MultiversX Sharp Edges

This skill alerts you to non-obvious behaviors, "gotchas," and platform-specific quirks that often lead to bugs.

1. Async Callbacks & Reverts

• •The Edge: When an Async Call fails, the #[callback] is executed.
• •The Sharp Logic:
  • •If you don't implement a callback, the funds return to the contract, but state changes in the original transaction are NOT reverted automatically.
  • •Mitigation: You must manually revert state in the callback if the sub-call failed (or use the synchronous back_transfers carefully).

2. Gas Limits & Out of Gas (OOG)

• •The Edge: OOG raises a specific error but can leave the system in a partial state if not handled.
• •The Sharp Logic:
  • •Cross-shard OOG: If the destination shard runs OOG, the transaction fails, but the sender shard has already processed the transfer. The callback is triggered with an error.

3. Storage Mappers vs Rust Types

• •The Edge: VecMapper is NOT a Vec.
• •The Sharp Logic:
  • •Vec loads everything into WASM memory (RAM spike).
  • •VecMapper loads nothing until you call get(i).
  • •Bug: Using Vec for a list of all users = OOG when userbase grows.

4. Token Decimal Precision

• •The Edge: ESDTs can have 0-18 decimals.
• •The Sharp Logic:
  • •Hardcoding 10^18 for "One Token" is wrong.
  • •Always fetch decimals or require standard 18-decimal tokens.

5. Upgradeability

• •The Edge: #[init] is NOT called on upgrade. #[upgrade] is called.
• •The Sharp Logic:
  • •If you add a new storage mapper, you must initialize it in #[upgrade].
  • •Changing the storage layout of existing mappers (e.g. struct field order) corrupts data.

6. Block Info inside Views

• •The Edge: get_block_timestamp() in a #[view] (off-chain simulation) might return 0 or a different value than on-chain.
• •The Sharp Logic:
  • •Don't rely on block info for critical off-chain view logic.