Sui Move does not support custom serialization; all objects use BCS (Binary Canonical Serialization) by default. You cannot implement custom serialization like Ethereum ABI. To optimize storage, use efficient data structures (e.g., Table, Bag) and minimize field count. For versioning, use the sui::versioned module or dynamic fields to manage schema upgrades safely. Nested objects should be modeled as separate owned or shared objects, linked via IDs, rather than embedded, to avoid size limits and improve gas efficiency. Cross-chain compatibility must be handled off-chain, as on-chain Move types are not ABI-compatible.

Custom Serialization in Sui Move: Advanced Implementation Guide

Core Serialization Strategies

1. Gas-Efficient Custom Encoding

module my_module::custom_serializer {
    use sui::bcs;
    use sui::object::{UID, ID};

    // Packed encoding for storage efficiency
    public fun serialize<T: store>(obj: &T): vector<u8> {
        let bytes = b"";
        // Custom field-by-field serialization
        bytes.append(bcs::to_bytes(&obj.field1));
        bytes.append(bcs::to_bytes(&obj.field2));
        // Compress repeating patterns
        compress(bytes)
    }
}

Versioned Schema Pattern

2. Backward-Compatible Versioning

struct VersionedData has key, store {
    id: UID,
    version: u64,
    // Union of all version payloads
    payload: vector<u8>
}

public fun migrate_v1_to_v2(old: VersionedData): VersionedData {
    let (id, version, payload) = old.destruct();
    let new_payload = if (version == 1) {
        deserialize_v1(payload).serialize_v2()
    } else { payload };
    VersionedData { id, version: 2, payload: new_payload }
}

Complex Data Handling

3. Nested Object Serialization

public fun serialize_nested(objs: &vector<MyObject>): vector<u8> {
    let result = b"";
    let header = 0;
    
    // Two-pass: first collect all object IDs
    foreach obj in objs {
        header := header << 8 | obj.id().bytes.length();
    };
    
    result.append(bcs::to_bytes(&header));
    
    // Then serialize actual data
    foreach obj in objs {
        result.append(serialize_obj(obj));
    };
    
    result
}

Security Hardening

4. Safe Deserialization

public fun safe_deserialize(bytes: vector<u8>): T {
    // 1. Size limit check
    assert!(bytes.length() <= MAX_ALLOWED_SIZE, EINVALID_INPUT);
    
    // 2. Version check
    let version = bytes[0];
    assert!(version <= CURRENT_VERSION, EUNSUPPORTED_VERSION);
    
    // 3. Structure validation
    let expected_len = get_expected_length(version);
    assert!(bytes.length() == expected_len, ECORRUPTED_DATA);
    
    deserialize_impl(bytes)
}

Performance Optimization Table

CLI Testing Tools

# Benchmark serialization costs
sui client call --gas-budget 100000000 \
  --module serializer \
  --function benchmark \
  --args \"0x::my_module::MyObject\"

To implement custom serialization in Sui Move objects while optimizing for gas cost, cross-chain compatibility, version control, and complex struct handling, you need to step beyond default bcs::serialize behavior and create a modular approach using byte vectors and manual encoding logic. Sui’s Move doesn’t support true custom serializers like Rust or Solidity, but you can emulate the same principles by designing deterministic layouts using vector<u8> fields and strictly controlled layout contracts. This allows you to reduce BCS overhead, embed versioning tags, and interoperate with other ecosystems like Ethereum by aligning with ABI-compatible formats.

To achieve gas efficiency, avoid storing full objects recursively. Instead, flatten deeply nested data into compact byte arrays where only essential data is serialized, and use hashing or indexing for references. Use numeric representations (u8, u16, etc.) where possible instead of storing full strings or enums. For versioning, prefix your serialized blob with a u8 or u16 schema version, and define deserialization logic that branches conditionally based on that version. Each version-specific deserializer should handle its layout, ensuring backward compatibility while allowing safe upgrades.

For example:

struct CustomData has store {
    raw: vector<u8>,
}

public fun encode(v: u64, flag: bool): vector<u8> {
    let mut out = vector::empty<u8>();
    vector::push_back(&mut out, 1); // version
    vector::append(&mut out, bcs::to_bytes(&v));
    vector::push_back(&mut out, if flag { 1 } else { 0 });
    out
}

public fun decode(raw: &vector<u8>): (u64, bool) {
    let version = vector::borrow(raw, 0);
    assert!(*version == 1, 100); // unsupported version
    let v = bcs::from_bytes<u64>(vector::sub_range(raw, 1, 9));
    let flag = *vector::borrow(raw, 9) == 1;
    (v, flag)
}

For nested and recursive structures, consider encoding them into separate vector<u8> chunks and referencing them using Sui object IDs. Avoid deeply nested recursive data unless flattened or normalized, as Sui's object-centric model favors storing subcomponents as individual objects with pointers (object IDs) rather than embedding them directly.

To secure your serialization:

• Validate all lengths and indexes before access to avoid panics or denial-of-service (DoS).
• Limit vector lengths and sizes.
• Use checksums or hash validation if storing externally generated bytes.
• Avoid malleability by clearly defining encoding order and format (e.g., always little-endian, always fixed-length).

You can test and deploy this pattern using the Sui CLI:

sui move build
sui client publish --gas-budget 100000000

And simulate encoding off-chain in TypeScript or Python using compatible BCS encoders before sending it to the contract.

For broader insights, check: https://docs.sui.io/build/move – Official Move for Sui guide https://github.com/MystenLabs/sui/tree/main/crates/sui-framework – Source for bcs, hash, and vector modules

Let me know if you need a full encoder/decoder library or cross-chain ABI serializer between Sui and EVM.

To implement custom serialization in Sui Move objects while meeting your goals such as reducing BCS overhead, enabling cross-chain compatibility, and supporting versioned schemas you need to manually manage how data is encoded into vector<u8> fields instead of relying solely on bcs::serialize. This approach gives you more control over how data is stored and interpreted, especially when working with nested or recursive structures and version-sensitive schemas. For optimal gas efficiency, encode only the essential fields using fixed-size primitives like u8 and u64, avoid storing large or deeply nested structs, and flatten objects into compact byte arrays whenever possible. You can also offload storage to auxiliary objects and reference them using object IDs to keep the parent structure lean.

To support versioning and backward-compatible upgrades, prefix your serialized data with a version byte (e.g., vector::push_back(&mut data, 1)) and include branching logic during deserialization that reads and interprets the structure based on this version. This makes it easier to evolve your object format over time without breaking compatibility with older versions. When handling complex nested objects or recursive types, avoid direct nesting inside the parent object. Instead, treat each component as an object with its own serialization logic and use object IDs to link them together. This not only simplifies the byte layout but also fits well with Sui’s object-centric model.

To prevent security risks, validate all input lengths and indices before reading from byte vectors to avoid panics or buffer overruns. Use strict type boundaries and format checks to ensure that malformed data doesn’t lead to unexpected behavior or denial-of-service vectors. Avoid malleability by defining a fixed order for serialized fields and normalizing any variable-length data (e.g., hashing variable-length inputs or enforcing canonical encodings). Use unit tests and fuzzing to ensure your serializer and deserializer are symmetrical and deterministic.

For CLI testing, deploy your module using:

sui move build
sui client publish --gas-budget 100000000

You can then pass serialized byte arrays via:

sui client call --function your_function --args 0xSerializedBytes --gas-budget 10000000

To learn more and reference actual code patterns, check: https://docs.sui.io/build/move – Official Sui Move documentation https://github.com/MystenLabs/sui/tree/main/crates/sui-framework – Explore source modules like vector, hash, and bcs

Implementing custom serialization in Sui Move objects is challenging but achievable with a careful blend of design patterns and Sui-native constraints. Here's a structured breakdown to meet your goals across performance, versioning, complexity, and security:

1. Performance & Cost

Most gas-efficient pattern:

Use flat structs with primitive types (u8, u64, fixed-size vector) instead of nested or dynamic fields.

Avoid Option and large recursive types—each abstraction layer adds BCS overhead.

For packed storage, consider using fixed-length vector to represent encoded data and implement manual parsers in Move.

Use the bcs::to_bytes only where necessary and cache pre-serialized blobs if the content is immutable.

2. Versioning & Upgrades

Encoding/decoding with versioning:

Define a top-level version: u8 field in your object to signal schema version.

Implement version-specific deserialization functions like:

public fun decode_v1(data: &vector): MyType { ... } public fun decode_v2(data: &vector): MyType { ... }

Use a dispatch function that selects the decoder based on the embedded version.

For upgrade paths, include a migration module that can convert from V1 -> V2 and register changes on-chain if needed.

3. Complex Data Handling

Best practices for nested/recursive types:

Flatten deeply nested types by introducing surrogate ID references or intermediate storage objects.

Store only object IDs and use on-chain references to actual nested values (reduces serialization depth).

For recursive structures, define a "parent ID" pattern or list of children IDs instead of raw embedded fields.

4. Security Considerations

Avoid pitfalls like malleability or DoS:

Always validate input size before parsing vectors—especially user-submitted vector blobs.

Implement length prefixes or bounds checks manually in custom decoding logic to guard against overflow attacks.

Reject any deserialization attempt that fails strict checks (type tag, magic number, schema hash).

Consider gas costs of malformed data—ensure that parsing fails fast and does not consume excessive computation.

Additional Tips

Use bcs::from_bytes only if your type has a predictable layout and you can control the format.

External encoding (like Ethereum ABI) should happen off-chain, but store ABI-compatible data in a way that minimizes transformation.

Write detailed unit tests for encoding/decoding flows and ensure schema changes are non-breaking.

If you're building something cross-chain or version-sensitive, combining fixed layout and manual deserialization is safer than relying on native BCS, especially if schema evolution is expected.