Cowork: gcpRegion validation rejects "eu"/"us" multi-region, but newest models are only available via eu multi-region on Vertex AI

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?

Claude Cowork Desktop App rejects "eu" and "us" as valid values for gcpRegion in the third-party inference configuration (Vertex AI), even though the newest Claude models (Sonnet 5, Fable 5, Opus 4.8, Opus 4.7) are exclusively available via the eu multi-region endpoint on Vertex AI — not in any specific European region like europe-west1.

What Should Happen?

Cowork should accept "eu" and "us" as valid gcpRegion values, consistent with how Claude Code CLI already handles multi-region endpoints (CLOUD_ML_REGION=eu). This would allow users to access the latest models via Vertex AI while maintaining EU data residency.

Error Messages/Logs

Invalid custom3p managed config: gcpRegion: must be a GCP region (e.g. us-central1) or 'global'
failingField: gcpRegion
checkedAt: 2026-07-01T08:45:26.581Z

Steps to Reproduce

1. Open Claude Cowork Desktop App
2. Go to Developer menu → Configure Third-party inference
3. Select Vertex AI as provider
4. Enter "eu" as the GCP region
5. Save → error banner appears immediately

Claude Model

None

Is this a regression?

No, this never worked

Last Working Version

_No response_

Claude Code Version

Claude 1.17377.1 (2b3ab4) 2026-06-30T20:11:42.000Z

Platform

Google Vertex AI

Operating System

Windows

Terminal/Shell

Windows Terminal

Additional Information

The Vertex AI model availability table confirms that Claude Sonnet 5, Fable 5, Opus 4.8 and Opus 4.7 are only available in the "eu" multi-region — not in any specific EU region like europe-west1. Using "global" is not acceptable for GDPR-compliant EU enterprise deployments. Related issue: #40526