diff --git a/packages/vertex-sdk/src/client.ts b/packages/vertex-sdk/src/client.ts
index 61f67582..06231649 100644
--- a/packages/vertex-sdk/src/client.ts
+++ b/packages/vertex-sdk/src/client.ts
@@ -114,6 +114,12 @@ export class AnthropicVertex extends Core.APIClient {
     url: string;
     timeout: number;
   } {
+    if (Core.isObj(options.body)) {
+      // create a shallow copy of the request body so that code that mutates it later
+      // doesn't mutate the original user-provided object
+      options.body = { ...options.body };
+    }
+
     if (Core.isObj(options.body)) {
       if (!options.body['anthropic_version']) {
         options.body['anthropic_version'] = DEFAULT_VERSION;