Now live on Rootstock Testnet, Union introduces a new model for Bitcoin interoperability based on verification rather than trust. By strengthening the security assumptions behind how BTC moves into programmable environments, Union reinforces Rootstock’s role as the Bitcoin-aligned smart contract platform. This first release is experimental and not production-ready, but it lays the foundation for a more open, verifiable, and trust-minimized system.
Most Bitcoin bridges today rely on trusted intermediaries, including federations, custodians, or multisig committees. While these models enabled early Bitcoin DeFi innovation, they also introduced counterparty risk and centralization. Such tradeoffs conflict with Bitcoin’s core ethos. As Bitcoin-native finance matures, the infrastructure securing BTC interoperability is becoming just as important as the applications built on top of it.
Today, Union Bridge enters its first public phase on Rootstock Testnet, introducing a new model for Bitcoin interoperability based on verification rather than trust.
Built on BitVMX, Union is designed to strengthen the security assumptions behind how BTC moves into programmable environments, helping Rootstock advance its role as the Bitcoin-aligned smart contract platform for DeFi, stablecoins, payments, and other onchain financial applications.
The Problem: Bridging Still Requires Trust
Bitcoin DeFi demand continues to grow, but access still largely depends on federated bridges, custodial wrapped assets, or emerging BitVM-based designs with governance layers. These approaches made Bitcoin interoperability possible, but they also introduced reliance on trusted operators and centralized coordination.
While newer BitVM-based bridge designs are beginning to emerge, many still depend on governance structures or committees to manage participation and security.
Union Bridge: From Trust to Verification
Union Bridge replaces trusted execution with cryptographic enforcement.
Built on BitVMX, it uses an optimistic model in which withdrawal transactions can be verified and challenged directly on Bitcoin. Instead of relying on a fixed group of signers, the system is designed around a dynamic participant model that can validate and challenge bridge operations.
Importantly, this expands what can be verified on Bitcoin without requiring changes to Bitcoin’s consensus rules.
The long-term vision is for Union to become the primary withdrawal verification layer within Rootstock’s bridge infrastructure, while Powpeg evolves into a complementary security mechanism for high-value transfers. This layered approach extends the defense-in-depth strategy that has guided Rootstock since its inception.
Union is also being developed in the open, with core components progressively open sourced to enable community review, testing, and contribution as the system evolves.
What Makes Union Different
Union introduces several structural advantages:
- Verifiable security: Disputes are resolved on Bitcoin through cryptographic proofs
- Credible decentralization: Reduced reliance on fixed signers over time
- Safe evolution: Designed to coexist with existing infrastructure like Powpeg during gradual rollout
- Future-proof design: Built to integrate advances in verifiable computation
- Open security: Developed transparently through open source and community review
- State-of-the-art architecture: Designed for collateral efficiency and censorship resistance, with low dispute costs and verification using standard Bitcoin transactions
Union on Testnet
Union Bridge is now available on Rootstock Testnet via Atlas, a unified interface for moving Bitcoin and other major assets into Rootstock. This is an early, experimental release focused on validating core bridge flows and usability.
What this phase includes
- A working, experimental MVP of Union Bridge
- Early peg-in and peg-out flows
- Open testing and feedback collection
Current limitations in V1.5
- Only basic flows (“happy path”) are available
- Dispute resolution and fraud-proof mechanisms are not yet implemented
- Limited reliability and monitoring
- No formal security audits yet
This phase is designed for testing and iteration, not production use.
What Comes Next
Union is rolling out progressively in phases:
- Testnet V2 (2026) will introduce dispute resolution and challenge-response mechanisms, enabling validation of the full security model.
- Mainnet (2027) will launch gradually with strict BTC transfer limits that increase over time as confidence in the system grows.
For Builders and Institutions
Union is designed for builders, protocols, and infrastructure providers who need more resilient BTC interoperability.
For institutions, the primary value is reducing reliance on counterparties and centralized bridge operators. This reduces operational dependency on centralized bridge operators and strengthens the long-term resilience of Bitcoin-based financial infrastructure.
Help Shape What Comes Next
Bitcoin bridging is evolving from trusted systems toward verifiable infrastructure. Union Bridge is an early implementation of that shift, and an early step toward a more open, resilient, and Bitcoin-aligned financial system.
This phase depends on participation. Test it, break it, and provide feedback. Union is being built in the open, and its evolution will be shaped by the ecosystem.
Access Union Bridge on Testnet via Atlas.
You can also review the current V1.5 limitations before submitting your feedback, or explore the open-source Union Bridge Client and Union Bridge Contracts repositories.
