feat: use systemInstruction for gemini-1.5-* models (#564)

src/client/vertexai.rs

@@ -262,7 +262,12 @@ pub fn gemini_build_chat_completions_body(
         stream: _,
     } = data;
 
-    patch_system_message(&mut messages);
+    let mut system_message = None;
+    if model.name().starts_with("gemini-1.5") {
+        system_message = extract_system_message(&mut messages);
+    } else {
+        patch_system_message(&mut messages);
+    };
 
     let mut network_image_urls = vec![];
     let contents: Vec<Value> = messages
@@ -333,6 +338,10 @@ pub fn gemini_build_chat_completions_body(
 
     let mut body = json!({ "contents": contents, "generationConfig": {} });
 
+    if let Some(v) = system_message {
+        body["systemInstruction"] = json!({ "parts": [{ "text": v }] });
+    }
+
     if let Some(v) = model.max_tokens_param() {
         body["generationConfig"]["maxOutputTokens"] = v.into();
     }