slack_send_message fails on externally shared (Slack Connect) channels with mcp_externally_shared_channel_restricted

[xplor-kriedling]

Bug / Feature Request

Attempting to send a message to a Slack Connect (externally shared) channel via the slack_send_message MCP tool fails with the following error:

MCP error -32602: Failed to run tool: execution_failed: mcp_externally_shared_channel_restricted

The channel is resolved correctly and the authenticated user has permission to post in it manually. The restriction appears to be enforced server-side by the hosted MCP server at mcp.slack.com/mcp.

Steps to Reproduce

1. Configure the Slack MCP server in Cursor (or any MCP client) per the README.
2. Authenticate via OAuth.
3. Use slack_search_channels to find a Slack Connect channel (one with is_ext_shared: true).
4. Call slack_send_message targeting that channel's ID.
5. Observe the -32602 error with reason mcp_externally_shared_channel_restricted.

Expected Behavior

The message should be sent successfully, consistent with the documentation which states the send message tool can "send messages to any type of conversation in Slack."

At minimum, if this is an intentional restriction, it should be:

• Documented in the MCP server overview and/or tool descriptions.
• Surfaced as a clear, documented error rather than an opaque execution_failed.

Actual Behavior

The request is rejected server-side with mcp_externally_shared_channel_restricted. This error string does not appear in any public Slack documentation.

Context

• MCP endpoint: https://mcp.slack.com/mcp
• Client: Cursor IDE
• Channel type: Slack Connect (externally shared) public channel
• The authenticated user can post to the same channel manually in the Slack client without issue.

Questions

1. Is this restriction intentional or a bug?
2. If intentional, is there a plan to make it admin-configurable (e.g., allowing workspace admins to opt in to MCP access for Connect channels)?
3. Could the error response include a human-readable explanation so MCP clients can surface actionable guidance?

[pxue]

interested in this.

[ksin]

Also looking to a fix for this as it inhibits some of our process

[ksin]

I took a pass through this repo to see whether the Slack Connect send restriction could be made configurable from the plugin side.

The short version: I do not think this is fixable in slackapi/slack-mcp-plugin as the code is currently structured. This repo packages the hosted Slack MCP endpoint for Claude/Cursor (https://mcp.slack.com/mcp) plus skills/docs, but it does not implement the MCP slack_send_message tool itself. The mcp_externally_shared_channel_restricted response is coming from Slack’s hosted MCP server, so an admin-configurable allowlist/policy would need to be implemented and enforced there.

A plugin-side config flag would not change behavior unless mcp.slack.com already accepted and honored that setting. A true client-side workaround would mean shipping a local MCP server or wrapper that intercepts sends and falls back to Slack Web API chat.postMessage, with separate token handling, scopes, admin approval, and tool parity. That would be a substantially different architecture from this plugin.

So I am closing my draft documentation-only PR and leaving this as a request for a hosted Slack MCP server change: make Slack Connect channel sends configurable by workspace/org admins, and return a documented actionable error when the policy blocks a send.