diff --git a/mempalace/config.py b/mempalace/config.py
index 4005779..2252a49 100644
--- a/mempalace/config.py
+++ b/mempalace/config.py
@@ -85,16 +85,21 @@ def sanitize_kg_value(value: str, field_name: str = "value") -> str:
 # (as_of, valid_from, valid_to, ended). Parameterized queries already
 # prevent SQL injection, but unvalidated date strings silently miss
 # every row — callers cannot distinguish "no fact at this time" from
-# "your date format was unrecognized." Accept YYYY, YYYY-MM, YYYY-MM-DD.
-_ISO_DATE_RE = re.compile(r"^\d{4}(?:-(?:0[1-9]|1[0-2])(?:-(?:0[1-9]|[12]\d|3[01]))?)?$")
+# "your date format was unrecognized." Require full YYYY-MM-DD: KG
+# queries compare TEXT dates lexicographically, so partials like "2026"
+# would re-introduce silent empty results (e.g. "2026-01-01" <= "2026"
+# is False), defeating the purpose of validation.
+_ISO_DATE_RE = re.compile(r"^\d{4}-(?:0[1-9]|1[0-2])-(?:0[1-9]|[12]\d|3[01])$")
 
 
 def sanitize_iso_date(value, field_name: str = "date"):
     """Validate an ISO-8601 date string, accepting None or empty as-is.
 
-    Accepts ``YYYY``, ``YYYY-MM``, or ``YYYY-MM-DD``. Raises ValueError
-    on any other non-empty input so the MCP layer can surface a clear
-    error to the caller instead of silently returning empty results.
+    Accepts only ``YYYY-MM-DD``. Raises ValueError on any other
+    non-empty input so the MCP layer can surface a clear error to the
+    caller instead of silently returning empty results. Partial dates
+    (``YYYY``, ``YYYY-MM``) are rejected because KG queries compare
+    TEXT dates lexicographically and would silently exclude valid facts.
     """
     if value is None or value == "":
         return value
@@ -103,8 +108,7 @@ def sanitize_iso_date(value, field_name: str = "date"):
     value = value.strip()
     if not _ISO_DATE_RE.match(value):
         raise ValueError(
-            f"{field_name}={value!r} is not a valid ISO-8601 date "
-            f"(expected YYYY, YYYY-MM, or YYYY-MM-DD)"
+            f"{field_name}={value!r} is not a valid ISO-8601 date " f"(expected YYYY-MM-DD)"
         )
     return value
 
diff --git a/tests/test_config.py b/tests/test_config.py
index f5064e2..204faae 100644
--- a/tests/test_config.py
+++ b/tests/test_config.py
@@ -223,12 +223,16 @@ def test_kg_value_rejects_over_length():
 # --- sanitize_iso_date ---
 
 
-def test_iso_date_accepts_year_only():
-    assert sanitize_iso_date("2026") == "2026"
+def test_iso_date_rejects_year_only():
+    # Partial dates re-introduce silent empty result sets via lexicographic
+    # TEXT comparison in KG queries (e.g. "2026-01-01" <= "2026" is False).
+    with pytest.raises(ValueError):
+        sanitize_iso_date("2026")
 
 
-def test_iso_date_accepts_year_month():
-    assert sanitize_iso_date("2026-03") == "2026-03"
+def test_iso_date_rejects_year_month():
+    with pytest.raises(ValueError):
+        sanitize_iso_date("2026-03")
 
 
 def test_iso_date_accepts_full_date():
diff --git a/tests/test_mcp_server.py b/tests/test_mcp_server.py
index 1b80f36..136b6f3 100644
--- a/tests/test_mcp_server.py
+++ b/tests/test_mcp_server.py
@@ -702,14 +702,21 @@ class TestKGTools:
         assert result["success"] is False
         assert "ended" in result["error"]
 
-    def test_kg_query_accepts_partial_iso_dates(self, monkeypatch, config, palace_path, seeded_kg):
+    def test_kg_query_rejects_partial_iso_dates(self, monkeypatch, config, palace_path, seeded_kg):
         _patch_mcp_server(monkeypatch, config, seeded_kg)
         from mempalace.mcp_server import tool_kg_query
 
-        # YYYY and YYYY-MM are valid ISO-8601 forms — must not be rejected.
-        for value in ("2026", "2026-03", "2026-03-15"):
+        # Partial ISO dates are rejected: KG queries compare TEXT dates
+        # lexicographically, so "2026-01-01" <= "2026" is False, which
+        # silently excludes facts. Reject at the boundary — only YYYY-MM-DD
+        # produces correct results.
+        for value in ("2026", "2026-03"):
             result = tool_kg_query(entity="Max", as_of=value)
-            assert "error" not in result, f"rejected valid date {value!r}: {result}"
+            assert "error" in result, f"accepted partial date {value!r}: {result}"
+
+        # Full ISO-8601 dates still pass.
+        result = tool_kg_query(entity="Max", as_of="2026-03-15")
+        assert "error" not in result, f"rejected valid date: {result}"
 
 
 # ── Diary Tools ─────────────────────────────────────────────────────────