diff --git a/CHANGELOG.md b/CHANGELOG.md
index 9b18e615c7..13ce934e72 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,12 @@ More expansive patch notes and explanations may be found in the specific [pathfi
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## Unreleased
+
+### Changed
+
+- BREAKING: JSON-RPC input no longer accepts hex strings that are missing the `0x` prefix, in line with the Starknet API spec.
+
 ## [0.22.2] - 2026-04-07
 
 ### Changed
diff --git a/crates/class-hash/src/lib.rs b/crates/class-hash/src/lib.rs
index 2400fe0097..d9818f8848 100644
--- a/crates/class-hash/src/lib.rs
+++ b/crates/class-hash/src/lib.rs
@@ -846,7 +846,7 @@ pub mod json {
             // fields (which we now need to ignore if empty).
 
             let expected = ComputedClassHash::Cairo(class_hash!(
-                "056b96c1d1bbfa01af44b465763d1b71150fa00c6c9d54c3947f57e979ff68c3"
+                "0x056b96c1d1bbfa01af44b465763d1b71150fa00c6c9d54c3947f57e979ff68c3"
             ));
 
             // Known contract which triggered a hash mismatch failure.
diff --git a/crates/common/src/consts.rs b/crates/common/src/consts.rs
index 31d44e3757..5fb9c9df81 100644
--- a/crates/common/src/consts.rs
+++ b/crates/common/src/consts.rs
@@ -4,10 +4,10 @@ use crate::macro_prelude::block_hash;
 use crate::BlockHash;
 
 pub const MAINNET_GENESIS_HASH: BlockHash =
-    block_hash!("047C3637B57C2B079B93C61539950C17E868A28F46CDEF28F88521067F21E943");
+    block_hash!("0x047C3637B57C2B079B93C61539950C17E868A28F46CDEF28F88521067F21E943");
 
 pub const SEPOLIA_TESTNET_GENESIS_HASH: BlockHash =
-    block_hash!("5c627d4aeb51280058bed93c7889bce78114d63baad1be0f0aeb32496d5f19c");
+    block_hash!("0x5c627d4aeb51280058bed93c7889bce78114d63baad1be0f0aeb32496d5f19c");
 
 pub const SEPOLIA_INTEGRATION_GENESIS_HASH: BlockHash =
-    block_hash!("19f675d3fb226821493a6ab9a1955e384bba80f130de625621a418e9a7c0ca3");
+    block_hash!("0x19f675d3fb226821493a6ab9a1955e384bba80f130de625621a418e9a7c0ca3");
diff --git a/crates/common/src/event.rs b/crates/common/src/event.rs
index 235c04332d..0eca48ab47 100644
--- a/crates/common/src/event.rs
+++ b/crates/common/src/event.rs
@@ -46,13 +46,13 @@ fn starkhash_to_dec_str(h: &Felt) -> String {
 
 /// A helper conversion function. Only use with __sequencer API related types__.
 fn starkhash_from_dec_str(s: &str) -> Result<Felt, anyhow::Error> {
-    match BigUint::from_str(s) {
-        Ok(b) => {
-            let h = Felt::from_be_slice(&b.to_bytes_be())?;
-            Ok(h)
-        }
+    // The order here matters because `Felt::from_hex_str` requires the '0x'
+    // prefix, so we'll never parse a hex string as a decimal string by mistake.
+    match Felt::from_hex_str(s) {
+        Ok(h) => Ok(h),
         Err(_) => {
-            let h = Felt::from_hex_str(s)?;
+            let b = BigUint::from_str(s)?;
+            let h = Felt::from_be_slice(&b.to_bytes_be())?;
             Ok(h)
         }
     }
diff --git a/crates/common/src/macros.rs b/crates/common/src/macros.rs
index cede112a7b..3d257374d9 100644
--- a/crates/common/src/macros.rs
+++ b/crates/common/src/macros.rs
@@ -304,6 +304,9 @@ macro_rules! felt {
                 Err(pathfinder_crypto::HexParseError::InvalidLength { .. }) => {
                     panic!("Too many hex digits")
                 }
+                Err(pathfinder_crypto::HexParseError::MissingPrefix) => {
+                    panic!("Missing '0x' prefix")
+                }
                 Err(pathfinder_crypto::HexParseError::Overflow) => panic!("Felt overflow"),
             };
         CONST_FELT
diff --git a/crates/crypto/src/algebra/field/felt.rs b/crates/crypto/src/algebra/field/felt.rs
index 2b01e12832..4d6295eabd 100644
--- a/crates/crypto/src/algebra/field/felt.rs
+++ b/crates/crypto/src/algebra/field/felt.rs
@@ -323,8 +323,8 @@ impl From<CurveOrderMontFelt> for Felt {
 impl Felt {
     /// A convenience function which parses a hex string into a [Felt].
     ///
-    /// Supports both upper and lower case hex strings, as well as an optional
-    /// "0x" prefix.
+    /// Supports both upper and lower case hex strings. The '0x' prefix is
+    /// mandatory.
     pub const fn from_hex_str(hex_str: &str) -> Result<Self, HexParseError> {
         const fn parse_hex_digit(digit: u8) -> Result<u8, HexParseError> {
             match digit {
@@ -336,19 +336,16 @@ impl Felt {
         }
 
         let bytes = hex_str.as_bytes();
-        let start = if bytes.len() >= 2 && bytes[0] == b'0' && bytes[1] == b'x' {
-            2
-        } else {
-            0
+        let len = match bytes {
+            [b'0', b'x', rest @ ..] if rest.len() > 64 => {
+                return Err(HexParseError::InvalidLength {
+                    max: 64,
+                    actual: rest.len(),
+                })
+            }
+            [b'0', b'x', rest @ ..] => rest.len(),
+            _ => return Err(HexParseError::MissingPrefix),
         };
-        let len = bytes.len() - start;
-
-        if len > 64 {
-            return Err(HexParseError::InvalidLength {
-                max: 64,
-                actual: bytes.len(),
-            });
-        }
 
         let mut buf = [0u8; 32];
 
@@ -358,7 +355,7 @@ impl Felt {
         // Handle a possible odd nibble remaining nibble.
         if len % 2 == 1 {
             let idx = len / 2;
-            buf[31 - idx] = match parse_hex_digit(bytes[start]) {
+            buf[31 - idx] = match parse_hex_digit(bytes[2]) {
                 Ok(b) => b,
                 Err(e) => return Err(e),
             };
@@ -461,6 +458,7 @@ impl Felt {
 pub enum HexParseError {
     InvalidNibble(u8),
     InvalidLength { max: usize, actual: usize },
+    MissingPrefix,
     Overflow,
 }
 
@@ -479,6 +477,7 @@ impl std::fmt::Display for HexParseError {
             Self::InvalidLength { max, actual } => {
                 f.write_fmt(format_args!("More than {} digits found: {}", *max, *actual))
             }
+            Self::MissingPrefix => f.write_str("Missing '0x' prefix"),
             Self::Overflow => f.write_str(OVERFLOW_MSG),
         }
     }
@@ -498,7 +497,7 @@ mod tests {
 
     #[test]
     fn view_bits() {
-        let one = Felt::from_hex_str("1").unwrap();
+        let one = Felt::from_hex_str("0x1").unwrap();
 
         let one = one.view_bits().to_bitvec();
 
@@ -566,7 +565,7 @@ mod tests {
 
         #[test]
         fn round_trip() {
-            let original = Felt::from_hex_str("abcdef0123456789").unwrap();
+            let original = Felt::from_hex_str("0xabcdef0123456789").unwrap();
             let bytes = original.to_be_bytes();
             let result = Felt::from_be_slice(&bytes[..]).unwrap();
 
@@ -575,7 +574,7 @@ mod tests {
 
         #[test]
         fn too_long() {
-            let original = Felt::from_hex_str("abcdef0123456789").unwrap();
+            let original = Felt::from_hex_str("0xabcdef0123456789").unwrap();
             let mut bytes = original.to_be_bytes().to_vec();
             bytes.push(0);
             Felt::from_be_slice(&bytes[..]).unwrap_err();
@@ -583,7 +582,7 @@ mod tests {
 
         #[test]
         fn short_slice() {
-            let original = Felt::from_hex_str("abcdef0123456789").unwrap();
+            let original = Felt::from_hex_str("0xabcdef0123456789").unwrap();
             let bytes = original.to_be_bytes();
             let result = Felt::from_be_slice(&bytes[24..]);
 
@@ -610,25 +609,25 @@ mod tests {
 
         #[test]
         fn debug() {
-            let hex_str = "1234567890abcdef000edcba0987654321";
+            let hex_str = "0x1234567890abcdef000edcba0987654321";
             let felt = Felt::from_hex_str(hex_str).unwrap();
             let result = format!("{felt:?}");
 
-            let mut expected = "0".repeat(64 - hex_str.len());
-            expected.push_str(hex_str);
-            let expected = format!("Felt({felt})");
+            let leading_zeros = "0".repeat(64 + 2 - hex_str.len());
+            let hex_upper_stripped = hex_str.strip_prefix("0x").unwrap().to_uppercase();
+            let expected = format!("Felt(0x{leading_zeros}{hex_upper_stripped})");
 
             assert_eq!(result, expected);
         }
 
         #[test]
         fn fmt() {
-            let hex_str = "1234567890abcdef000edcba0987654321";
+            let hex_str = "0x1234567890abcdef000edcba0987654321";
             let starkhash = Felt::from_hex_str(hex_str).unwrap();
             let result = format!("{starkhash:x}");
 
-            let mut expected = "0".repeat(64 - hex_str.len());
-            expected.push_str(hex_str);
+            let mut expected = "0".repeat(64 - hex_str.len() + 2);
+            expected.push_str(&hex_str[2..]);
 
             // We don't really care which casing is used by fmt.
             assert_eq!(result.to_lowercase(), expected.to_lowercase());
@@ -636,24 +635,24 @@ mod tests {
 
         #[test]
         fn lower_hex() {
-            let hex_str = "1234567890abcdef000edcba0987654321";
+            let hex_str = "0x1234567890abcdef000edcba0987654321";
             let starkhash = Felt::from_hex_str(hex_str).unwrap();
             let result = format!("{starkhash:x}");
 
-            let mut expected = "0".repeat(64 - hex_str.len());
-            expected.push_str(hex_str);
+            let mut expected = "0".repeat(64 - hex_str.len() + 2);
+            expected.push_str(&hex_str[2..]);
 
             assert_eq!(result, expected.to_lowercase());
         }
 
         #[test]
         fn upper_hex() {
-            let hex_str = "1234567890abcdef000edcba0987654321";
+            let hex_str = "0x1234567890abcdef000edcba0987654321";
             let starkhash = Felt::from_hex_str(hex_str).unwrap();
             let result = format!("{starkhash:X}");
 
-            let mut expected = "0".repeat(64 - hex_str.len());
-            expected.push_str(hex_str);
+            let mut expected = "0".repeat(64 - hex_str.len() + 2);
+            expected.push_str(&hex_str[2..]);
 
             assert_eq!(result, expected.to_uppercase());
         }
@@ -686,13 +685,6 @@ mod tests {
 
         #[test]
         fn simple() {
-            let (test_str, expected) = test_data();
-            let uut = Felt::from_hex_str(test_str).unwrap();
-            assert_eq!(uut, expected);
-        }
-
-        #[test]
-        fn prefix() {
             let (test_str, expected) = test_data();
             let uut = Felt::from_hex_str(&format!("0x{test_str}")).unwrap();
             assert_eq!(uut, expected);
@@ -700,13 +692,6 @@ mod tests {
 
         #[test]
         fn leading_zeros() {
-            let (test_str, expected) = test_data();
-            let uut = Felt::from_hex_str(&format!("000000000{test_str}")).unwrap();
-            assert_eq!(uut, expected);
-        }
-
-        #[test]
-        fn prefix_and_leading_zeros() {
             let (test_str, expected) = test_data();
             let uut = Felt::from_hex_str(&format!("0x000000000{test_str}")).unwrap();
             assert_eq!(uut, expected);
@@ -717,9 +702,24 @@ mod tests {
             assert_matches!(Felt::from_hex_str("0x123z").unwrap_err(), HexParseError::InvalidNibble(n) => assert_eq!(n, b'z'))
         }
 
+        #[test]
+        fn missing_prefix() {
+            assert_matches!(
+                Felt::from_hex_str("123").unwrap_err(),
+                HexParseError::MissingPrefix
+            )
+        }
+
         #[test]
         fn invalid_len() {
-            assert_matches!(Felt::from_hex_str(&"1".repeat(65)).unwrap_err(), HexParseError::InvalidLength{max: 64, actual: n} => assert_eq!(n, 65))
+            let invalid_str = "0".repeat(65);
+            assert_matches!(
+                Felt::from_hex_str(&format!("0x{invalid_str}")).unwrap_err(),
+                HexParseError::InvalidLength {
+                    max: 64,
+                    actual: 65
+                }
+            );
         }
 
         #[test]
diff --git a/crates/crypto/src/algebra/field/serde.rs b/crates/crypto/src/algebra/field/serde.rs
index c333aa3b52..b1889633c6 100644
--- a/crates/crypto/src/algebra/field/serde.rs
+++ b/crates/crypto/src/algebra/field/serde.rs
@@ -26,7 +26,7 @@ impl<'de> Deserialize<'de> for Felt {
             type Value = Felt;
 
             fn expecting(&self, formatter: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-                formatter.write_str("a hex string of up to 64 digits with an optional '0x' prefix")
+                formatter.write_str("a hex string of up to 64 digits with a mandatory '0x' prefix")
             }
 
             fn visit_str<E>(self, v: &str) -> Result<Self::Value, E>
@@ -53,7 +53,11 @@ mod tests {
 
     #[test]
     fn empty() {
-        assert_eq!(serde_json::from_str::<Felt>(r#""""#).unwrap(), Felt::ZERO);
+        assert!(serde_json::from_str::<Felt>(r#""""#)
+            .unwrap_err()
+            .to_string()
+            // Serde errors also include the line/column.
+            .starts_with("Missing '0x' prefix"));
         assert_eq!(serde_json::from_str::<Felt>(r#""0x""#).unwrap(), Felt::ZERO);
     }
 
diff --git a/crates/crypto/src/hash/pedersen/hash.rs b/crates/crypto/src/hash/pedersen/hash.rs
index 13348da54f..9eea97b065 100644
--- a/crates/crypto/src/hash/pedersen/hash.rs
+++ b/crates/crypto/src/hash/pedersen/hash.rs
@@ -52,15 +52,18 @@ mod tests {
     #[test]
     fn test_pedersen() {
         // Test vector from https://github.com/starkware-libs/crypto-cpp/blob/master/src/starkware/crypto/pedersen_hash_test.cc
-        let a =
-            Felt::from_hex_str("03d937c035c878245caf64531a5756109c53068da139362728feb561405371cb")
-                .unwrap();
-        let b =
-            Felt::from_hex_str("0208a0a10250e382e1e4bbe2880906c2791bf6275695e02fbbc6aeff9cd8b31a")
-                .unwrap();
-        let expected =
-            Felt::from_hex_str("030e480bed5fe53fa909cc0f8c4d99b8f9f2c016be4c41e13a4848797979c662")
-                .unwrap();
+        let a = Felt::from_hex_str(
+            "0x03d937c035c878245caf64531a5756109c53068da139362728feb561405371cb",
+        )
+        .unwrap();
+        let b = Felt::from_hex_str(
+            "0x0208a0a10250e382e1e4bbe2880906c2791bf6275695e02fbbc6aeff9cd8b31a",
+        )
+        .unwrap();
+        let expected = Felt::from_hex_str(
+            "0x030e480bed5fe53fa909cc0f8c4d99b8f9f2c016be4c41e13a4848797979c662",
+        )
+        .unwrap();
 
         let hash = pedersen_hash(a, b);
         assert_eq!(hash, expected);
diff --git a/crates/crypto/src/hash/poseidon/hash.rs b/crates/crypto/src/hash/poseidon/hash.rs
index 20ead3bedc..af13fc6ab0 100644
--- a/crates/crypto/src/hash/poseidon/hash.rs
+++ b/crates/crypto/src/hash/poseidon/hash.rs
@@ -170,8 +170,10 @@ mod tests {
     #[test]
     fn test_sponge() {
         let expected_result = MontFelt::from(
-            Felt::from_hex_str("07b8f30ac298ea12d170c0873f1fa631a18c00756c6e7d1fd273b9a239d0d413")
-                .unwrap(),
+            Felt::from_hex_str(
+                "0x07b8f30ac298ea12d170c0873f1fa631a18c00756c6e7d1fd273b9a239d0d413",
+            )
+            .unwrap(),
         );
 
         // Construct messages, the first few integers
diff --git a/crates/crypto/src/signature/ecdsa.rs b/crates/crypto/src/signature/ecdsa.rs
index ae67876149..6ddfa25776 100644
--- a/crates/crypto/src/signature/ecdsa.rs
+++ b/crates/crypto/src/signature/ecdsa.rs
@@ -215,10 +215,10 @@ mod tests {
 
     #[test]
     fn test_get_pk() {
-        let sk = felt_hex("03c1e9550e66958296d11b60f8e8e7a7ad990d07fa65d5f7652c4a6c87d4e3cc");
+        let sk = felt_hex("0x03c1e9550e66958296d11b60f8e8e7a7ad990d07fa65d5f7652c4a6c87d4e3cc");
 
-        let pk_x = felm_hex("77a3b314db07c45076d11f62b6f9e748a39790441823307743cf00d6597ea43");
-        let pk_y = felm_hex("54d7beec5ec728223671c627557efc5c9a6508425dc6c900b7741bf60afec06");
+        let pk_x = felm_hex("0x77a3b314db07c45076d11f62b6f9e748a39790441823307743cf00d6597ea43");
+        let pk_y = felm_hex("0x54d7beec5ec728223671c627557efc5c9a6508425dc6c900b7741bf60afec06");
 
         let pk = get_pk(sk).expect("valid sk");
         let pk_affine = AffinePoint::from_x(pk.into()).unwrap();
@@ -229,17 +229,17 @@ mod tests {
 
     #[test]
     fn from_x() {
-        let pk_x = felm_hex("77a3b314db07c45076d11f62b6f9e748a39790441823307743cf00d6597ea43");
-        let pk_y = felm_hex("54d7beec5ec728223671c627557efc5c9a6508425dc6c900b7741bf60afec06");
+        let pk_x = felm_hex("0x77a3b314db07c45076d11f62b6f9e748a39790441823307743cf00d6597ea43");
+        let pk_y = felm_hex("0x54d7beec5ec728223671c627557efc5c9a6508425dc6c900b7741bf60afec06");
         let pk_affine = AffinePoint::from_x(pk_x).unwrap();
         assert_eq!(pk_affine.y, pk_y);
     }
 
     #[test]
     fn get_partial_pk() {
-        let pk_x = felt_hex("77a3b314db07c45076d11f62b6f9e748a39790441823307743cf00d6597ea43");
+        let pk_x = felt_hex("0x77a3b314db07c45076d11f62b6f9e748a39790441823307743cf00d6597ea43");
 
-        let sk = felt_hex("03c1e9550e66958296d11b60f8e8e7a7ad990d07fa65d5f7652c4a6c87d4e3cc");
+        let sk = felt_hex("0x03c1e9550e66958296d11b60f8e8e7a7ad990d07fa65d5f7652c4a6c87d4e3cc");
         let pk = get_pk(sk).unwrap();
 
         assert_eq!(pk, pk_x);
@@ -247,8 +247,8 @@ mod tests {
 
     #[test]
     fn verify_partial() {
-        let sk = felt_hex("03c1e9550e66958296d11b60f8e8e7a7ad990d07fa65d5f7652c4a6c87d4e3cc");
-        let msg = felt_hex("397e76d1667c4454bfb83514e120583af836f8e32a516765497823eabe16a3f");
+        let sk = felt_hex("0x03c1e9550e66958296d11b60f8e8e7a7ad990d07fa65d5f7652c4a6c87d4e3cc");
+        let msg = felt_hex("0x397e76d1667c4454bfb83514e120583af836f8e32a516765497823eabe16a3f");
         let sig = ecdsa_sign(sk, msg).expect("can sign");
         let pk = get_pk(sk).expect("can get pk");
         assert!(ecdsa_verify_partial(pk, msg, sig.0, sig.1).is_ok());
@@ -257,12 +257,12 @@ mod tests {
     #[test]
     fn verify_inner() {
         // Test vector from https://github.com/starkware-libs/crypto-cpp/blob/master/src/starkware/crypto/ecdsa_test.cc
-        let pk_x = felm_hex("77a3b314db07c45076d11f62b6f9e748a39790441823307743cf00d6597ea43");
+        let pk_x = felm_hex("0x77a3b314db07c45076d11f62b6f9e748a39790441823307743cf00d6597ea43");
         let pk = ProjectivePoint::from_x(pk_x).unwrap();
 
-        let msg = felt_hex("397e76d1667c4454bfb83514e120583af836f8e32a516765497823eabe16a3f");
-        let r = felt_hex("173fd03d8b008ee7432977ac27d1e9d1a1f6c98b1a2f05fa84a21c84c44e882");
-        let w = felt_hex("1f2c44a7798f55192f153b4c48ea5c1241fbb69e6132cc8a0da9c5b62a4286e");
+        let msg = felt_hex("0x397e76d1667c4454bfb83514e120583af836f8e32a516765497823eabe16a3f");
+        let r = felt_hex("0x173fd03d8b008ee7432977ac27d1e9d1a1f6c98b1a2f05fa84a21c84c44e882");
+        let w = felt_hex("0x1f2c44a7798f55192f153b4c48ea5c1241fbb69e6132cc8a0da9c5b62a4286e");
 
         // Compute s=1/w
         let w = CurveOrderMontFelt::try_from(MontFelt::from(w)).unwrap();
@@ -274,31 +274,31 @@ mod tests {
     #[test]
     fn verify_bad() {
         // Changed last byte of pk from 3 to 4
-        let pk = felt_hex("77a3b314db07c45076d11f62b6f9e748a39790441823307743cf00d6597ea44");
-        let msg = felt_hex("397e76d1667c4454bfb83514e120583af836f8e32a516765497823eabe16a3f");
-        let r = felt_hex("173fd03d8b008ee7432977ac27d1e9d1a1f6c98b1a2f05fa84a21c84c44e882");
-        let s = felt_hex("1f2c44a7798f55192f153b4c48ea5c1241fbb69e6132cc8a0da9c5b62a4286e");
+        let pk = felt_hex("0x77a3b314db07c45076d11f62b6f9e748a39790441823307743cf00d6597ea44");
+        let msg = felt_hex("0x397e76d1667c4454bfb83514e120583af836f8e32a516765497823eabe16a3f");
+        let r = felt_hex("0x173fd03d8b008ee7432977ac27d1e9d1a1f6c98b1a2f05fa84a21c84c44e882");
+        let s = felt_hex("0x1f2c44a7798f55192f153b4c48ea5c1241fbb69e6132cc8a0da9c5b62a4286e");
         assert!(ecdsa_verify_partial(pk, msg, r, s).is_err());
 
         // Changed last byte of msg from f to 0
-        let pk = felt_hex("77a3b314db07c45076d11f62b6f9e748a39790441823307743cf00d6597ea43");
-        let msg = felt_hex("397e76d1667c4454bfb83514e120583af836f8e32a516765497823eabe16a30");
-        let r = felt_hex("173fd03d8b008ee7432977ac27d1e9d1a1f6c98b1a2f05fa84a21c84c44e882");
-        let s = felt_hex("1f2c44a7798f55192f153b4c48ea5c1241fbb69e6132cc8a0da9c5b62a4286e");
+        let pk = felt_hex("0x77a3b314db07c45076d11f62b6f9e748a39790441823307743cf00d6597ea43");
+        let msg = felt_hex("0x397e76d1667c4454bfb83514e120583af836f8e32a516765497823eabe16a30");
+        let r = felt_hex("0x173fd03d8b008ee7432977ac27d1e9d1a1f6c98b1a2f05fa84a21c84c44e882");
+        let s = felt_hex("0x1f2c44a7798f55192f153b4c48ea5c1241fbb69e6132cc8a0da9c5b62a4286e");
         assert!(ecdsa_verify_partial(pk, msg, r, s).is_err());
 
         // Changed last byte of r from 2 to 3
-        let pk = felt_hex("77a3b314db07c45076d11f62b6f9e748a39790441823307743cf00d6597ea43");
-        let msg = felt_hex("397e76d1667c4454bfb83514e120583af836f8e32a516765497823eabe16a3f");
-        let r = felt_hex("173fd03d8b008ee7432977ac27d1e9d1a1f6c98b1a2f05fa84a21c84c44e883");
-        let s = felt_hex("1f2c44a7798f55192f153b4c48ea5c1241fbb69e6132cc8a0da9c5b62a4286e");
+        let pk = felt_hex("0x77a3b314db07c45076d11f62b6f9e748a39790441823307743cf00d6597ea43");
+        let msg = felt_hex("0x397e76d1667c4454bfb83514e120583af836f8e32a516765497823eabe16a3f");
+        let r = felt_hex("0x173fd03d8b008ee7432977ac27d1e9d1a1f6c98b1a2f05fa84a21c84c44e883");
+        let s = felt_hex("0x1f2c44a7798f55192f153b4c48ea5c1241fbb69e6132cc8a0da9c5b62a4286e");
         assert!(ecdsa_verify_partial(pk, msg, r, s).is_err());
 
         // Changed last byte of s from e to f
-        let pk = felt_hex("77a3b314db07c45076d11f62b6f9e748a39790441823307743cf00d6597ea43");
-        let msg = felt_hex("397e76d1667c4454bfb83514e120583af836f8e32a516765497823eabe16a3f");
-        let r = felt_hex("173fd03d8b008ee7432977ac27d1e9d1a1f6c98b1a2f05fa84a21c84c44e882");
-        let s = felt_hex("1f2c44a7798f55192f153b4c48ea5c1241fbb69e6132cc8a0da9c5b62a4286f");
+        let pk = felt_hex("0x77a3b314db07c45076d11f62b6f9e748a39790441823307743cf00d6597ea43");
+        let msg = felt_hex("0x397e76d1667c4454bfb83514e120583af836f8e32a516765497823eabe16a3f");
+        let r = felt_hex("0x173fd03d8b008ee7432977ac27d1e9d1a1f6c98b1a2f05fa84a21c84c44e882");
+        let s = felt_hex("0x1f2c44a7798f55192f153b4c48ea5c1241fbb69e6132cc8a0da9c5b62a4286f");
         assert!(ecdsa_verify_partial(pk, msg, r, s).is_err());
     }
 
@@ -322,10 +322,10 @@ mod tests {
     #[test]
     fn openzeppelin_signature() {
         // From https://testnet.starkscan.co/tx/0x0630a81628900577eea4b4a677767e57a56dabd730c2f64772ecbbde22ff485a
-        let pk = felt_hex("792c60ec4fdfea7ce6409db046b8dde11f595911cb74906be02a87ae6a4f70d");
-        let msg = felt_hex("0630a81628900577eea4b4a677767e57a56dabd730c2f64772ecbbde22ff485a");
-        let r = felt_hex("176846ea9b114f4f27f0d4d3cefacb5f830513fbd2d2f69a1a4d7552ae040be");
-        let s = felt_hex("601a87d6bc3e3a6513bafa1d449ffb3a0dd6cbe72f927a8e7271af0e8dd1302");
+        let pk = felt_hex("0x792c60ec4fdfea7ce6409db046b8dde11f595911cb74906be02a87ae6a4f70d");
+        let msg = felt_hex("0x0630a81628900577eea4b4a677767e57a56dabd730c2f64772ecbbde22ff485a");
+        let r = felt_hex("0x176846ea9b114f4f27f0d4d3cefacb5f830513fbd2d2f69a1a4d7552ae040be");
+        let s = felt_hex("0x601a87d6bc3e3a6513bafa1d449ffb3a0dd6cbe72f927a8e7271af0e8dd1302");
         assert!(ecdsa_verify_partial(pk, msg, r, s).is_ok());
     }
 }
diff --git a/crates/gateway-client/src/lib.rs b/crates/gateway-client/src/lib.rs
index 440ae5fb7f..f9d5242aa6 100644
--- a/crates/gateway-client/src/lib.rs
+++ b/crates/gateway-client/src/lib.rs
@@ -866,7 +866,7 @@ mod tests {
             ) {
                 (
                     TransactionVersion::ONE,
-                    fee!("4F388496839"),
+                    fee!("0x4F388496839"),
                     vec![
                         transaction_signature_elem!(
                             "0x07dd3a55d94a0de6f3d6c104d7e6c88ec719a82f4e2bbc12587c8c187584d3d5"
@@ -882,10 +882,10 @@ mod tests {
                     vec![
                         call_param!("0x1"),
                         call_param!(
-                            "0677bb1cdc050e8d63855e8743ab6e09179138def390676cc03c484daf112ba1"
+                            "0x0677bb1cdc050e8d63855e8743ab6e09179138def390676cc03c484daf112ba1"
                         ),
                         call_param!(
-                            "0362398bec32bc0ebb411203221a35a0301193a96f317ebe5e40be9f60d15320"
+                            "0x0362398bec32bc0ebb411203221a35a0301193a96f317ebe5e40be9f60d15320"
                         ),
                         CallParam(Felt::ZERO),
                         call_param!("0x1"),
@@ -1114,11 +1114,11 @@ mod tests {
                         vec![
                             SelectorAndOffset {
                                 offset: byte_code_offset!("0x3a"),
-                                selector: entry_point!("0362398bec32bc0ebb411203221a35a0301193a96f317ebe5e40be9f60d15320"),
+                                selector: entry_point!("0x0362398bec32bc0ebb411203221a35a0301193a96f317ebe5e40be9f60d15320"),
                             },
                             SelectorAndOffset {
                                 offset: byte_code_offset!("0x5b"),
-                                selector: entry_point!("039e11d48192e4333233c7eb19d10ad67c362bb28580c604d67884c85da39695"),
+                                selector: entry_point!("0x039e11d48192e4333233c7eb19d10ad67c362bb28580c604d67884c85da39695"),
                             },
                         ],
                     ),
diff --git a/crates/gateway-test-fixtures/src/lib.rs b/crates/gateway-test-fixtures/src/lib.rs
index 70b5f132b1..38d36f3b67 100644
--- a/crates/gateway-test-fixtures/src/lib.rs
+++ b/crates/gateway-test-fixtures/src/lib.rs
@@ -125,7 +125,7 @@ pub mod class_definitions {
 
     pub const CONTRACT_DEFINITION: &[u8] = bytes_fixture!("contracts/contract_definition.json");
     pub const CONTRACT_DEFINITION_CLASS_HASH: ClassHash =
-        class_hash!("050b2148c0d782914e0b12a1a32abe5e398930b7e914f82c65cb7afce0a0ab9b");
+        class_hash!("0x050b2148c0d782914e0b12a1a32abe5e398930b7e914f82c65cb7afce0a0ab9b");
     pub const DUMMY_ACCOUNT: &[u8] = bytes_fixture!("contracts/dummy_account.json");
     pub const DUMMY_ACCOUNT_CLASS_HASH: ClassHash =
         class_hash!("0x0791563da22895f1e398b689866718346106c0cc71207a4ada68e6687ce1badf");
@@ -209,26 +209,26 @@ pub mod testnet {
     use pathfinder_crypto::Felt;
 
     pub const VALID_TX_HASH: TransactionHash =
-        transaction_hash!("0493d8fab73af67e972788e603aee18130facd3c7685f16084ecd98b07153e24");
+        transaction_hash!("0x0493d8fab73af67e972788e603aee18130facd3c7685f16084ecd98b07153e24");
     pub const INVALID_TX_HASH: TransactionHash =
-        transaction_hash!("0393d8fab73af67e972788e603aee18130facd3c7685f16084ecd98b07153e24");
+        transaction_hash!("0x0393d8fab73af67e972788e603aee18130facd3c7685f16084ecd98b07153e24");
     pub const VALID_CONTRACT_ADDR: ContractAddress =
-        contract_address!("06fbd460228d843b7fbef670ff15607bf72e19fa94de21e29811ada167b4ca39");
+        contract_address!("0x06fbd460228d843b7fbef670ff15607bf72e19fa94de21e29811ada167b4ca39");
     pub const INVALID_CONTRACT_ADDR: ContractAddress =
-        contract_address!("05fbd460228d843b7fbef670ff15607bf72e19fa94de21e29811ada167b4ca39");
+        contract_address!("0x05fbd460228d843b7fbef670ff15607bf72e19fa94de21e29811ada167b4ca39");
     pub const VALID_ENTRY_POINT: EntryPoint =
-        entry_point!("0362398bec32bc0ebb411203221a35a0301193a96f317ebe5e40be9f60d15320");
+        entry_point!("0x0362398bec32bc0ebb411203221a35a0301193a96f317ebe5e40be9f60d15320");
     pub const INVALID_ENTRY_POINT: EntryPoint = EntryPoint(Felt::ZERO);
     pub const VALID_KEY: StorageAddress =
-        storage_address!("0206F38F7E4F15E87567361213C28F235CCCDAA1D7FD34C9DB1DFE9489C6A091");
+        storage_address!("0x0206F38F7E4F15E87567361213C28F235CCCDAA1D7FD34C9DB1DFE9489C6A091");
     pub const VALID_KEY_DEC: &str =
         "916907772491729262376534102982219947830828984996257231353398618781993312401";
     pub const VALID_CALL_DATA: [CallParam; 1] = [call_param!("0x4d2")];
     /// Class hash for VALID_CONTRACT_ADDR
     pub const VALID_CLASS_HASH: ClassHash =
-        class_hash!("021a7f43387573b68666669a0ed764252ce5367708e696e31967764a90b429c2");
+        class_hash!("0x021a7f43387573b68666669a0ed764252ce5367708e696e31967764a90b429c2");
     pub const INVALID_CLASS_HASH: ClassHash =
-        class_hash!("031a7f43387573b68666669a0ed764252ce5367708e696e31967764a90b429c2");
+        class_hash!("0x031a7f43387573b68666669a0ed764252ce5367708e696e31967764a90b429c2");
 }
 
 pub mod traces {
diff --git a/crates/gateway-types/src/reply.rs b/crates/gateway-types/src/reply.rs
index e08b13a11e..3599c03fd4 100644
--- a/crates/gateway-types/src/reply.rs
+++ b/crates/gateway-types/src/reply.rs
@@ -2308,7 +2308,7 @@ pub mod add_transaction {
             let expected = InvokeResponse {
                 code: "TRANSACTION_RECEIVED".to_owned(),
                 transaction_hash: transaction_hash!(
-                    "0389dd0629f42176cc8b6c43acefc0713d0064ecdfc0470e0fc179f53421a38b"
+                    "0x0389dd0629f42176cc8b6c43acefc0713d0064ecdfc0470e0fc179f53421a38b"
                 ),
             };
             assert_eq!(expected, result);
@@ -2320,10 +2320,10 @@ pub mod add_transaction {
             let expected = DeployResponse {
                 code: "TRANSACTION_RECEIVED".to_owned(),
                 transaction_hash: transaction_hash!(
-                    "0296fb89b8a1c7487a1d4b27e1a1e33f440b05548e64980d06052bc089b1a51f"
+                    "0x0296fb89b8a1c7487a1d4b27e1a1e33f440b05548e64980d06052bc089b1a51f"
                 ),
                 address: contract_address!(
-                    "0677bb1cdc050e8d63855e8743ab6e09179138def390676cc03c484daf112ba1"
+                    "0x0677bb1cdc050e8d63855e8743ab6e09179138def390676cc03c484daf112ba1"
                 ),
             };
             assert_eq!(expected, result);
diff --git a/crates/merkle-tree/src/merkle_node.rs b/crates/merkle-tree/src/merkle_node.rs
index b539f702de..08788920b2 100644
--- a/crates/merkle-tree/src/merkle_node.rs
+++ b/crates/merkle-tree/src/merkle_node.rs
@@ -274,7 +274,7 @@ mod tests {
             // Note that the hash function must be exchanged for `async_stark_hash_func`,
             // otherwise it just uses some other test hash function.
             let expected = Felt::from_hex_str(
-                "0615bb8d47888d2987ad0c63fc06e9e771930986a4dd8adc55617febfcf3639e",
+                "0x0615bb8d47888d2987ad0c63fc06e9e771930986a4dd8adc55617febfcf3639e",
             )
             .unwrap();
             let left = felt!("0x1234");
@@ -300,7 +300,7 @@ mod tests {
             // Note that the hash function must be exchanged for `async_stark_hash_func`,
             // otherwise it just uses some other test hash function.
             let expected = Felt::from_hex_str(
-                "1d937094c09b5f8e26a662d21911871e3cbc6858d55cc49af9848ea6fed4e9",
+                "0x1d937094c09b5f8e26a662d21911871e3cbc6858d55cc49af9848ea6fed4e9",
             )
             .unwrap();
             let child = felt!("0x1234ABCD");
diff --git a/crates/pathfinder/src/state/block_hash.rs b/crates/pathfinder/src/state/block_hash.rs
index b223879e68..8298fd2520 100644
--- a/crates/pathfinder/src/state/block_hash.rs
+++ b/crates/pathfinder/src/state/block_hash.rs
@@ -221,7 +221,7 @@ mod meta {
     const MAINNET_METAINFO: BlockHashMetaInfo = BlockHashMetaInfo {
         first_0_7_block: BlockNumber::new_or_panic(833),
         fallback_sequencer_address: Some(sequencer_address!(
-            "021f4b90b0377c82bf330b7b5295820769e72d79d8acd0effa0ebde6e9988bc5"
+            "0x021f4b90b0377c82bf330b7b5295820769e72d79d8acd0effa0ebde6e9988bc5"
         )),
     };
 
diff --git a/crates/rpc/src/felt.rs b/crates/rpc/src/felt.rs
index 9f6a5354a3..91674fde9a 100644
--- a/crates/rpc/src/felt.rs
+++ b/crates/rpc/src/felt.rs
@@ -227,21 +227,16 @@ mod deserialization {
 
                 fn expecting(&self, formatter: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
                     formatter
-                        .write_str("a hex string of up to 64 digits with an optional '0x' prefix")
+                        .write_str("a hex string of up to 64 digits with a mandatory '0x' prefix")
                 }
 
                 fn visit_str<E>(self, v: &str) -> Result<Self::Value, E>
                 where
                     E: serde::de::Error,
                 {
-                    // Felt::from_hex_str currently does not enforce `0x` prefix, add it here to
-                    // prevent breaking other serde related code.
-                    match v.as_bytes() {
-                        &[b'0', b'x', ..] => pathfinder_crypto::Felt::from_hex_str(v)
-                            .map_err(|e| serde::de::Error::custom(e))
-                            .map(RpcFelt),
-                        _missing_prefix => Err(serde::de::Error::custom("Missing '0x' prefix")),
-                    }
+                    pathfinder_crypto::Felt::from_hex_str(v)
+                        .map_err(|e| serde::de::Error::custom(e))
+                        .map(RpcFelt)
                 }
             }
 
diff --git a/crates/rpc/src/lib.rs b/crates/rpc/src/lib.rs
index d337bb645c..22e48a2ff1 100644
--- a/crates/rpc/src/lib.rs
+++ b/crates/rpc/src/lib.rs
@@ -1504,9 +1504,9 @@ mod tests {
                 line!(),
                 r#"{"starting_block_hash":"0xa","starting_block_num":"0x1","current_block_hash":"0xb","current_block_num":"0x2","highest_block_hash":"0xc","highest_block_num":"0x3"}"#,
                 Syncing::Status(Status {
-                    starting: NumberedBlock::from(("a", 1)),
-                    current: NumberedBlock::from(("b", 2)),
-                    highest: NumberedBlock::from(("c", 3)),
+                    starting: NumberedBlock::from(("0xa", 1)),
+                    current: NumberedBlock::from(("0xb", 2)),
+                    highest: NumberedBlock::from(("0xc", 3)),
                 }),
             ),
         ];
diff --git a/crates/rpc/src/method/add_deploy_account_transaction.rs b/crates/rpc/src/method/add_deploy_account_transaction.rs
index de7d8aeb47..d7873f297e 100644
--- a/crates/rpc/src/method/add_deploy_account_transaction.rs
+++ b/crates/rpc/src/method/add_deploy_account_transaction.rs
@@ -423,22 +423,22 @@ mod tests {
                     max_fee: fee!("0xbf391377813"),
                     signature: vec![
                         transaction_signature_elem!(
-                            "07dd3a55d94a0de6f3d6c104d7e6c88ec719a82f4e2bbc12587c8c187584d3d5"
+                            "0x07dd3a55d94a0de6f3d6c104d7e6c88ec719a82f4e2bbc12587c8c187584d3d5"
                         ),
                         transaction_signature_elem!(
-                            "071456dded17015d1234779889d78f3e7c763ddcfd2662b19e7843c7542614f8"
+                            "0x071456dded17015d1234779889d78f3e7c763ddcfd2662b19e7843c7542614f8"
                         ),
                     ],
                     nonce: TransactionNonce::ZERO,
 
                     contract_address_salt: contract_address_salt!(
-                        "06d44a6aecb4339e23a9619355f101cf3cb9baec289fcd9fd51486655c1bb8a8"
+                        "0x06d44a6aecb4339e23a9619355f101cf3cb9baec289fcd9fd51486655c1bb8a8"
                     ),
                     constructor_calldata: vec![call_param!(
-                        "0677bb1cdc050e8d63855e8743ab6e09179138def390676cc03c484daf112ba1"
+                        "0x0677bb1cdc050e8d63855e8743ab6e09179138def390676cc03c484daf112ba1"
                     )],
                     class_hash: class_hash!(
-                        "01fac3074c9d5282f0acc5c69a4781a1c711efea5e73c550c5d9fb253cf7fd3d"
+                        "0x01fac3074c9d5282f0acc5c69a4781a1c711efea5e73c550c5d9fb253cf7fd3d"
                     ),
                 }),
             ),
diff --git a/crates/rpc/src/method/add_invoke_transaction.rs b/crates/rpc/src/method/add_invoke_transaction.rs
index 74e3971532..ef1c6e7020 100644
--- a/crates/rpc/src/method/add_invoke_transaction.rs
+++ b/crates/rpc/src/method/add_invoke_transaction.rs
@@ -355,20 +355,24 @@ mod tests {
                 max_fee: fee!("0x4F388496839"),
                 signature: vec![
                     transaction_signature_elem!(
-                        "07dd3a55d94a0de6f3d6c104d7e6c88ec719a82f4e2bbc12587c8c187584d3d5"
+                        "0x07dd3a55d94a0de6f3d6c104d7e6c88ec719a82f4e2bbc12587c8c187584d3d5"
                     ),
                     transaction_signature_elem!(
-                        "071456dded17015d1234779889d78f3e7c763ddcfd2662b19e7843c7542614f8"
+                        "0x071456dded17015d1234779889d78f3e7c763ddcfd2662b19e7843c7542614f8"
                     ),
                 ],
                 nonce: transaction_nonce!("0x1"),
                 sender_address: contract_address!(
-                    "023371b227eaecd8e8920cd429357edddd2cd0f3fee6abaacca08d3ab82a7cdd"
+                    "0x023371b227eaecd8e8920cd429357edddd2cd0f3fee6abaacca08d3ab82a7cdd"
                 ),
                 calldata: vec![
                     call_param!("0x1"),
-                    call_param!("0677bb1cdc050e8d63855e8743ab6e09179138def390676cc03c484daf112ba1"),
-                    call_param!("0362398bec32bc0ebb411203221a35a0301193a96f317ebe5e40be9f60d15320"),
+                    call_param!(
+                        "0x0677bb1cdc050e8d63855e8743ab6e09179138def390676cc03c484daf112ba1"
+                    ),
+                    call_param!(
+                        "0x0362398bec32bc0ebb411203221a35a0301193a96f317ebe5e40be9f60d15320"
+                    ),
                     call_param!("0x0"),
                     call_param!("0x1"),
                     call_param!("0x1"),
@@ -507,7 +511,7 @@ mod tests {
                     signature: vec![],
                     nonce: transaction_nonce!("0x2"),
                     sender_address: contract_address!(
-                        "03fdcbeb68e607c8febf01d7ef274cbf68091a0bd1556c0b8f8e80d732f7850f"
+                        "0x03fdcbeb68e607c8febf01d7ef274cbf68091a0bd1556c0b8f8e80d732f7850f"
                     ),
                     calldata: vec![],
                 },
diff --git a/crates/rpc/src/method/call.rs b/crates/rpc/src/method/call.rs
index 85be8a28cf..f0b557864d 100644
--- a/crates/rpc/src/method/call.rs
+++ b/crates/rpc/src/method/call.rs
@@ -829,7 +829,7 @@ mod tests {
                         // Entry point selector for the called contract
                         CallParam(EntryPoint::hashed(b"call").0),
                         // Length of the call data for the called contract
-                        call_param!("1"),
+                        call_param!("0x1"),
                         // Number of calls, but then no more data; leads to deserialization error
                         call_param!("0x1"),
                     ],
@@ -875,8 +875,8 @@ mod tests {
                     // Calldata length over the limit, the rest of the fields should not matter.
                     calldata: vec![call_param!("0x123"); CALLDATA_LIMIT + 5],
 
-                    contract_address: contract_address!("deadbeef"),
-                    entry_point_selector: entry_point!("deadbeef"),
+                    contract_address: contract_address!("0xdeadbeef"),
+                    entry_point_selector: entry_point!("0xdeadbeef"),
                 },
                 block_id: BlockId::Latest,
             };
@@ -898,7 +898,7 @@ mod tests {
 
         // Mainnet block number 5
         const BLOCK_5: BlockId = BlockId::Hash(block_hash!(
-            "00dcbd2a4b597d051073f40a0329e585bb94b26d73df69f8d72798924fd097d3"
+            "0x00dcbd2a4b597d051073f40a0329e585bb94b26d73df69f8d72798924fd097d3"
         ));
 
         // Data from transaction
@@ -906,14 +906,16 @@ mod tests {
         fn valid_mainnet_call() -> FunctionCall {
             FunctionCall {
                 contract_address: contract_address!(
-                    "020cfa74ee3564b4cd5435cdace0f9c4d43b939620e4a0bb5076105df0a626c6"
+                    "0x020cfa74ee3564b4cd5435cdace0f9c4d43b939620e4a0bb5076105df0a626c6"
                 ),
                 entry_point_selector: entry_point!(
-                    "03d7905601c217734671143d457f0db37f7f8883112abd34b92c4abfeafde0c3"
+                    "0x03d7905601c217734671143d457f0db37f7f8883112abd34b92c4abfeafde0c3"
                 ),
                 calldata: vec![
-                    call_param!("e150b6c2db6ed644483b01685571de46d2045f267d437632b508c19f3eb877"),
-                    call_param!("0494196e88ce16bff11180d59f3c75e4ba3475d9fba76249ab5f044bcd25add6"),
+                    call_param!("0xe150b6c2db6ed644483b01685571de46d2045f267d437632b508c19f3eb877"),
+                    call_param!(
+                        "0x0494196e88ce16bff11180d59f3c75e4ba3475d9fba76249ab5f044bcd25add6"
+                    ),
                 ],
             }
         }
diff --git a/crates/rpc/src/method/estimate_fee.rs b/crates/rpc/src/method/estimate_fee.rs
index b4afdf719a..9f44b85477 100644
--- a/crates/rpc/src/method/estimate_fee.rs
+++ b/crates/rpc/src/method/estimate_fee.rs
@@ -261,7 +261,7 @@ mod tests {
     fn declare_transaction(account_contract_address: ContractAddress) -> BroadcastedTransaction {
         let sierra_definition = include_bytes!("../../fixtures/contracts/storage_access.json");
         let sierra_hash =
-            class_hash!("0544b92d358447cb9e50b65092b7169f931d29e05c1404a2cd08c6fd7e32ba90");
+            class_hash!("0x0544b92d358447cb9e50b65092b7169f931d29e05c1404a2cd08c6fd7e32ba90");
         let casm_hash =
             casm_hash!("0x069032ff71f77284e1a0864a573007108ca5cc08089416af50f03260f5d6d4d8");
 
@@ -294,7 +294,7 @@ mod tests {
     ) -> BroadcastedTransaction {
         let max_fee = Fee::default();
         let sierra_hash =
-            class_hash!("0544b92d358447cb9e50b65092b7169f931d29e05c1404a2cd08c6fd7e32ba90");
+            class_hash!("0x0544b92d358447cb9e50b65092b7169f931d29e05c1404a2cd08c6fd7e32ba90");
 
         BroadcastedTransaction::Invoke(BroadcastedInvokeTransaction::V1(
             BroadcastedInvokeTransactionV1 {
@@ -312,7 +312,7 @@ mod tests {
                     CallParam(EntryPoint::hashed(b"deployContract").0),
                     // Length of the call data for the called contract, i.e.
                     // AccountCallArray::data_len
-                    call_param!("4"),
+                    call_param!("0x4"),
                     // classHash
                     CallParam(sierra_hash.0),
                     // salt
@@ -348,7 +348,7 @@ mod tests {
                     CallParam(EntryPoint::hashed(b"get_data").0),
                     // Length of the call data for the called contract, i.e.
                     // AccountCallArray::data_len
-                    call_param!("0"),
+                    call_param!("0x0"),
                 ],
             },
         ))
@@ -389,7 +389,7 @@ mod tests {
                     CallParam(EntryPoint::hashed(b"get_data").0),
                     // Length of the call data for the called contract, i.e.
                     // AccountCallArray::data_len
-                    call_param!("0"),
+                    call_param!("0x0"),
                 ],
                 nonce: transaction_nonce!("0x3"),
                 resource_bounds: ResourceBounds::default(),
@@ -694,7 +694,7 @@ mod tests {
                     CallParam(EntryPoint::hashed(b"deployContract").0),
                     // Length of the call data for the called contract, i.e.
                     // AccountCallArray::data_len
-                    call_param!("4"),
+                    call_param!("0x4"),
                     // classHash
                     CallParam(sierra_hash.0),
                     // salt
@@ -732,7 +732,7 @@ mod tests {
                     // Entry point selector for the called contract, i.e.
                     CallParam(EntryPoint::hashed(b"test_redeposits").0),
                     // Length of the call data for the called contract, i.e.
-                    call_param!("1"),
+                    call_param!("0x1"),
                     depth,
                 ],
                 nonce,
@@ -784,14 +784,14 @@ mod tests {
         let invoke_transaction = invoke_v3_transaction_with_data_gas(
             account_contract_address,
             transaction_nonce!("0x2"),
-            call_param!("7"),
+            call_param!("0x7"),
         );
         // Invoke once more to test that the execution state updates properly with L2
         // gas accounting aware code.
         let invoke_transaction2 = invoke_v3_transaction_with_data_gas(
             account_contract_address,
             transaction_nonce!("0x3"),
-            call_param!("7"),
+            call_param!("0x7"),
         );
 
         let input = Input {
@@ -839,14 +839,14 @@ mod tests {
         let invoke_transaction = invoke_v3_transaction_with_data_gas(
             account_contract_address,
             transaction_nonce!("0x2"),
-            call_param!("7"),
+            call_param!("0x7"),
         );
         // Invoke once more to test that the execution state updates properly with L2
         // gas accounting aware code.
         let invoke_transaction2 = invoke_v3_transaction_with_data_gas(
             account_contract_address,
             transaction_nonce!("0x3"),
-            call_param!("7"),
+            call_param!("0x7"),
         );
 
         let input = Input {
@@ -895,14 +895,14 @@ mod tests {
         let invoke_transaction = invoke_v3_transaction_with_data_gas(
             account_contract_address,
             transaction_nonce!("0x2"),
-            call_param!("7"),
+            call_param!("0x7"),
         );
         // Invoke once more to test that the execution state updates properly with L2
         // gas accounting aware code.
         let invoke_transaction2 = invoke_v3_transaction_with_data_gas(
             account_contract_address,
             transaction_nonce!("0x3"),
-            call_param!("7"),
+            call_param!("0x7"),
         );
 
         let input = Input {
@@ -949,7 +949,7 @@ mod tests {
                     // Entry point selector for the called contract, i.e.
                     CallParam(EntryPoint::hashed(b"bogus").0),
                     // Length of the call data for the called contract, i.e.
-                    call_param!("1"),
+                    call_param!("0x1"),
                     depth,
                 ],
                 nonce,
@@ -995,7 +995,7 @@ mod tests {
         let invoke_transaction = invoke_v3_transaction_with_invalid_entry_point(
             account_contract_address,
             transaction_nonce!("0x2"),
-            call_param!("7"),
+            call_param!("0x7"),
         );
 
         let input = Input {
@@ -1056,7 +1056,7 @@ mod tests {
         let invoke_transaction = invoke_v3_transaction_with_data_gas(
             account_contract_address,
             transaction_nonce!("0x2"),
-            call_param!("100000"),
+            call_param!("0x100000"),
         );
 
         let input = Input {
@@ -1087,7 +1087,7 @@ mod tests {
         let invoke_transaction = invoke_v3_transaction_with_data_gas(
             account_contract_address,
             transaction_nonce!("0x2"),
-            call_param!("100"),
+            call_param!("0x100"),
         );
 
         let input = Input {
diff --git a/crates/rpc/src/method/get_events.rs b/crates/rpc/src/method/get_events.rs
index 244bae1689..081b7a5c41 100644
--- a/crates/rpc/src/method/get_events.rs
+++ b/crates/rpc/src/method/get_events.rs
@@ -1000,7 +1000,7 @@ mod tests {
 
         let limit = EVENT_KEY_FILTER_LIMIT;
 
-        let keys = [vec![event_key!("01")]]
+        let keys = [vec![event_key!("0x01")]]
             .iter()
             .cloned()
             .cycle()
diff --git a/crates/rpc/src/method/simulate_transactions.rs b/crates/rpc/src/method/simulate_transactions.rs
index ee28c576c0..ddec1ab316 100644
--- a/crates/rpc/src/method/simulate_transactions.rs
+++ b/crates/rpc/src/method/simulate_transactions.rs
@@ -715,7 +715,7 @@ pub(crate) mod tests {
             include_bytes!("../../fixtures/contracts/cairo0_test.json");
 
         pub const CAIRO0_HASH: ClassHash =
-            class_hash!("02c52e7084728572ea940b4df708a2684677c19fa6296de2ea7ba5327e3a84ef");
+            class_hash!("0x02c52e7084728572ea940b4df708a2684677c19fa6296de2ea7ba5327e3a84ef");
 
         let contract_class = crate::types::ContractClass::from_definition_bytes(CAIRO0_DEFINITION)
             .unwrap()
@@ -897,7 +897,7 @@ pub(crate) mod tests {
         pub const SIERRA_DEFINITION: &[u8] =
             include_bytes!("../../fixtures/contracts/storage_access.json");
         pub const SIERRA_HASH: ClassHash =
-            class_hash!("0544b92d358447cb9e50b65092b7169f931d29e05c1404a2cd08c6fd7e32ba90");
+            class_hash!("0x0544b92d358447cb9e50b65092b7169f931d29e05c1404a2cd08c6fd7e32ba90");
         pub const CASM_HASH: CasmHash =
             casm_hash!("0x069032ff71f77284e1a0864a573007108ca5cc08089416af50f03260f5d6d4d8");
         pub const CASM_DEFINITION: &[u8] =
@@ -968,7 +968,7 @@ pub(crate) mod tests {
                             CallParam(EntryPoint::hashed(b"deployContract").0),
                             // Length of the call data for the called contract, i.e.
                             // AccountCallArray::data_len
-                            call_param!("4"),
+                            call_param!("0x4"),
                             // classHash
                             CallParam(SIERRA_HASH.0),
                             // salt
@@ -999,7 +999,7 @@ pub(crate) mod tests {
                             CallParam(EntryPoint::hashed(b"get_data").0),
                             // Length of the call data for the called contract, i.e.
                             // AccountCallArray::data_len
-                            call_param!("0"),
+                            call_param!("0x0"),
                         ],
                     },
                 ))
@@ -1037,7 +1037,7 @@ pub(crate) mod tests {
                             CallParam(EntryPoint::hashed(b"get_data").0),
                             // Length of the call data for the called contract, i.e.
                             // AccountCallArray::data_len
-                            call_param!("0"),
+                            call_param!("0x0"),
                         ],
                         proof_facts: vec![],
                         proof: Default::default(),
@@ -1082,7 +1082,7 @@ pub(crate) mod tests {
                             CallParam(EntryPoint::hashed(b"get_data").0),
                             // Length of the call data for the called contract, i.e.
                             // AccountCallArray::data_len
-                            call_param!("0"),
+                            call_param!("0x0"),
                         ],
                         proof_facts: vec![],
                         proof: Default::default(),
diff --git a/crates/serde/src/lib.rs b/crates/serde/src/lib.rs
index fd063d98a3..d96fdd2d2e 100644
--- a/crates/serde/src/lib.rs
+++ b/crates/serde/src/lib.rs
@@ -113,7 +113,7 @@ impl<'de> DeserializeAs<'de, H256> for H256AsNoLeadingZerosHexStr {
             type Value = H256;
 
             fn expecting(&self, formatter: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-                formatter.write_str("a hex string of up to 64 digits with an optional '0x' prefix")
+                formatter.write_str("a hex string of up to 64 digits with a mandatory '0x' prefix")
             }
 
             fn visit_str<E>(self, v: &str) -> Result<Self::Value, E>
@@ -323,13 +323,13 @@ pub fn starkhash_to_dec_str(h: &Felt) -> String {
 
 /// A helper conversion function. Only use with __sequencer API related types__.
 fn starkhash_from_dec_str(s: &str) -> Result<Felt, anyhow::Error> {
-    match BigUint::from_str(s) {
-        Ok(b) => {
-            let h = starkhash_from_biguint(b)?;
-            Ok(h)
-        }
+    // The order here matters because `Felt::from_hex_str` requires the '0x'
+    // prefix, so we'll never parse a hex string as a decimal string by mistake.
+    match Felt::from_hex_str(s) {
+        Ok(h) => Ok(h),
         Err(_) => {
-            let h = Felt::from_hex_str(s)?;
+            let b = BigUint::from_str(s)?;
+            let h = starkhash_from_biguint(b)?;
             Ok(h)
         }
     }
@@ -337,8 +337,8 @@ fn starkhash_from_dec_str(s: &str) -> Result<Felt, anyhow::Error> {
 
 /// A convenience function which parses a hex string into a byte array.
 ///
-/// Supports both upper and lower case hex strings, as well as an
-/// optional "0x" prefix.
+/// Supports both upper and lower case hex strings. The '0x' prefix is
+/// mandatory.
 fn bytes_from_hex_str<const N: usize>(hex_str: &str) -> Result<[u8; N], HexParseError> {
     fn parse_hex_digit(digit: u8) -> Result<u8, HexParseError> {
         match digit {
@@ -349,7 +349,9 @@ fn bytes_from_hex_str<const N: usize>(hex_str: &str) -> Result<[u8; N], HexParse
         }
     }
 
-    let hex_str = hex_str.strip_prefix("0x").unwrap_or(hex_str);
+    let Some(hex_str) = hex_str.strip_prefix("0x") else {
+        return Err(HexParseError::MissingPrefix);
+    };
     if hex_str.len() > N * 2 {
         return Err(HexParseError::InvalidLength {
             max: N * 2,
@@ -488,6 +490,7 @@ pub fn extract_program_and_entry_points_by_type(
 
 #[cfg(test)]
 mod tests {
+    use num_bigint::ParseBigIntError;
     use pretty_assertions_sorted::assert_eq;
 
     use super::*;
@@ -639,20 +642,21 @@ mod tests {
         // Regression: previously max in the error message was hard-coded at 64,
         // so try another buf size to make sure it is not anymore
         assert_eq!(
-            &format!("{}", bytes_from_hex_str::<1>("abc").unwrap_err()),
+            &format!("{}", bytes_from_hex_str::<1>("0xabc").unwrap_err()),
             "More than 2 digits found: 3"
         );
     }
 
     #[test]
     fn invalid_digit() {
-        starkhash_from_dec_str("123a").unwrap();
+        starkhash_from_dec_str("0x123a").unwrap();
         assert_eq!(
             starkhash_from_dec_str("123z")
                 .unwrap_err()
-                .downcast::<HexParseError>()
-                .unwrap(),
-            HexParseError::InvalidNibble(b'z')
+                .downcast::<ParseBigIntError>()
+                .unwrap()
+                .to_string(),
+            "invalid digit found in string"
         );
         assert_eq!(
             bytes_from_hex_str::<32>("0x123z"),
diff --git a/crates/storage/src/connection/block.rs b/crates/storage/src/connection/block.rs
index 57f157ddab..fa80ed1a09 100644
--- a/crates/storage/src/connection/block.rs
+++ b/crates/storage/src/connection/block.rs
@@ -695,7 +695,7 @@ mod tests {
             event_count: 40,
             l1_da_mode: L1DataAvailabilityMode::Blob,
             receipt_commitment: receipt_commitment_bytes!(b"receipt commitment genesis"),
-            state_diff_commitment: state_diff_commitment!("12"),
+            state_diff_commitment: state_diff_commitment!("0x12"),
             state_diff_length: 12,
         };
 
diff --git a/crates/storage/src/connection/ethereum.rs b/crates/storage/src/connection/ethereum.rs
index f3f8aae5ee..d895466600 100644
--- a/crates/storage/src/connection/ethereum.rs
+++ b/crates/storage/src/connection/ethereum.rs
@@ -101,9 +101,13 @@ mod tests {
     fn create_updates() -> [EthereumStateUpdate; 3] {
         (0..3usize)
             .map(|i| EthereumStateUpdate {
-                state_root: StateCommitment(Felt::from_hex_str(&"3".repeat(i + 1)).unwrap()),
+                state_root: Felt::from_hex_str(&format!("0x{}", &"3".repeat(i + 1)))
+                    .map(StateCommitment)
+                    .unwrap(),
                 block_number: BlockNumber::GENESIS + i as u64,
-                block_hash: BlockHash(Felt::from_hex_str(&"F".repeat(i + 1)).unwrap()),
+                block_hash: Felt::from_hex_str(&format!("0x{}", &"F".repeat(i + 1)))
+                    .map(BlockHash)
+                    .unwrap(),
             })
             .collect::<Vec<_>>()
             .try_into()
diff --git a/crates/storage/src/connection/trie.rs b/crates/storage/src/connection/trie.rs
index 141b7cb4e6..0227936d59 100644
--- a/crates/storage/src/connection/trie.rs
+++ b/crates/storage/src/connection/trie.rs
@@ -1277,14 +1277,14 @@ mod tests {
             &TrieUpdate {
                 nodes_added: vec![
                     (
-                        felt!("0"),
+                        felt!("0x0"),
                         Node::Binary {
                             left: NodeRef::Index(1),
                             right: NodeRef::Index(2),
                         },
                     ),
-                    (felt!("1"), Node::LeafBinary),
-                    (felt!("2"), Node::LeafBinary),
+                    (felt!("0x1"), Node::LeafBinary),
+                    (felt!("0x2"), Node::LeafBinary),
                 ],
                 nodes_removed: vec![],
                 root_commitment: Felt::ZERO,
@@ -1296,14 +1296,14 @@ mod tests {
             &TrieUpdate {
                 nodes_added: vec![
                     (
-                        felt!("3"),
+                        felt!("0x3"),
                         Node::Binary {
                             left: NodeRef::Index(1),
                             right: NodeRef::Index(2),
                         },
                     ),
-                    (felt!("4"), Node::LeafBinary),
-                    (felt!("5"), Node::LeafBinary),
+                    (felt!("0x4"), Node::LeafBinary),
+                    (felt!("0x5"), Node::LeafBinary),
                 ],
                 nodes_removed: vec![TrieStorageIndex(1)],
                 root_commitment: Felt::ZERO,
@@ -1315,14 +1315,14 @@ mod tests {
             &TrieUpdate {
                 nodes_added: vec![
                     (
-                        felt!("6"),
+                        felt!("0x6"),
                         Node::Binary {
                             left: NodeRef::Index(1),
                             right: NodeRef::Index(2),
                         },
                     ),
-                    (felt!("7"), Node::LeafBinary),
-                    (felt!("8"), Node::LeafBinary),
+                    (felt!("0x7"), Node::LeafBinary),
+                    (felt!("0x8"), Node::LeafBinary),
                 ],
                 nodes_removed: vec![],
                 root_commitment: Felt::ZERO,
@@ -1334,14 +1334,14 @@ mod tests {
             &TrieUpdate {
                 nodes_added: vec![
                     (
-                        felt!("9"),
+                        felt!("0x9"),
                         Node::Binary {
                             left: NodeRef::Index(1),
                             right: NodeRef::Index(2),
                         },
                     ),
-                    (felt!("10"), Node::LeafBinary),
-                    (felt!("11"), Node::LeafBinary),
+                    (felt!("0x10"), Node::LeafBinary),
+                    (felt!("0x11"), Node::LeafBinary),
                 ],
                 nodes_removed: vec![],
                 root_commitment: Felt::ZERO,
@@ -1357,14 +1357,14 @@ mod tests {
             &TrieUpdate {
                 nodes_added: vec![
                     (
-                        felt!("12"),
+                        felt!("0x12"),
                         Node::Binary {
                             left: NodeRef::Index(1),
                             right: NodeRef::Index(2),
                         },
                     ),
-                    (felt!("13"), Node::LeafBinary),
-                    (felt!("14"), Node::LeafBinary),
+                    (felt!("0x13"), Node::LeafBinary),
+                    (felt!("0x14"), Node::LeafBinary),
                 ],
                 nodes_removed: vec![],
                 root_commitment: Felt::ZERO,
@@ -1391,14 +1391,14 @@ mod tests {
             &TrieUpdate {
                 nodes_added: vec![
                     (
-                        felt!("0"),
+                        felt!("0x0"),
                         Node::Binary {
                             left: NodeRef::Index(1),
                             right: NodeRef::Index(2),
                         },
                     ),
-                    (felt!("1"), Node::LeafBinary),
-                    (felt!("2"), Node::LeafBinary),
+                    (felt!("0x1"), Node::LeafBinary),
+                    (felt!("0x2"), Node::LeafBinary),
                 ],
                 nodes_removed: vec![],
                 root_commitment: Felt::ZERO,
@@ -1410,14 +1410,14 @@ mod tests {
             &TrieUpdate {
                 nodes_added: vec![
                     (
-                        felt!("3"),
+                        felt!("0x3"),
                         Node::Binary {
                             left: NodeRef::Index(1),
                             right: NodeRef::Index(2),
                         },
                     ),
-                    (felt!("4"), Node::LeafBinary),
-                    (felt!("5"), Node::LeafBinary),
+                    (felt!("0x4"), Node::LeafBinary),
+                    (felt!("0x5"), Node::LeafBinary),
                 ],
                 nodes_removed: vec![TrieStorageIndex(1)],
                 root_commitment: Felt::ZERO,
@@ -1429,14 +1429,14 @@ mod tests {
             &TrieUpdate {
                 nodes_added: vec![
                     (
-                        felt!("6"),
+                        felt!("0x6"),
                         Node::Binary {
                             left: NodeRef::Index(1),
                             right: NodeRef::Index(2),
                         },
                     ),
-                    (felt!("7"), Node::LeafBinary),
-                    (felt!("8"), Node::LeafBinary),
+                    (felt!("0x7"), Node::LeafBinary),
+                    (felt!("0x8"), Node::LeafBinary),
                 ],
                 nodes_removed: vec![],
                 root_commitment: Felt::ZERO,
@@ -1448,14 +1448,14 @@ mod tests {
             &TrieUpdate {
                 nodes_added: vec![
                     (
-                        felt!("9"),
+                        felt!("0x9"),
                         Node::Binary {
                             left: NodeRef::Index(1),
                             right: NodeRef::Index(2),
                         },
                     ),
-                    (felt!("10"), Node::LeafBinary),
-                    (felt!("11"), Node::LeafBinary),
+                    (felt!("0x10"), Node::LeafBinary),
+                    (felt!("0x11"), Node::LeafBinary),
                 ],
                 nodes_removed: vec![],
                 root_commitment: Felt::ZERO,
@@ -1468,14 +1468,14 @@ mod tests {
             &TrieUpdate {
                 nodes_added: vec![
                     (
-                        felt!("12"),
+                        felt!("0x12"),
                         Node::Binary {
                             left: NodeRef::Index(1),
                             right: NodeRef::Index(2),
                         },
                     ),
-                    (felt!("13"), Node::LeafBinary),
-                    (felt!("14"), Node::LeafBinary),
+                    (felt!("0x13"), Node::LeafBinary),
+                    (felt!("0x14"), Node::LeafBinary),
                 ],
                 nodes_removed: vec![],
                 root_commitment: Felt::ZERO,
@@ -1514,14 +1514,14 @@ mod tests {
             &TrieUpdate {
                 nodes_added: vec![
                     (
-                        felt!("0"),
+                        felt!("0x0"),
                         Node::Binary {
                             left: NodeRef::Index(1),
                             right: NodeRef::Index(2),
                         },
                     ),
-                    (felt!("1"), Node::LeafBinary),
-                    (felt!("2"), Node::LeafBinary),
+                    (felt!("0x1"), Node::LeafBinary),
+                    (felt!("0x2"), Node::LeafBinary),
                 ],
                 nodes_removed: vec![],
                 root_commitment: Felt::ZERO,
@@ -1533,14 +1533,14 @@ mod tests {
             &TrieUpdate {
                 nodes_added: vec![
                     (
-                        felt!("3"),
+                        felt!("0x3"),
                         Node::Binary {
                             left: NodeRef::Index(1),
                             right: NodeRef::Index(2),
                         },
                     ),
-                    (felt!("4"), Node::LeafBinary),
-                    (felt!("5"), Node::LeafBinary),
+                    (felt!("0x4"), Node::LeafBinary),
+                    (felt!("0x5"), Node::LeafBinary),
                 ],
                 nodes_removed: vec![],
                 root_commitment: Felt::ZERO,
@@ -1552,14 +1552,14 @@ mod tests {
             &TrieUpdate {
                 nodes_added: vec![
                     (
-                        felt!("6"),
+                        felt!("0x6"),
                         Node::Binary {
                             left: NodeRef::Index(1),
                             right: NodeRef::Index(2),
                         },
                     ),
-                    (felt!("7"), Node::LeafBinary),
-                    (felt!("8"), Node::LeafBinary),
+                    (felt!("0x7"), Node::LeafBinary),
+                    (felt!("0x8"), Node::LeafBinary),
                 ],
                 nodes_removed: vec![],
                 root_commitment: Felt::ZERO,
@@ -1577,14 +1577,14 @@ mod tests {
             &TrieUpdate {
                 nodes_added: vec![
                     (
-                        felt!("3"),
+                        felt!("0x3"),
                         Node::Binary {
                             left: NodeRef::Index(1),
                             right: NodeRef::Index(2),
                         },
                     ),
-                    (felt!("4"), Node::LeafBinary),
-                    (felt!("5"), Node::LeafBinary),
+                    (felt!("0x4"), Node::LeafBinary),
+                    (felt!("0x5"), Node::LeafBinary),
                 ],
                 nodes_removed: vec![1, 2, 3].into_iter().map(TrieStorageIndex).collect(),
                 root_commitment: Felt::ZERO,
@@ -1596,14 +1596,14 @@ mod tests {
             &TrieUpdate {
                 nodes_added: vec![
                     (
-                        felt!("6"),
+                        felt!("0x6"),
                         Node::Binary {
                             left: NodeRef::Index(1),
                             right: NodeRef::Index(2),
                         },
                     ),
-                    (felt!("7"), Node::LeafBinary),
-                    (felt!("8"), Node::LeafBinary),
+                    (felt!("0x7"), Node::LeafBinary),
+                    (felt!("0x8"), Node::LeafBinary),
                 ],
                 nodes_removed: vec![],
                 root_commitment: Felt::ZERO,
@@ -1633,14 +1633,14 @@ mod tests {
                 &TrieUpdate {
                     nodes_added: vec![
                         (
-                            felt!("0"),
+                            felt!("0x0"),
                             Node::Binary {
                                 left: NodeRef::Index(1),
                                 right: NodeRef::Index(2),
                             },
                         ),
-                        (felt!("1"), Node::LeafBinary),
-                        (felt!("2"), Node::LeafBinary),
+                        (felt!("0x1"), Node::LeafBinary),
+                        (felt!("0x2"), Node::LeafBinary),
                     ],
                     nodes_removed: vec![],
                     root_commitment: Felt::ZERO,
diff --git a/crates/storage/src/test_utils.rs b/crates/storage/src/test_utils.rs
index 6babd39b1c..fda0edfbd6 100644
--- a/crates/storage/src/test_utils.rs
+++ b/crates/storage/src/test_utils.rs
@@ -29,9 +29,13 @@ pub(crate) fn create_blocks(n_blocks: usize) -> Vec<BlockHeader> {
     (0..n_blocks)
         .map(|block_number| {
             let storage_commitment =
-                StorageCommitment(Felt::from_hex_str(&"b".repeat(block_number + 3)).unwrap());
+                Felt::from_hex_str(&format!("0x{}", "b".repeat(block_number + 3)))
+                    .map(StorageCommitment)
+                    .unwrap();
             let class_commitment =
-                ClassCommitment(Felt::from_hex_str(&"c".repeat(block_number + 3)).unwrap());
+                Felt::from_hex_str(&format!("0x{}", "c".repeat(block_number + 3)))
+                    .map(ClassCommitment)
+                    .unwrap();
             let index_as_felt = Felt::from_be_slice(&[block_number as u8]).unwrap();
 
             BlockHeader::builder()
@@ -42,9 +46,11 @@ pub(crate) fn create_blocks(n_blocks: usize) -> Vec<BlockHeader> {
                 .sequencer_address(SequencerAddress(index_as_felt))
                 .transaction_commitment(TransactionCommitment(index_as_felt))
                 .event_commitment(EventCommitment(index_as_felt))
-                .finalize_with_hash(BlockHash(
-                    Felt::from_hex_str(&"a".repeat(block_number + 3)).unwrap(),
-                ))
+                .finalize_with_hash(
+                    Felt::from_hex_str(&format!("0x{}", "a".repeat(block_number + 3)))
+                        .map(BlockHash)
+                        .unwrap(),
+                )
         })
         .collect::<Vec<_>>()
 }
@@ -63,22 +69,28 @@ pub(crate) fn create_transactions_and_receipts(
     );
     let transactions = (0..n_transactions).map(|i| match i % transactions_per_block {
         x if x < INVOKE_TRANSACTIONS_PER_BLOCK => Transaction {
-            hash: TransactionHash(Felt::from_hex_str(&"4".repeat(i + 3)).unwrap()),
+            hash: Felt::from_hex_str(&format!("0x{}", "4".repeat(i + 3)))
+                .map(TransactionHash)
+                .unwrap(),
             variant: TransactionVariant::InvokeV0(InvokeTransactionV0 {
-                calldata: vec![CallParam(Felt::from_hex_str(&"0".repeat(i + 3)).unwrap())],
-                sender_address: ContractAddress::new_or_panic(
-                    Felt::from_hex_str(&"1".repeat(i + 3)).unwrap(),
-                ),
-                entry_point_selector: EntryPoint(Felt::from_hex_str(&"2".repeat(i + 3)).unwrap()),
+                calldata: vec![Felt::from_hex_str(&format!("0x{}", "0".repeat(i + 3)))
+                    .map(CallParam)
+                    .unwrap()],
+                sender_address: Felt::from_hex_str(&format!("0x{}", "1".repeat(i + 3)))
+                    .map(ContractAddress::new_or_panic)
+                    .unwrap(),
+                entry_point_selector: Felt::from_hex_str(&format!("0x{}", "2".repeat(i + 3)))
+                    .map(EntryPoint)
+                    .unwrap(),
                 entry_point_type: Some(if i & 1 == 0 {
                     EntryPointType::External
                 } else {
                     EntryPointType::L1Handler
                 }),
                 max_fee: Fee::ZERO,
-                signature: vec![TransactionSignatureElem(
-                    Felt::from_hex_str(&"3".repeat(i + 3)).unwrap(),
-                )],
+                signature: vec![Felt::from_hex_str(&format!("0x{}", "3".repeat(i + 3)))
+                    .map(TransactionSignatureElem)
+                    .unwrap()],
             }),
         },
         x if (INVOKE_TRANSACTIONS_PER_BLOCK
@@ -86,33 +98,46 @@ pub(crate) fn create_transactions_and_receipts(
             .contains(&x) =>
         {
             Transaction {
-                hash: TransactionHash(Felt::from_hex_str(&"9".repeat(i + 3)).unwrap()),
+                hash: Felt::from_hex_str(&format!("0x{}", "9".repeat(i + 3)))
+                    .map(TransactionHash)
+                    .unwrap(),
                 variant: TransactionVariant::DeployV0(DeployTransactionV0 {
-                    contract_address: ContractAddress::new_or_panic(
-                        Felt::from_hex_str(&"5".repeat(i + 3)).unwrap(),
-                    ),
-                    contract_address_salt: ContractAddressSalt(
-                        Felt::from_hex_str(&"6".repeat(i + 3)).unwrap(),
-                    ),
-                    class_hash: ClassHash(Felt::from_hex_str(&"7".repeat(i + 3)).unwrap()),
-                    constructor_calldata: vec![ConstructorParam(
-                        Felt::from_hex_str(&"8".repeat(i + 3)).unwrap(),
-                    )],
+                    contract_address: Felt::from_hex_str(&format!("0x{}", "5".repeat(i + 3)))
+                        .map(ContractAddress::new_or_panic)
+                        .unwrap(),
+                    contract_address_salt: Felt::from_hex_str(&format!("0x{}", "6".repeat(i + 3)))
+                        .map(ContractAddressSalt)
+                        .unwrap(),
+                    class_hash: Felt::from_hex_str(&format!("0x{}", "7".repeat(i + 3)))
+                        .map(ClassHash)
+                        .unwrap(),
+                    constructor_calldata: vec![Felt::from_hex_str(&format!(
+                        "0x{}",
+                        "8".repeat(i + 3)
+                    ))
+                    .map(ConstructorParam)
+                    .unwrap()],
                 }),
             }
         }
         _ => Transaction {
-            hash: TransactionHash(Felt::from_hex_str(&"e".repeat(i + 3)).unwrap()),
+            hash: Felt::from_hex_str(&format!("0x{}", "e".repeat(i + 3)))
+                .map(TransactionHash)
+                .unwrap(),
             variant: TransactionVariant::DeclareV0(DeclareTransactionV0V1 {
-                class_hash: ClassHash(Felt::from_hex_str(&"a".repeat(i + 3)).unwrap()),
+                class_hash: Felt::from_hex_str(&format!("0x{}", "a".repeat(i + 3)))
+                    .map(ClassHash)
+                    .unwrap(),
                 max_fee: Fee::ZERO,
-                nonce: TransactionNonce(Felt::from_hex_str(&"b".repeat(i + 3)).unwrap()),
-                sender_address: ContractAddress::new_or_panic(
-                    Felt::from_hex_str(&"c".repeat(i + 3)).unwrap(),
-                ),
-                signature: vec![TransactionSignatureElem(
-                    Felt::from_hex_str(&"d".repeat(i + 3)).unwrap(),
-                )],
+                nonce: Felt::from_hex_str(&format!("0x{}", "b".repeat(i + 3)))
+                    .map(TransactionNonce)
+                    .unwrap(),
+                sender_address: Felt::from_hex_str(&format!("0x{}", "c".repeat(i + 3)))
+                    .map(ContractAddress::new_or_panic)
+                    .unwrap(),
+                signature: vec![Felt::from_hex_str(&format!("0x{}", "d".repeat(i + 3)))
+                    .map(TransactionSignatureElem)
+                    .unwrap()],
             }),
         },
     });
@@ -144,12 +169,16 @@ pub(crate) fn create_transactions_and_receipts(
             };
             let events = if i % transactions_per_block < EVENTS_PER_BLOCK {
                 vec![pathfinder_common::event::Event {
-                    from_address: ContractAddress::new_or_panic(
-                        Felt::from_hex_str(&"2".repeat(i + 3)).unwrap(),
-                    ),
-                    data: vec![EventData(Felt::from_hex_str(&"c".repeat(i + 3)).unwrap())],
+                    from_address: Felt::from_hex_str(&format!("0x{}", "2".repeat(i + 3)))
+                        .map(ContractAddress::new_or_panic)
+                        .unwrap(),
+                    data: vec![Felt::from_hex_str(&format!("0x{}", "c".repeat(i + 3)))
+                        .map(EventData)
+                        .unwrap()],
                     keys: vec![
-                        EventKey(Felt::from_hex_str(&"d".repeat(i + 3)).unwrap()),
+                        Felt::from_hex_str(&format!("0x{}", "d".repeat(i + 3)))
+                            .map(EventKey)
+                            .unwrap(),
                         event_key!("0xdeadbeef"),
                     ],
                 }]