diff --git a/crates/core/src/decode/json_to_scval.rs b/crates/core/src/decode/json_to_scval.rs
new file mode 100644
index 00000000..656455ed
--- /dev/null
+++ b/crates/core/src/decode/json_to_scval.rs
@@ -0,0 +1,311 @@
+use anyhow::{anyhow, Result};
+use serde_json::Value;
+use std::str::FromStr;
+use stellar_xdr::curr::{
+    ContractExecutable, Hash, Int128Parts, ScAddress, ScBytes, ScContractInstance, ScError,
+    ScErrorCode, ScMap, ScMapEntry, ScNonceKey, ScString, ScVal, ScVec, StringM, UInt128Parts,
+};
+
+const MAX_SCVAL_DEPTH: usize = 100;
+
+/// Parses a `serde_json::Value` back into an `ScVal`.
+pub fn json_to_scval(value: &Value) -> Result<ScVal> {
+    convert(value, 0)
+}
+
+fn convert(value: &Value, depth: usize) -> Result<ScVal> {
+    if depth > MAX_SCVAL_DEPTH {
+        return Err(anyhow!(
+            "max recursion depth ({}) exceeded",
+            MAX_SCVAL_DEPTH
+        ));
+    }
+
+    match value {
+        Value::Null => Ok(ScVal::Void),
+        Value::Bool(b) => Ok(ScVal::Bool(*b)),
+        Value::Number(num) => {
+            if let Some(i) = num.as_i64() {
+                if i >= 0 && i <= u32::MAX as i64 {
+                    Ok(ScVal::U32(i as u32))
+                } else if i >= i32::MIN as i64 && i <= i32::MAX as i64 {
+                    Ok(ScVal::I32(i as i32))
+                } else if i < 0 {
+                    Ok(ScVal::I64(i))
+                } else {
+                    Ok(ScVal::U64(i as u64))
+                }
+            } else if let Some(u) = num.as_u64() {
+                if u <= u32::MAX as u64 {
+                    Ok(ScVal::U32(u as u32))
+                } else {
+                    Ok(ScVal::U64(u))
+                }
+            } else {
+                Err(anyhow!(
+                    "unsupported float or out of bounds number: {}",
+                    num
+                ))
+            }
+        }
+        Value::String(s) => convert_string(s),
+        Value::Array(arr) => {
+            // Check if this is the lossless map fallback: [{"key": ..., "value": ...}, ...]
+            let is_lossless_map = !arr.is_empty()
+                && arr.iter().all(|v| {
+                    v.as_object().map_or(false, |obj| {
+                        obj.len() == 2 && obj.contains_key("key") && obj.contains_key("value")
+                    })
+                });
+
+            if is_lossless_map {
+                let mut entries = Vec::with_capacity(arr.len());
+                for item in arr {
+                    let obj = item.as_object().unwrap();
+                    let key = convert(obj.get("key").unwrap(), depth + 1)?;
+                    let val = convert(obj.get("value").unwrap(), depth + 1)?;
+                    entries.push(ScMapEntry { key, val });
+                }
+                return Ok(ScVal::Map(Some(ScMap(
+                    entries.try_into().map_err(|_| anyhow!("map too large"))?,
+                ))));
+            }
+
+            // Normal Array
+            let mut items = Vec::with_capacity(arr.len());
+            for item in arr {
+                items.push(convert(item, depth + 1)?);
+            }
+            Ok(ScVal::Vec(Some(ScVec(
+                items.try_into().map_err(|_| anyhow!("array too large"))?,
+            ))))
+        }
+        Value::Object(obj) => {
+            if obj.contains_key("__truncated__") {
+                return Err(anyhow!("cannot convert truncated JSON back to ScVal"));
+            }
+
+            // Check for explicit markers
+            if obj.contains_key("type") && obj.contains_key("code") && obj.len() == 2 {
+                if let Ok(err) = parse_error(obj) {
+                    return Ok(ScVal::Error(err));
+                }
+            }
+
+            if obj.contains_key("nonce") && obj.len() == 1 {
+                if let Some(nonce_val) = obj.get("nonce").and_then(|v| v.as_i64()) {
+                    return Ok(ScVal::LedgerKeyNonce(ScNonceKey { nonce: nonce_val }));
+                }
+            }
+
+            if obj.contains_key("executable") && obj.contains_key("storage") {
+                if let Ok(instance) = parse_contract_instance(obj, depth) {
+                    return Ok(ScVal::ContractInstance(instance));
+                }
+            }
+
+            // Normal Map with stringified keys
+            let mut entries = Vec::with_capacity(obj.len());
+            for (k, v) in obj {
+                let key_val = convert_string(k)?;
+                let val_val = convert(v, depth + 1)?;
+                entries.push(ScMapEntry {
+                    key: key_val,
+                    val: val_val,
+                });
+            }
+            Ok(ScVal::Map(Some(ScMap(
+                entries.try_into().map_err(|_| anyhow!("map too large"))?,
+            ))))
+        }
+    }
+}
+
+fn convert_string(s: &str) -> Result<ScVal> {
+    if s == "LedgerKeyContractInstance" {
+        return Ok(ScVal::LedgerKeyContractInstance);
+    }
+    if let Some(hex) = s.strip_prefix("0x") {
+        if let Ok(bytes) = hex::decode(hex) {
+            return Ok(ScVal::Bytes(ScBytes(
+                bytes.try_into().map_err(|_| anyhow!("bytes too large"))?,
+            )));
+        }
+    }
+    if s.starts_with('G') && s.len() == 56 {
+        if let Ok(pubkey) = stellar_strkey::ed25519::PublicKey::from_string(s) {
+            return Ok(ScVal::Address(ScAddress::Account(
+                stellar_xdr::curr::AccountId(stellar_xdr::curr::PublicKey::PublicKeyTypeEd25519(
+                    stellar_xdr::curr::Uint256(pubkey.0),
+                )),
+            )));
+        }
+    }
+    if s.starts_with('C') && s.len() == 56 {
+        if let Ok(contract) = stellar_strkey::Contract::from_string(s) {
+            return Ok(ScVal::Address(ScAddress::Contract(Hash(contract.0))));
+        }
+    }
+
+    // Try parsing large numbers (u128, i128). We skip 256-bit parsing here as a simplification,
+    // falling back to String if they exceed u128.
+    if let Ok(u) = u128::from_str(s) {
+        if u > u64::MAX as u128 {
+            let hi = (u >> 64) as u64;
+            let lo = u as u64;
+            return Ok(ScVal::U128(UInt128Parts { hi, lo }));
+        }
+    }
+    if let Ok(i) = i128::from_str(s) {
+        if i < i64::MIN as i128 || i > i64::MAX as i128 {
+            let hi = (i >> 64) as i64;
+            let lo = (i & 0xFFFFFFFFFFFFFFFF) as u64;
+            return Ok(ScVal::I128(Int128Parts { hi, lo }));
+        }
+    }
+
+    // Default to String. (Could be Symbol, but we can't infer that purely from a JSON string).
+    Ok(ScVal::String(ScString(
+        StringM::try_from(s.as_bytes().to_vec()).map_err(|_| anyhow!("string too long"))?,
+    )))
+}
+
+fn parse_error(obj: &serde_json::Map<String, Value>) -> Result<ScError> {
+    let err_type = obj.get("type").and_then(|v| v.as_str()).unwrap_or("");
+    let code_val = obj.get("code").unwrap();
+
+    let parse_code = |c: &Value| -> Result<ScErrorCode> {
+        let code_str = c.as_str().ok_or_else(|| anyhow!("code must be string"))?;
+        match code_str {
+            "UnexpectedType" => Ok(ScErrorCode::UnexpectedType),
+            "UnexpectedSize" => Ok(ScErrorCode::UnexpectedSize),
+            "MissingValue" => Ok(ScErrorCode::MissingValue),
+            "InternalError" => Ok(ScErrorCode::InternalError),
+            "ExceededLimit" => Ok(ScErrorCode::ExceededLimit),
+            "InvalidAction" => Ok(ScErrorCode::InvalidAction),
+            "InvalidInput" => Ok(ScErrorCode::InvalidInput),
+            _ => Err(anyhow!("unknown error code variant {}", code_str)),
+        }
+    };
+
+    match err_type {
+        "Contract" => {
+            let code = code_val
+                .as_u64()
+                .ok_or_else(|| anyhow!("Contract code must be u32"))? as u32;
+            Ok(ScError::Contract(code))
+        }
+        "WasmVm" => Ok(ScError::WasmVm(parse_code(code_val)?)),
+        "Context" => Ok(ScError::Context(parse_code(code_val)?)),
+        "Storage" => Ok(ScError::Storage(parse_code(code_val)?)),
+        "Object" => Ok(ScError::Object(parse_code(code_val)?)),
+        "Crypto" => Ok(ScError::Crypto(parse_code(code_val)?)),
+        "Events" => Ok(ScError::Events(parse_code(code_val)?)),
+        "Budget" => Ok(ScError::Budget(parse_code(code_val)?)),
+        "Value" => Ok(ScError::Value(parse_code(code_val)?)),
+        "Auth" => Ok(ScError::Auth(parse_code(code_val)?)),
+        _ => Err(anyhow!("Unknown ScError type: {}", err_type)),
+    }
+}
+
+fn parse_contract_instance(
+    obj: &serde_json::Map<String, Value>,
+    depth: usize,
+) -> Result<ScContractInstance> {
+    let exec_obj = obj
+        .get("executable")
+        .and_then(|v| v.as_object())
+        .ok_or_else(|| anyhow!("executable must be object"))?;
+
+    let exec_type = exec_obj.get("type").and_then(|v| v.as_str()).unwrap_or("");
+    let executable = if exec_type == "Wasm" {
+        let hash_str = exec_obj
+            .get("wasmHash")
+            .and_then(|v| v.as_str())
+            .unwrap_or("");
+        let hex = hash_str.strip_prefix("0x").unwrap_or(hash_str);
+        let bytes = hex::decode(hex)?;
+        ContractExecutable::Wasm(Hash(
+            bytes.try_into().map_err(|_| anyhow!("invalid hash size"))?,
+        ))
+    } else if exec_type == "StellarAsset" {
+        ContractExecutable::StellarAsset
+    } else {
+        return Err(anyhow!("Unknown executable type"));
+    };
+
+    let storage_val = obj.get("storage").unwrap();
+    let storage = if storage_val.is_null() {
+        None
+    } else {
+        match convert(storage_val, depth + 1)? {
+            ScVal::Map(Some(m)) => Some(m),
+            _ => return Err(anyhow!("storage must map to ScMap")),
+        }
+    };
+
+    Ok(ScContractInstance {
+        executable,
+        storage,
+    })
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::decode::scval_to_json;
+
+    #[test]
+    fn test_roundtrip_primitives() {
+        let cases = vec![
+            ScVal::Void,
+            ScVal::Bool(true),
+            ScVal::Bool(false),
+            ScVal::U32(42),
+            ScVal::I32(-7),
+            ScVal::U64(u64::MAX),
+            ScVal::I64(i64::MIN),
+            ScVal::String(ScString(StringM::try_from(b"hello".to_vec()).unwrap())),
+            ScVal::Bytes(ScBytes(vec![0xDE, 0xAD, 0xBE, 0xEF].try_into().unwrap())),
+        ];
+
+        for case in cases {
+            let json = scval_to_json(&case);
+            let parsed = json_to_scval(&json).expect("should parse");
+
+            // For strings, scval_to_json might have been passed a Symbol, but json_to_scval will parse it back as String.
+            // Since we pass ScVal::String above, it matches exactly.
+            assert_eq!(parsed, case, "failed on {:?}", case);
+        }
+    }
+
+    #[test]
+    fn test_lossless_map_roundtrip() {
+        // ScVal::U32(7) and ScVal::String("7") both stringify to the JSON key "7".
+        let map = ScVal::Map(Some(ScMap(
+            vec![
+                ScMapEntry {
+                    key: ScVal::U32(7),
+                    val: ScVal::String(ScString(
+                        StringM::try_from(b"from_number".to_vec()).unwrap(),
+                    )),
+                },
+                ScMapEntry {
+                    key: ScVal::String(ScString(StringM::try_from(b"7".to_vec()).unwrap())),
+                    val: ScVal::String(ScString(
+                        StringM::try_from(b"from_string".to_vec()).unwrap(),
+                    )),
+                },
+            ]
+            .try_into()
+            .unwrap(),
+        )));
+
+        let json = scval_to_json(&map);
+        // This should be the fallback array mode
+        assert!(json.is_array());
+
+        let parsed = json_to_scval(&json).unwrap();
+        assert_eq!(parsed, map);
+    }
+}
diff --git a/crates/core/src/decode/mod.rs b/crates/core/src/decode/mod.rs
index 63e0c9fd..9fddd5d7 100644
--- a/crates/core/src/decode/mod.rs
+++ b/crates/core/src/decode/mod.rs
@@ -13,6 +13,7 @@ pub mod event_walker;
 pub mod fee_analyzer;
 pub mod function_call_decoder;
 pub mod host_error;
+pub mod json_to_scval;
 pub mod mappings;
 /// Envelope-level decoding that emits one diagnostic report per operation.
 pub mod multi_op_decoder;
@@ -31,6 +32,7 @@ pub use auth_address_nonce::AddressWithNonce;
 pub use chain_analyzer::{analyze_call_chain, CallChain, ChainAnalyzer, ChainFrame, FrameRole};
 pub use deepest_error::{find_deepest_error, DeepestError, DeepestErrorFinder};
 pub use function_call_decoder::{DecodedArgument, DecodedFunctionCall, FunctionCallDecoder};
+pub use json_to_scval::json_to_scval;
 pub use multi_op_decoder::{decode_transaction_with_op_filter, MultiOpDecoder};
 pub use resource_analyzer::{
     MetricDiagnostic, MetricKind, ResourceDiagnostics, ResourceUsageAnalyzer, TransactionResultMeta,