Slack MCP (mcp.slack.com) fails: does not support dynamic client registration

Description

The official Slack MCP server at https://mcp.slack.com/mcp cannot authenticate in Claude Code (CLI or desktop). Both the programmatic OAuth flow (mcp__slack__authenticate tool) and the /mcp UI dialog fail with the same error:

SDK auth failed: Incompatible auth server: does not support dynamic client registration

Steps to reproduce

1. Add the Slack MCP server:

``
claude mcp add --transport http slack https://mcp.slack.com/mcp
``

1. Run claude mcp list — shows slack: ✗ Failed to connect
2. Try /mcp → select Slack → select "Authenticate" → same error
3. Try calling mcp__slack__authenticate tool programmatically → same error

Expected behavior

The OAuth browser flow should open, allow the user to authorize their Slack workspace, and connect the MCP server — similar to how Atlassian (mcp.atlassian.com) and other HTTP MCP servers handle auth.

Environment

• Claude Code CLI (also reproduces in desktop app)
• macOS (Darwin 25.4.0)
• Slack MCP URL: https://mcp.slack.com/mcp

Analysis

Slack's MCP OAuth server does not implement RFC 7591 Dynamic Client Registration, which Claude Code's SDK requires for HTTP MCP server authentication. Other MCP servers (e.g., Atlassian) apparently support this, so Slack is the outlier — but Claude Code could also add a fallback for servers that use static client registration or pre-registered clients.

Workaround

None for the mcp.slack.com endpoint. The slack-by-salesforce plugin provides partial Slack functionality via skills as an alternative.