diff --git a/tests/test_required_substring_prefilter_contract.py b/tests/test_required_substring_prefilter_contract.py
new file mode 100644
index 00000000..8491056b
--- /dev/null
+++ b/tests/test_required_substring_prefilter_contract.py
@@ -0,0 +1,81 @@
+"""Security-contract tests for the required-substring scanner prefilter."""
+
+import re
+from unittest.mock import patch
+
+import pytest
+
+from scanner.cli.appguardrail import _scan_file
+
+_RULE_ID = "required-substring-prefilter-contract"
+
+
+def _rule(required_substrings, pattern):
+    """Build one controlled scanner rule with the production prefilter field."""
+
+    return {
+        "id": _RULE_ID,
+        "pattern": pattern,
+        "severity": "HIGH",
+        "message": "controlled scanner finding [CWE-20 - Improper Input Validation]",
+        "extensions": [".py"],
+        "required_substrings": tuple(required_substrings),
+    }
+
+
+def _scan(tmp_path, content, required_substrings, pattern):
+    """Execute the real file-scanner boundary with one controlled rule."""
+
+    source_file = tmp_path / "source.py"
+    source_file.write_text(content, encoding="utf-8")
+    with patch(
+        "scanner.cli.appguardrail.SCAN_RULES",
+        [_rule(required_substrings, pattern)],
+    ):
+        return _scan_file(source_file, tmp_path)
+
+
+@pytest.mark.parametrize(
+    "required_substrings",
+    [
+        ("missing", "present_a", "present_b"),
+        ("present_a", "missing", "present_b"),
+        ("present_a", "present_b", "missing"),
+    ],
+)
+def test_required_substring_prefilter_skips_rule_when_any_literal_is_missing(
+    tmp_path, required_substrings
+):
+    """A missing first, middle, or last required literal must skip regex work."""
+
+    class ExplodingPattern:
+        def finditer(self, _content):
+            raise AssertionError("regex must not run after prefilter rejection")
+
+    findings = _scan(
+        tmp_path,
+        "present_a = 1\npresent_b = danger_call()\n",
+        required_substrings,
+        ExplodingPattern(),
+    )
+
+    assert findings == []
+
+
+def test_required_substring_prefilter_preserves_finding_when_all_literals_exist(tmp_path):
+    """All-present prefiltering must preserve the no-prefilter finding exactly."""
+
+    content = "present_a = 1\npresent_b = danger_call()\n"
+    pattern = re.compile(r"danger_call\(\)")
+
+    no_prefilter = _scan(tmp_path, content, (), pattern)
+    with_prefilter = _scan(
+        tmp_path,
+        content,
+        ("present_a", "present_b", "danger_call"),
+        pattern,
+    )
+
+    assert len(no_prefilter) == 1
+    assert with_prefilter == no_prefilter
+    assert with_prefilter[0]["rule_id"] == _RULE_ID