[Bug] plugin:slack OAuth flow never triggers — SSE endpoint 302-redirects instead of returning 401

Summary

The official Slack plugin (slack@claude-plugins-official) fails to connect with Failed to reconnect to plugin:slack:slack and the OAuth browser flow is never triggered. Root cause appears to be that Slack's MCP SSE endpoint returns a 302 redirect instead of a 401 with WWW-Authenticate, so the Claude Code MCP client never reaches OAuth discovery.

Environment

• Claude Code: 2.1.114
• Platform: macOS (Darwin 24.3.0)
• Plugin: slack@claude-plugins-official (commit 96276205880a)
• Plugin .mcp.json:

``json
{ "slack": { "type": "sse", "url": "https://mcp.slack.com/sse" } }
``

Steps to Reproduce

1. claude plugin install slack@claude-plugins-official
2. Restart Claude Code
3. /mcp → select slack → attempt Authenticate

Expected

Browser opens for Slack OAuth consent.

Actual

• /mcp shows Failed to reconnect to plugin:slack:slack
• Browser never opens
• claude mcp list reports plugin:slack:slack: https://mcp.slack.com/sse (SSE) - ✗ Failed to connect

Diagnosis

Requesting the SSE endpoint directly:

$ curl -sI https://mcp.slack.com/sse
HTTP/2 302
location: https://mcp-9827.slack.com/sse

The redirect target (mcp-9827.slack.com, team "My Cooking Party") returns a Slack sign-in HTML page, not an MCP response. OAuth discovery endpoints are reachable and valid:

• https://mcp.slack.com/.well-known/oauth-authorization-server ✓
• https://mcp.slack.com/.well-known/oauth-protected-resource ✓

So the OAuth metadata is published correctly, but the MCP client doesn't reach the unauthenticated-request challenge because the SSE endpoint redirects instead of returning 401 + WWW-Authenticate. The result: OAuth flow never starts, browser never opens.

Suggested Fix

Either:

• The plugin's .mcp.json should point at an endpoint that returns the proper 401 challenge, or
• The Claude Code MCP client should fall back to /.well-known/oauth-* discovery when an SSE endpoint responds with a non-401, non-200 status.

Workaround

None locally. Plugin disabled via claude plugin disable.