[BUG] Claude.ai fails to connect to Nia remote MCP server (OAuth) - "error connecting to the MCP server"

Bug Description

Claude.ai fails to connect to Nia's remote MCP server (https://apigcp.trynia.ai/mcp) via the custom connector interface. The OAuth flow does not initiate — instead, an immediate error banner appears.

Error Message

"There was an error connecting to the MCP server. Please check your server URL and make sure your server handles auth correctly. If this persists, share this reference with support: a3f5497cb3ce3f7e"

A second error also appears:

"There was an error connecting to Nia. Please check your server URL and make sure your server handles auth correctly. If this persists, share this reference with support: a3f5497cb3ce3f7e"

Steps to Reproduce

1. Go to Claude.ai → Settings → Connectors
2. Add custom MCP server with URL: https://apigcp.trynia.ai/mcp
3. Click "Connect"
4. Error banner appears immediately — OAuth flow never initiates

Expected Behavior

OAuth authorization flow should open, allowing the user to authenticate with Nia and grant Claude.ai access to the MCP server.

Environment

• Claude.ai plan: Max
• Browser: macOS Safari/Chrome (both fail)
• Nia account: Active, with valid API key
• Same Nia MCP server works in: Claude Code (plugin), Claude Desktop (local stdio transport)

Related Issues

• #11814 — Same symptoms (OAuth-protected MCP server fails to connect), closed as duplicate and locked. Filing new issue as instructed by the bot.

Notes

• The Nia MCP server uses OAuth authentication and works correctly in Claude Code and Claude Desktop
• The error reference a3f5497cb3ce3f7e is consistent across both error banners
• Nia's remote endpoint (https://apigcp.trynia.ai/mcp) supports Streamable HTTP transport