Xcode 26.3 RC - Claude Agent returns "Your request couldn't be completed"

Environment:

• Xcode 26.3 RC
• macOS 26.3 (25D125) (Apple Silicon / arm64)

Setup:

• Xcode > Settings > Intelligence > Claude Agent: Signed In (account status shows "Signed In")
• Model: Default

Steps to reproduce:

1. Open a new chat in Xcode's Coding Assistant
2. Select "Claude Agent" from the agent dropdown (instead of "Claude Sonnet 4.5")
3. Send any message (e.g. "HI")

Expected result: Claude Agent responds normally.

Actual result: The message is sent but immediately returns the error: "Your request couldn't be completed."

🚩I have an active Claude Code subscription with remaining usage. Running claude in Terminal works perfectly, confirming the subscription and quota are valid.