[BUG] Slack MCP integration: message-related tools return internal error while profile tools work

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?

The Claude.ai-hosted Slack MCP integration partially fails — profile/channel-search tools work, but all message-reading and message-searching tools return [Tool result missing due to internal error].

This has been persistent for 2+ days and survives Slack integration disconnect/reconnect from https://claude.ai/settings/integrations.

Working vs Broken Tools

| Tool | Status | Notes |
|------|--------|-------|
| slack_read_user_profile | ✅ Works | Returns full profile data |
| slack_search_channels | ✅ Works | Returns results normally |
| slack_read_channel | ❌ Internal error | Any channel, any parameters |
| slack_read_thread | ❌ Internal error | Any thread |
| slack_search_public | ❌ Internal error | Any query |

Key Observations

1. OAuth connection is valid — slack_read_user_profile successfully authenticates and returns data
2. Pattern: All tools that read message content fail; tools that read metadata only (profiles, channel list) work
3. Persists across sessions — restarting Claude Code doesn't help
4. Persists across reconnect — disconnecting and reconnecting Slack integration from claude.ai/settings/integrations doesn't help
5. Other users on the same Slack workspace report the integration works for them
6. Account-specific — suggests a per-user token/scope issue rather than a server-wide outage

Error Messages/Logs

Every message-related tool call returns:

[Tool result missing due to internal error]

No additional error details are provided to the client.

Steps to Reproduce

1. Open Claude Code CLI (v2.1.79)
2. Ensure Slack integration is connected via claude.ai/settings/integrations
3. Call slack_read_user_profile → works fine
4. Call slack_read_channel with any valid channel_id → internal error
5. Call slack_read_thread with any valid channel_id + message_ts → internal error
6. Call slack_search_public with any query → internal error

Attempted Remediation

• Disconnect + reconnect Slack from claude.ai/settings/integrations (multiple times)
• Restart Claude Code sessions
• None resolved the issue

Claude Model

Opus

Is this a regression?

Yes — this was working previously

Claude Code Version

2.1.79

Platform

Claude Code CLI (with claude.ai hosted MCP integrations)

Operating System

macOS (Darwin 25.2.0)

Terminal/Shell

zsh (inside tmux)

Additional Information

• The mcp__claude_ai_Slack__ prefix confirms these are Claude.ai-hosted MCP tools, not locally configured
• The selective failure pattern (metadata OK, messages broken) strongly suggests a Slack OAuth scope issue — possibly channels:history and search:read scopes were lost or revoked for this specific user's token while basic scopes remain valid
• Slack workspace: sendbird