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

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?

Start by reviewing the MCP server overview and the tool descriptions, then reproduce the request against https://mcp.slack.com/mcp using a Slack Connect channel with is_ext_shared: true. Determine whether mcp_externally_shared_channel_restricted is intentional and whether the restriction or error response is controlled by this project; done means the behavior is resolved or its limitation and actionable error are documented.