Developers Fabian Jahr and Dan Gould presented the PISA prototype on October 23, 2026, at the Plan B Week and Forum in Lugano, Switzerland. The project integrates Payjoin, as defined in BIP77 and BIP78, with Cross-Input Signature Aggregation (CISA) to reduce the size and costs of collaborative Bitcoin transactions.
CISA allows for the aggregation of multiple signatures across different inputs, which typically occupy significant space in a transaction. By combining this with Payjoin, the PISA prototype aims to maintain privacy while reducing transaction footprints. Estimates suggest that CISA could decrease transaction size and fees by approximately 7–26%.
The initiative is a collaborative effort involving Brink and the Payjoin Dev Kit, with support from Cypher Tank. The team has made the code and specifications publicly available on the project website. However, the underlying proposals, BIP 458 and BIP 459, remain in the research phase and have not yet been activated on the Bitcoin network.
For infrastructure teams and wallet developers, PISA serves as a reference point for evaluating potential protocol changes. The practical implementation of these fee savings depends on the future adoption of the draft proposals. Currently, the project functions as a research tool rather than a live feature for Bitcoin wallets.
