diff --git a/docs/development/testing-and-quality.md b/docs/development/testing-and-quality.md
index 50c23129d2..c15ff84ba2 100644
--- a/docs/development/testing-and-quality.md
+++ b/docs/development/testing-and-quality.md
@@ -729,6 +729,16 @@ and a global `loopx` may resolve to a different installed source tree.
 与测试依赖；能否自动下载 Python 取决于下载配置与网络。系统 `python3` 可能过旧，
 全局 `loopx` 也可能指向另一个已安装版本。
 
+Fixtures that model a child without LoopX must isolate that child from an
+inherited source-checkout `PYTHONPATH`. Check imports with the same interpreter
+mode and environment as the command under test: an isolated `python -I` probe
+does not qualify a later ordinary child process. Scope environment cleanup to
+the fixture; retain the real negative command, recovery and settlement checks.
+
+模拟未安装 LoopX 的子进程时，fixture 必须隔离继承的源码 `PYTHONPATH`。
+导入探测应使用被测命令相同的解释器模式与环境：`python -I` 探测失败不能证明
+后续普通子进程也无法导入。环境清理仅限该 fixture，保留真实失败命令、恢复及结算断言。
+
 ```bash
 uv sync --extra test
 uv run --extra test python -m ruff check tests loopx/canary loopx/control_plane loopx/domain_packs loopx/presentation
diff --git a/tests/test_native_steward_reports.py b/tests/test_native_steward_reports.py
index 8183d5add0..85df255170 100644
--- a/tests/test_native_steward_reports.py
+++ b/tests/test_native_steward_reports.py
@@ -990,10 +990,13 @@ def embedded_report_python(tmp_path, monkeypatch):
     import os
     from types import SimpleNamespace
     from loopx.capabilities.native_chat import steward_reports
+    # A checkout-wide source bootstrap would make this empty child import
+    # LoopX even though the real embedding host has not installed it.
+    monkeypatch.delenv("PYTHONPATH", raising=False)
     folder = tmp_path / "embedded-python"
     subprocess.run([sys.executable, "-m", "venv", "--without-pip", str(folder)], check=True)
     python = folder / ("Scripts/python.exe" if os.name == "nt" else "bin/python")
-    missing = subprocess.run([str(python), "-I", "-c", "import loopx"],
+    missing = subprocess.run([str(python), "-c", "import loopx"],
         cwd=tmp_path, capture_output=True, text=True)
     assert missing.returncode != 0 and "ModuleNotFoundError" in missing.stderr
     monkeypatch.setattr(steward_reports, "sys", SimpleNamespace(executable=str(python)))