Cardano Browser Node Experiments Advance in TypeScript and Haskell

Cardano developers have reported new results from two independent node experiments. Harmonic Labs expanded Gerolamo testing beyond an empty block, while Seungheon Oh demonstrated browser execution of the Haskell node stack through WebAssembly. Both remain experimental, and neither establishes a production block producer operating entirely inside a browser.

Gerolamo Produces a Second Cardano Preview Block

Gerolamo is an alternative Cardano node implementation written in TypeScript and running through the Bun runtime. Harmonic Labs is developing the project under founder Michele Nuzzi, with its source code publicly available.

The first block attributed to Gerolamo was accepted on Cardano Preview on September 12. It was empty and provided an initial test of whether the alternative implementation could produce a block accepted under Cardano’s protocol rules.

A second block followed on September 14 and contained three transactions. According to information published by the team, they included a smart contract execution and an on-chain message. The test therefore progressed from an empty block to one carrying transaction activity.

Gerolamo has also synchronized the Preview chain from genesis to the current tip while validating chain data. Its server implementation supports Cardano node to node protocols, optional local client communication, Mithril bootstrap and a subset of Blockfrost style API functions.

Available documentation does not describe full Praos leadership as a completed part of the project. Harmonic Labs has not published a complete technical account of the forging path used for the two Preview blocks. The network accepted the blocks and the team attributed them to Gerolamo, but public evidence does not establish that the block producer operated inside a web browser.

Cardano Gains Two Different Browser Node Approaches

Alongside Gerolamo, Harmonic Labs is developing Gerolamino, a lighter implementation intended for web browsers and a Chrome extension. Gerolamino is not the same software instance that produced the Preview blocks.

The public demonstration allows a browser to communicate with a Cardano relay through the Ouroboros Handshake, ChainSync and BlockFetch mini protocols. It can follow headers and blocks, process CBOR data and use IndexedDB and WebWorkers for local storage and computation.

A WebSocket to TCP bridge currently connects the browser with an external Preprod relay because web applications cannot open standard TCP connections directly to the Cardano network.

Planned development includes Mithril support for faster initial synchronization, local UTxO indexing and a communication interface resembling existing Cardano wallet standards. That model could allow web applications to verify more information on the user’s device instead of requesting every response from a remote API.

Separately from Harmonic Labs, developer Seungheon Oh demonstrated a Haskell Cardano node running inside a browser through the GHC WebAssembly backend. The demonstration included following the chain tip, displaying an ADA balance and submitting a transaction from the browser.

The Cardano Hard Fork Working Group recorded the demonstration on September 15 as a development related to node diversity. Oh has not released the source code, and the demonstration did not include block production. The prototype therefore cannot yet be independently reviewed or compared in detail with the production Haskell cardano-node software.

Local Verification Could Change Cardano dApp Infrastructure

Cardano wallets and decentralized applications frequently use remote services to access blockchain data, locate UTxOs, check balances and submit transactions. This infrastructure simplifies development but creates dependencies on API providers, indexers and their continued availability.

Browser node functionality could move part of that process onto the user’s device. An application could follow the Cardano chain locally, process blocks and verify the information it uses before preparing or submitting a transaction. Local access could also reduce the number of addresses and data requests sent to centralized API services.

For developers, this architecture could support Cardano applications that remain functional when an individual indexer or backend provider becomes unavailable. Multiple independently developed access paths can strengthen application resilience when they validate data according to the same Cardano protocol rules.

The approach may also allow wallets and decentralized applications to verify UTxOs and ADA balances closer to the user instead of treating a remote service as the only source of blockchain state. Gerolamo adds an independently developed TypeScript implementation, while Oh’s work explores how the established Haskell stack can operate through WebAssembly.

The current prototypes do not remove the need for external infrastructure. Gerolamino still uses a WebSocket bridge and an external relay, while broader local synchronization must account for browser storage, memory, bandwidth and indexing limits. Mithril snapshots, network peers and transaction submission routes also remain part of the architecture.

Publicly demonstrated progress now covers a TypeScript node that produced Preview blocks, a browser light node using Ouroboros mini protocols and a separate Haskell prototype capable of following the chain and submitting transactions. A production relay or block producer operating entirely inside a browser has not been documented. The immediate change is narrower: Cardano applications can begin moving part of blockchain verification from remote services onto the user’s device.