diff --git a/README.md b/README.md
index 9421235..e020237 100644
--- a/README.md
+++ b/README.md
@@ -199,6 +199,7 @@ example (`promptproof run examples/multiturn.yaml --mock`).
 | `max_chars`    | `max_chars: 500`                          | the response is at most N characters                     |
 | `min_chars`    | `min_chars: 50`                           | the response is at least N characters                    |
 | `equals`       | `equals: "yes"`                           | the response matches exactly (whitespace-trimmed)        |
+| `not_equals`   | `not_equals: "unknown"`                   | the response differs (whitespace-trimmed)                |
 | `starts_with`  | `starts_with: "Summary:"`                 | the response begins with the literal prefix              |
 | `ends_with`    | `ends_with: "done."`                      | the response ends with the literal suffix                |
 | `llm_judge`    | `llm_judge: "the response is polite"`     | a judge model decides the response meets the criterion  |
diff --git a/src/promptproof/assertions.py b/src/promptproof/assertions.py
index 4b7ceef..64473ea 100644
--- a/src/promptproof/assertions.py
+++ b/src/promptproof/assertions.py
@@ -12,8 +12,9 @@
       - json_schema: {type: object, required: [sentiment]}
       - llm_judge: "the response is polite and answers the question"
 
-``equals`` compares after stripping leading/trailing whitespace (models love
-trailing newlines); ``starts_with`` / ``ends_with`` match literally.
+``equals`` and ``not_equals`` compare after stripping leading/trailing
+whitespace (models love trailing newlines); ``starts_with`` / ``ends_with``
+match literally.
 
 ``not_regex`` is the mirror of ``regex``: it passes when the pattern does
 *not* match anywhere in the response — handy for banning PII shapes,
@@ -207,6 +208,14 @@ def _check_equals(response: str, expected: Any) -> AssertionResult:
     )
 
 
+def _check_not_equals(response: str, expected: Any) -> AssertionResult:
+    wanted = str(expected).strip()
+    ok = response.strip() != wanted
+    return AssertionResult(
+        "not_equals", ok, "" if ok else f"response equals forbidden {wanted!r}"
+    )
+
+
 def _check_starts_with(response: str, expected: Any) -> AssertionResult:
     prefix = str(expected)
     ok = response.startswith(prefix)
@@ -274,6 +283,7 @@ def _check_llm_judge(response: str, expected: Any, judge: LLMJudge) -> Assertion
     "max_chars": _check_max_chars,
     "min_chars": _check_min_chars,
     "equals": _check_equals,
+    "not_equals": _check_not_equals,
     "starts_with": _check_starts_with,
     "ends_with": _check_ends_with,
     "json_schema": _check_json_schema,
diff --git a/tests/test_assertions.py b/tests/test_assertions.py
index 252b949..f2a7a98 100644
--- a/tests/test_assertions.py
+++ b/tests/test_assertions.py
@@ -68,6 +68,39 @@ def test_equals():
     assert not only("yes please", {"equals": "yes"}).passed
 
 
+def test_not_equals():
+    result = only("yes please", {"not_equals": "yes"})
+    assert result.kind == "not_equals"
+    assert result.passed
+    assert result.detail == ""
+    result = only("yes", {"not_equals": "yes"})
+    assert not result.passed
+    assert "yes" in result.detail
+
+
+def test_not_equals_trims_both_sides():
+    assert not only(" \tyes\n", {"not_equals": " yes "}).passed
+    assert not only("\n", {"not_equals": ""}).passed
+    assert only("Yes", {"not_equals": "yes"}).passed
+
+
+def test_not_equals_explicit_form():
+    assert only("no", {"type": "not_equals", "value": "yes"}).passed
+    assert not only("42\n", {"type": "not_equals", "value": 42}).passed
+
+
+@pytest.mark.parametrize(
+    "assertion, message",
+    [
+        ({"type": "not_equals"}, "missing its 'value'"),
+        ({"not_equals": "yes", "contains": "no"}, "exactly one key"),
+    ],
+)
+def test_not_equals_malformed_config(assertion, message):
+    with pytest.raises(AssertionConfigError, match=message):
+        check_assertions("no", [assertion])
+
+
 def test_starts_with():
     assert only("Summary: great keyboard", {"starts_with": "Summary:"}).passed
     assert not only("In summary: great", {"starts_with": "Summary:"}).passed
diff --git a/tests/test_runner.py b/tests/test_runner.py
index b97daa3..f277fcc 100644
--- a/tests/test_runner.py
+++ b/tests/test_runner.py
@@ -61,6 +61,34 @@ def test_json_suite_files_are_supported(tmp_path):
     assert report.ok
 
 
+def test_not_equals_from_yaml_suite(tmp_path):
+    suite_file = tmp_path / "not-equals.yaml"
+    suite_file.write_text(
+        """
+name: not-equals
+prompt: "Answer the question"
+cases:
+  - name: different answer
+    mock_response: "yes please"
+    assertions:
+      - not_equals: "yes"
+  - name: same answer
+    mock_response: " yes "
+    assertions:
+      - type: not_equals
+        value: "yes"
+""",
+        encoding="utf-8",
+    )
+    report = run_suite(load_suite(suite_file), mock=True)
+    assert report.passed_count == 1
+    assert report.failed_count == 1
+    assert not report.ok
+    assert [case.assertions[0].kind for case in report.cases] == [
+        "not_equals", "not_equals"
+    ]
+
+
 def test_mock_rule_regex_matching(tmp_path):
     suite_file = tmp_path / "suite.yaml"
     suite_file.write_text(