Common Prefix is formally verifying the $XRP Ledger Lending Protocol to mathematically prove that it cannot be drained, become insolvent, or break its rules.

The work focuses on the Lending Protocol introduced through XLS-66. Common Prefix explained its approach in a six-part series, including why it chose Lean 4 for the verification process.

Moving Beyond Conventional Testing for $XRP Ledger

Common Prefix said formal verification goes beyond normal software testing by using mathematics to prove that a system works correctly in all possible situations.

For the $XRP Ledger lending protocol, the goal is to prove that it cannot be drained, become insolvent, or break its defined rules.

Unlike standard testing, which checks selected scenarios, formal verification aims to prove that these properties always hold within the system.

$XRP Ledger validator Vet, also known as Hussein Zangana, said formal verification is already used in high-risk systems such as military technology, air traffic software, flight controls, and nuclear power plants.

He explained that the approach uses mathematics to show a system remains valid across all possible inputs, not just the situations developers have tested.

Formal Verification is used on the $XRP Ledger and particularly the Lending Protocol XLS-66.

It's also used in high assurance military systems, air traffic software, flight control systems, nuclear power plants and in all places where you want to sure the worst case is with a… https://t.co/aHODpwliX8

— Vet (@Vet_X0) September 17, 2026

Why Common Prefix Chose Lean 4

Common Prefix considered several tools, including Dafny, Lean 4, TLA+, and P. The team decided that TLA+ and P were not a good fit for the specific questions it needed to answer about the lending protocol.

One reason it chose Lean 4 was that it does not rely on an SMT solver. Common Prefix found that Dafny’s solver could sometimes time out when handling the complex arithmetic needed for the verification.

Lean requires more work by hand, but this also makes errors easier for developers to find and fix.

The team also said Lean is more flexible. While Dafny uses preconditions and postconditions to describe how a system should behave, Lean uses theorems, giving Common Prefix more flexibility for its verification work.

Cross-Checking the Real Implementation

Another reason Common Prefix chose Lean is that it works well with the C programming language. Lean can compile to C and connect with C-based software. This allows the team to compare its mathematical model with the actual lending protocol.

Instead of assuming the model and real system work the same way, the team can test the Lean model against the real implementation. Common Prefix said this is an important part of its verification process.

Lean’s Ecosystem

Common Prefix also chose Lean because it has active development, good tools, and a growing community. The team said Lean’s large community means many technical problems have likely already been solved by other developers.

Vet said using formal verification for the $XRP Ledger Lending Protocol (XLS-66) is important because it helps prevent serious failures.

The Lending Protocol supports lending on the $XRP Ledger, so verification helps ensure it follows its financial and operational rules under different conditions.

This is different from regular testing or a traditional audit. Instead, Common Prefix is using mathematical proofs to show that the protocol’s key properties hold under the conditions defined in its model.