diff --git a/Agent.md b/Agent.md
index 63fbdb0f..541afb13 100644
--- a/Agent.md
+++ b/Agent.md
@@ -118,7 +118,7 @@ Community needs voiced in HN agent-UI discussions map directly to EMRG's design:
 pkill -f "emrg.server"; rm -f ~/.emrg/emrgd.port; python -m emrg
 ```
 
-Python: `uv run pytest tests/ -v` (773) — import check: `uv run python -c "from emrg.client.app import run_client"`
+Python: `uv run pytest tests/ -v` (777) — import check: `uv run python -c "from emrg.client.app import run_client"`
 GUI: `cd emrg/gui && npm test` (232: 44 daemon_client + 19 conn-manager + 22 app-commands + 110 renderer smoke + 15 i18n + 7 integration + 3 commands + 5 build-config + 7 gui-state) — syntax: `node --check main.js preload.js daemon_client.js renderer/js/*.js`
 CI: `uv run pytest` (ubuntu + **windows-2025 matrix** — Windows pytest 回归在 PR CI 即失败，v0.2.29 教训 #725) + GUI tests + **actionlint workflow lint** (`rhysd/actionlint@v1.7.12` gate, #444 — workflow 解析错误在 PR CI 即失败，如 `if:` secrets 上下文)
 Re-trigger: `scripts/re-trigger-ci.sh [branch]` (workflow_dispatch, #527 — 替代空 commit 重触发：Actions outage 会整段丢弃 push 事件，dispatch 走 API 路径不受影响)
diff --git a/emrg/server/scheduler.py b/emrg/server/scheduler.py
index 1b090614..05c19eb3 100644
--- a/emrg/server/scheduler.py
+++ b/emrg/server/scheduler.py
@@ -1105,12 +1105,25 @@ class TaskScheduler:
 
     def __init__(self, identity: InstanceIdentity) -> None:
         self.identity = identity
-        self._tasks_file = config_dir() / "tasks.yml"
+        self._tasks_file: Path | None = None
         self._handlers: list[TaskHandler] = []
         self._coros: list[asyncio.Task] = []
         # cfg (from tasks.yml) used to start each handler — for hot-reload diffing.
         self._handler_cfgs: dict[str, dict] = {}
 
+    @property
+    def _tasks_file(self) -> Path:
+        """tasks.yml path — resolved lazily so config_dir() patches work
+        (hermeticity guard #738: tests patch config_dir after construction,
+        and tests may override _tasks_file directly with a tmp path)."""
+        if self.__tasks_file is None:
+            self.__tasks_file = config_dir() / "tasks.yml"
+        return self.__tasks_file
+
+    @_tasks_file.setter
+    def _tasks_file(self, value: Path | None) -> None:
+        self.__tasks_file = value
+
     def _start_handler_for(self, cfg: dict) -> TaskHandler:
         """Create + start a handler for a task cfg; returns the handler."""
         template_path = _resolve_task_template(cfg["type"])
diff --git a/tests/conftest.py b/tests/conftest.py
new file mode 100644
index 00000000..b59820fc
--- /dev/null
+++ b/tests/conftest.py
@@ -0,0 +1,53 @@
+"""Shared pytest fixtures — hermeticity guards.
+
+2026-08-13 incident: the full test suite intermittently wrote pytest
+temp paths into the real ~/.emrg/projects.yml. The mechanism is a
+feedback loop — once a stale entry lands in the file (seeded by the
+workspace self-heal family), subsequent tests re-read it and re-write
+it, advancing the pytest-N counter on every full run. The daemon's
+per-cycle repair (#734) eventually cleans it, but until then the host's
+GUI project picker shows a dead path.
+
+CI finding (PR #738): test_ws_e2e._boot_server only patched
+daemon/connect config_dir, but EmrgServer.serve() builds a real
+TaskScheduler whose load_and_start() → _ensure_self_evolution_task()
+writes config_dir()/projects.yml AND tasks.yml via scheduler.py's own
+(unpatched) config_dir → on a fresh runner this hits the real
+~/.emrg/ files.
+
+This autouse fixture makes any write to the REAL ~/.emrg/projects.yml
+or ~/.emrg/tasks.yml a hard test failure: the offending test is named
+immediately instead of the pollution being discovered later (precedent:
+#583 assertPortFileInTmp sandbox guard for the emrgd.port file).
+"""
+from __future__ import annotations
+
+from pathlib import Path
+
+import pytest
+
+_REAL_CONFIG_FILES = (
+    (Path.home() / ".emrg" / "projects.yml").resolve(),
+    (Path.home() / ".emrg" / "tasks.yml").resolve(),
+)
+
+
+@pytest.fixture(autouse=True)
+def _guard_real_config_files(monkeypatch):
+    """Fail any test that writes the real ~/.emrg/projects.yml / tasks.yml."""
+    import emrg.server.daemon as daemon_mod
+    import emrg.server.scheduler as sched_mod
+
+    orig = sched_mod.atomic_write_yaml
+    assert orig is daemon_mod.atomic_write_yaml, "both modules must share atomic_write_yaml"
+
+    def guarded(data, path, **kwargs):
+        if Path(path).resolve() in _REAL_CONFIG_FILES:
+            raise AssertionError(
+                "test attempted to write a real ~/.emrg config file; "
+                f"keep tests hermetic (target={path!r})"
+            )
+        return orig(data, path, **kwargs)
+
+    monkeypatch.setattr(sched_mod, "atomic_write_yaml", guarded)
+    monkeypatch.setattr(daemon_mod, "atomic_write_yaml", guarded)
diff --git a/tests/test_daemon.py b/tests/test_daemon.py
index 0dfa8d50..780aa383 100644
--- a/tests/test_daemon.py
+++ b/tests/test_daemon.py
@@ -216,7 +216,12 @@ def _migrate_wrapper():
 
 def _make_server() -> EmrgServer:
     """Create a minimal EmrgServer for testing."""
-    return EmrgServer(LlmConfig(base_url="http://localhost", api_key="test"))
+    server = EmrgServer(LlmConfig(base_url="http://localhost", api_key="test"))
+    # Point the project log at a tmp file by default so context/message tests
+    # can never write the real ~/.emrg/projects.yml (2026-08-13 leak). Tests
+    # that exercise _projects_log override it explicitly afterwards.
+    server._projects_log = Path(tempfile.mkdtemp()) / "projects.yml"
+    return server
 
 
 def test_context_section_no_files(tmp_path):
diff --git a/tests/test_hermeticity_guard.py b/tests/test_hermeticity_guard.py
new file mode 100644
index 00000000..efe03b9e
--- /dev/null
+++ b/tests/test_hermeticity_guard.py
@@ -0,0 +1,58 @@
+"""Hermeticity guard tests (2026-08-13 ~/.emrg/projects.yml leak).
+
+Verifies the autouse conftest fixture is wired (both server modules
+carry the guarded wrapper) and that the discriminator is reliable in
+BOTH states (#455 lesson):
+- tmp projects.yml writes pass through unchanged (negative state);
+- the real ~/.emrg/projects.yml / tasks.yml paths raise AssertionError
+  (positive state) — the raise happens before any write, so this test
+  is safe.
+"""
+from __future__ import annotations
+
+from pathlib import Path
+
+import pytest
+
+
+def test_guard_installed_in_both_modules():
+    """Both scheduler and daemon carry the guarded atomic_write_yaml."""
+    import emrg.server.daemon as daemon_mod
+    import emrg.server.scheduler as sched_mod
+
+    wrapped = sched_mod.atomic_write_yaml
+    assert wrapped is daemon_mod.atomic_write_yaml
+    assert wrapped.__name__ == "guarded"
+
+
+def test_guard_allows_tmp_projects_yml(tmp_path):
+    """Writes to a tmp projects.yml pass through to the original writer."""
+    import emrg.server.scheduler as sched_mod
+
+    target = tmp_path / "projects.yml"
+    sched_mod.atomic_write_yaml(
+        [{"name": "x", "path": "p", "last_active": "t"}],
+        target,
+        prefix=".projects_",
+    )
+    assert target.exists()
+    data = target.read_text(encoding="utf-8")
+    assert "name: x" in data
+
+
+def test_guard_rejects_real_projects_yml():
+    """Writing the real ~/.emrg/projects.yml is a hard error."""
+    import emrg.server.scheduler as sched_mod
+
+    real = (Path.home() / ".emrg" / "projects.yml").resolve()
+    with pytest.raises(AssertionError, match="hermetic"):
+        sched_mod.atomic_write_yaml([], real, prefix=".projects_")
+
+
+def test_guard_rejects_real_tasks_yml():
+    """Writing the real ~/.emrg/tasks.yml is a hard error (same class)."""
+    import emrg.server.scheduler as sched_mod
+
+    real = (Path.home() / ".emrg" / "tasks.yml").resolve()
+    with pytest.raises(AssertionError, match="hermetic"):
+        sched_mod.atomic_write_yaml([], real, prefix=".tasks_")
diff --git a/tests/test_ws_e2e.py b/tests/test_ws_e2e.py
index 7035f6f8..574abd14 100644
--- a/tests/test_ws_e2e.py
+++ b/tests/test_ws_e2e.py
@@ -73,13 +73,16 @@ async def _boot_server(tmp: Path):
     wait for the port file to appear. Returns (server, serve_task).
     """
     import emrg.server.daemon as daemon_mod
+    import emrg.server.scheduler as sched_mod
     import emrg.connect as connect_mod
 
     _orig_daemon_cfg = daemon_mod.config_dir
+    _orig_sched_cfg = sched_mod.config_dir
     _orig_connect_cfg = connect_mod.config_dir
 
-    # Isolate config dir to tmp (port file, tasks.yml, etc.)
+    # Isolate config dir to tmp (port file, tasks.yml, projects.yml, etc.)
     daemon_mod.config_dir = lambda: tmp
+    sched_mod.config_dir = lambda: tmp  # scheduler builds its own projects_file (#738)
     connect_mod.config_dir = lambda: tmp
 
     server = daemon_mod.EmrgServer(_make_config())
@@ -108,6 +111,7 @@ async def _cleanup():
         except asyncio.CancelledError:
             pass
         daemon_mod.config_dir = _orig_daemon_cfg
+        sched_mod.config_dir = _orig_sched_cfg
         connect_mod.config_dir = _orig_connect_cfg
 
     return server, task, _cleanup