PISA demo shows how Payjoin and CISA could shrink Bitcoin transactions

Bitcoin logo (public domain, Grayliptrot) via Wikimedia Commons

A prototype unveiled at Plan ₿ Week in Lugano pairs Payjoin privacy with cross-input signature aggregation to cut fees on collaborative payments

Privacy on Bitcoin has always come with a surcharge. A new prototype called PISA is trying to lower the bill.

Developers demonstrated PISA on October 23, 2026, during the Plan ₿ Week and Forum events in Lugano, Switzerland. The project combines Payjoin with Cross-Input Signature Aggregation, better known as CISA, to make collaborative Bitcoin transactions smaller and cheaper.

Start with the two ingredients. Payjoin, defined in BIP77 and BIP78, is a method for collaborative transactions with built-in privacy features.

Collaborative transactions pull in inputs from more than one source. That makes them harder for outside observers to read, which is the privacy benefit.

The catch is size. Every input in a Bitcoin transaction normally carries its own signature, and every signature takes up space.

CISA attacks that problem directly. It lets multiple signatures across different inputs be aggregated together, rather than each one sitting in the transaction separately.

PISA stitches these two ideas into a single working prototype. The goal is to keep Payjoin’s privacy while using CISA to trim the transaction’s footprint.

The savings are not trivial. CISA proposals could cut transaction size and fees by approximately 7–26%, depending on the type of transaction.

The biggest wins should land on multi-input payments. Those are exactly the transactions common in tools like CoinJoin and Payjoin.

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Fabian Jahr and Dan Gould are the primary developers on PISA. Both emphasized the potential for meaningful fee savings in cooperative transactions.

The project is a collaborative effort involving the Payjoin Dev Kit and Brink. Cypher Tank supported the Lugano demonstration.

The team did not keep the work behind a curtain. Developers showcased the code and specifications during the event and made them publicly available through a dedicated project website at pisa.payjoin.org.

Discussion on social media following the demo focused on user experience improvements.

Now for the fine print. The CISA proposals behind PISA remain in the research and specification phase.

The relevant draft Bitcoin Improvement Proposals, BIP 458 and BIP 459, have not been activated on the Bitcoin network. Until they are, the savings PISA demonstrates live in a prototype, not in your wallet.

Work on improving collaborative transactions has continued since the Taproot activation in 2021, with developers looking for ways to boost privacy while reducing the costs of using these newer transaction methods.

PISA is one of the more concrete outputs of that effort. It turns a set of specifications into something people can actually run and inspect.

For wallet developers and infrastructure teams, PISA offers a reference point. Public code and specifications give builders something tangible to evaluate ahead of any protocol change.

For investors, the near-term picture is more modest. PISA is protocol research, and its direct impact depends on whether BIP 458 and BIP 459 move from draft to activation.

Progress on BIP 458 and BIP 459, developer feedback on the published PISA code, and any wallet teams experimenting with the prototype will signal whether this demo becomes a building block or stays a conference highlight.