docs: add Vertex AI Model Garden section to Google provider docs

Assisted-By: docker-agent

Author: dgageot

docs/providers/google/index.md

@@ -111,3 +111,46 @@ models:
 | `google_search`  | Enables Google Search grounding for up-to-date info  |
 | `google_maps`    | Enables Google Maps grounding for location queries   |
 | `code_execution` | Enables server-side code execution for computations  |
+
+## Vertex AI Model Garden
+
+You can use non-Gemini models (e.g. Claude, Llama) hosted on Google Cloud's
+[Vertex AI Model Garden](https://cloud.google.com/vertex-ai/generative-ai/docs/partner-models/use-partner-models)
+through the `google` provider. When a `publisher` is specified in `provider_opts`,
+requests are routed through Vertex AI's OpenAI-compatible endpoint instead of the
+Gemini SDK.
+
+### Authentication
+
+Vertex AI uses Google Cloud Application Default Credentials (ADC). Make sure you
+are authenticated:
+
+```bash
+gcloud auth application-default login
+```
+
+### Configuration
+
+```yaml
+models:
+  claude-on-vertex:
+    provider: google
+    model: claude-sonnet-4-20250514
+    provider_opts:
+      project: my-gcp-project       # GCP project ID (or set GOOGLE_CLOUD_PROJECT)
+      location: us-east5             # GCP region (or set GOOGLE_CLOUD_LOCATION)
+      publisher: anthropic           # Model publisher (anthropic, meta, etc.)
+```
+
+| Option      | Description                                                                          |
+| ----------- | ------------------------------------------------------------------------------------ |
+| `project`   | GCP project ID. Falls back to `GOOGLE_CLOUD_PROJECT` env var                         |
+| `location`  | GCP region (e.g. `us-east5`, `us-central1`). Falls back to `GOOGLE_CLOUD_LOCATION`   |
+| `publisher`  | Model publisher (e.g. `anthropic`, `meta`, `mistral`). Must not be `google`          |
+
+<div class="callout callout-info">
+<div class="callout-title">ℹ️ Gemini models on Vertex AI
+</div>
+  <p>Setting <code>publisher: google</code> (or omitting <code>publisher</code>) uses the native Gemini SDK path. The Model Garden endpoint is only used for non-Google publishers.</p>
+
+</div>