diff --git a/tests/mem_os/test_format_utils.py b/tests/mem_os/test_format_utils.py
new file mode 100644
index 000000000..cd9666862
--- /dev/null
+++ b/tests/mem_os/test_format_utils.py
@@ -0,0 +1,75 @@
+"""
+Test suite for src/memos/mem_os/utils/format_utils.py
+
+Focus: clean_json_response function defensive behavior
+Related issue: #1525
+"""
+
+import pytest
+
+from memos.mem_os.utils.format_utils import clean_json_response
+
+
+class TestCleanJsonResponse:
+    """Test clean_json_response function with various inputs."""
+
+    def test_clean_json_response_with_none_raises_value_error(self):
+        """Test that passing None raises ValueError with diagnostic message."""
+        with pytest.raises(ValueError) as exc_info:
+            clean_json_response(None)
+
+        error_message = str(exc_info.value)
+        assert "clean_json_response received None" in error_message
+        assert "upstream LLM call" in error_message
+        assert "timed_with_status" in error_message or "generate()" in error_message
+
+    def test_clean_json_response_removes_json_code_block(self):
+        """Test removal of ```json markers."""
+        input_str = "```json\n{\"key\": \"value\"}\n```"
+        expected = "{\"key\": \"value\"}"
+        assert clean_json_response(input_str) == expected
+
+    def test_clean_json_response_removes_plain_code_block(self):
+        """Test removal of ``` markers without json keyword."""
+        input_str = "```\n{\"key\": \"value\"}\n```"
+        expected = "{\"key\": \"value\"}"
+        assert clean_json_response(input_str) == expected
+
+    def test_clean_json_response_strips_whitespace(self):
+        """Test that leading/trailing whitespace is stripped."""
+        input_str = "  \n  {\"key\": \"value\"}  \n  "
+        expected = "{\"key\": \"value\"}"
+        assert clean_json_response(input_str) == expected
+
+    def test_clean_json_response_handles_plain_json(self):
+        """Test that plain JSON without markdown is unchanged (except strip)."""
+        input_str = "{\"key\": \"value\"}"
+        expected = "{\"key\": \"value\"}"
+        assert clean_json_response(input_str) == expected
+
+    def test_clean_json_response_handles_empty_string(self):
+        """Test that empty string is handled correctly."""
+        assert clean_json_response("") == ""
+
+    def test_clean_json_response_with_complex_json(self):
+        """Test with realistic LLM response containing nested JSON."""
+        input_str = """```json
+{
+    "queries": [
+        {"query": "test", "weight": 1.0},
+        {"query": "example", "weight": 0.5}
+    ]
+}
+```"""
+        result = clean_json_response(input_str)
+        assert "```json" not in result
+        assert "```" not in result
+        assert '"queries"' in result
+        assert result.strip() == result  # No leading/trailing whitespace
+
+    def test_clean_json_response_preserves_internal_backticks(self):
+        """Test that backticks inside JSON content are preserved."""
+        input_str = '```json\n{"code": "`example`"}\n```'
+        result = clean_json_response(input_str)
+        assert '`example`' in result
+        assert result.count('`') == 2  # Only internal backticks remain