Slack MCP plugin OAuth fails with "Please specify client_id" despite client_id being sent correctly

Title: Slack MCP plugin OAuth fails with "Please specify client_id" despite client_id being sent correctly

Environment:

• Claude Code (CLI)
• Plugin: slack@claude-plugins-official, version 77a10794d6fc
• Platform: Linux (Home Assistant OS container)

Steps to reproduce:

1. Have the slack MCP plugin installed and enabled (shows "needs authentication" in MCP status).
2. Trigger authentication via mcp__plugin_slack_slack__authenticate (or the equivalent /mcp auth flow).
3. Open the returned authorization URL in a browser:

https://slack.com/oauth/v2_user/authorize?response_type=code&client_id=1601185624273.8899143856786&code_challenge=...&code_challenge_method=S256&redirect_uri=http%3A%2F%2Flocalhost%3A3118%2Fcallback&state=...&scope=...&resource=https%3A%2F%2Fmcp.slack.com%2F

1. Slack shows: "Something's gone wrong while authorising this app. Try going back and authorising again. If problems persist, contact support for help. Error details: Please specify client_id"

Investigation done:

• Confirmed local plugin config (.mcp.json for the slack plugin) has oauth.clientId set to 1601185624273.8899143856786, matching what's sent.
• Checked MCP debug logs (mcp-logs-plugin-slack-slack/*.jsonl) — confirmed the generated authorization URL includes client_id=1601185624273.8899143856786 correctly in the query string sent to https://slack.com/oauth/v2_user/authorize.
• The request appears correctly formed on the client side; the error is returned by Slack's own OAuth authorization endpoint, suggesting the issue is with the Slack app registration for this client_id (e.g., deactivated app, misconfigured redirect/resource, or Slack-side outage) rather than the local Claude Code plugin configuration.

Expected: Slack OAuth consent screen should load normally and let the user authorize the integration.

Actual: Slack immediately errors with "Please specify client_id" even though client_id is present in the request URL.