diff --git a/rust/Cargo.lock b/rust/Cargo.lock
index a73e5ad..b2350bc 100644
--- a/rust/Cargo.lock
+++ b/rust/Cargo.lock
@@ -208,6 +208,7 @@ dependencies = [
  "logos-blockchain-circuits-build",
  "logos-blockchain-circuits-common",
  "logos-blockchain-circuits-types",
+ "serde_json",
 ]
 
 [[package]]
@@ -217,6 +218,7 @@ dependencies = [
  "logos-blockchain-circuits-build",
  "logos-blockchain-circuits-common",
  "logos-blockchain-circuits-types",
+ "serde_json",
 ]
 
 [[package]]
@@ -226,6 +228,7 @@ dependencies = [
  "logos-blockchain-circuits-build",
  "logos-blockchain-circuits-common",
  "logos-blockchain-circuits-types",
+ "serde_json",
 ]
 
 [[package]]
@@ -253,6 +256,12 @@ dependencies = [
  "libc",
 ]
 
+[[package]]
+name = "memchr"
+version = "2.8.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6b947ae49db0d222b1dbc6b113ce7248a3fc3a6ca21b696717bfc000ba4484d8"
+
 [[package]]
 name = "miniz_oxide"
 version = "0.8.9"
@@ -387,6 +396,48 @@ dependencies = [
  "untrusted",
 ]
 
+[[package]]
+name = "serde"
+version = "1.0.228"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9a8e94ea7f378bd32cbbd37198a4a91436180c5bb472411e48b5ec2e2124ae9e"
+dependencies = [
+ "serde_core",
+]
+
+[[package]]
+name = "serde_core"
+version = "1.0.228"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "41d385c7d4ca58e59fc732af25c3983b67ac852c1a25000afe1175de458b67ad"
+dependencies = [
+ "serde_derive",
+]
+
+[[package]]
+name = "serde_derive"
+version = "1.0.228"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d540f220d3187173da220f885ab66608367b6574e925011a9353e4badda91d79"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+]
+
+[[package]]
+name = "serde_json"
+version = "1.0.150"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e8014e44b4736ed0538adeecded0fce2a272f22dc9578a7eb6b2d9993c74cfb9"
+dependencies = [
+ "itoa",
+ "memchr",
+ "serde",
+ "serde_core",
+ "zmij",
+]
+
 [[package]]
 name = "shlex"
 version = "1.3.0"
@@ -612,3 +663,9 @@ name = "zeroize"
 version = "1.8.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "b97154e67e32c85465826e8bcc1c59429aaaf107c1e4a9e53c8d8ccd5eff88d0"
+
+[[package]]
+name = "zmij"
+version = "1.0.21"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b8848ee67ecc8aedbaf3e4122217aff892639231befc6a1b58d29fff4c2cabaa"
diff --git a/rust/Cargo.toml b/rust/Cargo.toml
index 4c72d98..e76662a 100644
--- a/rust/Cargo.toml
+++ b/rust/Cargo.toml
@@ -36,6 +36,7 @@ dirs = "^6"
 fd-lock = "^4"
 flate2 = "^1.1"
 libc = "^0.2"
+serde_json = "^1"
 tar = "^0.4"
 ureq = "^3.3"
 bytes = "^1.11"
diff --git a/rust/logos-blockchain-circuits-poc-sys/Cargo.toml b/rust/logos-blockchain-circuits-poc-sys/Cargo.toml
index a3824ed..910cd03 100644
--- a/rust/logos-blockchain-circuits-poc-sys/Cargo.toml
+++ b/rust/logos-blockchain-circuits-poc-sys/Cargo.toml
@@ -19,5 +19,8 @@ workspace = true
 lbc-types = { workspace = true }
 lbc-common = { workspace = true }
 
+[dev-dependencies]
+serde_json = { workspace = true }
+
 [build-dependencies]
 lbc-build = { workspace = true }
diff --git a/rust/logos-blockchain-circuits-poc-sys/src/native.rs b/rust/logos-blockchain-circuits-poc-sys/src/native.rs
index 9933d7a..bbc6527 100644
--- a/rust/logos-blockchain-circuits-poc-sys/src/native.rs
+++ b/rust/logos-blockchain-circuits-poc-sys/src/native.rs
@@ -75,11 +75,9 @@ mod tests {
     #[test]
     fn test_generate_witness_constraint_violation_returns_err() {
         let json = std::fs::read_to_string(&*INPUTS).unwrap();
-        let bad_json = json.replace(
-            "\"voucher_root\": \"20810875415353676096192834577269613981524168537821543897016159330974871397924\"",
-            "\"voucher_root\": \"1\"",
-        );
-        let input = PocWitnessInput::new(bad_json).unwrap();
+        let mut inputs: serde_json::Value = serde_json::from_str(&json).unwrap();
+        inputs["voucher_root"] = serde_json::json!("1");
+        let input = PocWitnessInput::new(serde_json::to_string(&inputs).unwrap()).unwrap();
         assert!(generate_witness(&input).is_err());
     }
 
diff --git a/rust/logos-blockchain-circuits-pol-sys/Cargo.toml b/rust/logos-blockchain-circuits-pol-sys/Cargo.toml
index 08481ff..03f3fa9 100644
--- a/rust/logos-blockchain-circuits-pol-sys/Cargo.toml
+++ b/rust/logos-blockchain-circuits-pol-sys/Cargo.toml
@@ -19,5 +19,8 @@ workspace = true
 lbc-types = { workspace = true }
 lbc-common = { workspace = true }
 
+[dev-dependencies]
+serde_json = { workspace = true }
+
 [build-dependencies]
 lbc-build = { workspace = true }
diff --git a/rust/logos-blockchain-circuits-pol-sys/src/native.rs b/rust/logos-blockchain-circuits-pol-sys/src/native.rs
index ca465b9..c177e0c 100644
--- a/rust/logos-blockchain-circuits-pol-sys/src/native.rs
+++ b/rust/logos-blockchain-circuits-pol-sys/src/native.rs
@@ -75,11 +75,9 @@ mod tests {
     #[test]
     fn test_generate_witness_constraint_violation_returns_err() {
         let json = std::fs::read_to_string(&*INPUTS).unwrap();
-        let bad_json = json.replace(
-            "\"ledger_aged\": \"9907496234164738674719754286318998202143315407023653151112941050435603056651\"",
-            "\"ledger_aged\": \"1\"",
-        );
-        let input = PolWitnessInput::new(bad_json).unwrap();
+        let mut inputs: serde_json::Value = serde_json::from_str(&json).unwrap();
+        inputs["ledger_aged"] = serde_json::json!("1");
+        let input = PolWitnessInput::new(serde_json::to_string(&inputs).unwrap()).unwrap();
         assert!(generate_witness(&input).is_err());
     }
 
diff --git a/rust/logos-blockchain-circuits-poq-sys/Cargo.toml b/rust/logos-blockchain-circuits-poq-sys/Cargo.toml
index 3e29099..e4230c1 100644
--- a/rust/logos-blockchain-circuits-poq-sys/Cargo.toml
+++ b/rust/logos-blockchain-circuits-poq-sys/Cargo.toml
@@ -19,5 +19,8 @@ workspace = true
 lbc-types = { workspace = true }
 lbc-common = { workspace = true }
 
+[dev-dependencies]
+serde_json = { workspace = true }
+
 [build-dependencies]
 lbc-build = { workspace = true }
diff --git a/rust/logos-blockchain-circuits-poq-sys/src/native.rs b/rust/logos-blockchain-circuits-poq-sys/src/native.rs
index 2e51eb2..0189e09 100644
--- a/rust/logos-blockchain-circuits-poq-sys/src/native.rs
+++ b/rust/logos-blockchain-circuits-poq-sys/src/native.rs
@@ -75,13 +75,9 @@ mod tests {
     #[test]
     fn test_generate_witness_constraint_violation_returns_err() {
         let json = std::fs::read_to_string(&*INPUTS).unwrap();
-        // Swap core_root for a wrong value; the Merkle path no longer verifies,
-        // so is_registered.out = 0 and the constraint at circom line 108 fires.
-        let bad_json = json.replace(
-            "\"core_root\": \"20423847801203321296654759690878714805328777188198550442842378955293864405749\"",
-            "\"core_root\": \"1\"",
-        );
-        let input = PoqWitnessInput::new(bad_json).unwrap();
+        let mut inputs: serde_json::Value = serde_json::from_str(&json).unwrap();
+        inputs["core_root"] = serde_json::json!("1");
+        let input = PoqWitnessInput::new(serde_json::to_string(&inputs).unwrap()).unwrap();
         assert!(generate_witness(&input).is_err());
     }