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claude-code - 💡(How to fix) Fix [BUG] Slack MCP OAuth fails with "Invalid client_id parameter"

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Error Message

flow fails with the following error from Slack: ▎ Error details: Invalid client_id parameter

Error Messages/Logs

Code Example

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.
RAW_BUFFERClick to expand / collapse

Preflight Checklist

  • I have searched existing issues and this hasn't been reported yet
  • This is a single bug report (please file separate reports for different bugs)
  • 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

extent analysis

TL;DR

The most likely fix is to update the hardcoded client_id in the MCP server to a valid and recognized value by Slack.

Guidance

  • Verify that the client_id (1601185624273.8899143856786) is correctly configured on Anthropic's side and recognized by Slack.
  • Check the Slack API documentation to ensure the client_id format is correct and has not changed.
  • Attempt to use a different client_id or register a new one with Slack to test if the issue is specific to the current client_id.
  • Review the MCP server configuration to ensure it is using the correct client_id and that there are no typos or formatting issues.

Example

No code snippet is provided as the issue seems to be related to configuration rather than code.

Notes

The issue may be specific to the client_id used in the MCP server, and updating it to a valid value may resolve the issue. However, without more information about the Slack API and the MCP server configuration, it is difficult to provide a more specific solution.

Recommendation

Apply workaround: Update the hardcoded client_id in the MCP server to a valid and recognized value by Slack, as this is the most likely cause of the issue.

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