diff --git a/api/v1alpha1/llmprovider_types.go b/api/v1alpha1/llmprovider_types.go
index e95ba0cf..0e129781 100644
--- a/api/v1alpha1/llmprovider_types.go
+++ b/api/v1alpha1/llmprovider_types.go
@@ -72,6 +72,20 @@ type AnthropicConfig struct {
 	URL string `json:"url,omitempty"`
 }
 
+// GoogleCloudVertexModelProvider selects which model provider stack talks to Vertex AI.
+//
+// +kubebuilder:validation:Enum=Anthropic;Google;OpenAI
+type GoogleCloudVertexModelProvider string
+
+const (
+	// GoogleCloudVertexModelProviderAnthropic uses Anthropic models on Vertex (Claude SDK).
+	GoogleCloudVertexModelProviderAnthropic GoogleCloudVertexModelProvider = "Anthropic"
+	// GoogleCloudVertexModelProviderGoogle uses Google models on Vertex (GenAI SDK).
+	GoogleCloudVertexModelProviderGoogle GoogleCloudVertexModelProvider = "Google"
+	// GoogleCloudVertexModelProviderOpenAI uses OpenAI-compatible models on Vertex (OpenAI SDK).
+	GoogleCloudVertexModelProviderOpenAI GoogleCloudVertexModelProvider = "OpenAI"
+)
+
 // GoogleCloudVertexConfig contains configuration for the Google Cloud Vertex AI provider.
 type GoogleCloudVertexConfig struct {
 	// credentialsSecret references a Secret in the operator namespace
@@ -100,6 +114,13 @@ type GoogleCloudVertexConfig struct {
 	// +kubebuilder:validation:XValidation:rule="self.matches('^[a-z][a-z0-9-]*[a-z0-9]$')",message="region must contain only lowercase letters, digits, and hyphens, start with a letter, and not end with a hyphen"
 	Region string `json:"region,omitempty"`
 
+	// modelProvider selects which model provider stack talks to Vertex AI (required).
+	// "Anthropic" uses Anthropic models on Vertex; "Google" uses Google models on Vertex;
+	// "OpenAI" uses OpenAI-compatible models on Vertex.
+	// Enum is defined on GoogleCloudVertexModelProvider.
+	// +required
+	ModelProvider GoogleCloudVertexModelProvider `json:"modelProvider"`
+
 	// url is an optional override for the Vertex AI API endpoint.
 	// Only needed for custom deployments or API proxies.
 	// Must be a valid HTTP or HTTPS URL with a hostname. Paths and query
@@ -312,6 +333,7 @@ type LLMProviderSpec struct {
 //	      name: llm-credentials
 //	    projectID: my-gcp-project
 //	    region: us-central1
+//	    modelProvider: Anthropic
 type LLMProvider struct {
 	metav1.TypeMeta `json:",inline"`
 
diff --git a/config/crd/bases/agentic.openshift.io_llmproviders.yaml b/config/crd/bases/agentic.openshift.io_llmproviders.yaml
index 18509a78..d9167138 100644
--- a/config/crd/bases/agentic.openshift.io_llmproviders.yaml
+++ b/config/crd/bases/agentic.openshift.io_llmproviders.yaml
@@ -36,7 +36,8 @@ spec:
           AI provider (model specified on Agent, not here):\n\n\tapiVersion: agentic.openshift.io/v1alpha1\n\tkind:
           LLMProvider\n\tmetadata:\n\t  name: vertex-ai\n\tspec:\n\t  type: GoogleCloudVertex\n\t
           \ googleCloudVertex:\n\t    credentialsSecret:\n\t      name: llm-credentials\n\t
-          \   projectID: my-gcp-project\n\t    region: us-central1"
+          \   projectID: my-gcp-project\n\t    region: us-central1\n\t    modelProvider:
+          Anthropic"
         properties:
           apiVersion:
             description: |-
@@ -268,6 +269,17 @@ spec:
                     required:
                     - name
                     type: object
+                  modelProvider:
+                    description: |-
+                      modelProvider selects which model provider stack talks to Vertex AI (required).
+                      "Anthropic" uses Anthropic models on Vertex; "Google" uses Google models on Vertex;
+                      "OpenAI" uses OpenAI-compatible models on Vertex.
+                      Enum is defined on GoogleCloudVertexModelProvider.
+                    enum:
+                    - Anthropic
+                    - Google
+                    - OpenAI
+                    type: string
                   projectID:
                     description: |-
                       projectID is the Google Cloud Project ID where Vertex AI is enabled.
@@ -316,6 +328,7 @@ spec:
                       rule: '!self.contains(''#'')'
                 required:
                 - credentialsSecret
+                - modelProvider
                 - projectID
                 - region
                 type: object
diff --git a/controller/proposal/reconciler_test.go b/controller/proposal/reconciler_test.go
index 21fd84b6..dcedece2 100644
--- a/controller/proposal/reconciler_test.go
+++ b/controller/proposal/reconciler_test.go
@@ -106,6 +106,7 @@ func testLLM(name string) *agenticv1alpha1.LLMProvider {
 				CredentialsSecret: agenticv1alpha1.SecretReference{Name: "llm-secret"},
 				ProjectID:         "test-project",
 				Region:            "us-central1",
+				ModelProvider:     agenticv1alpha1.GoogleCloudVertexModelProviderAnthropic,
 			},
 		},
 	}
diff --git a/examples/setup/00-llm-providers.yaml b/examples/setup/00-llm-providers.yaml
index 237804fa..5366ef46 100644
--- a/examples/setup/00-llm-providers.yaml
+++ b/examples/setup/00-llm-providers.yaml
@@ -20,3 +20,4 @@ spec:
       name: llm-credentials
     projectID: my-gcp-project
     region: us-central1
+    modelProvider: Anthropic
diff --git a/test/e2e/helpers_test.go b/test/e2e/helpers_test.go
index 59a5e2a4..7bbbad05 100644
--- a/test/e2e/helpers_test.go
+++ b/test/e2e/helpers_test.go
@@ -134,6 +134,7 @@ func createFixtures(t *testing.T, c client.Client) *e2eFixtures {
 					CredentialsSecret: agenticv1alpha1.SecretReference{Name: "e2e-llm-secret"},
 					ProjectID:         "e2e-project",
 					Region:            "us-central1",
+					ModelProvider:       agenticv1alpha1.GoogleCloudVertexModelProviderAnthropic,
 				},
 			},
 		},