Overview
Kuber is a free developer toolkit that lets programmers create blockchain transactions by describing them in JSON, a plain text format for structured data. A developer lists who pays, who receives, and any tokens or smart contract scripts involved. Kuber then works out the fee, picks which coins to spend, and returns a transaction that only needs signing1. It suits teams that are building wallets, marketplaces, and other apps and would rather not assemble transaction details by hand.
Kuber builds transactions for Cardano and sits among the directory's transaction builders. It comes from Dquadrant, a software company that specializes in Haskell development and also provides smart contract auditing and quality testing2. Kuber ships in two forms: a Haskell code library and a ready-to-run server that other apps talk to over the web. The code is published under the MIT license, a permissive open-source license3.
Key Features
- Balanced transactions from a short description. Kuber reads a JSON request listing outputs, tokens, and scripts, then calculates fees, selects coins, and adds collateral, ADA-only inputs consumed only if a smart-contract script fails1.
- Answers about the network on request. Kuber's web API, a set of addresses other programs call, reports the coins held at an address, current network settings, and conversions between clock time and network time slots4.
- Cost estimates before sending. Separate endpoints calculate the minimum fee for a transaction and the computing budget its scripts need, known as execution units, before anything is sent4.
- One server for the main chain and Hydra. kuber-hydra adds commands to open, fund, close, and pay out a Hydra head, an off-chain state channel shared by a group of participants5, while still serving main-chain requests6.
- Drop-in replacement for standard submission. Kuber includes a submit endpoint compatible with Cardano's standard submit API, so tools already built around that service can send transactions through Kuber instead7.
What to Expect
Developers who want their own Kuber server start from the GitHub repository. A bundled docker-compose file starts Kuber alongside a Cardano node, the program that connects to the network. Official images are on GitHub's container registry. The first run takes a while, because the node has to catch up with the network before Kuber answers requests1.
JavaScript and TypeScript developers reach Kuber through kuber-client, a client library maintained by the software company Sireto8 in its kuber-client-js repository9. The client offers the same interface for Cardano's main chain and for Hydra1. For a working reference, the README points to Dquadrant's marketplace example1, a simple NFT buy-and-sell app built with Kuber10.
Anyone who wants to try Kuber without installing anything can use KuberIDE, a browser playground built by Sireto and Dquadrant. After connecting a wallet, you can compose transactions in JSON, convert address and hex data formats, and compile Plutus scripts, Cardano's smart contract code, without setting up tools locally11.
Builders exploring layer 2 scaling get a separate Kuber-Hydra guide. Its example runs three sample participants on a local test network and walks through opening a Hydra head, adding funds, sending a transfer, then closing and paying out12. The Hydra integration was funded through Project Catalyst Fund 13, Cardano's community grant program13.
