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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchProof of Transfer (PoX) is Stacks’ consensus and block-production mechanism. Instead of asking miners to burn electricity to win a proof-of-work race, PoX has them transfer already-mined BTC through Bitcoin transactions to compete for the right to produce Stacks blocks. STX holders can lock their tokens as Stackers, validate and sign those blocks, and receive BTC rewards. Bitcoin supplies the commitment, leader-selection inputs and anchoring; Stacks remains a separate execution layer with its own blocks, state and Clarity smart contracts.
The Nakamoto upgrade, a 2024 Stacks hard fork, changed the timing: a Bitcoin sortition selects a miner for a tenure in which that miner can produce multiple Stacks blocks, rather than only one block for each Bitcoin block. Stackers sign and validate the blocks during the tenure.
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PoX in plain English
PoX links Stacks’ economy to Bitcoin without making Stacks a Bitcoin sidechain or requiring Stacks miners to mine Bitcoin. A miner commits BTC in the Bitcoin layer. That commitment enters a cryptographic, bid-weighted leader-election process. The selected miner assembles and publishes Stacks blocks, executes Stacks transactions and smart contracts, and can earn newly minted STX plus transaction fees.
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STX holders have a separate role. By locking STX at the protocol level, they can participate as Stackers (also called signers), checking blocks, signing valid blocks and propagating them. The BTC committed by miners is directed to participating Stackers as rewards. Locked STX stays in the holder’s wallet, but the protocol prevents it from being spent while the lock is active.
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Thus, “transfer” in PoX means a transfer of BTC as an economic commitment. It does not mean that miners transfer computing work to Bitcoin, and it does not mean that Stacks transactions execute on Bitcoin.
How a Stacks PoX round works
1. Miners register and commit BTC
A prospective miner prepares a Bitcoin-layer commitment transaction. The BTC is not consumed as electricity or converted into a proof-of-work hash. It is transferred according to PoX rules and recorded in a way that Stacks can verify. The amount and timing of commitments affect a miner’s chance in the selection process, but a large commitment is not an unconditional guarantee of winning.
PoX uses a verifiable random function (VRF) and bid-weighted selection. Bitcoin activity supplies an externally verifiable input, while the protocol’s selection logic determines which eligible miner wins a sortition. The mechanism therefore combines a cryptographic lottery with an economic bid.
2. Bitcoin provides the sortition input
Bitcoin blocks establish the ordering and confirmation context for miner commitments. Stacks observes the relevant Bitcoin-layer events and uses them to determine a winner. This is the point at which Bitcoin participates directly in leader election: it provides the settlement history and an input that miners cannot privately rewrite on the Stacks network.
3. The selected miner builds Stacks blocks
Once selected, the miner gathers pending Stacks transactions, runs Clarity smart-contract calls, updates Stacks state and publishes a Stacks block. The block is a Stacks object, not a Bitcoin block, and its execution occurs on Stacks nodes.
After Nakamoto, the selected miner can build multiple Stacks blocks during a single tenure between Bitcoin sortitions. This removes the old one-to-one rhythm in which Stacks block production followed each Bitcoin block. The miner’s tenure continues while the protocol accepts its blocks and Stackers validate them.
4. Stackers validate, sign and propagate
Stackers lock STX and perform consensus duties. They verify that a proposed block follows Stacks rules, sign blocks they consider valid and propagate those signatures and blocks to other participants. Their signatures provide an additional validity signal during a miner’s tenure; the miner is not the only party involved in confirming the chain’s progress.
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Stacking is not a custody transfer. The STX remains in the holder’s wallet, but protocol-level locking makes it unavailable for spending until the relevant lock period and conditions are complete. The documentation describes no slashing mechanism in PoX. A participant who fails required signer duties can nevertheless lose expected BTC rewards and may be unable to unlock STX as intended, so “no slashing” does not mean “no consequences.”
5. Rewards flow in two directions
- Miners: may receive newly minted STX and Stacks transaction fees for producing accepted blocks.
- Stackers: receive BTC committed by participating miners, subject to the protocol’s participation and reward rules.
- Bitcoin miners: do not receive a special PoX role merely because they mine Bitcoin. PoX miners are Stacks participants who make Bitcoin transactions and compete under Stacks rules.
What Bitcoin does—and does not do—in PoX
| Function | Bitcoin’s role | What remains on Stacks |
|---|---|---|
| Leader election | Bitcoin transactions and block ordering provide verifiable commitment inputs for the PoX sortition. | Stacks applies the VRF and bid-weighted rules, then identifies the miner allowed to produce blocks. |
| Execution | Bitcoin does not execute Stacks transactions or Clarity contracts. | Stacks nodes execute transactions and maintain Stacks state. |
| Block production | Bitcoin blocks establish the timing context for sortitions. | The selected miner produces Stacks blocks during its tenure. |
| Validation | Bitcoin confirms the underlying BTC transactions under Bitcoin’s own consensus. | Stackers and Stacks nodes validate and sign Stacks blocks. |
| Rewards | BTC is the asset transferred by miners and paid to eligible Stackers. | Miners can earn STX issuance and Stacks transaction fees. |
| Anchoring | Bitcoin history gives Stacks an external reference that is difficult to alter without controlling relevant Bitcoin conditions. | Stacks still has its own chain history, state transitions and reorganization rules. |
This distinction matters. Calling Stacks “Bitcoin” would be inaccurate: Stacks has a separate virtual machine, separate blocks and separate application state. Calling it completely independent of Bitcoin would also miss PoX’s central design, because Bitcoin-layer commitments and sortitions determine who gets to produce Stacks blocks.
What the Nakamoto upgrade changed
The Nakamoto release was a hard fork in the fourth quarter of 2024. Before it, Stacks block production generally tracked Bitcoin’s block cadence, with a one-to-one relationship between a Bitcoin block event and a Stacks block opportunity. Under the post-Nakamoto design, a Bitcoin sortition chooses a miner for a tenure, and that miner can produce multiple Stacks blocks before the next sortition.
Faster block cadence
Because one tenure can contain several Stacks blocks, applications do not have to wait for a fresh Bitcoin block for every Stacks block. The change was designed to increase transaction throughput and reduce confirmation delay on the Stacks layer.
Signer responsibilities become central
Stackers validate and sign the sequence of blocks produced during the tenure. Their signatures are part of the protocol’s finality and validity process, rather than a peripheral reward activity.
Bitcoin-linked finality and reorganization resistance
Stacks documentation describes the upgrade as providing “100% Bitcoin finality,” alongside resistance to Bitcoin-miner extractable value (MEV) affecting PoX and greater difficulty for chain reorganizations. That wording is the project’s description of the design, not an unconditional guarantee. The actual security properties depend on the stated Bitcoin-mining and Stacker assumptions, network operation and correct protocol implementation.
Reward cycles and the Bitcoin-block clock
The PoX overview describes a reward cycle of 2,100 Bitcoin blocks:
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- 2,000 blocks: the reward phase, during which the cycle’s reward activity takes place.
- 100 blocks: the prepare phase, used to prepare the next cycle.
These are Bitcoin-block counts, not fixed calendar periods. A cycle’s elapsed time changes with Bitcoin’s actual block-production rate, so a statement such as “one cycle lasts exactly X days” would be misleading. The published overview lists PoX 4 with a date of April 22, 2024; contract versions and cycle parameters can change, so anyone making an operational decision should check the current Stacks documentation and deployed contract.
What locking STX as a Stacker involves
Locking is protocol-level, not a wallet handoff
Stacking does not require sending STX to a miner or surrendering ownership to a third party. The tokens remain in the holder’s wallet address. While locked, however, they cannot be spent through the protocol. The holder must satisfy the cycle’s participation and unlock conditions before the tokens become available again.
Signing is an active responsibility
A Stacker is expected to monitor proposed blocks, validate them and sign when they meet the rules. A holder who locks STX but does not perform the required signer work should not assume that BTC rewards are automatic. Documentation notes that missed duties can result in lost BTC rewards and inability to unlock STX as expected.
There is no PoX slashing rule
The official Stacking documentation states that PoX has “no concept of slashing.” That means the protocol does not apply the familiar proof-of-stake penalty in which a portion of a validator’s stake is destroyed for equivocation. It does not eliminate operational risk: missed signatures, invalid behavior, software failures or an inability to meet protocol conditions can still affect rewards and access to locked tokens.
Security and liveness assumptions
Bitcoin anchoring strengthens the relationship between Stacks history and Bitcoin history, but it is not an assumption-free security guarantee. Stacks’ technical specifications identify specific failure scenarios:
- Bitcoin mining threat: the documentation says that control of 51% of malicious Bitcoin mining power could reorganize the Stacks chain or enable a double-spend against it. This is a published threat-model assumption, not a measurement that such an attack is occurring.
- Stacker liveness threshold: the documentation says the chain can halt if Stackers cannot reach 70% consensus on block validity. A halt protects against accepting blocks without sufficient agreement, but it interrupts normal transaction processing until the condition is resolved.
These assumptions explain why “secured by Bitcoin” should be read precisely. Bitcoin confirms the transactions used by PoX and anchors Stacks’ history, while Stacks still relies on its miners, Stackers, software and network communication. The design can make certain reversals harder without making every failure mode impossible.
PoX compared with other consensus designs
| Question | Stacks PoX | Bitcoin proof-of-work | Typical proof-of-stake system |
|---|---|---|---|
| What participants commit? | Miners transfer BTC; Stackers lock STX. | Miners expend electricity and hardware capacity. | Validators lock the system’s native stake or a designated asset. |
| How is a leader selected? | A Bitcoin-linked, VRF-based, bid-weighted sortition. | The miner that finds a valid proof first wins the block race. | A protocol-specific stake-weighted or randomized selection. |
| Where is execution? | On the separate Stacks layer, including Clarity contracts. | On the Bitcoin network, within Bitcoin’s transaction and script rules. | On that system’s own execution layer. |
| Who validates blocks? | Stacks nodes and Stackers; the selected miner proposes blocks. | Bitcoin nodes verify proof-of-work and protocol rules. | Validators and full nodes under that protocol’s rules. |
| How do rewards flow? | Miners may earn STX and fees; Stackers receive BTC rewards from miner commitments. | Bitcoin miners earn block subsidy and transaction fees. | Validators generally earn the native asset and fees, subject to that protocol’s rules. |
| Main stated assumptions | Bitcoin-mining security, sufficient Stacker agreement and correct Stacks operation. | Honest-majority mining assumptions and Bitcoin’s own network conditions. | Stake distribution, validator honesty, networking and protocol-specific liveness assumptions. |
These systems are not interchangeable. PoX is distinctive because the asset transferred by block producers is BTC while the chain they produce is Stacks, and because STX holders provide a separate signing function.
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How the mechanism is implemented and inspected
Clarity contracts
Stacks documentation identifies PoX as implemented through a Clarity smart contract and identifies pox-4 as the stacking contract. Contract names and deployments are versioned; use the currently deployed contract rather than assuming that an older PoX version remains active.
Node and API data
The Stacks API reference documents a /v2/pox endpoint for PoX information. Node data includes fields such as Bitcoin burn-chain height and PoX consensus information. These fields help an operator inspect the Bitcoin height that Stacks is using and the protocol’s current PoX state.
A Stacks documentation update dated February 26, 2026 announced additional endpoints for querying PoX-related Bitcoin transactions by block or address. The update establishes their availability in that release documentation; it does not by itself promise a particular uptime, rate limit or service-level guarantee. Check the live API reference before building production software around any endpoint.
Common misconceptions and practical checks
“Stacks miners mine Bitcoin.”
No. They submit BTC commitments through Bitcoin transactions, but they do not perform Bitcoin proof-of-work as part of producing a Stacks block. A participant may separately operate Bitcoin-mining equipment, but that is not required by PoX.
“Stacks transactions are Bitcoin transactions.”
No. Bitcoin records the commitment and anchoring events. Stacks nodes execute Stacks transactions and Clarity contracts, and the resulting state lives on Stacks.
“A larger BTC bid guarantees the next block.”
No. PoX uses bid weighting in a cryptographic selection process. A larger commitment can affect probability, but the result remains a protocol selection rather than a purchase of a guaranteed block slot.
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“Locked STX has been deposited with a custodian.”
No. The tokens remain in the holder’s wallet, although protocol-level locking prevents spending until the lock requirements are satisfied.
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“No slashing means nothing can go wrong.”
No. PoX does not use slashing, but missed signer duties can remove BTC rewards and interfere with unlocking. Operators should treat signing, monitoring and key management as active responsibilities.
“Bitcoin anchoring makes reorganizations impossible.”
No. The protocol documentation describes stronger Bitcoin-linked finality and more difficult reorganizations after Nakamoto, while its technical specification still identifies a 51% malicious Bitcoin-mining-power scenario that could reorg Stacks or double-spend. Security claims must be read with those assumptions attached.
What PoX means for different participants
For application users
You are using a Stacks execution layer, not directly invoking Bitcoin’s scripting environment. Transaction timing, fees, contract behavior and application state follow Stacks rules, even though Bitcoin events influence consensus.
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Mining requires Bitcoin-layer transactions, BTC capital, Stacks-node operation and software capable of constructing valid blocks during a tenure. Returns depend on selection, accepted blocks, STX issuance, fees, BTC commitments and operating costs; PoX is not a fixed-yield program.
For Stackers
Participation requires STX that can be locked for the relevant protocol period, reliable signer infrastructure and a clear understanding of reward and unlock conditions. The absence of slashing should not be treated as an absence of technical or opportunity cost.
For developers and operators
Use the current pox-4 deployment and API documentation, record the Bitcoin burn-chain height and PoX state your software relies on, and plan for endpoint or contract changes. Reward-cycle boundaries are measured in Bitcoin blocks, so software should not hard-code a calendar duration.
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