diff --git a/barretenberg/cpp/src/barretenberg/ecc/fields/field_conversion.hpp b/barretenberg/cpp/src/barretenberg/ecc/fields/field_conversion.hpp
index 9d389c986389..51eadd4242b5 100644
--- a/barretenberg/cpp/src/barretenberg/ecc/fields/field_conversion.hpp
+++ b/barretenberg/cpp/src/barretenberg/ecc/fields/field_conversion.hpp
@@ -250,7 +250,14 @@ class U256Codec {
         BB_ASSERT_EQ(vec.size(), calc_num_fields<T>());
         if constexpr (IsAnyOf<T, bool>) {
             return static_cast<bool>(vec[0]);
-        } else if constexpr (IsAnyOf<T, uint32_t, uint64_t, uint256_t, bb::fr, fq>) {
+        } else if constexpr (IsAnyOf<T, bb::fr>) {
+            BB_ASSERT_LT(
+                vec[0], uint256_t(bb::fr::modulus), "Non-canonical scalar field element: value >= fr::modulus");
+            return static_cast<T>(vec[0]);
+        } else if constexpr (IsAnyOf<T, fq>) {
+            BB_ASSERT_LT(vec[0], uint256_t(fq::modulus), "Non-canonical base field element: value >= fq::modulus");
+            return static_cast<T>(vec[0]);
+        } else if constexpr (IsAnyOf<T, uint32_t, uint64_t, uint256_t>) {
             return static_cast<T>(vec[0]);
         } else if constexpr (IsAnyOf<T, bn254_commitment, grumpkin_commitment>) {
             using BaseField = typename T::Fq;
diff --git a/barretenberg/cpp/src/barretenberg/ecc/fields/field_conversion.test.cpp b/barretenberg/cpp/src/barretenberg/ecc/fields/field_conversion.test.cpp
index a3ad4c20d4b8..c2ae83f1fced 100644
--- a/barretenberg/cpp/src/barretenberg/ecc/fields/field_conversion.test.cpp
+++ b/barretenberg/cpp/src/barretenberg/ecc/fields/field_conversion.test.cpp
@@ -271,4 +271,88 @@ TEST_F(FieldConversionTest, RejectPointNotOnCurve)
 }
 #endif
 
+// ============================================================================
+// U256Codec canonical checks
+// ============================================================================
+
+/**
+ * @brief Test that U256Codec accepts canonical field elements.
+ */
+TEST_F(FieldConversionTest, U256CodecAcceptsCanonicalFr)
+{
+    // Zero
+    {
+        std::vector<uint256_t> vec = { uint256_t(0) };
+        auto result = U256Codec::deserialize_from_fields<bb::fr>(vec);
+        EXPECT_EQ(result, bb::fr(0));
+    }
+    // One
+    {
+        std::vector<uint256_t> vec = { uint256_t(1) };
+        auto result = U256Codec::deserialize_from_fields<bb::fr>(vec);
+        EXPECT_EQ(result, bb::fr(1));
+    }
+    // modulus - 1 (largest canonical value)
+    {
+        std::vector<uint256_t> vec = { uint256_t(bb::fr::modulus) - 1 };
+        auto result = U256Codec::deserialize_from_fields<bb::fr>(vec);
+        EXPECT_EQ(result, bb::fr(bb::fr::modulus - 1));
+    }
+}
+
+/**
+ * @brief Test that U256Codec accepts canonical fq elements.
+ */
+TEST_F(FieldConversionTest, U256CodecAcceptsCanonicalFq)
+{
+    using fq = grumpkin::fr;
+    std::vector<uint256_t> vec = { uint256_t(0) };
+    auto result = U256Codec::deserialize_from_fields<fq>(vec);
+    EXPECT_EQ(result, fq(0));
+
+    vec = { uint256_t(fq::modulus) - 1 };
+    result = U256Codec::deserialize_from_fields<fq>(vec);
+    EXPECT_EQ(result, fq(fq::modulus - 1));
+}
+
+/**
+ * @brief Test that U256Codec rejects non-canonical field elements (>= modulus).
+ * @details Security-critical: prevents Fiat-Shamir challenge grinding in Keccak transcript.
+ */
+#ifndef __wasm__
+TEST_F(FieldConversionTest, U256CodecRejectsNonCanonicalFr)
+{
+    // fr::modulus itself (v + 0*p where v=0 but encoded as p)
+    {
+        std::vector<uint256_t> vec = { uint256_t(bb::fr::modulus) };
+        EXPECT_THROW_OR_ABORT(U256Codec::deserialize_from_fields<bb::fr>(vec), "Non-canonical");
+    }
+    // fr::modulus + 1
+    {
+        std::vector<uint256_t> vec = { uint256_t(bb::fr::modulus) + 1 };
+        EXPECT_THROW_OR_ABORT(U256Codec::deserialize_from_fields<bb::fr>(vec), "Non-canonical");
+    }
+    // 2 * fr::modulus (another alias for 0)
+    {
+        std::vector<uint256_t> vec = { uint256_t(bb::fr::modulus) * 2 };
+        EXPECT_THROW_OR_ABORT(U256Codec::deserialize_from_fields<bb::fr>(vec), "Non-canonical");
+    }
+}
+
+TEST_F(FieldConversionTest, U256CodecRejectsNonCanonicalFq)
+{
+    using fq = grumpkin::fr;
+    // fq::modulus itself
+    {
+        std::vector<uint256_t> vec = { uint256_t(fq::modulus) };
+        EXPECT_THROW_OR_ABORT(U256Codec::deserialize_from_fields<fq>(vec), "Non-canonical");
+    }
+    // fq::modulus + 1
+    {
+        std::vector<uint256_t> vec = { uint256_t(fq::modulus) + 1 };
+        EXPECT_THROW_OR_ABORT(U256Codec::deserialize_from_fields<fq>(vec), "Non-canonical");
+    }
+}
+#endif
+
 } // namespace bb::field_conversion_tests