diff --git a/Gradata/src/gradata/hooks/session_close.py b/Gradata/src/gradata/hooks/session_close.py
index 8e663326..cf0e4970 100644
--- a/Gradata/src/gradata/hooks/session_close.py
+++ b/Gradata/src/gradata/hooks/session_close.py
@@ -278,9 +278,21 @@ def _run_cloud_sync(brain_dir: str, data: dict) -> None:
     Claude Code never calls ``brain.end_session()`` directly, so
     ``_cloud_sync_session`` never fired from IDE sessions before this hook
     path existed. Gated on GRADATA_API_KEY — no key, no sync, no network.
+
+    As of #194 day 4, Brain.correct() write-through is the default cloud
+    sync path. The session_close hook tick is redundant when write-through
+    is active and was double-writing every correction. Set
+    GRADATA_DISABLE_WRITE_THROUGH=1 to fall back to this legacy path.
     """
     if not os.environ.get("GRADATA_API_KEY"):
         return
+    # Default behavior: write-through covers it. Skip the legacy tick.
+    if os.environ.get("GRADATA_DISABLE_WRITE_THROUGH") != "1":
+        _log.info(
+            "write-through enabled, skipping legacy cloud sync from session_close "
+            "(set GRADATA_DISABLE_WRITE_THROUGH=1 to restore legacy behavior)"
+        )
+        return
     try:
         from gradata._core import cloud_sync_tick
 
diff --git a/Gradata/tests/test_session_close_write_through_gate.py b/Gradata/tests/test_session_close_write_through_gate.py
new file mode 100644
index 00000000..9add19d9
--- /dev/null
+++ b/Gradata/tests/test_session_close_write_through_gate.py
@@ -0,0 +1,42 @@
+"""Test the GRADATA_DISABLE_WRITE_THROUGH gate on session_close cloud sync.
+
+#194 day 4: Brain.correct() write-through is the default cloud sync path.
+The session_close hook's legacy cloud_sync_tick is now skipped by default
+to avoid double-writes. Only runs when GRADATA_DISABLE_WRITE_THROUGH=1.
+"""
+
+from __future__ import annotations
+
+from unittest.mock import patch
+
+from gradata.hooks import session_close
+
+
+def test_run_cloud_sync_skipped_when_write_through_default(monkeypatch):
+    """Default behavior: write-through covers it, session_close tick is skipped."""
+    monkeypatch.setenv("GRADATA_API_KEY", "gd_live_test")
+    monkeypatch.delenv("GRADATA_DISABLE_WRITE_THROUGH", raising=False)
+    with patch("gradata._core.cloud_sync_tick") as mock_tick:
+        session_close._run_cloud_sync("/tmp/fake-brain", {"session_number": 1})
+    (
+        mock_tick.assert_not_called(),
+        ("session_close must NOT call cloud_sync_tick when write-through is on"),
+    )
+
+
+def test_run_cloud_sync_invoked_when_write_through_disabled(monkeypatch):
+    """Opt-out via GRADATA_DISABLE_WRITE_THROUGH=1: legacy path runs."""
+    monkeypatch.setenv("GRADATA_API_KEY", "gd_live_test")
+    monkeypatch.setenv("GRADATA_DISABLE_WRITE_THROUGH", "1")
+    with patch("gradata._core.cloud_sync_tick") as mock_tick:
+        session_close._run_cloud_sync("/tmp/fake-brain", {"session_number": 1})
+    mock_tick.assert_called_once_with("/tmp/fake-brain", 1)
+
+
+def test_run_cloud_sync_skipped_when_no_api_key(monkeypatch):
+    """No API key: never call cloud regardless of write-through state."""
+    monkeypatch.delenv("GRADATA_API_KEY", raising=False)
+    monkeypatch.setenv("GRADATA_DISABLE_WRITE_THROUGH", "1")
+    with patch("gradata._core.cloud_sync_tick") as mock_tick:
+        session_close._run_cloud_sync("/tmp/fake-brain", {"session_number": 1})
+    mock_tick.assert_not_called()