[BUG] [Slack MCP] "Sent using @Claude" attribution footer appears twice on some messages

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

When posting messages to Slack via the Slack MCP connector and a Routine, the "Sent using @Claude" attribution footer occasionally appears duplicated — showing twice at the bottom of the same message. It does not happen on every post; the behavior is intermittent.

<img width="2000" height="938" alt="Image" src="https://github.com/user-attachments/assets/ae9148d5-0801-4b88-bb4b-4aad8e54d4c8" />

What Should Happen?

The attribution footer appears at most once per message.

Error Messages/Logs

N/A

Steps to Reproduce

1. Configure a scheduled Claude routine that posts a structured multi-line message to a Slack channel using the Slack MCP connector
2. Observe the posted message in Slack
3. On some (not all) runs, the "Sent using @Claude" line appears twice at the bottom of the message

Claude Model

Sonnet (default)

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.218

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

• The routine prompt does not include any footer text — the duplication is entirely from the connector
• The issue is intermittent; the same prompt/message structure sometimes produces one footer and sometimes two
• Suspected cause: the connector may be appending the attribution to both the Slack message text field and a blocks element, causing both to render visibly when the message hits certain length or structure thresholds