diff --git a/src/aipass/aipass/docs/probe_hygiene.md b/src/aipass/aipass/docs/probe_hygiene.md
new file mode 100644
index 00000000..8223cd7a
--- /dev/null
+++ b/src/aipass/aipass/docs/probe_hygiene.md
@@ -0,0 +1,42 @@
+# Probe Hygiene SOP
+
+Standard operating procedure for throwaway test installs of AIPass.
+
+## Principle
+
+Temporary environments are used, then deleted, gone. Nothing permanent may ever point at a temp path.
+
+## Rules
+
+- Install probes and throwaway test installs live ONLY in throwaway directories (system temp dir, `/tmp`, Claude Code scratchpad dirs).
+- Used = deleted = GONE. Delete the probe directory immediately after the test completes.
+- NOTHING permanent may ever point at a temporary path: no global settings (`~/.claude/settings.json` `env.AIPASS_HOME`), no symlinks, no registry entries.
+- `aipass install` now refuses throwaway homes automatically. The `--force-global-home` flag is the explicit unsafe override, for probe use only.
+- `aipass doctor` now detects a hijacked global `AIPASS_HOME` (nonexistent or temp path) and flags it as an error with fix guidance.
+
+## What the defenses do
+
+1. **`is_throwaway_path()`** (bootstrap.py) — detects paths under `tempfile.gettempdir()`, `/tmp` (POSIX), or containing `scratchpad`. Shared gate used by both install and bootstrap.
+2. **`run_install()` gate** (install.py) — refuses to proceed when the resolved home is throwaway. Prints a loud `REFUSED` message with guidance. `--force-global-home` overrides.
+3. **`_claude_settings()` gate** (bootstrap.py) — refuses to write `env.AIPASS_HOME` into project settings when the detected home is throwaway. Defense-in-depth behind the install gate.
+4. **`_check_global_aipass_home()`** (doctor.py) — reads `~/.claude/settings.json` and flags `env.AIPASS_HOME` pointing at a nonexistent or throwaway path as an error.
+
+## Correct probe workflow
+
+```bash
+# 1. Create throwaway dir
+cd /tmp && mkdir aipass_probe && cd aipass_probe
+
+# 2. Run the probe (install will refuse — this is correct)
+aipass install --here
+# → REFUSED: '/tmp/aipass_probe' is a temporary/scratchpad path.
+
+# 3. If you genuinely need a temp install (testing only):
+aipass install --here --force-global-home
+
+# 4. IMMEDIATELY after testing, delete the probe
+rm -rf /tmp/aipass_probe
+
+# 5. Verify global settings are clean
+aipass doctor
+```
diff --git a/src/aipass/aipass/tests/test_bootstrap.py b/src/aipass/aipass/tests/test_bootstrap.py
index 9efc5556..8d793c66 100644
--- a/src/aipass/aipass/tests/test_bootstrap.py
+++ b/src/aipass/aipass/tests/test_bootstrap.py
@@ -278,8 +278,12 @@ def test_init_project_claude_settings_content(tmp_path):
     assert "deny" in data["permissions"]
 
 
-def test_init_project_settings_no_hooks(tmp_path):
+def test_init_project_settings_no_hooks(tmp_path, monkeypatch):
     """.claude/settings.json has no hooks — all hooks fire from provider level."""
+    monkeypatch.setattr(
+        "aipass.aipass.apps.handlers.init.bootstrap.is_throwaway_path",
+        lambda _: False,
+    )
     target = tmp_path / "proj"
     target.mkdir()
 
@@ -451,8 +455,12 @@ def test_update_project_return_dict_structure(tmp_path):
     assert isinstance(result["skipped_files"], list)
 
 
-def test_update_project_already_current_after_init(tmp_path):
+def test_update_project_already_current_after_init(tmp_path, monkeypatch):
     """Running update immediately after init reports all managed files as already current."""
+    monkeypatch.setattr(
+        "aipass.aipass.apps.handlers.init.bootstrap.is_throwaway_path",
+        lambda _: False,
+    )
     target = tmp_path / "proj"
     target.mkdir()
     init_project(target, project_name="fresh")
@@ -463,8 +471,12 @@ def test_update_project_already_current_after_init(tmp_path):
     assert len(result["already_current"]) >= 5
 
 
-def test_update_project_idempotent(tmp_path):
+def test_update_project_idempotent(tmp_path, monkeypatch):
     """Running update twice in a row produces no changes on second run."""
+    monkeypatch.setattr(
+        "aipass.aipass.apps.handlers.init.bootstrap.is_throwaway_path",
+        lambda _: False,
+    )
     target = tmp_path / "proj"
     target.mkdir()
     init_project(target, project_name="idem")
@@ -571,8 +583,12 @@ def test_init_project_returns_aipass_home(tmp_path):
     assert result["aipass_home"] is None or isinstance(result["aipass_home"], str)
 
 
-def test_init_project_settings_has_aipass_home_when_detected(tmp_path):
+def test_init_project_settings_has_aipass_home_when_detected(tmp_path, monkeypatch):
     """When AIPASS_HOME is detected, settings.json includes env.AIPASS_HOME."""
+    monkeypatch.setattr(
+        "aipass.aipass.apps.handlers.init.bootstrap.is_throwaway_path",
+        lambda _: False,
+    )
     target = tmp_path / "proj"
     target.mkdir()
 
@@ -598,8 +614,12 @@ def test_update_project_returns_aipass_home(tmp_path):
     assert result["aipass_home"] is None or isinstance(result["aipass_home"], str)
 
 
-def test_update_project_adds_aipass_home_if_missing(tmp_path):
+def test_update_project_adds_aipass_home_if_missing(tmp_path, monkeypatch):
     """update_project injects AIPASS_HOME into settings.json if env section is absent."""
+    monkeypatch.setattr(
+        "aipass.aipass.apps.handlers.init.bootstrap.is_throwaway_path",
+        lambda _: False,
+    )
     target = tmp_path / "proj"
     target.mkdir()
     init_project(target, project_name="addenv")
diff --git a/src/aipass/aipass/tests/test_init_flow.py b/src/aipass/aipass/tests/test_init_flow.py
index 545a2b69..5d3213b6 100644
--- a/src/aipass/aipass/tests/test_init_flow.py
+++ b/src/aipass/aipass/tests/test_init_flow.py
@@ -261,6 +261,11 @@ def _bypass_preflight():
 
 
 class TestRunInit:
+    @pytest.fixture(autouse=True)
+    def _isolate_cwd(self, tmp_path, monkeypatch):
+        """Avoid _guard_init rejecting the real cwd when it has .trinity/."""
+        monkeypatch.chdir(tmp_path)
+
     def _patch_all_stages(self):
         """Context manager that patches all 10 stage functions to no-ops."""
         stage_names = [