Overview

For the purposes of a demo video that utilizes Slack MCP and Cartesia MCP, I needed to be able to attach files to messages through MCP, so I made the following changes:

Slack API

• Added File Upload
• Added Open Conversation

MCP

• Updated the addMessageParams to include a potential attachmentURL and attachmentName
  • (Optional) attachmentURL can be a local file or publicly accessible file URL. The file at this URL will be downloaded in and used as the body for the file upload API
  • (Optional) attachmentName the name to use for the uploaded file
• Updated the ConversationsAddMessageHandler to incorporate a file attachment path
  • If an attachment_url is specified, we interpret local vs remote files , download, and then call the Slack API for uploading files (with an optional initial comment + thread timestamp)
  • If no attachment_url is specified, the previous behavior of calling the Slack API to post a message works as it did before

Testing

The tweet linked above exhibits the functionality in action.

1. I ran make build to build the binary
2. I pointed the Claude Desktop config to the binary with the --transport stdio arguments and the relevant Slack tokens
3. I ran Cartesia's MCP
4. I had it generate me a script, TTS'd the script into an audio file using Cartesia's MCP, and then sent it to myself afterwards directly over slack. Screenshot below: