diff --git a/loopx/contract.py b/loopx/contract.py
index 992f39746a..4f14268ef6 100644
--- a/loopx/contract.py
+++ b/loopx/contract.py
@@ -117,20 +117,28 @@ def _prefilter_fold(text: str) -> str:
             "author" + "ization:",
         ),
     ),
+    # The private roots take the host's own separator, so a Windows host
+    # writes the same path class natively with backslash separators. Allowing
+    # a run of separators also covers serialized text, where a JSON or source
+    # string literal escapes each backslash into two.
     "local_private_path": LeakRule(
         pattern=re.compile(
-            "("
-            + "/"
+            r"([\\/]+"
             + "Users"
-            + "/"
-            + r"[^/\s]+/(?:Documents|code"
+            + r"[\\/]+[^\\/\s]+[\\/]+(?:Documents|code"
             + "-"
-            + r"reading)|"
-            + "/ext"
-            + "_data/"
-            + ")"
+            + r"reading)|[\\/]+ext"
+            + "_data"
+            + r"[\\/]+)"
+        ),
+        # Cheap necessary condition: every private root is followed by at
+        # least one separator, whatever the host or serialization chose.
+        required_literals=(
+            "users/",
+            "users\\",
+            "ext" + "_data/",
+            "ext" + "_data\\",
         ),
-        required_literals=("/" + "users/", "/ext" + "_data/"),
     ),
     "internal_task_id": LeakRule(
         pattern=re.compile(r"\bt-" + r"20\d{12}-[a-z0-9]+\b"),
diff --git a/loopx/semantics/project_registry_io_manifest_v1.json b/loopx/semantics/project_registry_io_manifest_v1.json
index 2e48944626..e2dd4d7a27 100644
--- a/loopx/semantics/project_registry_io_manifest_v1.json
+++ b/loopx/semantics/project_registry_io_manifest_v1.json
@@ -1047,7 +1047,7 @@
     },
     {
       "site": "loopx/contract.py::<module>.check_contract::codec_read:load_registry#1",
-      "line": 1099,
+      "line": 1107,
       "column": 20,
       "kind": "codec_read",
       "api": "load_registry",
diff --git a/tests/test_contract_public_boundary_prefilter.py b/tests/test_contract_public_boundary_prefilter.py
index d8ab82eeb7..5682f0510b 100644
--- a/tests/test_contract_public_boundary_prefilter.py
+++ b/tests/test_contract_public_boundary_prefilter.py
@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+from itertools import product
 from pathlib import Path
 import subprocess
 from typing import Any
@@ -70,6 +71,15 @@ def test_explicit_scan_file_keeps_unsupported_suffix(tmp_path: Path) -> None:
         ("local_private_path", "/" + "Users/alice/Documents/example.md"),
         ("local_private_path", "/" + "Users/alice/code-reading/example.md"),
         ("local_private_path", "/ext" + "_data/example.md"),
+        ("local_private_path", "\\" + "Users\\alice\\Documents\\example.md"),
+        ("local_private_path", "\\" + "Users\\alice\\code-reading\\example.md"),
+        ("local_private_path", "\\ext" + "_data\\example.md"),
+        # Either separator at each junction: a Windows host mixes them when
+        # it appends a POSIX-style segment to a native home directory, and a
+        # serialized document double-escapes each backslash.
+        ("local_private_path", "\\" + "Users/alice/Documents/example.md"),
+        ("local_private_path", "/Users" + "\\alice\\Documents\\example.md"),
+        ("local_private_path", "C:" + "\\\\" + "Users" + "\\\\" + "alice" + "\\\\" + "Documents" + "\\\\" + "example.md"),
         ("internal_task_id", "ticket t-" + "20260828123456-example"),
         ("private_ip", "host 10" + ".1.2.3"),
         ("private_ip", "host 172" + ".31.2.3"),
@@ -86,6 +96,22 @@ def test_every_authoritative_leak_pattern_has_a_matching_prefilter(
     assert rule.is_candidate(contract._prefilter_fold(line))
 
 
+def test_prefilter_is_a_necessary_condition_for_every_separator_spelling() -> None:
+    # The prefilter may be looser than the pattern but never stricter: every
+    # root spelling the authoritative pattern classifies, whatever separator
+    # or escape run the host or serializer chose, must remain a candidate.
+    rule = contract.LEAK_RULES["local_private_path"]
+    separators = ("/", "\\", "//", "\\\\")
+    for outer, inner in product(separators, repeat=2):
+        line = f"C:{outer}Users{inner}alice{inner}Documents{inner}report.md"
+        assert rule.pattern.search(line), line
+        assert rule.is_candidate(contract._prefilter_fold(line)), line
+    for separator in separators:
+        line = f"Mounted at {separator}ext_data{separator}report.md"
+        assert rule.pattern.search(line), line
+        assert rule.is_candidate(contract._prefilter_fold(line)), line
+
+
 @pytest.mark.parametrize(
     ("rule_name", "line"),
     [
@@ -225,3 +251,33 @@ def search(self, line: str) -> None:
     (tmp_path / "sample.md").write_text("\n".join(lines), encoding="utf-8")
     assert contract.scan_public_boundary([tmp_path])["ok"] is True
     assert pattern.lines == ["actual a|b", "actual 10."]
+
+
+@pytest.mark.parametrize(
+    "line",
+    [
+        "See C:" + "\\" + "Users\\alice\\Documents\\report.md",
+        "See C:" + "\\" + "Users\\alice\\code-reading\\report.md",
+        "Mounted at " + "\\ext" + "_data\\report.md",
+        "See C:" + "\\" + "Users/alice/Documents/report.md",
+        'json: {"path": "C:' + "\\\\" + 'Users\\\\alice\\\\Documents\\\\report.md"}',
+    ],
+)
+def test_native_windows_private_paths_are_blocked(line: str, tmp_path: Path) -> None:
+    # The same private path class must be blocked when a Windows host spells it
+    # with the native separator, not only in its POSIX form.
+    (tmp_path / "sample.md").write_text(line + "\n", encoding="utf-8")
+
+    payload = contract.scan_public_boundary([tmp_path])
+
+    assert payload["ok"] is False
+    assert payload["hits"] == ["sample.md:1: local_private_path"]
+
+
+def test_windows_paths_outside_the_private_roots_stay_public(tmp_path: Path) -> None:
+    # Accepting the Windows separator must not widen the private roots the rule
+    # names, so an unrelated profile subdirectory stays public.
+    line = "Cache at C:" + "\\" + "Users\\alice\\AppData\\Local\\Temp\\report.md"
+    (tmp_path / "sample.md").write_text(line + "\n", encoding="utf-8")
+
+    assert contract.scan_public_boundary([tmp_path])["ok"] is True