Vertex AI multi-region location (us/eu) produces invalid hostname

[ethan-spitz-viasat-com]

Description

When the Vertex AI provider's location is set to a continental multi-region value like us or eu, opencode builds an invalid API hostname:

Invalid hostname: us-aiplatform.googleapis.com

Only global is treated as not needing a region prefix on the hostname; us and eu need the same treatment (they're multi-region, not single-region like us-central1). This affects both the OpenAI-compatible endpoint template in opencode's own code, and the native @ai-sdk/google-vertex SDK call (which has the identical "only special-cases global" bug baked into its own default baseURL).

I have a fix ready and will open a PR referencing this issue.

Plugins

None

OpenCode version

1.18.1

Steps to reproduce

1. Configure the google-vertex provider:
   "provider": {
   "google-vertex": {
   "options": { "project": "", "location": "us" }
   }
   }
2. Select a Gemini model
3. Send a message — request fails with "Invalid hostname: us-aiplatform.googleapis.com"

Screenshot and/or share link

Operating System

macOS 26.5.1

Terminal

iTerm2