diff --git a/backend/main.py b/backend/main.py
index d6a64c9..c9e2ce0 100644
--- a/backend/main.py
+++ b/backend/main.py
@@ -154,15 +154,27 @@ def _body_detail(s: int) -> str:
 
 
 def _derive_grade(score: float) -> str:
+    # 1. Type validation checking: Reject anything that isn't an integer or float
+    if not isinstance(score, (int, float)) or isinstance(score, bool):
+        raise ValueError(
+            f"Invalid input type: {type(score)}. Score must be a numeric float/int."
+        )
+
+    # 2. Scale boundaries validation: Must reside strictly between 0.0 and 100.0 inclusive
+    if score < 0 or score > 100:
+        raise ValueError(f"Score {score} is out of valid scale range (0.0 - 100.0).")
+
+    # 3. Proceed safely with the evaluation hierarchy
     if score >= 92:
         return "A+"
-    if score >= 80:
+    elif score >= 80:
         return "A"
-    if score >= 65:
+    elif score >= 65:
         return "B"
-    if score >= 50:
+    elif score >= 50:
         return "C"
-    return "D"
+    else:
+        return "D"
 
 
 def _to_db_grade(grade: str) -> str:
diff --git a/backend/tests/test_ci.py b/backend/tests/test_ci.py
index 7cd902d..f8b05ac 100644
--- a/backend/tests/test_ci.py
+++ b/backend/tests/test_ci.py
@@ -11,6 +11,21 @@
 # Ensure the backend directory is on the path
 sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
 
+# ---------------------------------------------------------------------------
+# INTERCEPT MOCKS (Must run BEFORE importing any app modules)
+# ---------------------------------------------------------------------------
+from unittest.mock import MagicMock
+
+# Inject dummy modules into sys.modules to stop Python from loading heavy/broken packages
+mock_supabase = MagicMock()
+sys.modules['supabase'] = mock_supabase
+
+mock_inference = MagicMock()
+mock_inference.load_models = MagicMock()
+mock_inference.predict_stream_a = MagicMock()
+mock_inference.predict_stream_b = MagicMock()
+sys.modules['inference'] = mock_inference
+
 import numpy as np
 
 
@@ -100,3 +115,80 @@ def test_dev_bypass_constants_are_readable():
     assert hasattr(auth, "DEV_BYPASS_TOKEN")
     assert isinstance(auth.DEV_BYPASS_AUTH, bool)
     assert isinstance(auth.DEV_BYPASS_TOKEN, str)
+
+
+# ---------------------------------------------------------------------------
+# main.py — Grade Derivation Logic & Validation Checks
+# ---------------------------------------------------------------------------
+
+
+
+
+# Now we can safely import from main without needing the actual package installed!
+import pytest
+from main import _derive_grade
+
+# 1. Test Boundary Values and Valid Grade Ranges
+@pytest.mark.parametrize("score, expected_grade", [
+    (100, "A+"),
+    (92, "A+"),    # Exact boundary for A+
+    (91.9, "A"),   # Just below A+ boundary
+    (80, "A"),     # Exact boundary for A
+    (79.9, "B"),   # Just below A boundary
+    (65, "B"),     # Exact boundary for B
+    (64.9, "C"),   # Just below B boundary
+    (50, "C"),     # Exact boundary for C
+    (49.9, "D"),   # Just below C boundary
+    (0, "D"),      # Minimum standard boundary
+])
+def test_derive_grade_boundaries(score, expected_grade):
+    assert _derive_grade(score) == expected_grade
+
+
+# 2. Test Out-of-Scale Inputs (Expected to raise ValueError)
+@pytest.mark.parametrize("invalid_score", [
+    (-1),         # Negative scale boundary
+    (-50.5),      # Extreme negative
+    (101),        # Just over maximum scale
+    (200.0),      # Extreme over-scale
+])
+def test_derive_grade_out_of_scale(invalid_score):
+    with pytest.raises(ValueError):
+        _derive_grade(invalid_score)
+
+
+# 3. Test Incorrect Data Types (Expected to raise ValueError)
+@pytest.mark.parametrize("bad_type", [
+    ("95"),         # String containing numbers
+    ("fresh_fish"), # Regular text string
+    (None),         # None Type
+    ([],),          # List/Iterable object
+])
+def test_derive_grade_invalid_types(bad_type):
+    with pytest.raises(ValueError):
+        _derive_grade(bad_type)
+
+
+
+# ---------------------------------------------------------------------------
+# main.py — Grade Derivation Logic & Validation Checks
+# ---------------------------------------------------------------------------
+
+
+from unittest.mock import MagicMock
+
+# 1. Trick Python into thinking 'supabase' is already loaded
+mock_supabase = MagicMock()
+sys.modules['supabase'] = mock_supabase
+
+# 2. Trick Python into bypassing 'inference' entirely to avoid the broken PyTorch DLLs
+mock_inference = MagicMock()
+# Provide dummy mock functions so main.py can unpack them during import
+mock_inference.load_models = MagicMock()
+mock_inference.predict_stream_a = MagicMock()
+mock_inference.predict_stream_b = MagicMock()
+sys.modules['inference'] = mock_inference
+
+# Now we can safely import from main in total isolation!
+import pytest
+