diff --git a/src/tether/validate.py b/src/tether/validate.py
index 9862cda..49ce63f 100644
--- a/src/tether/validate.py
+++ b/src/tether/validate.py
@@ -20,6 +20,7 @@ class ValidationResult:
     num_elements: int
     threshold: float
     details: str
+    cosine_sim: float = float("nan")
 
     def to_dict(self) -> dict:
         return {
@@ -27,6 +28,7 @@ def to_dict(self) -> dict:
             "max_abs_diff": round(self.max_abs_diff, 6),
             "mean_abs_diff": round(self.mean_abs_diff, 6),
             "max_rel_diff": round(self.max_rel_diff, 6),
+            "cosine_sim": round(self.cosine_sim, 6),
             "num_elements": self.num_elements,
             "threshold": self.threshold,
             "details": self.details,
@@ -63,6 +65,17 @@ def validate_outputs(
     max_abs = float(abs_diff.max())
     mean_abs = float(abs_diff.mean())
 
+    reference_flat = reference.ravel().astype(np.float64, copy=False)
+    candidate_flat = candidate.ravel().astype(np.float64, copy=False)
+    reference_norm = float(np.linalg.norm(reference_flat))
+    candidate_norm = float(np.linalg.norm(candidate_flat))
+    if reference_norm == 0.0 or candidate_norm == 0.0:
+        cosine_sim = 0.0
+    else:
+        cosine_sim = float(
+            np.dot(reference_flat, candidate_flat) / (reference_norm * candidate_norm)
+        )
+
     denom = np.maximum(np.abs(reference), 1e-8)
     rel_diff = abs_diff / denom
     max_rel = float(rel_diff.max())
@@ -70,7 +83,7 @@ def validate_outputs(
     passed = max_abs < threshold
     details = (
         f"{name}: max_abs={max_abs:.6f}, mean_abs={mean_abs:.6f}, "
-        f"max_rel={max_rel:.4f}, threshold={threshold}"
+        f"max_rel={max_rel:.4f}, cosine_sim={cosine_sim:.6f}, threshold={threshold}"
     )
     if passed:
         logger.info("PASS %s", details)
@@ -85,6 +98,7 @@ def validate_outputs(
         num_elements=int(reference.size),
         threshold=threshold,
         details=details,
+        cosine_sim=cosine_sim,
     )
 
 
diff --git a/tests/test_validate.py b/tests/test_validate.py
index f0fb4f9..bae2484 100644
--- a/tests/test_validate.py
+++ b/tests/test_validate.py
@@ -13,6 +13,7 @@ def test_identical_outputs_pass(self):
         result = validate_outputs(ref, ref, threshold=0.01)
         assert result.passed
         assert result.max_abs_diff == 0.0
+        assert result.cosine_sim == pytest.approx(1.0)
 
     def test_small_diff_passes(self):
         ref = np.array([1.0, 2.0, 3.0])
@@ -27,14 +28,52 @@ def test_large_diff_fails(self):
         assert not result.passed
         assert result.max_abs_diff == pytest.approx(0.5)
 
+    def test_opposite_direction_reports_negative_cosine(self):
+        ref = np.array([1.0, 0.0])
+        candidate = np.array([-1.0, 0.0])
+        result = validate_outputs(ref, candidate, threshold=0.01)
+        assert not result.passed
+        assert result.cosine_sim == pytest.approx(-1.0)
+
+    def test_orthogonal_vectors_report_zero_cosine(self):
+        ref = np.array([1.0, 0.0])
+        candidate = np.array([0.0, 1.0])
+        result = validate_outputs(ref, candidate, threshold=2.0)
+        assert result.cosine_sim == pytest.approx(0.0)
+
+    def test_identical_tiny_outputs_report_unit_cosine(self):
+        ref = np.array([1e-6, 0.0])
+        result = validate_outputs(ref, ref, threshold=0.01)
+        assert result.cosine_sim == pytest.approx(1.0)
+
+    def test_zero_norm_outputs_report_zero_cosine(self):
+        ref = np.array([0.0, 0.0])
+        result = validate_outputs(ref, ref, threshold=0.01)
+        assert result.passed
+        assert result.cosine_sim == 0.0
+
     def test_shape_mismatch_fails(self):
         ref = np.array([1.0, 2.0])
         candidate = np.array([1.0, 2.0, 3.0])
         result = validate_outputs(ref, candidate)
         assert not result.passed
+        assert np.isnan(result.cosine_sim)
 
     def test_torch_tensor_input(self):
         ref = torch.tensor([1.0, 2.0, 3.0])
         candidate = torch.tensor([1.001, 2.001, 3.001])
         result = validate_outputs(ref, candidate, threshold=0.01)
         assert result.passed
+
+    def test_to_dict_includes_rounded_cosine(self):
+        result = ValidationResult(
+            passed=True,
+            max_abs_diff=0.0,
+            mean_abs_diff=0.0,
+            max_rel_diff=0.0,
+            num_elements=3,
+            threshold=0.01,
+            details="output",
+            cosine_sim=0.123456789,
+        )
+        assert result.to_dict()["cosine_sim"] == 0.123457