diff --git a/tests/test_sense.py b/tests/test_sense.py
index 9a7bd373a..fa4f0e167 100644
--- a/tests/test_sense.py
+++ b/tests/test_sense.py
@@ -210,6 +210,33 @@ def test_process_log_writer_thread_safe(tmp_path, monkeypatch):
     assert len([line for line in lines if line]) == 50
 
 
+def test_process_log_writer_pins_journal_root_at_init(tmp_path, monkeypatch):
+    """Env-var drift between construction and flush must not redirect writes."""
+    from think import runner
+
+    journal_a = tmp_path / "a"
+    journal_b = tmp_path / "b"
+    journal_a.mkdir()
+    journal_b.mkdir()
+
+    monkeypatch.setenv("_SOLSTONE_JOURNAL_OVERRIDE", str(journal_a))
+    monkeypatch.setattr(runner, "_current_day", lambda: "20241101")
+
+    ref = "test_ref"
+    writer = ProcessLogWriter(ref, "echo")
+
+    # Drift: env var changes and day changes before the next flush.
+    monkeypatch.setenv("_SOLSTONE_JOURNAL_OVERRIDE", str(journal_b))
+    monkeypatch.setattr(runner, "_current_day", lambda: "20241102")
+
+    writer.write("hello\n")
+    writer.close()
+
+    leaked_paths = list(journal_b.rglob("*"))
+    assert not leaked_paths, f"writes leaked into drifted journal: {leaked_paths}"
+    assert list(journal_a.rglob("*.log")) or list(journal_a.rglob("*echo*"))
+
+
 def test_handler_process_cleanup():
     """Test HandlerProcess cleanup joins threads and closes logger."""
     mock_managed = MagicMock()
diff --git a/think/runner.py b/think/runner.py
index 9b531feb8..29075fc30 100644
--- a/think/runner.py
+++ b/think/runner.py
@@ -43,12 +43,12 @@ def _current_day() -> str:
     return datetime.now().strftime("%Y%m%d")
 
 
-def _day_health_log_path(day: str, ref: str, name: str) -> Path:
+def _day_health_log_path(journal_root: Path, day: str, ref: str, name: str) -> Path:
     """Build path to day health log.
 
     Returns: journal/chronicle/{day}/health/{ref}_{name}.log
     """
-    return _get_journal_path() / CHRONICLE_DIR / day / "health" / f"{ref}_{name}.log"
+    return journal_root / CHRONICLE_DIR / day / "health" / f"{ref}_{name}.log"
 
 
 def _atomic_symlink(link_path: Path, target: str) -> None:
@@ -98,11 +98,14 @@ class DailyLogWriter:
     - journal/health/{name}.log -> chronicle/{YYYYMMDD}/health/{ref}_{name}.log (journal-level)
 
     When the day changes, automatically closes old file, opens new file, and updates symlinks.
+    The journal root is resolved once at construction time and pinned for the
+    lifetime of the writer.
     """
 
     def __init__(self, ref: str, name: str, day: str | None = None):
         self._ref = ref
         self._name = name
+        self._journal_root: Path = _get_journal_path()
         self._pinned = day is not None
         self._lock = threading.Lock()
         self._current_day = day or _current_day()
@@ -111,13 +114,15 @@ class DailyLogWriter:
 
     def _open_log(self):
         """Open log file for current day."""
-        log_path = _day_health_log_path(self._current_day, self._ref, self._name)
+        log_path = _day_health_log_path(
+            self._journal_root, self._current_day, self._ref, self._name
+        )
         log_path.parent.mkdir(parents=True, exist_ok=True)
         return log_path.open("a", encoding="utf-8")
 
     def _update_symlinks(self) -> None:
         """Update day-level and journal-level symlinks to point to current log."""
-        journal = _get_journal_path()
+        journal = self._journal_root
         day_health = journal / CHRONICLE_DIR / self._current_day / "health"
         log_filename = f"{self._ref}_{self._name}.log"
 
@@ -167,7 +172,9 @@ class DailyLogWriter:
     @property
     def path(self) -> Path:
         """Get current log file path."""
-        return _day_health_log_path(self._current_day, self._ref, self._name)
+        return _day_health_log_path(
+            self._journal_root, self._current_day, self._ref, self._name
+        )
 
 
 @dataclass