are limited to 10 MB; text attachments are truncated at 100,000 characters. PDFs are limited to 20 MB, 50 pages, and 100,000 extracted characters.

Related MCP server: Slack MCP Server

Tools

Agent message prefix

The write tools post through the account's own browser session, so Slack shows the account name on them. To keep agent-written messages distinguishable, the text of every posted message is prefixed with 🤖: by default:

🤖: deployed revision 4f2c1a9

The MCP transport does not report which model is calling, so a write tool call can pass model with the model it is running; the prefix then names it:

🤖 claude-sonnet-4-5: deployed revision 4f2c1a9

The prefix is added once: text that already starts with 🤖 is sent unchanged, so editing a prefixed message does not stack prefixes. A file upload without an initial_comment — or with only whitespace — gets no comment rather than a bare prefix, and model has no effect then.

slack_edit_message can also replace a message with text that is not agent-written, such as a correction to the user's own message: pass agent_authored: false and the text is stored as given, without the prefix. model has no effect when agent_authored is false.

Setup

1. Obtain browser session tokens

• xoxc: Slack web localStorage token (from your logged-in browser session).

• xoxd: Slack cookie named d.

These may need refreshing when Slack rotates/invalidates the session.

2. Store the secrets

Save each token as a single-line file:

~/.config/yt-slack-mcp/secrets/slack-xoxc.txt
~/.config/yt-slack-mcp/secrets/slack-xoxd.txt

The server reads these paths by default. Override them in the config file described below if your MCP host manages secrets elsewhere.

3. (Optional) Configure allowed write channels and paths

All configuration lives in a JSON config file at ~/.config/yt-slack-mcp/config.json:

{
  "secretsDir": "~/.config/yt-slack-mcp/secrets",
  "xoxcPath": "~/.config/yt-slack-mcp/secrets/slack-xoxc.txt",
  "xoxdPath": "~/.config/yt-slack-mcp/secrets/slack-xoxd.txt",
  "allowedWriteChannels": ["C0123456789", "C0987654321"],
  "userAgent": "Mozilla/5.0"
}

By default the write tools (slack_reply_to_thread, slack_add_reaction, slack_remove_reaction, slack_edit_message) and slack_upload_files are locked down — no channel is writable. Each uploaded file is limited to 10 MB, and a call accepts at most 10 files. Every field is optional; a missing file just means defaults.

Configuration

Configuration comes from the JSON config file, with built-in defaults for absent keys.

Environment-variable configuration has been removed. Move any environment-supplied values into ~/.config/yt-slack-mcp/config.json; the config path is fixed for the CLI. allowedWriteChannels accepts an array or a comma-separated string, so an old comma-separated value can be copied as-is.

MCP host configuration

Configure your MCP host to run yt-slack-mcp over stdio. The host-specific configuration varies, but the command is:

npx yt-slack-mcp

For example, Claude Desktop uses:

{
  "mcpServers": {
    "slack": {
      "command": "npx",
      "args": ["yt-slack-mcp"]
    }
  }
}

Install as a skill

skills/slack-mcp/SKILL.md packages this server as an Agent Skill so an agent knows when to read Slack, when a write is allowed, and which tool fits the request. Copy or symlink the skills/slack-mcp directory into your agent's skills directory (for example .agents/skills/slack-mcp); the server still needs the setup above.

License

MIT