Bitcoin Covenants — Why CTV and CHECKSIGFROMSTACK Could Unlock Smart Contracts Without Ethereum

Bitcoin's scripting language is deliberately limited. Satoshi Nakamoto designed Script to be non-Turing-complete — capable of expressing conditions for spending a transaction output, but not capable of arbitrary computation. This design choice has been Bitcoin's most durable feature and its most persistent limitation simultaneously. Covenant proposals attempt to extend Script's expressiveness in a targeted direction: enabling a transaction output to constrain not just who can spend it, but how the resulting spend transaction must be constructed. The implications for Bitcoin smart contracts, if implemented, are significant.

## What Is a Covenant?

In property law, a covenant is a restriction that runs with the land — a limitation on how property can be used that binds future owners, not just the current one. In Bitcoin, a covenant is a mechanism that allows a UTXO (unspent transaction output) to restrict how the transaction that spends it must be structured.

Current Bitcoin Script can enforce conditions like "spendable by key A after block 800,000" or "requires two of three signatures from keys A, B, and C." What it cannot do is enforce conditions like "when spent, the output must send exactly this amount to this address" or "when spent, the resulting UTXO must itself be governed by this same script." That second category — the ability to impose conditions on future transaction structure, not just on who can spend the current output — is what covenants enable.

## OP_CHECKTEMPLATEVERIFY (BIP 119)

The most discussed covenant proposal is OP_CHECKTEMPLATEVERIFY (CTV), developed by Bitcoin developer Jeremy Rubin and specified in Bitcoin Improvement Proposal 119. CTV works by committing to a hash of the spending transaction's template: its version, locktime, input count, output structure, and sequence values. A script containing `<hash> OP_CTV` succeeds only if the spending transaction matches that pre-committed hash.

This limited expressiveness is intentional. CTV cannot enforce arbitrary conditions — it can only verify that a spending transaction matches a specific pre-defined template. This makes it useful for specific high-value applications:

**Congestion control**: A single on-chain transaction can commit to paying many recipients later, with the actual payments batching when fees are lower. The recipients can verify their payment is committed without an immediate on-chain transaction.

**Vaults**: A vault construction using CTV allows a UTXO to require that any spending transaction first move funds to a "warm wallet" with a time delay, during which the original owner can recover funds if the spending was unauthorised. This creates a recovery window after a key compromise.

**Payment channels**: CTV enables more efficient Lightning Network channel constructions and could support channel factories — shared channel constructions that reduce the on-chain footprint of multi-party payment networks.

## CHECKSIGFROMSTACK (CSFS)

CHECKSIGFROMSTACK is a more expressive opcode that verifies a signature against an arbitrary message that is on the script execution stack — as opposed to the current OP_CHECKSIG, which signs only the transaction itself. CSFS enables Bitcoin Script to verify signatures over data external to the transaction, opening up more complex conditional constructions.

The combination of CTV and CSFS enables what developers call "introspection" — a transaction output can inspect the properties of the transaction spending it and enforce conditions based on those properties. This approaches the expressiveness of Ethereum's smart contract environment while remaining far more constrained in scope.

## The Activation Debate

Bitcoin protocol changes require broad community consensus and a formal activation mechanism. The most recent major upgrade, Taproot, activated in November 2021 after several years of discussion and a Speedy Trial activation process. Covenant proposals have not yet achieved comparable consensus.

The technical debate centres on whether CTV's limited expressiveness is a feature or a bug. Critics argue that even limited covenants enable "coin control" — the ability of a UTXO creator to constrain future owners of those coins — which has implications for fungibility and financial censorship. Proponents argue that covenants are opt-in constructions that users choose to create and that they enable security improvements that benefit all Bitcoin users.

As of 2026, CTV has not been activated on Bitcoin mainnet. Development focus has shifted toward Taproot-based constructions and the broader covenants conversation has expanded to include multiple competing proposals including ANYPREVOUT and OP_CAT. The conservative Bitcoin upgrade process makes near-term activation uncertain, but the technical case for some form of covenant functionality continues to gain supporters among core developers.

Bitcoin Covenants — Why CTV and CHECKSIGFROMSTACK Could Unlock Smart Contracts Without Ethereum

// COMMENTS

ON THIS PAGE