Vertex (anthropic): can't use continental multi-region locations #28348

Description

When using Google Vertex AI with continental multi-region locations (eu, us), the @ai-sdk/google-vertex SDK constructs endpoint URLs like https://eu-aiplatform.googleapis.com. Unlike standard regional endpoints (e.g. europe-west1-aiplatform.googleapis.com), these continental hostnames do not resolve correctly — they either return 404 or fail to connect, depending on the network environment.

This is a significant issue for enterprise users whose GCP projects provision Anthropic model quota exclusively in continental multi-regions (eu, us) rather than specific regions like europe-west1. In these setups, setting VERTEX_LOCATION=eu is required to reach the quota pool, but the current URL construction makes the endpoint unreachable.

Plugins

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OpenCode version

1.15.4

Steps to reproduce

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Screenshot and/or share link

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Operating System

mac 0S

Terminal

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