In a development that reignites discussions about Bitcoin’s potential evolution beyond its original scope, Input Output (IO), the company responsible for Cardano, has introduced a new compilation pipeline. This pipeline enables Cardano smart contracts to run on Bitcoin without altering its consensus code.
“A groundbreaking compilation pipeline from Input | Output allows a Cardano smart contract, written in modern languages like Python, TypeScript, or Scala, to operate on Bitcoin without any modifications to Bitcoin itself,” the company announced on X late Tuesday night, accompanied by a detailed six-post technical thread explaining the process.
Cardano Smart Contracts Arrive on Bitcoin
The thread outlines a sequence of transformations that start with high-level source code and culminate in Bitcoin Script. Initially, Cardano’s toolchain converts the contract into Untyped Plutus Core (UPLC), a simplified functional bytecode. This UPLC is then serialized and processed by a compact interpreter known as the Control–Environment–Continuation (CEK) machine. Instead of requiring Bitcoin to directly interpret UPLC, IO compiles the CEK machine into RISC-V, a straightforward CPU instruction set. The resulting RISC-V program is the payload that Bitcoin eventually verifies, with the serialized UPLC provided at runtime.
In a blog post authored by software engineer Riley Kilgore, the design is summarized succinctly: “By integrating a serialized smart contract format (Untyped Plutus Core – UPLC), an innovative interpreter architecture (CEK machine), and a widely supported open-source reduced instruction set architecture (RISC-V), IO is realizing this concept.” Because the RISC-V code is deterministic and its execution trace can be verified—or contested—within Bitcoin Script, no soft-fork is necessary. Bitcoin remains unchanged, with expressiveness transferred to an auxiliary virtual CPU whose behavior can be examined step by step.
The security mechanism for these examinations is BitVMX, an evolution of Robin Linus’s 2023 BitVM proposal for “verifiable computation” on Bitcoin. IO’s press release describes BitVMX as an “innovative, trust-minimized protocol” that “relies on a 1-out-of-n honest-operator model” and allows developers to “pay transaction fees in BTC directly from their chosen wallet.” BitVMX adopts the optimistic-roll-up approach of BitVM—off-chain execution and on-chain fraud proofs—yet replaces BitVM’s custom circuits with a full RISC-V core, enabling unaltered binaries compiled by standard toolchains.
The aim is to integrate Cardano’s eUTxO-based smart-contract ecosystem—Marlowe, Aiken, and others—into Bitcoin’s extensive liquidity network. Practically, this could facilitate lending, swaps, and even NFT-backed loans secured by BTC collateral, all while avoiding custodial control by a sidechain.
For Bitcoin enthusiasts, this advancement is both familiar and controversial. Taproot in 2021 enhanced Script with Schnorr signatures and key-path spending, introducing the network to complex smart-contract logic, while BitVM in 2023 demonstrated that arbitrary computation could be verified instead of executed by Bitcoin nodes. IO’s pipeline advances these concepts further while maintaining the base layer unchanged—a design principle deemed essential for Bitcoin’s social contract by BitVM’s creators.
The compilation process is still under development; IO has yet to announce a main-net release date or publish gas-cost benchmarks for standard contracts. However, it has successfully demonstrated a theoretically comprehensive bridge between two philosophically distinct blockchains. If engineering challenges—such as transaction sizes, dispute latency, and user-experience issues—can be managed, Cardano’s software stack might soon be fully integrated with Bitcoin.
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