OAuth failure - "does not support dynamic client registration"

[lucas-ifd]

Environment

Plan: Slack Enterprise
Claude Code version: Latest, native install
OS: macOS 26.3.1

Setup

• Installed Slack plugin from the official Anthropic plugin marketplace in Claude Code via /plugin > Discover
• Other MCP connectors work fine in Claude Code

Steps to Reproduce

• Install Claude Code Slack Plugin
• Start Claude Code
• Run /mcp
• Select "Authenticate" from the menu
• No browser window opens
• An error message is displayed

/mcp Output

Plugin:slack:slack MCP Server
Status: ❌ failed
Auth: ❌ not authenticated
URL: https://mcp.slack.com/mcp
Config location: Dynamically configured

Error: Incompatible auth server: does not support dynamic client registration

Possible Issue

The MCP OAuth server metadata returned from the discovery endpoint at https://mcp.slack.com/.well-known/oauth-authorization-server reports an issuer "https://slack.com" which mismatches the expected issuer "https://mcp.slack.com" used to perform the discovery.

The OAuth authorization server metadata at https://mcp.slack.com/.well-known/oauth-authorization-server must contain an issuer field exactly equal to the discovery issuer URI:

{
  ...
  "issuer": "https://mcp.slack.com",
  ...
}

Related Issues

• [BUG] Slack plugin fails to authenticate - "does not support dynamic client registration" anthropics/claude-code#18009

[stevengill]

Hey! I think this got resolved now. Can you confirm if you are still having the error?

[goneri]

No this is still broken. I use 2.1.19 (Claude Code).
Nevermind, it works fine with v2.1.79.

[alexsku]

Still broken on v2.1.80 (Claude Code), macOS.

The mcp.json workaround with explicit clientId does not work for me - same error:

Error: Incompatible auth server: does not support dynamic client registration

The root cause appears to be the issuer mismatch - it's still present as of today. The metadata at https://mcp.slack.com/.well-known/oauth-authorization-server returns:

"issuer": "https://slack.com"

Per RFC 8414, this must exactly match the discovery URI, so it should be:

"issuer": "https://mcp.slack.com"

[alexsku]

Update: Got it working. Steps:

1. claude mcp remove slack (removed manual MCP entry)
2. claude plugin install slack (reinstalled via plugin system)

The plugin's OAuth flow triggered successfully after this. The manual mcp.json approach with explicit clientId did not work on v2.1.80 - the plugin system appears to handle pre-configured OAuth credentials differently. It seems the issue is with Slack's MCP configuration or Slack's MCP conflicting with the plugin system.

[lucas-ifd]

Hey @stevengill , I missed your message. Looking at it now.

Claude version: 2.1.69 (Claude Code)

I tried some paths:

1. Following the direct guide on Claude Code docs

I ended up on the same error as before:

Plugin:slack:slack MCP Server
Status: ❌ failed
Auth: ❌ not authenticated
URL: https://mcp.slack.com/mcp
Config location: Dynamically configured

Error: Incompatible auth server: does not support dynamic client registration

2. Adding the clientid workaround

    "slack": {
      "type": "http",
      "url": "https://mcp.slack.com/mcp",
      "oauth": {
        "clientId": "1601185624273.8899143856786",
        "callbackPort": 3118
      }
    }

Authentication failed with:

redirect_uri did not match any configured URIs. Passed URI: http://localhost:3118/callback

3. Installing the MCP via Plugin System

I hit the same error as Path 1, but I also noticed some differences in the configuration:

• mcp url is using /sse instead of /mcp
• the plugin is installed with user scope and not the project scope

│Plugin:slack:slack MCP Server                                                
                                                                              
 Status: ✘ failed                                                             
 Auth: ✘ not authenticated                                                    
 URL: https://mcp.slack.com/sse                                               
 Config location: Dynamically configured                                      
                                                                              
 Error: Incompatible auth server: does not support dynamic client             
 registration                                                                            

[lucas-ifd]

I updated Claude Code to 2.1.84, still failing.