Overview

Rendezvous is a fuzzer for Clarity smart contracts that finds vulnerabilities through stateful, randomized testing, all in Clarity.

Smart contracts on Stacks are immutable. Bugs are forever. Test early. Test often. Fuzzing finds edge cases that unit tests often miss.

What is Fuzz Testing?

Fuzzing hits your code with random inputs. It helps uncover unexpected behavior and subtle bugs. Unlike unit tests, it explores paths you didn't think of.

What is Rendezvous?

Rendezvous (rv) is a Clarity fuzzer. It supports:

Property-Based Testing

You extract properties about your smart contract using Clarity. Rendezvous checks them multiple times with random inputs, in a stateful manner (the smart contract's state is not refreshed during the run).

What is a property?

A property is a universal truth about your smart contract's state, functions, etc.

How to extract a property?

Say that your smart contract has a function that reverses a list of uints. In this case, one property can be that "reversing a list twice returns the original list". You write the property directly inside your contract, marked as simnet-only code:

;; #[env(simnet)]
(define-private (test-reverse-list (seq (list 127 uint)))
  (begin
    (asserts!
      (is-eq seq
        (reverse-uint
          (reverse-uint seq)
        )
      )
      (err u999)
    )
    (ok true)
  )
)

Making your property valid for Rendezvous

For a property to be considered valid by Rendezvous, it has to comply with the following rules:

• Function lives in the contract file, annotated with ;; #[env(simnet)]
• Function name starts with test-
• Function is declared as private
• Test passes when it returns (ok true)
• Test would be discarded if it returned (ok false)
• Test fails if it returns an error or throws an exception

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Invariant Testing

You define read-only conditions in Clarity that must always hold true. Rendezvous attempts to create state transitions in your smart contract and continuously checks the conditions you defined to hold.

What is an invariant?

An invariant is a general truth regarding your smart contract's internal state. It will not be able to mutate the state, its role being solely to check the integrity of the state.

How to extract an invariant?

Say that you have a counter contract, having functions to increment and decrement. In this case, you could use the Rendezvous context to extract an invariant regarding your smart contract's internal state:

;; #[env(simnet)]
(define-read-only (invariant-counter-gt-zero)
  (let
    (
      (increment-num-calls
        (default-to u0 (get called (map-get? context "increment")))
      )
      (decrement-num-calls
        (default-to u0 (get called (map-get? context "decrement")))
      )
    )
    (if
      (<= increment-num-calls decrement-num-calls)
      true
      (> (var-get counter) u0)
    )
  )
)

Making your invariant valid for Rendezvous

For an invariant to be considered valid by Rendezvous, it has to comply with the following ruleset:

• Function lives in the contract file, annotated with ;; #[env(simnet)]
• Function name starts with invariant-
• Function is declared as read-only
• Function returns a boolean value (true if the invariant holds, false if violated)
• The test can use the special context map to access execution history

Invariant testing requires the context map and a update-context private function in your contract (both annotated with ;; #[env(simnet)]). Rendezvous uses them to track how many times each public function has been called. See the Rendezvous Reference for the exact definitions.

Why Test in Clarity?

1

Same constraints as production

Tests operate under the exact same constraints as production code.

2

Better understanding of Clarity

Writing tests in Clarity improves your familiarity with the language and its semantics.

3

No need to expose internals

You don't have to expose internal functions as public solely for testing.

4

Fewer tools to manage

Running tests in Clarity reduces the number of external tools and integrations you need to maintain.

Getting Started

Write your tests directly inside your contract, marked as simnet-only code with the ;; #[env(simnet)] annotation. Clarinet strips this code when deploying to real networks, so it never reaches mainnet — and Rendezvous picks it up automatically during testing.

my-project/
├── Clarinet.toml
├── contracts/
│   └── my-contract.clar       # Contract + its #[env(simnet)] tests
└── settings/
    └── Devnet.toml

Installation

To install Rendezvous as a dependency in your project, use npm:

npm install @stacks/rendezvous

This will add Rendezvous to your project's node_modules and update your package.json.

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Additional Resources

• [GitHub] Rendezvous repository
• [Rendezvous Book] Full documentation
• [Library API] Drive the testing loop yourself from TypeScript
• [YouTube @jofawole] How to Use Rendezvous to Fuzz Clarity Contracts