diff --git a/agent/gemini_native_adapter.py b/agent/gemini_native_adapter.py
index a0f8e9df548..d8e6c63c48e 100644
--- a/agent/gemini_native_adapter.py
+++ b/agent/gemini_native_adapter.py
@@ -330,7 +330,7 @@ def _build_gemini_contents(messages: List[Dict[str, Any]]) -> tuple[List[Dict[st
     system_instruction = None
     joined_system = "\n".join(part for part in system_text_parts if part).strip()
     if joined_system:
-        system_instruction = {"parts": [{"text": joined_system}]}
+        system_instruction = {"role": "system", "parts": [{"text": joined_system}]}
     return contents, system_instruction
 
 
diff --git a/tests/agent/test_gemini_native_adapter.py b/tests/agent/test_gemini_native_adapter.py
index 4f894c512a6..703428d4eb7 100644
--- a/tests/agent/test_gemini_native_adapter.py
+++ b/tests/agent/test_gemini_native_adapter.py
@@ -328,6 +328,25 @@ def test_stream_event_translation_keeps_identical_calls_in_distinct_parts():
     assert tool_chunks[0].choices[0].delta.tool_calls[0].id != tool_chunks[1].choices[0].delta.tool_calls[0].id
 
 
+def test_system_instruction_includes_role_field_and_stays_out_of_contents():
+    from agent.gemini_native_adapter import build_gemini_request
+
+    request = build_gemini_request(
+        messages=[
+            {"role": "system", "content": "You are a helpful assistant."},
+            {"role": "user", "content": "Hello"},
+        ],
+        tools=[],
+        tool_choice=None,
+    )
+
+    assert request["systemInstruction"] == {
+        "role": "system",
+        "parts": [{"text": "You are a helpful assistant."}],
+    }
+    assert all(content.get("role") != "system" for content in request["contents"])
+
+
 def test_max_tokens_none_defaults_to_gemini_output_ceiling():
     """max_tokens=None must send the model's full output ceiling, not omit it.