diff --git a/packages/opencode/src/session/system.ts b/packages/opencode/src/session/system.ts
index 300943881..c962cadb5 100644
--- a/packages/opencode/src/session/system.ts
+++ b/packages/opencode/src/session/system.ts
@@ -19,11 +19,17 @@ import PROMPT_CODEX from "./prompt/codex.txt"
 import type { Provider } from "@/provider/provider"
 
 export namespace SystemPrompt {
-  export function header(providerID: string) {
-    if (providerID.includes("anthropic")) return [PROMPT_ANTHROPIC_SPOOF.trim()]
+  function providerSpoof(providerID: string) {
+    if (providerID !== "google-vertex-anthropic" && providerID.includes("anthropic")) {
+      return [PROMPT_ANTHROPIC_SPOOF.trim()]
+    }
     return []
   }
 
+  export function header(providerID: string) {
+    return [...providerSpoof(providerID)]
+  }
+
   export function provider(model: Provider.Model) {
     if (model.api.id.includes("gpt-5")) return [PROMPT_CODEX]
     if (model.api.id.includes("gpt-") || model.api.id.includes("o1") || model.api.id.includes("o3"))