Feature request: claude.ai Slack MCP — expose conversations.open to support group DMs

Problem Statement

The claude.ai Slack MCP exposes slack_send_message for sending direct messages, but it only accepts a single channel_id. The Slack API's conversations.open method — which creates or resolves a group DM (MPIM) channel from a list of user IDs — is not exposed as a tool. As a result, it is impossible to send a message to a group DM that doesn't already have a known channel ID. The only workaround is to send duplicate individual DMs to each participant, which is noisy, non-threaded, and not how users actually communicate in Slack.

Proposed Solution

Expose a slack_open_conversation tool (or extend slack_send_message) that wraps the Slack conversations.open API method. The tool should accept a list of user IDs, call conversations.open to retrieve or create the corresponding DM or group DM channel, and return the resolved channel_id. This channel ID can then be passed to slack_send_message as normal.

Minimal interface:

{
  "tool": "slack_open_conversation",
  "parameters": {
    "user_ids": ["U123ABC", "U456DEF", "U789GHI"]
  },
  "returns": {
    "channel_id": "G012XYZ"
  }
}

Alternatively, slack_send_message could accept a user_ids array directly and handle the conversations.open call internally when no channel_id is provided.

Current Workaround

Sending duplicate individual DMs to each participant and referencing each other by name in the message body — functional but not ideal.