`slack_send_message_draft` returns success when a draft already exists (should error `draft_already_exists`)

[TomZaiger]

Summary

Calling slack_send_message_draft a second time for the same DM/channel (when an attached draft already exists) can return a successful response that looks like a new draft was created, but the existing draft text is not replaced. Agents then believe they edited the draft when nothing changed.

The tool schema documents draft_already_exists for this case and says only one attached draft is allowed per channel — but that error is not always returned.

Steps to Reproduce

1. Connect to the Slack MCP server at https://mcp.slack.com/mcp (e.g. from Cursor).
2. Call slack_send_message_draft for a DM with message text A.
   • Observe: success, includes a draft_id (e.g. Dr…).
3. Without deleting that draft, call slack_send_message_draft again for the same channel with different message text B.
4. Open Drafts in the Slack client.

Observed

• Second call returns success ("Draft message is created") and a new widget_id.
• Second call often omits draft_id (first call had one; second did not).
• The draft in Slack still contains text A — text B was never applied.

Expected

• Second call should fail with documented error draft_already_exists, or
• Clearly document and implement replace-in-place semantics (update the existing draft to text B and return its draft_id).

Silent success without replacing content is the worst of both: agents cannot detect the no-op.

Environment

• MCP Server: https://mcp.slack.com/mcp
• Client: Cursor IDE (Slack plugin / plugin-slack-slack)
• Tool: slack_send_message_draft
• Date observed: 2026-07-14

Why it matters

AI agents revise draft wording often (tone, typos, user feedback). Today the only safe revise path is: user deletes the draft in the Slack UI → agent creates a new one. A false success makes agents report “updated” while the user still sees the old text.

Suggested fix

1. When an attached draft already exists for channel_id, return draft_already_exists (match the published tool schema).
2. Optionally add a dedicated update tool (see companion feature request) so revise-without-send is possible without manual delete.

Related

• Tool schema documents: draft_already_exists — “A draft already exists for this channel (user should edit or delete the existing draft first)”
• Only one attached draft per channel (tool notes)
• Sibling draft bug: slack_send_message_draft strips newlines — draft renders as a single unbroken line #18 (slack_send_message_draft strips newlines)

This repository is the public Slack-maintained surface for the hosted MCP server. The fix requires a change on mcp.slack.com.

[vegeris]

Hi @TomZaiger,

Thanks for reporting this! We're looking into it internally!