Slack MCP: slack_send_message fails with MCP error -32603 (Internal Server Error) while slack_schedule_message works

Description

slack_send_message consistently fails with MCP error -32603: Internal Server Error even after granting chat:write permissions via Claude Desktop → Settings → Connectors → Slack → Configure.

slack_schedule_message works correctly on the same channel with the same credentials, suggesting the issue is specific to the instant send code path.

Steps to Reproduce

1. Connect Slack via Claude Desktop → Settings → Connectors → Slack
2. Enable write permissions in the connector settings
3. Disconnect and reconnect to refresh the OAuth token
4. Attempt to send a message using slack_send_message to any channel (public or private)

Expected Behavior

Message is posted to the channel immediately.

Actual Behavior

MCP error -32603: Internal Server Error

The error is the same regardless of:

• Channel (tried both channel name and channel ID C0AP7PS4P6H)
• Message length (tested with "test" as well as longer messages)
• Whether the bot has been invited to the channel

Workaround

slack_schedule_message works and can be used to schedule messages for the near future as a workaround.

Environment

• Claude Code version: latest
• OS: macOS Darwin 24.5.0
• Slack connector: claude_ai_Slack
• Tools affected: slack_send_message, slack_send_message_draft
• Tools working: slack_schedule_message, slack_read_channel, slack_search_public_and_private