support token caching for anthropic via openrouter

packages/opencode/src/provider/transform.ts

@@ -0,0 +1,24 @@
+import type { CoreMessage } from "ai"
+
+export namespace ProviderTransform {
+  export function message(
+    msg: CoreMessage,
+    index: number,
+    providerID: string,
+    modelID: string,
+  ) {
+    if (
+      (providerID === "anthropic" || modelID.includes("anthropic")) &&
+      index < 4
+    ) {
+      msg.providerOptions = {
+        ...msg.providerOptions,
+        anthropic: {
+          cacheControl: { type: "ephemeral" },
+        },
+      }
+    }
+
+    return msg
+  }
+}