diff --git a/rust/logos-blockchain-circuits-poc-sys/src/native.rs b/rust/logos-blockchain-circuits-poc-sys/src/native.rs
index 959926f..9933d7a 100644
--- a/rust/logos-blockchain-circuits-poc-sys/src/native.rs
+++ b/rust/logos-blockchain-circuits-poc-sys/src/native.rs
@@ -60,6 +60,29 @@ mod tests {
     static INPUTS: LazyLock<PathBuf> =
         LazyLock::new(|| PathBuf::from(env!("CARGO_MANIFEST_DIR")).join("sample.input.json"));
 
+    #[test]
+    fn test_generate_witness_invalid_json_returns_err() {
+        let input = PocWitnessInput::new("{".to_owned()).unwrap();
+        assert!(generate_witness(&input).is_err());
+    }
+
+    #[test]
+    fn test_generate_witness_missing_inputs_returns_err() {
+        let input = PocWitnessInput::new("{}".to_owned()).unwrap();
+        assert!(generate_witness(&input).is_err());
+    }
+
+    #[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();
+        assert!(generate_witness(&input).is_err());
+    }
+
     #[test]
     fn test_generate_witness() {
         let dat = LIB_DIR.join("witness_generator");
diff --git a/rust/logos-blockchain-circuits-pol-sys/src/native.rs b/rust/logos-blockchain-circuits-pol-sys/src/native.rs
index 62c3d43..ca465b9 100644
--- a/rust/logos-blockchain-circuits-pol-sys/src/native.rs
+++ b/rust/logos-blockchain-circuits-pol-sys/src/native.rs
@@ -60,6 +60,29 @@ mod tests {
     static INPUTS: LazyLock<PathBuf> =
         LazyLock::new(|| PathBuf::from(env!("CARGO_MANIFEST_DIR")).join("sample.input.json"));
 
+    #[test]
+    fn test_generate_witness_invalid_json_returns_err() {
+        let input = PolWitnessInput::new("{".to_owned()).unwrap();
+        assert!(generate_witness(&input).is_err());
+    }
+
+    #[test]
+    fn test_generate_witness_missing_inputs_returns_err() {
+        let input = PolWitnessInput::new("{}".to_owned()).unwrap();
+        assert!(generate_witness(&input).is_err());
+    }
+
+    #[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();
+        assert!(generate_witness(&input).is_err());
+    }
+
     #[test]
     fn test_generate_witness() {
         let dat = LIB_DIR.join("witness_generator");
diff --git a/rust/logos-blockchain-circuits-poq-sys/src/native.rs b/rust/logos-blockchain-circuits-poq-sys/src/native.rs
index 1570c7e..2e51eb2 100644
--- a/rust/logos-blockchain-circuits-poq-sys/src/native.rs
+++ b/rust/logos-blockchain-circuits-poq-sys/src/native.rs
@@ -60,6 +60,31 @@ mod tests {
     static INPUTS: LazyLock<PathBuf> =
         LazyLock::new(|| PathBuf::from(env!("CARGO_MANIFEST_DIR")).join("sample.input.json"));
 
+    #[test]
+    fn test_generate_witness_invalid_json_returns_err() {
+        let input = PoqWitnessInput::new("{".to_owned()).unwrap();
+        assert!(generate_witness(&input).is_err());
+    }
+
+    #[test]
+    fn test_generate_witness_missing_inputs_returns_err() {
+        let input = PoqWitnessInput::new("{}".to_owned()).unwrap();
+        assert!(generate_witness(&input).is_err());
+    }
+
+    #[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();
+        assert!(generate_witness(&input).is_err());
+    }
+
     #[test]
     fn test_generate_witness() {
         let dat = LIB_DIR.join("witness_generator");
diff --git a/rust/logos-blockchain-circuits-signature-sys/src/native.rs b/rust/logos-blockchain-circuits-signature-sys/src/native.rs
index 62f710b..c2c847f 100644
--- a/rust/logos-blockchain-circuits-signature-sys/src/native.rs
+++ b/rust/logos-blockchain-circuits-signature-sys/src/native.rs
@@ -65,6 +65,18 @@ mod tests {
     static INPUTS: LazyLock<PathBuf> =
         LazyLock::new(|| PathBuf::from(env!("CARGO_MANIFEST_DIR")).join("sample.input.json"));
 
+    #[test]
+    fn test_generate_witness_invalid_json_returns_err() {
+        let input = SignatureWitnessInput::new("{".to_owned()).unwrap();
+        assert!(generate_witness(&input).is_err());
+    }
+
+    #[test]
+    fn test_generate_witness_missing_inputs_returns_err() {
+        let input = SignatureWitnessInput::new("{}".to_owned()).unwrap();
+        assert!(generate_witness(&input).is_err());
+    }
+
     #[test]
     fn test_generate_witness() {
         let dat = LIB_DIR.join("witness_generator");
diff --git a/rust/logos-blockchain-circuits-tests/tests/conflicts.rs b/rust/logos-blockchain-circuits-tests/tests/conflicts.rs
index 5d58d5d..446718e 100644
--- a/rust/logos-blockchain-circuits-tests/tests/conflicts.rs
+++ b/rust/logos-blockchain-circuits-tests/tests/conflicts.rs
@@ -22,4 +22,23 @@ mod tests {
         let poq_result = lbc_poq_sys::generate_witness(&inputs_json);
         assert!(poq_result.is_ok());
     }
+
+    #[test]
+    fn test_concurrent_poq_calls() {
+        let inputs_json_raw = std::fs::read_to_string(inputs::POQ.as_path()).unwrap();
+
+        let handles: Vec<_> = (0..4)
+            .map(|_| {
+                let json = inputs_json_raw.clone();
+                std::thread::spawn(move || {
+                    let input = PoqWitnessInput::new(json).unwrap();
+                    lbc_poq_sys::generate_witness(&input)
+                })
+            })
+            .collect();
+
+        for h in handles {
+            assert!(h.join().unwrap().is_ok());
+        }
+    }
 }