Tachys targets one second blocks for Cardano partner chains as Apex Fusion sets out a three-chain model for scale

Tachys is a new consensus protocol designed to reduce transaction latency and bring fast finality to Cardano partner chains. It is built to improve the user experience in environments where waiting for confirmation is not acceptable, while preserving the engineering discipline associated with the Ouroboros lineage.

Tachys is being built in collaboration with five organisations: Well-Typed, Ensurable Systems, PNSol, Apex Fusion Foundation, and Hal8.

The intention is open source delivery, with the ambition that Tachys can ultimately land in the Cardano node itself, available for broad use and contribution.

Filip Blagojević, Head of Ecosystem at Apex Fusion Foundation, put it simply. “Tachys is a consensus protocol, which just means it sets the rules on how blockchain works. How do we agree? What is the true blockchain.”

Tachys is positioned as an Ouroboros variant that uses a public slot leader schedule. The design makes a deliberate trade-off, aiming to prioritise speed of finality and a smoother user experience in the environments it targets.

Tachys is not intended for the Cardano mainnet. It is intended for partner chains, including application chains, where smaller or federated validator sets can make that trade-off a pragmatic choice.

Apex Fusion in full: Prime, Vector and Nexus

Among those partner chains is Apex Fusion, which operates a multi-chain environment with three blockchains: Prime, Vector, and Nexus.

Prime is the settlement layer and layer one. It is built for decentralisation, resilience and security, with geographically dispersed validators and a slower block rhythm designed to support robustness over speed. Prime is closely aligned to Cardano’s technology, carrying across lessons learned into a dedicated settlement chain.

Vector is the execution layer where application logic lives. Applications do not run on Prime. UTXO-based applications run on Vector, which is designed to feel responsive for both builders and end users. The aim is practical performance without undermining the integrity of the underlying settlement layer.

Nexus is the third chain in the ecosystem and supports the EVM side of Apex Fusion, providing an additional execution environment alongside Prime and Vector.

Where Tachys fits

Tachys is designed to push performance on the execution side towards one-second blocks and near instant finality, so that a transaction completes in the time it takes a user to move to the next action.

The architecture is intentional. Prime carries the weight of decentralised settlement. Vector carries the workload of application execution. Nexus extends reach to EVM developers. Tachys exists to close the latency gap that users feel when systems are secure but slow.

For developers and users, the discussion around performance often gets stuck on raw throughput. Tachys is framed around the practical experience of time.

Neil Davies of PNSol offered a builder-friendly analogy for block density and the waiting time it reduces. “Think of this as a transport schedule. You walk to the end of the street to catch the bus, but how long will you wait or how big is the bus. One part of that is the schedule, how long before the next bus arrives. The second part is how many people can get on that bus at one time. Is there going to be space on the next bus, or do you have to wait for a bus with space? Having a large bus only helps with the latter. If the schedule is not one you need you still have annoying waiting times.”

That distinction matters because it separates two different levers. One is how quickly blocks arrive. The other is how much each block can carry.

Tachys and Leios: a clean mental model

Tachys is not presented as an alternative to Ouroboros Leios. It is positioned as complementary, addressing a different constraint.

Philipp Kant from Ensurable Systems set out the right mental model. “Leios and Tachys are complementary in two ways. Leios aims at increasing the transaction throughput, how many transactions the system can handle per unit of time. Tachys on the other hand reduces the latency for individual transactions, how long it takes between submitting a transaction and its inclusion in a block.”

He tied that to user experience and system capacity. “Throughput determines how many users can use the system simultaneously without overwhelming it, while low latency ensures that an individual user can get their transactions in quickly, and gets a good user experience.”

He also drew a practical boundary between where they belong. “Leios is intended to run on Cardano mainnet, to improve the overall capacity of the system. Tachys is aiming for app- and partner chains that enable applications that need low latency in order to deliver a decent user experience.”

The decentralisation argument

Apex Fusion links its design choices back to decentralisation. Systems are simple when control is centralised. Decentralised systems are harder because decision-making, oversight and governance become part of the technical challenge.

The position is that decentralisation is not only about running nodes. It is also about who decides, how decisions are made, and how accountability is maintained, including the practical question of who watches the watchers. Governance remains an unsolved problem, and complexity grows as robustness and decentralisation increase.

What success unlocks

The long-term argument for Tachys is that it makes Cardano-based partner chains easier to launch, easier to tailor, and materially better on latency, throughput and cost, while remaining connected to Cardano mainnet as the value hub.

Kevin Hammond from Ensurable Systems framed the upside in ecosystem terms. “If Tachys succeeds, Cardano becomes the hub for achieving massive levels of scalability, capable of processing all the world’s needs for financial settlement, for example. Builders will be able to quickly spin up new Cardano-based partner chains with much higher throughput and faster finality, as well as greatly reduced per-transaction costs.”

He emphasised the link back to mainnet value. “These chains will link with Cardano mainnet, exchanging value and increasing TVL. Builders will be able to easily customise partner chains to meet the needs of their own applications. Tachys is not a one-size-fits-all solution.”

Blagojević stressed the motivation to contribute back to the open source stack. “We are happy to build on Cardano. We are happy to put in time, effort, money, to build on Cardano, because we as Apex Fusion Foundation use Cardano technology, which is open source, which is there to be used. We want to add to it. We want to continue building on it.”

About

Hal8 is one of the software development companies contributing to the wider Apex Fusion ecosystem. Structured as a for-profit company, HAL8 is delivering work such as network launch, integration, tooling and blockchain explorers.

Apex Fusion Foundation is a non-profit foundation, with no members or beneficiaries. Its formal governance structure includes executive leadership and a seven-member council and supports progressive decentralisation of the protocol and wider Apex Fusion infrastructure.