Relay error "No stable connected client is registered in Unity" is thrown when configuring an external MCP client

How to reproduce:

1. Open the attached project “IN-142368”
2. Ensure the relay binary is present at ~/.unity/relay/relay_mac_arm64.app/Contents/MacOS/relay_mac_arm64
3. Go to Project Settings > AI > Assistant MCP Extensions > Server Configuration > Config File > Open
4. Configure an external MCP client (e.g., Cursor) to connect via the direct relay with args: ["--mcp"] (See internal notes)
5. Enter and run a prompt to invoke any Unity tool through the direct MCP connection
6. Observe the console

Actual result: The connection briefly appears then drops with "Transport closed" on the client side and an error is thrown (see internal notes)
Expected result: The direct MCP connection successfully relays tool calls between the external client and Unity

Reproducible with: 1.0.0-pre.2 (6000.0.60f1), 2.7.0-pre.3 (6000.3.15f1)

Reproducible on: macOS 15 (Apple Silicon / ARM64) (reporter), macOS Tahoe 26.1
Not reproducible on: Windows 11 x64

Workaround: Use the Unity Assistant cloud/gateway connection path instead of the direct MCP relay

Notes:

• Not reproducible with built-in Assistant tools
• The error occurs in the relay binary's bundled JavaScript (minified variable 'U' is undefined at the point where '.name' is accessed)
• The issue persists across relay restarts, Unity restarts, client restarts, and re-approval attempts
• Reproducible after removing the older open-source MCP server