feat(vertex_ai): Model Garden OpenAPI for publisher model ids

- Route publisher/model ids (e.g. xai/grok) to .../endpoints/openapi; keep model in JSON body
- Add model_prices keys for vertex_ai/openai/xai/grok-*
- Document xAI Grok on vertex_partner (aligned with GPT-OSS)
- Add tests for create_vertex_url and body-model heuristic

Made-with: Cursor

litellm/llms/vertex_ai/vertex_model_garden/main.py

@@ -27,6 +27,17 @@ from ..common_utils import VertexAIError, get_vertex_base_model_name
 from ..vertex_llm_base import VertexBase
 
 
+def _vertex_model_garden_model_id_in_json_body(model: str) -> bool:
+    """
+    Vertex catalog / publisher models are addressed as publisher/model (e.g.
+    xai/grok-4.1-fast-reasoning) on the shared OpenAPI URL, with the id in the JSON body.
+
+    Deployed Model Garden endpoints are typically a single segment (often numeric)
+    and use .../endpoints/{ENDPOINT_ID}/chat/completions with an empty model field.
+    """
+    return "/" in model
+
+
 def create_vertex_url(
     vertex_location: str,
     vertex_project: str,
@@ -34,8 +45,13 @@ def create_vertex_url(
     model: str,
     api_base: Optional[str] = None,
 ) -> str:
-    """Return the base url for the vertex garden models"""
+    """Return the api base for vertex model garden (without /chat/completions)."""
     base_url = get_vertex_base_url(vertex_location)
+    if _vertex_model_garden_model_id_in_json_body(model):
+        return (
+            f"{base_url}/v1/projects/{vertex_project}/locations/{vertex_location}"
+            "/endpoints/openapi"
+        )
     return f"{base_url}/v1beta1/projects/{vertex_project}/locations/{vertex_location}/endpoints/{model}"
 
 
@@ -129,7 +145,10 @@ class VertexAIModelGardenModels(VertexBase):
                 vertex_location=vertex_location or "us-central1",
                 vertex_api_version="v1beta1",
             )
-            model = ""
+            # Publisher/catalog models: model id must be sent in the JSON body (OpenAPI route).
+            # Single-segment endpoint ids: model is encoded in the URL path; body model stays empty.
+            if not _vertex_model_garden_model_id_in_json_body(model):
+                model = ""
             return openai_like_chat_completions.completion(
                 model=model,
                 messages=messages,

model_prices_and_context_window.json

@@ -33391,6 +33391,72 @@
         "source": "https://console.cloud.google.com/vertex-ai/publishers/openai/model-garden/gpt-oss-120b-maas",
         "supports_reasoning": true
     },
+    "vertex_ai/openai/xai/grok-4.1-fast-non-reasoning": {
+        "cache_read_input_token_cost": 5e-08,
+        "input_cost_per_token": 2e-07,
+        "litellm_provider": "vertex_ai",
+        "max_input_tokens": 2000000,
+        "max_output_tokens": 2000000,
+        "max_tokens": 2000000,
+        "mode": "chat",
+        "output_cost_per_token": 5e-07,
+        "source": "https://docs.x.ai/docs/models (Vertex AI Model Garden)",
+        "supports_function_calling": true,
+        "supports_response_schema": true,
+        "supports_tool_choice": true,
+        "supports_vision": true,
+        "supports_web_search": true
+    },
+    "vertex_ai/openai/xai/grok-4.1-fast-reasoning": {
+        "cache_read_input_token_cost": 5e-08,
+        "input_cost_per_token": 2e-07,
+        "litellm_provider": "vertex_ai",
+        "max_input_tokens": 2000000,
+        "max_output_tokens": 2000000,
+        "max_tokens": 2000000,
+        "mode": "chat",
+        "output_cost_per_token": 5e-07,
+        "source": "https://docs.x.ai/docs/models (Vertex AI Model Garden)",
+        "supports_function_calling": true,
+        "supports_reasoning": true,
+        "supports_response_schema": true,
+        "supports_tool_choice": true,
+        "supports_vision": true,
+        "supports_web_search": true
+    },
+    "vertex_ai/openai/xai/grok-4.20-non-reasoning": {
+        "cache_read_input_token_cost": 2e-07,
+        "input_cost_per_token": 2e-06,
+        "litellm_provider": "vertex_ai",
+        "max_input_tokens": 2000000,
+        "max_output_tokens": 2000000,
+        "max_tokens": 2000000,
+        "mode": "chat",
+        "output_cost_per_token": 6e-06,
+        "source": "https://docs.x.ai/docs/models (Vertex AI Model Garden)",
+        "supports_function_calling": true,
+        "supports_response_schema": true,
+        "supports_tool_choice": true,
+        "supports_vision": true,
+        "supports_web_search": true
+    },
+    "vertex_ai/openai/xai/grok-4.20-reasoning": {
+        "cache_read_input_token_cost": 2e-07,
+        "input_cost_per_token": 2e-06,
+        "litellm_provider": "vertex_ai",
+        "max_input_tokens": 2000000,
+        "max_output_tokens": 2000000,
+        "max_tokens": 2000000,
+        "mode": "chat",
+        "output_cost_per_token": 6e-06,
+        "source": "https://docs.x.ai/docs/models (Vertex AI Model Garden)",
+        "supports_function_calling": true,
+        "supports_reasoning": true,
+        "supports_response_schema": true,
+        "supports_tool_choice": true,
+        "supports_vision": true,
+        "supports_web_search": true
+    },
     "vertex_ai/qwen/qwen3-235b-a22b-instruct-2507-maas": {
         "input_cost_per_token": 2.5e-07,
         "litellm_provider": "vertex_ai-qwen_models",

tests/test_litellm/llms/vertex_ai/test_vertex_model_garden_openapi.py

@@ -0,0 +1,41 @@
+"""Vertex Model Garden: OpenAPI base URL for publisher/model ids vs per-endpoint path."""
+
+import pytest
+
+from litellm.llms.vertex_ai.vertex_model_garden.main import (
+    _vertex_model_garden_model_id_in_json_body,
+    create_vertex_url,
+)
+
+
+@pytest.mark.parametrize(
+    "model,expect_openapi_base",
+    [
+        ("xai/grok-4.1-fast-reasoning", True),
+        ("openai/foo/bar", True),
+        ("5464397967697903616", False),
+        ("gpt-oss-20b-maas", False),
+    ],
+)
+def test_create_vertex_url_openapi_vs_deployed_endpoint(
+    model: str, expect_openapi_base: bool
+) -> None:
+    url = create_vertex_url(
+        vertex_location="us-central1",
+        vertex_project="my-project",
+        stream=False,
+        model=model,
+    )
+    if expect_openapi_base:
+        assert "/v1/projects/my-project/locations/us-central1/endpoints/openapi" in url
+    else:
+        assert (
+            "/v1beta1/projects/my-project/locations/us-central1/endpoints/"
+            f"{model}" in url
+        )
+        assert "openapi" not in url
+
+
+def test_model_id_in_json_body_heuristic() -> None:
+    assert _vertex_model_garden_model_id_in_json_body("xai/grok-4.1-fast-reasoning") is True
+    assert _vertex_model_garden_model_id_in_json_body("5464397967697903616") is False