[BUG] Slack MCP OAuth fails with "Invalid client_id parameter"

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 attempting to authorize the Slack MCP server in Claude Code, the OAuth
flow fails with the following error from Slack:

▎ Something went wrong when authorizing this app.
▎ Error details: Invalid client_id parameter

Steps to reproduce:

1. Open Claude Code
2. Attempt to authenticate the Slack MCP tool (via mcp__slack__authenticate)
3. Open the generated authorization URL in a browser
4. Slack immediately returns "Invalid client_id parameter"

Expected behavior:
The OAuth flow completes successfully and the Slack MCP tools become
available.

Actual behavior:
Slack rejects the authorization request with Invalid client_id parameter,
indicating the client_id hardcoded in the MCP server
(1601185624273.8899143856786) is either invalid, deleted, or no longer
recognized by Slack.

Environment:

• Claude Code (claude-sonnet-4-6)
• macOS Darwin 25.2.0
• MCP server: mcp.slack.com/mcp

Notes:
This is not a workspace restriction issue — workspace permissions were
verified. The client_id appears to be misconfigured on Anthropic's side.

What Should Happen?

Expected behavior:
The OAuth flow completes successfully and the Slack MCP tools become
available.

Error Messages/Logs

Actual behavior:                                                              
  Slack rejects the authorization request with Invalid client_id parameter,
  indicating the client_id hardcoded in the MCP server                          
  (1601185624273.8899143856786) is either invalid, deleted, or no longer
  recognized by Slack.

Steps to Reproduce

Steps to reproduce:

1. Open Claude Code
2. Attempt to authenticate the Slack MCP tool (via mcp__slack__authenticate)
3. Open the generated authorization URL in a browser
4. Slack immediately returns "Invalid client_id parameter"

Claude Model

Sonnet (default)

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

• Claude Code (claude-sonnet-4-6)

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_