[BUG] Notion Connector Fails with "Unknown OAuth Client"

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Description
The Notion connector fails to authenticate with the error "Unknown OAuth client" on both claude.ai (web) and Claude Desktop.

Environment

• Claude plan: Max
• Affected surfaces: claude.ai web AND Claude Desktop (latest version)
• OS: macOS
• Notion workspace type: Free plan
• Other connectors tested: Gmail ✓ working, Google Calendar ✓ working, Notion ✗ failing

Workaround (Successful)

• Used mcp-remote (npx -y mcp-remote https://mcp.notion.com/mcp) as a workaround — this successfully connects to Notion, but requires periodic re-authentication

Already Tried

• The "Unknown OAuth client" error was present before any workarounds were attempted
• Attempted to initiate connection from Notion's side (Settings → Connections)
• Verified no stale or duplicate Claude connections exist in Notion workspace settings
• Uninstalled and reinstalled the Notion connector via Settings → Connectors
• Full quit and relaunch of Claude Desktop
• Confirmed Claude Desktop is on the latest version
• Removed mcp-remote config and cleared ~/.mcp-remote cache to retest the native connector — error persists

What Should Happen?

Notion OAuth flow completes successfully and the connector becomes active.

Error Messages/Logs

Unknown OAuth client

Steps to Reproduce

1. Go to Settings → Connectors on either claude.ai or Claude Desktop
2. Attempt to connect the Notion connector
3. OAuth flow initiates and redirects to Notion's authorization page
4. Authentication fails with error: "Unknown OAuth client"

Claude Model

Sonnet (default)

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.119

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_