Acai

Inspiration

The Acai contract was inspired by a recurring issue in Bitcoin-based minting systems — frontrunning and RBF (Replace-by-Fee) attacks, especially in open public mempools. Existing solutions often require fully private infrastructure or custom transaction types. I wanted to explore a new design space: can we discourage public mempools economically instead of technically forbidding them?

The project also builds on the vision of making Bitcoin's L1 more programmable via WASM smart contracts — particularly in the Alkanes ecosystem.

What it does

Acai is a RBF-resistant smart contract for minting tokens on Bitcoin L1 using Alkanes WASM. It introduces two key protections:

• Economic discouragement of public mempool use: Minting through a public mempool has only a 14–20% chance of success, while using Rebar Shield ensures 100%.
• Coinbase miner tag validation: The contract parses the coinbase transaction's scriptSig to ensure the block was mined by a Rebar partner.

It also includes creator enforcement logic: minting is only valid if a portion of the output is paid to the contract creator — ensuring economic alignment and sustainability.

How we built it

The project was built as a modification of the open-source free-mint WASM smart contract provided by Alkanes. The changes included:

• Extending validation logic to parse the scriptSig of the coinbase input
• Implementing a list of approved miner tags
• Adding a rule that checks for required output to the creator’s address
• Deploying and testing the logic via the fartane.com interface

The tooling used included Rust, the Alkanes SDK, and test deployments via Rebar-connected mining pools.

Challenges we ran into

• Lack of standardization in coinbase scriptSig: Miner tags are arbitrary, variable, and often obfuscated.
• Testing against real blocks: Verifying miner tags against actual Rebar-partnered blocks required manual inspection and iteration.

Accomplishments that we're proud of

• First fully functioning MEV-resistant WASM smart contract on Bitcoin L1
• Successfully processed over 317 mint transactions through Rebar Shield
• On-chain enforcement of creator compensation without breaking standard Bitcoin rules
• Proof-of-concept that a contract can reshape mempool dynamics

What we learned

• Bitcoin’s coinbase structure can be leveraged for protocol logic without soft forks or consensus changes
• WASM on Bitcoin via Alkanes opens the door to a whole new class of programmable interactions
• Economic alignment (e.g., incentivizing use of Rebar) can be more powerful than hard restrictions

What's next for Acai

• Generalizing the validation pattern for other sensitive operations beyond minting — including auctions, lending triggers, and fair token distributions
• Proposing a standard format for miner tags in coinbase scriptSig to simplify future contract validations
• Optimizing for fuel cost and expanding developer tooling
• Potentially integrating with other dApps on Alkanes that need trust-minimized transaction ordering

Built With

• alkanes
• fartane.com
• metashrew
• oyl-sdk
• rust
• rust-bitcoin
• shield
• wasm