One-click Slack MCP install: register official OAuth client

Summary

cmd mcp add --transport http slack https://mcp.slack.com/mcp cannot complete OAuth. Slack rejects the fallback client with Invalid client_id.

Root cause

Slack's MCP server (mcp.slack.com) does not advertise a Dynamic Client Registration endpoint in its OAuth authorization-server metadata:

{"issuer":"https://mcp.slack.com","authorization_endpoint":"https://slack.com/oauth/v2_user/authorize","token_endpoint":"https://slack.com/api/oauth.v2.user.access",...}

(compare Notion: mcp.notion.com advertises registration_endpoint and auto-registers fine).

When DCR is unavailable, the CLI falls back to a generic pre-registered client — clientId: "command-code" — which Slack rejects with Invalid client_id at https://slack.com/oauth/v2_user/authorize.

Expected behavior

One-click install like Claude/ChatGPT: user runs cmd mcp add slack https://mcp.slack.com/mcp, browser opens Slack's authorization page against an official, pre-registered Command Code Slack app, and completes.

Suggestion

Register an official Slack app (Slack "MCP connector" or custom app) with:

• OAuth redirect URL: http://127.0.0.1:8085/callback (the CLI's local callback)
• User scopes per Slack's MCP server scopes_supported (search:read., chat:write, channels:history, groups:history, im:history, mpim:history, canvases:read/write, users:read, reactions:, emoji:read, files:read/write, channels:read/write, groups:read/write, im:write, mpim:write, lists:read/write)

Then either ship clientId/clientSecret in the CLI for the well-known slack URL, or support server-keyed pre-registered credentials like other editors do.

Impact

Blocked install for the second most-requested MCP integration after Notion. Workaround today requires users to create their own Slack app and mcp add-json the client credentials.

Start at the cmd mcp add --transport http OAuth flow and compare Slack's authorization-server metadata with the existing Notion DCR path. Trace where the fallback command-code client is selected and how the local http://127.0.0.1:8085/callback redirect is configured. Done means the well-known Slack URL opens Slack authorization with an official client and completes installation without user-created credentials.