fix: uses empty system prompt by default (#290)

fix #289

Author: yokomotod

src/claude_agent_sdk/_internal/transport/subprocess_cli.py

@@ -97,7 +97,7 @@ def _build_command(self) -> list[str]:
         cmd = [self._cli_path, "--output-format", "stream-json", "--verbose"]
 
         if self._options.system_prompt is None:
-            pass
+            cmd.extend(["--system-prompt", ""])
         elif isinstance(self._options.system_prompt, str):
             cmd.extend(["--system-prompt", self._options.system_prompt])
         else:

tests/test_transport.py

@@ -46,6 +46,8 @@ def test_build_command_basic(self):
         assert "stream-json" in cmd
         assert "--print" in cmd
         assert "Hello" in cmd
+        assert "--system-prompt" in cmd
+        assert cmd[cmd.index("--system-prompt") + 1] == ""
 
     def test_cli_path_accepts_pathlib_path(self):
         """Test that cli_path accepts pathlib.Path objects."""