diff --git a/src/proxy/middleware/request/add-key.ts b/src/proxy/middleware/request/add-key.ts
index ef149b9..23431ed 100644
--- a/src/proxy/middleware/request/add-key.ts
+++ b/src/proxy/middleware/request/add-key.ts
@@ -60,6 +60,7 @@ export const addKey: ProxyRequestMiddleware = (proxyReq, req) => {
   // TODO: KeyProvider should assemble all necessary headers
   if (assignedKey.service === "anthropic") {
     proxyReq.setHeader("X-API-Key", assignedKey.key);
+    proxyReq.setHeader("anthropic-version", "2023-01-01");
   } else if (assignedKey.service === "openai") {
     const key: OpenAIKey = assignedKey as OpenAIKey;
     if (key.organizationId) {