Vyper struct ABI encoding vs Solidity caller interoperability

Answer

1. No, Vyper’s current external‑return struct encoding is not Solidity‑compatible. Vyper packs struct members tightly (no 32‑byte word padding), while Solidity’s ABI expects each member to start on a 32‑byte boundary.

2. There is no compiler flag to make Vyper emit Solidity‑aligned tuples. The packed layout is intentional and will remain until a breaking change is introduced in a major release.

3. Production contracts should pin Vyper to a specific patch version (e.g., 0.3.10) and run an integration test against a Solidity consumer on a forked mainnet.

Explanation

Vyper’s ABI follows the packed‑tuple rule. For a struct like

struct Data { uint128 a; address b; uint256 c; }

Vyper returns 48 bytes (16 + 20 + 32). Solidity’s abi.decode however expects 96 bytes because each field is padded to a full 32‑byte word. The decoder will therefore read the wrong values or revert if the data is too short.

Because the layout is not governed by semantic‑versioning guarantees, a minor or patch release could alter the packing rules without a major bump. The Vyper team has not announced any plan to align with Solidity’s tuple encoding.

Practical Steps for Your Case

1. Verify the raw return data. Deploy the Vyper contract and call the external function with cast call or an eth_call RPC. Confirm the byte length equals the sum of member sizes (packed).
2. Wrap the struct in a fixed‑size bytes array. In Vyper, return bytes[96] and pack the fields manually using concat and convert:

   def get() -> bytes[96]:
       return concat(convert(a, bytes[16]), convert(b, bytes[20]), convert(c, bytes[32]))

3. Decode on the Solidity side with explicit offsets. Use abi.decode on the 96‑byte array, or decode manually with assembly to avoid the padding mismatch.
4. Include an integration test. In your CI, compile the Vyper contract with the pinned version, deploy to a forked testnet, and call from a Solidity contract that logs each field. Verify the logged values match the expected ones.

Version Pinning Recommendation

Pin the Vyper compiler to a specific patch (e.g., 0.3.10) in your pyproject.toml or requirements.txt. Add a comment explaining that the packed‑tuple behavior is intentional and may change in a future minor release. Continue to run the integration test after every patch upgrade to catch silent drift.

Ask for Diagnostic Detail

If your Solidity consumer already performs padding (e.g., by using abi.encode on the struct), let us know so we can adjust the workaround accordingly.